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FIG. 323. PLANO-CONVEX LENS WITH LEAST ABERRATION, §809

FIG. 322. PLANO-CONVEX LENS WITH GREATEST ABERRATION,

FIG. 320. CROSSING OF LIGHT RAYS WITH GREATEST ABERRATION,

FIG. 32.1A. LIGHT CONK WITH THE RADIANT ABOVE THE OPTIC Axis, §57

OPTIC PROJECTION

PRINCIPLES, INSTALLATION AND USE

OF THE

MAGIC LANTERN
PROJECTION MICROSCOPE
REFLECTING LANTERN
MOVING PICTURE MACHINE

FULLY ILLUSTRATED WITH PLATES AND WITH
OVER 400 TEXT-FIGURES

By SIMON HENRY GAGE

Professor of Histology and Embryology, Emeritus, Cornell University

HENRY PHELPS GAGE, Ph.D.

ITHACA, NEW YORK

COMSTOCK PUBLISHING COMPANY

1914

COMSTOCK PUBLISHING CO.,

ITHACA, N. Y.

JACOB GOULD SCHURMAN

IN GRATEFUL RECOGNITION OF HIS ABLE AND
DEVOTED SERVICE TO CORNELL UNIVERSITY, OF
HIS BREADTH OF SYMPATHY FOR ALL ART AND
ALL SCIENCE, AND OF THE ENCOURAGEMENT
AND SUPPORT WHICH HAVE MADE THE PRESENT
WORK POSSIBLE, THIS VOLUME IS DEDICATED.

PREFACE

OUR aim in the preparation of this work on Optic Projection
has been to explain the underlying principles on which the
art depends, and to give such simple and explicit directions
that any intelligent person can succeed in all the fields of projec-
tion; and our hope is that the book will serve to make more
general this graphic art by means of which many persons can be
appealed to at the same time and in the most striking manner.
Furthermore we believe that this art has great, undeveloped
possibilities for giving pleasure, arousing interest and kindling
enthusiasm, in that it provides for the rapid demonstration of
maps, diagrams and pictures of all kinds, the structure and develop-
ment of animals and plants, many of the actual phenomena of
physics and chemistry, and finally scenes from nature and from life,
even with their natural motions and colors.

The authors have received most generous aid from many indi-
viduals and many manufacturers; and most loyal service from
those who have helped to put the book in its present actual form.

Manufacturers have not only answered our numerous questions,
but have put at our disposal valuable apparatus for experiment.
They have also loaned us electrotypes of their apparatus.

We feel especially indebted to the Department of Physics of
Cornell University for the help given by different members of the
staff, and for the use of a research room and apparatus for the
numerous photometric and other determinations required. Pro-
fessor George S. Molcr of that department read over the manu-
script, and gave us many valuable hints derived from his experience
of over 40 years with all kinds of projection apparatus.

While we have both joined in the preparation of the entire
work, each holds himself especially responsible for certain chapters
as follows:

The senior author for 10 chapters (I-V, VII-X and XII).

The junior author for 5 chapters (VI, XI, XIII-XV).

SIMON HENRY GAGE,
October 4, 1914. HENRY PHELPS GAGE.

CONTENTS

Introduction pp. 3-7

CHAPTER I
Magic Lantern with Direct Current

Fig. 1-38; § 1-99, PP. 9-67

Apparatus and material for Ch. I, § i; Works of reference, § 2; Magic
Lanterns, § 3-19; Perfection and brilliancy of the screen image, § 20;
Suggestions for the lecturer or demonstrator, § 21-25; Suggestions to the
operator, § 26-41 ; Projection of horizontal objects with a vertical objec-
tive, § 42; Projection with multiple lanterns, § 43-46; Moving slides for
single lanterns, § 47-49; Stereoscopic screen images, § 50; Centering the
parts of the magic lantern, §51-54; Correct separation of the parts, § 55-
56; Optical test for centering, § 57-58; Centering the vertical objective,
§ 59-60; Troubles with the magic lantern, § 61-93; Breaking of conden-
ser lenses, § 94-97; Examples of American magic lanterns, § 99, fig. 32-
38; Summary of Ch. I, § 99,.

CHAPTER II
Magic Lantern with Alternating Current

Fig. 39-40; § 100-119 PP- 68-77

Apparatus and material required, § 100; Comparison of direct and alternat-
ing current, § 102-103; Installation with alternating current, § 104-113;
Use of the magic lantern with alternating current, § 114-115; Troubles
with alternating current lanterns, § 116-118; Summary of Ch. II, § 119.

CHAPTER III
Magic Lantern for Use on the House Electric Lighting System

Fig. 41-55; § 120-148 pp. 78-99

Apparatus and material, § 120; Magic lantern with small current for home
and laboratory, § 122-126; Arc lamps for the house circuit, § 127-131:
Turning the arc lamp on and off, § 132-135; Magic lantern with mazda
lamp, §136-139; Magic lanterns with Nernst lamp, § 140-146; Troubles
in Ch. Ill, § 147; Summary of Ch. Ill, § 148.

Magic Lantern with the Lime Light

Fig. 56-63; § 150-186 pp. 100-118

Apparatus and material, § 150; Lime light with oxygen and hydrogen, § 152-
158; Management of the lime light lantern, § 159-173; The lime light
with oxygen and illuminating gas, § 174-176; The lime light with oxygen
and ether vapors, § 177-179; Troubles with the lime light, § 180-185;
Summary of Chapter IV, § 186.

CHAPTER V

Magic Lantern with a Petroleum Lamp, with Gas, Acetylene, and Alcohol

Lamps

Fig. 64-73; § 190-224 pp. 119-137

Apparatus and material, § 190; Oil and gas lamps, § 192-195; Magic lantern
with kerosene lamp, § 196-202; Lantern with mantle gas lamps, § 203-
207; Lantern with acetylene lamp, § 208-213; Lantern with alcohol
lamp and mantle, § 214-219; Troubles in Ch. V, § 220-223; Summary
of Ch. V, § 224.

CHAPTER VI
Magic Lantern with Sunlight; Heliostats

Fig. 74-87; § 230-265 pp. 138-165

Apparatus and material for Ch. VI, § 230; Light from the sun, and
heliostats, § 232-233; Installation and use of hcliostats, § 234-238, 239-
248; Heliostats for the southern hemisphere, § 249-255; Condenser for
sunlight, § 256-258; Conduct of an exhibition with sunlight, § 259-262;
Troubles with sunlight, § 263-264; Summary of Ch. VI, § 265.

CHAPTER VII
Projection of Images of Opaque Objects

Fig. 88-1 ii ; § 270-297 pp. 166-199

Apparatus and material for Ch. VII, § 270; Images of opaque objects, § 272;
Comparison of the projection of opaque and transparent objects, § 273-
280; Combined projections, § 281-282; Opaque projection demonstra-
tions, § 283-292 ; Erect images with opaque projection, § 293-295 ; Troubles
with opaque projections, § 296; Summary of Ch. VII, § 297.

CONTENTS vn

CHAPTER VIII
Preparation of Lantern Slides

Fig, 112-120; § 310-340 pp. 200-220

Apparatus and material for Ch. VIII, § 310; Sizes of lantern slides, and
necessary condensers, § 312-314; Making lantern slides direct, § 315-324;
Photographic lantern slides, marking, mounting and coloring them, § 325-
337; Storing lantern slides, § 338; Troubles in making lantern slides,
§ 339; Summary of Ch. VIII, § 340.

CHAPTER IX
The Projection Microscope

Fig. 121-178; § 350-441 pp. 221-318

Apparatus and references, § 350-351; General on micro-projection, § 352-
354; Objectives, amplifiers, oculars, visibility of objects, diopter, § 355-
359; Room and screen, §360; Arc lamp and wiring, fine adjustment, con-
denser, water-cell stage, mechanical stage, § 361-369; Blackening appara-
tus, § 370-371 ; Hoods for objectives and shield for stray light, § 372-373;
Centering the projection microscope, § 374-376; Table of candle power,
§ 377! Use of the projection microscope, § 379-390; Magnification of
screen images, § 391-392; Projection with an ordinary microscope, § 393-
396; Projection of horizontal objects with a vertical microscope, § 397;
Sample objects for micro-projection, § 399; Conduct of an exhibition,
§ 400; Demonstration with high powers, § 401-411; Alternating
current for micro-projection, § 412-416; Micro-projection with the house-
current, § 417-418; Micro-projection with sunlight, § 419-421; Micro-
projection with lime light, § 422-423; Home-made projection apparatus,
§ 424-431; Combined micro-projection and lantern slide projection,
§ 432; Projection microscopes on the market, § 433-434; Troubles in
micro-projection, § 435-440; Summary of Ch. IX, § 441.

CHAPTER X
Drawing and Photography with Projection Apparatus

Fig. 179-220; § 450-549 pp. 3 ! 9-389

Apparatus and material, § 450; Drawing with projection apparatus, general,
§ 452; Room for drawing, § 453-455; Projection apparatus for draw-
ing. § 456-460; Light for drawing, § 461-463; Drawing with the magic
lantern, § 464-468; Drawing with the reflecting lantern, § 469-470;
Drawing with a photographic camera, § 471; Drawing with the projec-
tion microscope, § 472-485; Drawing with the house current, § 486-491;
Microscope to use with the house current, § 492-503; Avoidance of heat,

via OPTIC PROJECTION

§ 504; Stray light, § 505-506; Magnification of drawings, § 507-508;
509-510; Drawings for models, § 511; Erect images in the drawings,
§ 512-516; 517-526; Drawings for publication with projection appara-
tus, § 527-530; Drawings and their lettering, § 531; Photography with
projection apparatus, § 532-547; Troubles met in Ch. X, § 548; Sum
mary of Ch. X, § 549.

CHAPTER XI
Moving Pictures

Fig. 221-236; § 550-599 •• .. .pp. 390-438

Apparatus and material, § 550; Introduction, § 552; Auditorium, screen
and operating room, § 553-557; Current, lamps and moving picture
machine, § 558-574; Installation of a moving picture outfit, § 575;
Optics of moving picture projection, § 576-578; Magic lantern with the
moving picture machine, § 579; Management of the lamp, moving pic-
ture machine mechanism, § 580-589; flicker, §590-592; General precau-
tions, § 593; Splicing films, § 594; Winding and rewinding, § 595;
Danger of fire, § 596; Conduct of an exhibition, § 597; Home projectors
and advertising magic lanterns, § 598; Troubles with moving pictures,
§ 599; Summary of Ch. XI, § 5991.

CHAPTER XII
Projection Rooms and Screens

Fig. 237-251 ; § 600-642 pp. 439-473

Apparatus and material for Ch. XII, § 600; Suitable room for projection, and
its lighting, § 602-611; Position of the projection apparatus in the room,
§ 612-620; Screen for the image, § 621-628; 629-632; Size of screens and
screen images, § 633-640; Troubles with rooms and screens, § 641;
Summary of Ch. XII, § 642.

CHAPTER XIII

Electric Currents and their Measurement; Arc Lamps, Wiring and Control;
Candle-Power of Arc Lamps for Projection

Fig. 252-308; § 650-782 pp. 474-571

Apparatus and material for Ch. XIII, § 650; Electric currents, kinds and
comparison, § 652-653; Electric units, § 654-661; Electric measure-
ments and apparatus, § 662-671 ; 672-674; Power factor, cycle, frequency,
§ 675-677; Special dynamo for arc lamps, § 678-680; Current rectifiers,
§ 681-683; 135 and 25 cycle currents for projection, § 684-685; Wiring
for arc lamp from dynamo back to dynamo, § 686; Amperages for
different purposes, short circuit, ground, insulation of wires, § 687-690;

CONTENTS ix

Regulations for wiring, § 691-692; 693-700; Polarity tests for direct
current, § 701-703; Wiring the three-wire automatic lamp, § 704; Wiring
for alternating current, § 705-710; Switches, circuit breakers and fuses
§ 711-722; Resistors or rheostats, § 723-735; Reactors, inductors,
choke-coils, etc., § 736-738; Transformer, § 739; The electric arc, § 740-
743; The use of ballast (rheostats, etc.), § 744 — 748; The arc lamp, light
and heat from, § 749-752 ; Carbons and their position, § 753 ; Table show-
ing size and wear of carbons, § 753a; Candle power of arc lamps, § 754-
762; Candle power, and energy required, § 763-768; Distribution of light
intensity in different directions, § 769-771; Intrinsic brilliancy of the
crater, § 773; Visible and invisible radiation, § 774; Radiant efficiency
of arc lamps, §775-776; Energy required for moving picture projection,
§ 779-78i ; Effect of opacity in the film, § 782.

CHAPTER XIV
Optics of Projection

Fig- 309-349; § 790-865 pp. 572-620

Reflection and refraction, § 792-801; Lenses, § 802-808; Spherical and
chromatic aberration in lenses, § 809-810; Image formation with the
magic lantern, § 811-817; Focus of Condenser and objective, § 818;
Types of condensers, § 819-821; Image formation with moving pictures,
§ 822-832; Image formation with the projection microscope, § 833-838;
Light losses, § 839-843; Energy losses, § 844-854; Effect of aperture,
§ 855-856; Brightness of the screen image, § 857; Microscopic image
and aperture, § 858-863; Koehler method of illumination, § 864-865.

CHAPTER XV
Uses of Projection in Physics; Normal and Defective Vision

Fig. 350-402; § 875-932 pp. 621-672

Apparatus and material for Ch. XV, § 875; Introduction, § 877-878; Experi-
ments with polarized light, § 879-884; Projection of spectra, § 885-900;
Absorption spectra, § 901-902; Emission spectra, § 903-905; Ultra-
violet light, photography, § 906-908; Abbe diffraction theory, § 909-911;
Dark ground illumination, striae, § 912-915; Normal vision and eye
defects, § 916-932.

Appendix. Brief Historical Statement on the Origin and Develop-
ment of Projection Apparatus 673

Projection Apparatus and Accessories in the Open Market; Manu-
facturers 688

Bibliography on Projectior 693

Index of names and subjects 705

INTRODUCTION

IN THE following pages, Projection Apparatus of various forms
and with various sources of light have been considered from
a three-fold standpoint:

(1) The standpoint of the actual user of the apparatus.

(2) The standpoint of the manufacturer.

(3 ) The standpoint of the student for whom an understanding

of the principles involved is of fundamental importance.

From the first and second standpoints simple "rule of thumb"
would answer, and in many cases has answered to bring about
fairly good results. For example, the toy magic lanterns so much
in evidence at Christmas time, are almost exact copies of the first
magic lantern shown by Walgenstein in 1665. The only striking
difference is that instead of a candle or lamp without a chimney
such as he used, there is now a small petroleum lamp with a glass
chimney.

But for adapting projection apparatus to new conditions and
applying it to new uses with the greatest efficiency, the user and
the manufacturer must comprehend the fundamental optical and
mechanical principles involved. In a word, to make good projec-
tion apparatus and to produce good projection in the different
fields, the manufacturer and the user must know the principles,
and then they must build and must use the apparatus in accordance
with those principles.

Besides the optical and mechanical principles involved in the
apparatus, it seems to the authors that the physiology of vision
should have prime consideration, because, after all, it is not only
the possibility of producing a brilliant screen image that must be
thought of, but also the possibility that the observer get a satis-
factory impression of that image. With the magic lantern and
arc light it is very easy to get screen images as brilliant as daylight
scenes in nature. These brilliant images are best seen when the
eyes of the observers are adapted to daylight vision. If now, as is

4 INTRODUCTION

possible with modern combined apparatus, the brilliant screen
image of the transparency is replaced by a relatively dim image
projected by the opaque lantern, it will appear exceedingly dim
until the eyes can be adjusted to twilight vision. If the operation
is reversed after the eyes are adapted to dim light, the brilliant
screen image of the transparency will dazzle the eyes.

It is then, not only the dead machine that must be considered,
but also the living machine — the eye. It is for the eye that all the
work is done, and perfection can be gained only by understanding
the workings of the two machines, and adapting the dead machine
to the physiologic laws governing the living machine.

Our aim in writing this book then has been to show how good
results can be most easily and certainly obtained in all the forms of
projection by obeying the laws of physiology as well as those of
optics and mechanics.

Naturally, most users of projection apparatus will employ that
which is regularly manufactured, but in many institutions not all
of the desired apparatus can be afforded. Furthermore, every one
who is to do any special work in projection must be capable ot
combining and adapting apparatus for those special needs. Hence,
we have indicated how home-made apparatus can be got up, and
how apparatus designed for one purpose can be utilized for other
purposes. We have done this for two reasons, first, because we
feel sure that a great gain in efficiency can be made in teaching by
the use of the magic lantern, the projection microscope and other
forms of projection apparatus, and secondly, because the con-
struction or adaptation of projection apparatus gives one an
intimate and working knowledge which more than pays for all the
time and trouble.

In examining the apparatus of many different makers we have
been impressed with the general excellence of the apparatus and
also with certain general defects.

The defects seem to us almost wholly due to the fact that the
manufacturers of apparatus and the users of the same are not
intimately enough associated, and, therefore, are not so mutually
helpful as seems desirable.

INTRODUCTION 5

The manufacturer naturally advertises the possibilities of his
apparatus as if he expected it to be used under the most favorable
conditions, and operated by men skilled in the use of optical instru-
ments, and the results to be judged by persons of experience who
do not expect the impossible.

For example, if one reads the statements concerning the projec-
tion of pictures in books, photographs, postal cards and actual
objects, the impression would be very strong that the screen pic-
tures so produced were every bit as satisfactory as those of lantern
slides, and just as easily produced. In speaking with many
individuals we have found the belief is very general that with the
new apparatus nothing is simpler than to get good screen images
of objects, pictures, etc., with all their natural colors, and that the
expense of lantern slides can be wholly done away with. But we
have yet to find the actual user of such apparatus who found his
sanguine expectations fully realized.

Modern opaque projection is marvelous in its accomplishments,
but what is gained in the use of actual objects, books, etc., is lost in
the relative dimness of the screen image, in the expense and diffi-
culty of managing the apparatus, and in the large electric current
needed to give even tolerable screen images.

Judging from our observations the manufacturers have not fully
realized the lack of optical and mechanical knowledge and instinct
in many users of projection apparatus. Naturally, the user of the
apparatus wants results, and he wants the apparatus to give the
results without trouble.

Perhaps the most striking, as also it seems to us the most easily
obviated defect, is, that with many parts of the apparatus, it is just
as possible to insert them in the wrong position as in the right
position. For example, in most of the apparatus we have examined
the condenser is so mounted that it can be put with either end
facing the arc lamp. So with many other parts, they can be put in
a wrong position just as easily as in a right position.

In our opinion there are five fundamental rules in the production
of projection apparatus that the manufacturers should follow:

6 INTRODUCTION

1. The optical parts should be arranged on one longitudinal

axis and fixed in that position, except that the projection
objective must be movable along the axis for focusing.

2. The radiant or source of light should be adjustable in every

direction to insure proper centering of the light along the
optic axis, and to insure the proper relative position of the
source of light and the condenser.

3 . The object carrier for lantern slides is preferably fixed in one

position; but the stage of the microscope and the object
holder of most other kinds of apparatus should be movable
along the longitudinal axis so that the object can be put in
the cone of light where it will be fully and most brilliantly
lighted.

4. The parts should be constructed so that either (a) it makes

no difference how they arc placed or (b) so that they can-
not be put together wrong. (See footnotes to 4-5, p. 7).

5. Every part of the apparatus should be dull black to avoid

reflections.

Of course for experimental apparatus the more adjustable each
part is the greater are its possibilities, but for apparatus to use for
definite purposes we believe that no unnecessary adjustments
should be possible.

The custom followed by many manufacturers of sending an
illustrated pamphlet giving instructions for installing and using
their apparatus, is wholly commendable. In addition it would be
advisable in some cases to attach tags to the different parts, stating
their purpose and connections (fig. 45).

All of the apparatus and all of the experiments discussed in this
book have been personally tested or observed by us to make sure
that they will work; and we have tried to give directions and
methods which arc intelligible, and which will most easily produce
the desired results.

Finally, the authors of this book most earnestly advise any one
who is to use projection apparatus to go to some place where the
facilities arc abundant, and where there is someone skillful in
using them. This will give him a standard of what can be accom-

INTRODUCTION 7

plished and what can reasonably be expected. The learner will
find that in such a place the apparatus, the room, the screen and
the light are all adapted to the purpose to be served.

Good projection, like any other skilled operation, requires
knowledge, facilities and experience.

There is a very trenchant expression used in shops and in laboratories which
seems to us to cover the ground. It is: "Fool Proof."

From the testimony of many who are especially skilled in machinery and in
the use of apparatus, and from our own personal experience, the "fool proof"
construction of apparatus is not only necessary for the careless and unskilled,
but much appreciated by the most skilled and careful. When one is absorbed
in the principles and complexities which some experiment is meant to elucidate,
it is a great advantage to have the apparatus which is to be used so constructed
that it will go together in the right way with the least conscious effort on the
part of the user. The user ought not to be compelled to make a special study
of the apparatus every time it is assembled. It is the business of the manu-
facturer to put thought into the construction of the apparatus, and it is the
user's business to work out problems with it.

From time immemorial it has been the habit of mankind to make tools,
implements and more elaborate apparatus with smooth and glistening surfaces,
bright colors often being added to heighten the effect. The microscope and
other optical apparatus naturally followed the fashion.

While to many workers in optics there early came the fundamental apprecia-
tion that the clearest images were possible only when absolutely no light
reached the eye except from the image field, still polished brass and nickel
finish persisted, and the dazzling reflections when bright lights were used, often
overwhelmed the image which it was the sole purpose of the apparatus to make
visible.

During the last few years the knowledge of the best conditions for clear
images has asserted itself more and more, and the mirror surfaces of optical
apparatus have gradually disappeared. At first the dull black apparatus was
prepared only for the few who could demand and pay for a special finish. The
advantage of the dull finish of optical apparatus is so apparent when once seen
and used that now it is becoming very common.

The great advantage of such dull black, non-reflecting surfaces for the out-
side as well as for the inside of optical instruments became apparent to the
senior author by the accident of a laboratory fire (1900) during which the
lacquer of his best microscope was blackened by the dense smoke.

The ordinary point of view ten to fifteen years ago that optical apparatus
should of course have a 'bright brass or nickel finish is well illustrated by this
incident: The senior author was having, by special contract, a microscope
with all its accessories made dull black. A visitor, interested in optical goods,
going through the factory noticed this lone, black microscope among the
brilliant array and asked: "When are you going to bury that one?"

CHAPTER I

THE MAGIC LANTERN WITH DIRECT CURRENT ARC
LAMP AND ITS USE.

§ 1. Apparatus and Material for Chapter I:

Suitable projection room with screen (Ch. XII) ; Magic lantern
(§ 3~I9) I Arc lamp, automatic or hand-feed, with fine adjustments,
lamp-house and wiring for current up to 25 amperes (fig. 3);
Cored carbons adapted to the current (§ 7 53 a) ; Rheostat; Lantern
table (Ch. XII); Double-pole, knife switch (§ 8); Ammeter (§ 7);
Incandescent lamp or flashlight (§ 14-15); Gloves with asbestos
patches (§ 27); Lantern slides; Opera-glasses (§ 38); Testing
incandescent lamp (§ 6 1 , fig, 21); Fuses ; Extra condenser lenses to
replace cracked ones (§ 94) ; Screw driver and pliers.

§ 2. Historical Summary and Works of Reference:

For a historical summary of the invention and use of the Magic
Lantern, see the Appendix.

The reader will find many good hints in the following works on
Projection. For the full titles, see the Bibliography.

R. C. Bayley. — Modern Magic Lanterns and their Management.

H. Fourtier. — La Pratique des Projections.

Hassack and Rosenberg. — Die Projektionsapparate.

T. C. Hepworth. — The Book of the Lantern.

R. Neuhauss. — Lehrbuch der Projektion.

C. G. Norton. — The Lantern and How to Use It.

F. P. Wimmer. — Praxis der Makro — und Mikro-Projektion.

Lewis Wright. — Optical Projection.

The latest information and many useful hints may be found in
the catalogues of the manufacturers (see Appendix).

MAGIC LANTERN

§ 3. The Magic Lantern as the standard for projection appara-
tus.— The magic lantern may be taken as the standard example of
projection apparatus, for it is in the most common use and is the
simplest instrument for image projection.

MAGIC LANTERN WITH DIRECT CURRENT [Cfl. I

Condenser

Arc La

FIG. i. SIMPLEST FORM OF MODERN MAGIC LANTERN WITH ARC

LAMP

It consists of an arc lamp with suitable connections to the current supply,
a rheostat and a table switch; a double condenser, lantern-slide holder and
projection objective.

Arc Lamp This is a mechanism for holding and feeding the carbons.

h c Horizontal (upper) and

v c Vertical (lower) carbons.

5 5 Set screws for holding the carbons in place.

In In Insulation between the carbon holders, and the rest of the lamp to
prevent a short circuit.

/ 5 Feeding mechanism for moving the carbons.

d Clamp for fixing the lamp in any position on its vertical support.

SW Supply wires to the lamp socket or wall receptacle.

So Lamp socket.

K Key of the socket switch.

5 — P Separable attachment plug.

L W Supply wires from the cap of the attachment plug to the table switch.

K S Double-pole knife switch on the table for turning the current off and
on the arc lamp.

Rheostat for controlling the current. It is inserted in one wire.

Condenser In this simple form it is composed of two plano-convex lenses
with the convexities facing each other.

I 2 The two elements of the condenser.

L S Lantern slide close to the plane face of the 2d condenser lens.

Axis Axis The straight line passing from the source of light along the
optic axis of the condenser and the objective to the image screen.

Objective The projection objective for giving a clear image of the lantern
slide on the screen.

c The center of the objective where the rays from the condenser should
cross.

Image Screen The white screen upon which the image of the lantern slide
is projected by the objective.

If the principles governing the magic lantern are mastered, and
one gains skill in handling it, the more difficult forms of projection
will offer no great obstacles.

CH. I] MAGIC LANTERN WITH DIRECT CURRENT 1 1

§ 4. Standard source of light. — With all forms of present day
projection the direct current arc light is taken as the standard
because, next to the sun, it is the most perfect light source available.
In many places it is to be had during the entire twenty-four hours,
and is the safest and most easily managed light capable of furnish-
ing sufficient illumination for use with all kinds of apparatus, from
the simplest magic lantern to the moving picture machine and the
compound microscope.

MAGIC LANTERN WITH DIRECT CURRENT ARC LIGHT

Except the projection table, the room and screen, (for which see
§ 424 and Ch. XII,) the essential elements of a magic lantern and
their arrangement are shown in fig. i, 2, 3. They are as follows:

§ 5. Wires for the electric current. — There must be two wires
for carrying the current extending from the main line to the electric
lamp. One wire, the positive ( + ), conveys the current to the
upper carbon of the lamp, and the other, the negative ( — ), conveys
the current from the lower carbon back to the main line (fig. 1,2)
(see also Ch. XIII).

§ 6. Rheostat. — This device must be placed in the circuit along
either the positive or the negative wire, it makes no difference
which. In figures i and 2 it is placed in the negative wire.
It serves as a balance, and [limits the amount of current pas-
sing through the lamp (§ 744-748).

§ 7. Ammeter. — This indicates the amount of current flowing.
It is not necessary, like the rheostat, but is very desirable, for with
the information it gives, the operator can determine whether any
defects in the brightness of the screen image are due to the lack of
current, or whether something else is at fault (see Troubles.
§ 61-95.)

The ammeter is placed along one wire the same as the rheostat
(fig. i, 2).

In case no ammeter is used the rheostat can be calibrated and
marked when the apparatus is installed (see § 729).

12 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I

§ 8. Double-pole switch. — It is important to have a double-pole
switch near the lamp. By its means the operator can at any time
turn the current on or off the lamp. When the switch is open no
current can reach the lamp (fig. 1-3).

§ 9. Arc Lamp; automatic type. — The lamp is needed to hold
the carbons, and to provide a mechanism for moving them toward
each other as they burn away (see §12). The lamp may be of the
automatic type in which there is a magnetic release or motor for the
mechanism, so that the carbons are brought nearer together when-
ever the arc gets too long. If it is properly designed and con-
structed, the lamp will burn continuously as long as the switch is
closed, and the carbons last. There should also be a hand-feed
mechanism in these arc lamps, so that slight modifications may be
made by hand when necessary ; furthermore, there must be arrange-
ments for moving one or both carbons separately to correct any
irregularity in the wasting away of the carbons.

§ 10. Fine adjustments. — There must be adjusting screws by
means of which the lamp can be slightly raised or lowered, or moved
to the right or to the left, to enable the operator to keep the crater
of the positive carbon exactly in the axis. This is to compensate
for the slight change in position of the crater as the carbons burn
away (fig. 3).

§ 11. Arc lamp, hand-feed type. — In this form of arc lamp the
operator must work the mechanism by hand. The carbons usually
have to be moved nearer together every four or five minutes. As
with the automatic type, one or both carbons should be movable
independently, and there should be fine adjustments (§ 9, 10).

§ 12. Carbon Terminals. As a light source for projection,
carbon terminals or electrodes are used in the arc lamp.. With a
direct current the carbons burn away unequally, the upper, positive
carbon, wasting about twice as fast as the lower, negative carbon.
If the carbons are of equal size and quality, the mechanism of the
lamp must move the upper carbon about twice as fast as the lower
one. Some times a lamp with equal motion for the upper and lower

CH. II

MAGIC LATERN WITH DIRECT CURRENT

Condenser

H C

FIG. 2. MAGIC LANTERN WITH TRIPLE CONDENSER AND
WATER-CELL.

H C, V C Horizontal or upper and vertical or lower carbon of an arc lamp.
The upper carbon furnishes the light.

D + C Supply wires for the electric current. The positive wire (+) goes
to the upper carbon (H C), and the negative wire ( — ) comes from the lower
carbon (V C). The arrows indicate the direction of the electric current.

F Fuses where the supply wires for the lamp connect with the main line.

L Incandescent lamp with wire guard. It is connected with the supply
wires before the table switch (S) and the resistor (R), hence it can be used
while the arc lamp is running or when it is turned off (See also fig. 4).

S Double-pole, knife switch for turning the current on or off the arc lamp.

R Rheostat for controlling the current. It is inserted in one wire.

A Ammeter to indicate the amount of current being used. It is inserted
in one wire.

Condenser This consists of a meniscus next the arc light, and of two plano-
convex lenses with a water-cell between them. The lenses must be arranged
as here indicated.

W Water-cell placed between the plano-convex lenses of the condenser.
It absorbs much of the radiant heat.

L S Lantern slide close to the condenser.

Axis Axis The straight line passing from the source of light along the
optic axis of the condenser and the objective to the screen.

Objective Projection objective serving to give a clear image of the lantern
slide on the screen.

C Center of the objective where the rays from the condenser should cross.

Screen Image The image of the lantern slide formed by the objective on
the white screen.

carbons is used and the upper carbon is enough larger than the
lower one, so that the two shorten at the same rate.

In our experience it is more satisfactory to have both carbons
with soft cores, but some advocate and use a large soft-cored carbon
above and a smaller solid carbon below (fig. 299).

U MAGIC LANTERN WITH DIRECT CURRENT [Cn. I

§ 13. Lamp-house. — This is a metal box in which the arc lamp
is enclosed. It should be of good size, and be well ventilated by
means of openings at the bottom, and a flue at the top. There
should be one or more large doors, so that the lamp can be reached
for changing the carbons and making any necessary adjustments.
Opposite the crater at the end of the positive carbon there should
be a window about 2 to 3 cm. (2 in.) square so that the ends of the
carbons can be observed when the lamp is burning without opening
the door. This window should be provided with a combination of
red and green, or red and blue glass, or with smoky mica or with
deeply tinted glass so that the eyes will not be injured when look-
ing at the crater (fig. 133, 147).

§ 14. Incandescent lamp. — If experiments are to be made it is
desirable to have an incandescent lamp with wire guard to use in
connection with the lantern. It should have a flexible cord of
sufficient length so that it can be carried to any desired position.
This lamp must be connected with the supply wires before the
rheostat is inserted ; then it will burn brightly while the arc lamp
is going. By consulting fig. 2, it will be seen that the two wires for
this lamp are connected one with each of the supply wires. That
is the incandescent lamp is not connected with one wire like the
rheostat and the ammeter but with both wires.

§ 15. Electric flash-light. — An electric flash-light is a great
convenience about a lantern; and is almost a necessity when an
incandescent light (fig. 1,2) is absent. It should lock, so that it
will burn continuously ; then carbons may be changed by its light
and other corrections made. It is an absolutely safe light also.

§ 16. Incandescent lamp to burn when the arc lamp is turned
off. — To avoid the great darkness in the room when the arc lamp is
turned out, it is advantageous to have an incandescent lamp con-
nected with the line, as indicated in fig. 4.

§ 17. Condenser. --This collects the light from the arc lamp
and directs it through the objective. In passing from the con-
denser to the objective it passes through the lantern slide or other
object whose image is to be projected (fig. 1,2 4).

CH. I]

MAGIC LANTERN WITH DIRECT CURRENT

FIG. 3. ARC LAMP FOR PROJECTION, WITH WIRING, SWITCHES
AND FUSES

Supply Wires The conductors from the supply to the outlet box.

Outlet Box. An iron box receiving the supply wires at one end and giving
exit to them from the other.

Fuses & Switch Two cartridge fuses in the circuit and a double-pole knife
switch beyond the fuses. The fuses are present to avoid accident in case of a
short circuit and the switch to turn the current on or off as desired.

P W R Polarized wall receptacle. This is composed of two parts as
shown, the part on the wall to receive the supply wires from the outlet box,
and the cap to connect with the table switch. The metal connections of the
cap with the receptacle arc in planes at right angles so that the cap can be put
in place only in one way, hence the polarity is always the same.

Arc Supply The wires connecting the cap of the wall receptacle and the
table switch.

Switch The double-pole, knife switch for turning the current on and off the
arc lamp.

Wi The wire extending from the switch to the upper carbon of the arc
lamp.

W2 The wire extending from the switch to the rheostat.

If j The wire extending from the rheostat to the lower carbon of the arc
lamp.

1 6 MAGIC LANTERN WITH DIRECT CURRENT [CH. I

Rheostat This is for controlling the current. It is inserted in one -wire.

Arc Lamp The mechanism for holding and feeding the carbons.

F S Feeding screws for moving the carbons closer together or farther
apart. The carbons can be moved separately or both at once.

V A Fine adjustment screw for moving the carbons up or down.

L A Fine adjustment screw for moving the carbons to the right or left.
in in Insulation between the carbon holders and the rest of the lamp. This
is to prevent the current from leaving the carbons and making a short circuit
through the metal part of the lamp.

.s ^ Set screws for holding the carbons in place.

Lamp-House The metal box enclosing the arc lamp. The feeding screws
(F S) and the line adjustments (V A, L A) should project through the wall of
the lamp-house.

Condenser A condenser composed of three lenses with a water-cell in the
parallel beam between the plano-convex lenses.

/ The first element of the triple condenser is composed of a meniscus lens
next the arc lamp, and a plano-convex lens next the water-cell.

2 The second element of this condenser is a plano-convex lens. The con-
vex surfaces of the plano-convex lenses face each other as in the double con-
denser (fig. i).

Block i. The block supporting the arc lamp. It is movable back and forth
along the track on the base-board. The socket and set screws permit the
adjustment of the lamp.

Block 2. The block holding the condenser. It is movable along the track
cm the base-board. The socket and set screw (S) enable one to adjust the
position of the condenser.

Base Board The board on which all the parts of the projection apparatus
rest (see fig. 158-159).

The condenser is of two or of three lenses. If of three lenses the
first lens, which is nearest the arc lamp, is usually of meniscus form,
with the concavity next the lamp. The second lens is a plano-
convex, as is also the third (fig. 2). If the condenser is of two
lenses both are usually plano-convex with the convex surfaces fac-
ing each other and the plane faces looking toward the radiant and
toward the lantern slide (fig. i).

The two condensers appear alike in form and relation of the
lenses except that in the three-lens type a meniscus has been added.

In the three-lens type the meniscus and first plano-convex
together render the diverging light from the lamp parallel, and the
third lens or second element renders this parallel beam converging,
bringing it to a focus at the center of the projection objective when
the condenser and objective are properly proportioned to each
other (fig. 1-2).

AVith the two-lens condenser the usual practice is to bring the
condenser closer to the lam]) than the focal length of the first lens.

CH. I] MAGIC LANTERN WITH DIRECT CURRENT

FIG. 4. MAGIC LANTERN WITH INCANDESCENT LAMP IN THE CIRCUIT
AFTER THE RHEOSTAT (Compare fig. 2).

W W Supply wires.

F Fuses in the supply wires (see fig. 3).

R R Rheostat for controlling the current.

A Ammeter for indicating the amount of current.

p p The two binding posts of the knife switch. The two wires of the
incandescent lamp are connected at these points.

b s The incandescent bulb and the key switch of the lamp socket. From
the connections of the supply wires to the incandescent lamp it will shine
whenever the socket key is closed whether the knife switch to the arc lamp is
opened or closed. When the arc lamp is burning the incandescent lamp will
be very dim and when the arc lamp is out it will shine with full brilliance.

S The table, knife switch.

L The source of light.

The +'s, — 's and arrows indicate the polarity and course of the electric
current.

Condenser A two-lens condenser with water-cell (W).

L S Lantern slide.

Axis The principal optic axis of the condenser and of the objective.

Objective The objective for projecting an image of the lantern slide upon a
screen.

Screen Image. The image projected on the screen by the objective.

This gives a somewhat diverging beam between the two lenses.
The second lens brings this diverging beam to a focus beyond its
own principal focus.

This condenser is sometimes placed so that the crater of the arc
lamp is at the principal focus of the first lens and the center of the
projection objective at the focus of the second lens, as in fig. 2.

Whatever the form of the condenser, the lenses must be so
mounted that there is freedom for expansion; and they must be
so arranged that the proper lens is next the radiant (see fig. 2, 3,
36 B).

1 8 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I

§ 18. Water-cell. This is a vessel of water with parallel, glass
sides, placed in the beam of light from the lamp, before the light
reaches the lantern slide or other object. The water-cell absorbs
most of the radiant heat from the lamp and thus protects the
objects from over-heating (fig. 2-3).

The water-cell is especially needed for opaque lantern slides like
those of dark scenes or colored slides made by the Autochrome
process. It sometimes happens that in an exhibition as many as
10 to 30 per cent, of the slides are cracked by the heat, if no water-
cell is used.

Unfortunately the water-cell is oftener absent than present in
magic lanterns. (For a further discussion of the avoidance of heat
see § 364, § 854).

§ 19. Projection objective. — This forms an image of the lan-
tern slide upon the screen. If the instrument is in proper adjust-
ment the objective will transmit to the screen the rays of light from
the condenser which pass through the lantern slide or other semi-
transparent object. These rays reflected from the screen to the
eye give rise to a picture with all the gradations of light and shade
and color of the lantern slide or other object used (see fig. 1,2, and
§811).

PERFECTION AND BRILLIANCY OF THE SCREEN IMAGE

§ 20. The quality of the screen image depends upon :

1 . The accurate centering along one axis of the source of light,

the condenser, and the projection objective (fig. 1,2).

2. The amount and intensity of the light used.

3. The excellence of the condenser.

4. The aperture and perfection of the objective.

5. The proper proportion of the objective and the condenser to

each other and to the size of the room. (See fig. i, 2,
§ 634-636).

6. The perfection and transparency of the lantern slides or

other objects imaged on the screen.

7. The accuracy of the focus of the image on the screen.

8. The reflecting qualities of the screen (sec § 621).

CH. I] MAGIC LANTERN WITH DIRECT CURRENT 19

9. The darkness of the projection room (see §608).
10. The proper adjustment of the eyes of the spectators to
either daylight or twilight vision (§ 281).

USE OF A MAGIC LANTERN FOR EXHIBITIONS AND FOR
DEMONSTRATIONS

SUGGESTIONS TO THE LECTURER OR DEMONSTRATOR!

§ 21. Order of the lantern slides. — The lecturer or demon-
strator should have his slides in the exact order in which they are
to be shown. They should not only be in the exact order of exhibi
tion, but they should all be in the same relative position so that
the operator can insert them correctly without the trouble of
looking at them individually.

§ 22. Duplication of lantern slides. — It frequently happens
that the same slide, for example, of a map or some other general
subject, should be shown at two or more stages of a lecture. There
is always difficulty in doing this unless the operator is carefully
instructed, and the slide is marked to be repeated, and a slip of
paper inserted in the pile of slides at the proper level. With a
small audience, and for an informal talk the difficulty is, perhaps,
not great; but for a large audience and anything like a formal
presentation, the repetition of the same slide almost always causes
confusion and delay.

To avoid this confusion, one can have duplicate lantern slides.
Then the slides can be put exactly in order, and no confusion is
possible.

If a person has ever exhibited lantern slides for a friend, and one
or more of the slides had to be shown two or three times, he can
understand the troubles of the operator when the same slide must
be shown more than once, and will agree that it is better to have
the slide duplicated.

§ 23. Marking or "spotting" lantern slides. — In order that
lantern slides may be inserted in the carrier by the operator
correctly, and without hesitation or worry, the slides must be
marked or "spotted" in some conspicuous way (fig. 7, 8, 13).

20 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I

If the slides are not marked, and the correct position must be
determined for each individual slide during the exhibition, even
the most expert operator is liable to make mistakes, especially
when the slides are shown rapidly.

§ 24. Inspection of the room and lantern by the lecturer. — It

is highly desirable that the lecturer make himself acquainted with
the room in which he is to speak, and inspect the lantern himself
before the lecture hour. If the operator is with him it gives
opportunity to establish pleasant relations, and to stimulate the
operator to make the best exhibition possible. It also gives oppor-
tunity and time to make any slight changes necessary to insure a
good exhibition. Foresight is always more satisfactory in its
results than hindsight.

§ 25. Directions for the operator. — The lecturer should in-
struct the operator how he wishes the slides shown. There must
be some signal for changing the slides. Preferably the signaling
device is some form of electric signal on the operator's table, then
he can see or hear it, but the audience will not be distracted by it,
as when the lecturer has to speak to the operator, or hammer on
the floor with the pointer, etc. (For signaling devices see the list
of apparatus in the appendix).

The lecturer should direct the operator to light the lantern
before the room lights are extinguished, and give ample warning.
The operator should also be told to leave the lantern burning
until the room lights are turned on.

SUGGESTIONS TO THE OPERATOR

§ 26. Testing the lantern. — Before every exhibition or demon-
stration the operator should make sure that the lantern is in good
working order. This is only fair to the speaker who depends upon
his illustrations which he has taken so much trouble and expense to
prepare. If the slides are not well shown it injures the effect of the
lecture or demonstration and makes it difficult or impossible for
the speaker to make clear the subject he is treating. It also dis-
quiets the audience; and should make the operator uncomfortable.

CH. I] MAGIC LANTERN WITH DIRECT CURRENT 21

In testing the lantern the following points should be especially
looked to :

(A) That there is voltage in the supply line. This is easily
determined by turning on the incandescent lamp (fig. 2), or by
trying to light the arc lamp.

(B) That the arc lamp is in working order and has carbons long
enough to last during the exhibition. By closing the switch and
bringing the carbons in contact and slightly separating them the
arc light should be established almost instantly (see also § 30).
It takes a certain amount of experience to tell whether the carbons
are long enough to last during the exhibition. If there is any
doubt, put a new pair in position.

From the high temperature of the carbons, and the lamp gener-
ally, after the current has been on some time, it is not easy to put
in new carbons in the midst of a demonstration. It also makes an
embarrassing break in the exercises (see § 27).

§ 27. Gloves with asbestos patches. — In spite of all precau-
tions it is sometimes necessary to work about the arc lamp after it
has been running, and is therefore very hot. By the use of suitable
pliers or tongs one can usually manage to do the things necessary ;
but for certainty and rapidity one always needs to be able to use
the hands directly. This is rendered possible by the use of gloves
with asbestos patches in the places which come in direct contact
with the hot metal or carbons. The gauntlet form of gloves is best
for then the wrists also are protected.

The asbestos patches may be of asbestos cloth, or preferably of
quilted asbestos paper. The asbestos cloth is very thick and
clumsy. The asbestos paper of about half a millimeter thickness
(Vso in.) quilted between thin cotton or linen cloth answers well.
The quilting stitches should be long and extend obliquely in two
directions (fig. 5). The object of the quilting is to overcome the
weakness and easy tearing of the asbestos paper.

For most work a patch on the thumb and index finger is sufficient
but as it is often convenient to grasp a hot carbon between the
index and middle finger, it is well to have a patch on the middle
finger also (fig. 5).

22 MAGIC LANTERN WITH DIRECT CURRENT ICH I

FIG. 5. GLOVES WITH ASBESTOS PATCHES, PALM SIDE UP.

Left glove, p. i. m. The pollex or thumb (/>), the index or fore finger
(i), and the medius or middle finger (m), have the patches on the palmar sur-
face and sides.

c Carbon held pincer-like between the index and medius.

Right glove. The asbestos patches are as in the left. Above the correspond-
ing digits (i, 2, 3) are patterns of suitable patches drawn to the same scale as
the gloves.

With the hands protected by such gloves, one can grasp the hot
carbons within two or three centimeters (i in.) of the hot tips with
entire safety. The asbestos being a non-conductor of electricity
as well as of heat, makes it safe also to work about the lantern when
the current is on (§ 2 7 a).

§ 27a. Old leather gloves answer very well if one does not wish to sacri-
fice a new pair. New cloth gloves with gauntlets can be had for 20 cents.
These answer fairly, but are not so good as the leather gloves, and there is
no danger of the leather gloves being motheaten or catching fire. It is easier
to sew the patches on the cloth gloves, however.

Asbestos mittens are to be had of dealers in chemicals and chemical
apparatus. They are of asbestos cloth but are so thick and clumsy that they
are not adapted for working about the lantern.

CH. I]

MAGIC LANTERN WITH DIRECT CURRENT

§ 28. See if the lantern is centered. — Make sure that the
different elements of the lantern are centered along one longitudinal
axis (fig. 1,2). Then and then only will a perfect screen image be
produced. If the apparatus was installed correctly in the begin-
ning the only part liable to be out of line is the crater of the positive
carbon. In burning the carbons frequently so wear away that the
crater is at one side of the axis. Slight decentering of the crater
can be easily corrected by using the fine adjustment designed for
the purpose (§ 10, fig. 3, see also Troubles § 79).

§ 29. Slide-carrier. — Be sure that the slide-carrier works
properly and easily. The "push-through" form (fig. 6), is very
convenient, for while one slide is on exhibition the one previously
shown can be removed and another put in place, and it can be
instantly put in front of the condenser when the lecturer signals.

S1 ~ -'

eujiJi-j eoiBeyg euj8}e~|

/JvV

U^|

!r.V,.\.V.V«

4">^~.^,-~---

_ — • -aia

FIG. 6. "PUSH-THROUGH" OR DOUBLE SLIDE-CARRIER.

/ The frame which remains in one position in front of the condenser and
serves as a container and guide for the "push-through" part.

2 The movable slide-holder or "push-through" part of the carrier. It
moves easily to the right and to the left. It contains two slides in the proper
position for an erect image on a vertical opaque screen (see fig. 7,8}.

3 3 Notches in the movable part to enable one to grasp the slide easily.

4 Elevator serving to lift the slide when at either end. In some forms the
elevator lifts the entire slide from the middle instead of tilting one end.

5 6 Inclined planes at each end. These raise the elevator when the carrier
is moved to either end of the base.

S i Lantern slide in the carrier in front of the condenser.

S 2 Lantern slide in the carrier at the left end. It is of the first magic
lantern (1665) and is in position to be removed or to be pushed to the right for
exhibition.

§ 30. To start the arc light. — Turn on the current by closing
the switch (fig. 1-4). If the lam]) is of the automatic type the

24 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I

magnetic release will allow the carbons to come in contact and
separate slightly so that the arc will be of the correct length.

If the lamp is of the hand-feed type the operator must start it
by bringing the carbons in contact and then separating them a
short distance (3 to 4 mm.; }/% in.). This is done by turning the
feed screws by hand (fig. 3, F. S.).

§ 31. Managing the arc lamp during the exhibition. — For an

automatic lamp, the operator has only to close the switch to start,
and to open the switch to stop the lamp. The automatic mechan-
ism is supposed to keep the lamp burning in the best manner.
From the uneven burning of the carbons it is sometimes necessary
to make slight adjustments by hand even with automatic lamps.
This is easily accomplished by turning the proper screws present
for the purpose (fig. 3, F. S., L. A., V. A.).

For the hand-feed lamp the operator must bring the carbons
closer together every four to five minutes or oftener by turning the
feed screws. If this is not done the distance between the carbons
soon becomes too great for the current to pass, and the lamp will go
out. Allowing the lamp to go out when it should not is one of
the things for the operator to avoid.

§ 32. Amount of current to use. — This depends upon the kind
of arc lamp used (Ch. XIII), the screen distance, and the character
of the lantern slides. For dark lantern slides or long distances
more current must be used than for clear lantern slides and short
distances.

For a screen distance up to 10 meters (33 ft.) and a right-angled
arc lamp (fig. 1-3) one will rarely need more than 12 amperes.
For a screen distance of 1 5 to 25 meters (50 to 80 ft.), 1 5 or at most
20 amperes should suffice. If more than 20 amperes are needed to
give the proper brilliancy to the screen images something is wrong
with the slides, the room, or the lantern itself, or more probably
with the management of the lantern. (See under Ammeter § 7).

§ 33. When to light the lamp. — The room should never be
totally dark during an exhibition. The incandescent lamp men-

CH. I]

MAGIC LANTERN WITH DIRECT CURRENT

tioned above (§ 16) will avoid this; and furthermore, the operator
should start the arc lamp before the lecturer turns off the room
lights (§ 25).

§ 34. When to put the lamp out. — The operator should not turn
out the arc lamp until the lecturer turns on the room lights. The
intervals of total darkness so common in exhibitions can be avoided
by keeping in mind the suggestions in this and the previous section.

It is also a good plan for the operator to remove the last slide
when the lecturer is through with it, and show a blank disc of light.
This will inform the lecturer that all the slides have been exhibited
and give him the hint to turn on the room lights.

To determine how a lantern slide
should be placed in the carrier to give
an erect image on the screen :

Look through the lantern slide toward
something light. Turn it until the
picture is right side up and the print
reads right, as in this model.

Then turn the slide so that the bottom
edge is uppermost like the next model.

FIG. 7. STANDARD AMERICAN LANTERN SLIDE, FULL SIZE, WITH
DIRECTIONS FOR INSERTING IT IN THE CARRIER so THAT
THE SCREEN IMAGE WILL BE ERECT.

26 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I

FIG. 8. STANDARD AMERICAN LANTERN SLIDE, FULL SIZE, WITH

DIRECTIONS FOR MARKING IT, AND INSERTING IT IN THE

CARRIER so THAT THE SCREEN IMAGE WILL BE ERECT.

§ 35. Correct position of the lantern slide in the carrier. — In

order that the image on the screen may be right side up and like the
original in every way, the lantern slide must be put into the carrier
in the following manner to counterbalance the inverting effect of
the projection objective (fig. i).

1 . A lantern slide with any printing upon it must have the side

which reads correctly face the lamp, if the screen is of

ordinary form.

If the screen is translucent like ground glass and the picture is
viewed from the back of the screen, then the printing must face the
screen, not the lamp.

2. In all cases the slide must be put into the holder with the

bottom edge up (fig. 6, 8).

CH. I] MAGIC LANTERN WITH DIRECT CURRENT
1O CENTIMETER RULE

The upper edge ts m millimeters, the lower in centimeters.

FIG. 9. SCREEN IMAGE OF A LANTERN SLIDE CORRECTLY INSERTED
IN THE CARRIER (FiG. 6-8).

ni j9/v\O[ aq; 'sj3j9tni[[im ui si aSpa aaddn

FIG. 10. LANTERN SLIDE IMAGE, WRONG EDGE UP.
3.IUH H3T31/:iTtt3O OI

ni iswol ?d) .aisJarnillim ni zi sg
FIG. ii. LANTERN SLIDE IMAGE, FACING IN THE WRONG DIRECTION.
JJJG nbbei. eqSc 12 lu mi]]inj&(Gi.3' rpG JOMGL in CGnfiraGfGi.3'

FIG. 12. LANTERN SLIDE IMAGE, WRONG EDGE UP AND FACING IN
THE WRONG DIRECTION.

FIGURES 9-10-11-12. LANTERN SLIDES OF A METRIC RULE FULL SIZE.

The figures show the image as it appears on an opaque vertical screen in each
of the four possible ways of inserting the slide in the carrier.

For a translucent screen, or when a mirror is used, the slide in fig. 11 would
appear erect.

28 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I

3. In the slide-changer of the Spencer Lens Co.'s magic lan-
terns (Delineascopes) , the slide is laid flat, with the face
up, i. e., so it will be toward the condenser when ready for
projection. The edge which is to be uppermost in the
ordinary vertical carrier, is toward the screen. Now when
this slide-changer is used it turns the slide up in the ver-
tical position so that it is in precisely the same position as
with the ordinary slide carrier.

§ 36. Possible ways of inserting American lantern slides in the
slide - carrier. — The standard American lantern slide is oblong
(10x8.2 cm.; 4x3^ in.), and the carriers are constructed to
receive them lengthwise. While they would never be inserted
with the short edge up, they can be inserted with either long edge
up, and facing in either direction. This gives four possible posi-
tions in the carrier, only one of which is correct. That is, there are
three wrong ways of inserting the slide in the carrier with the
corresponding wrong images on the screen. It is not very uncom-
mon for an audience to see all possible images of the same slide, and
occasionally the wrong ones repeated once or twice. This is as
inexcusable as it is unnecessary (fig. 10-12).

§ 37. Possible ways of inserting the square English lantern
slides. — These slides are 8.3 x 8.3 cm. (3^x3^ in.), and being
square they may be put into the carrier with any of the four edges
up, and of course with either face toward the lamp. This gives
eight possible ways of insertion, seven of which are wrong. Square
slides must have two "spots," (see fig. 13).

§ 38. Focusing the image on the screen. — When the lantern
slide is in the correct position before the condenser (fig. 1-2) the
objective must be at such a distance from the slide that the screen
image will be sharp, and show clearly the printed matter and all
the details of the picture. With the usual magic lantern the
objective is nearly in the right position all of the time. But for
any necessary final focusing there is a rack and pinion on the
objective, or it is mounted in a tube with spiral movement. By
turning the milled head of the pinion, or by turning the objective

CH. I] MAGIC LANTERN WITH DIRECT CURRENT 29

FIG. 13. SQUARE ENGLISH LANTERN SLIDE FULL SIZE.

This figure shows the method of "spotting" or marking by the English
Photographic Club. That is, there are two marks on the upper front margin
of the slide. Two marks are necessary for square slides, while a single one
answers for oblong slides.

The picture on the lantern slide is of a retouching frame to hold the slides
while being colored.

in its spiral casing the image may be made perfectly sharp, provided
that the light is good and the objective also good. With an
imperfectly corrected objective the margins of the screen image are
liable to be lacking in sharpness although the middle may be good.

It may be necessary to focus slightly for each individual slide,
but ordinarily if one slide is in perfect focus those following will
also give good images.

If the screen distance is small (three to five meters ; 10 to 16 feet)
it may be necessary to focus slightly for each slide if the sharpest

30 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I

images are desired. When, however, the screen distance is 10
meters (30 ft.) or over, it is not usually necessary to focus for each
slide.

If the screen distance is very great (20 meters; 65 ft. or more)
the operator cannot tell by his eye alone when the screen image is
perfectly sharp. In such a case he must have an assistant stand
near the screen to tell him when the image is sharp, or he can use
good opera-glasses and determine for himself.

When the focus is once found for these long distances it is well
to mark in some way the exact position of the objective ; then in
future the operator can be sure of good screen images in the same
position provided the lantern has not been moved.

§ 39. Hints on running the lantern for a demonstration lecture.

—It frequently happens that in a demonstration lecture, slides are
to be shown at several different times. Ordinarily the arc lamp is
turned out during the intervals ; but to make sure that the desired
slide can be shown without delay, the arc lamp can be left burning
all the time, and to avoid lighting the screen a mask can be put in
front of the objective (fig. 14). A "push-through" carrier (fig. 6)
should be used, and the next slide to be shown put in one of the
compartments. The other compartment is left vacant, and this
empty compartment is put in front of the condenser. If the slide
were left in position all the time it might become over heated and
break.

Whenever the slide is called for it is pushed into position and
the mask turned aside. This will bring the picture on the screen
almost instantly.

A mask or shield for the objective is much more important for
the slow starting lights like the Nernst, than for the arc (§ 146,
169, 202, 217).

§ 40. Collecting and arranging the lantern slides at the close
of an exhibition. — After the exhibition is over be sure to remove
the last lantern slide from the slide-carrier. It not infrequently
happens that the last slide is left in the carrier, and the lecturer's
set is thus rendered incomplete.

CH. I] MAGIC LANTERN WITH DIRECT CURRENT

It should be a part of regular routine to look in the slide carrier
at the close of'everv exhibition to make sure that the last lantern

tr- ^ ^

slide has-been removed.

FIG. 14. SHIELD FOR THE OBJECTIVE IN INTER-
MITTENT PROJECTION WITH SLOW-
LIGHTING RADIANTS.

Sl Shield raised to allow the light to pass from the objective to the screen.

S Shield down in front of the objective to cut off the light from the screen.

The shield should be of a concave form and in front of the objective a short
distance to avoid heating. It should be made of metal or asbestos and be
hinged so that it can be easily turned up or down.

This is also the best time to arrange the slides in the box or a
pile exactly as they were at the beginning of the exhibition ; then
the set will be ready for use at the next lecture or demonstration.

§ 41. Lantern slides permanently fixed in individual carriers. —

Originally lantern slides were mounted in wooden frames. Each
slide then had its own carrier, which was inserted in a special
opening for it next the condenser (fig. 15, 32). This method of
mounting slides still prevails for some purposes. If one wishes to
use them in the ordinary lantern the common slide-carrier (fig. 6)
is removed entirely ; then each slide in its carrier is inserted in
order during the exhibition. This method of mounting is
admirable for a small collection of slides, as the wooden frame pro-
tects them, but for a large collection they take too much space and
arc too expensive.

MAGIC LANTERN WITH DIRECT CURRENT [Cn. I

FIG. 15.

LANTERN SLIDE IN PERMANENT WOODEN
CARRIER; ONE-HALF SIZE.

/ Face view of the carrier and its slide.

2 Sectional view of the carrier, showing the shelf on which the slide rests,
and the wire spring above.

The slide is usually cut in circular form, and fitted into a circular opening
in the frame. A hole of the desired size is first made in the middle of the
carrier, but not going clear through ; then a slightly smaller hole goes entirely
through. This leaves a narrow shelf for supporting the slide. Above the
slide is placed a cover-glass, and then a wire spring to hold the glass in position.

PROJECTION OF HORIZONTAL OBJECTS

§ 42. The ordinary magic lantern is in a horizontal position
(fig. i), but the lantern slide must then be vertical. Objects in
liquids, and some other objects cannot be put in a vertical position,
hence the necessity of a rearrangement of the lantern parts so that
the object may be placed horizontally. This is accomplished by
placing the second or terminal part of the condenser, in a horizontal
position, and the projection objective is made vertical. By means
of a plane mirror in the path of the beam of light from the first part
of the condenser, the light is reflected vertically upward. The
object is placed horizontally just above the second element of the
condenser. The vertical projection objective would give a picture

CH. I] MAGIC LANTERN WITH DIRECT CURRENT

on the ceiling above, but by means of another mirror at 45 degrees
or a prism this vertically directed light is reflected horizontally to
the ordinary vertical screen (fig. 16, § 4aa). (For projection with
the vertical microscope see § 397).

FIG. 1 6. ARRANGEMENT OF THE MAGIC LANTERN FOR HORIZONTAL
OBJECTS.

(Cut loaned by C. H. Stoelting Co.).

Commencing at the left the parts are :
L Hand-feed lamp with right-angled carbons.
H Lamp -house cut away to show the lamp within.
/ 2 Adjusting screws to move the carbons,
j 4 Screws for centering the crater.

5 Adjusting screw for moving the lamp toward and from the condenser.
C The plano-convex lens of the condenser next the radiant. It here gives
a parallel beam.

T Water-cell in the path of the parallel beam.

§ 42a. In England and America this is often called vertical projection from
the position of the objective; in Germany it is called horizontal projection
from the position of the object.

34 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I

Af, 45 degree mirror to reflect the parallel beam vertically.

C, Second element of the condenser in a horizontal position. The lantern
slide is put just above it.

O Projection objective in a vertical position for opaque projection.

M 45 degree mirror above the objective to reflect the light horizontally
to the screen.

G Vertical support for the condenser, objective and mirror.

E Lantern front holding the objective.

E2 Set screws for holding the objective in position when once centered.

M Mirror in horizontal position. When raised 45° it serves to reflect the
horizontal beam down upon an opaque object.

C2 Second element of the condenser used in projection with the microscope
or lantern objective with the object in the ordinary vertical position.

S Opening for the lantern slide carrier.

Z>, Objective and its holder.

O Projection objective for lantern slides.

FFF Supports of the condenser, etc.

N Platform on which opaque objects are placed.

J5i; 2 Legs or supports of the prismatic rod serving as an optical bench.

PROJECTION WITH MULTIPLE LANTERNS

In the period before the common use of the moving picture
machine, when the pictorial effect was dependent wholly on the
magic lantern, two and even more lanterns were run simultaneously
i. c., both were going all the time.

§ 43. Composition of multiple lanterns.—

1. Each lantern must be complete in itself.

2. The size of image of each lantern must be exactly the same.

3 . The lanterns must be so placed and so inclined toward each

other that the light discs on the screen exactly coincide.
They are now usually placed one above the other (fig. 17).

§ 44. Wiring for multiple lanterns. — Each lantern must have
its own electric lamp. When the supply is no volts or less each
lamp must be separately wired, and each lamp must also have its
own rheostat and double-pole knife switch (fig. 2,3).

In case the supply is 220 volts, each lamp may be separately
wired as just described; or both lamps may be put in series, i. e.,
along one wire, on one system of wiring, and use but a single
rheostat.

§ 45. Use of multiple lanterns. — By the use of two lanterns
there is not shown first one slide and then another simply, but one

CH. I] " MAGIC LANTERN WITH DIRECT CURRENT 35

slide seems to melt into the other, hence the name "dissolving
views." This is brought about by a shutter gradually uncovering
one objective and at the same time obscuring the other; or, as in
the figure here shown (fig. 17), by the closing of the iris diaphragm
of one objective while the other opens.

FIG. 17. MULTIPLE LANTKKX FOR DISSOLVING
VIEWS.

(Cut loaned by the Bausch & Lomb Optical Company).

Each lantern must have its own arc lamp and rheostat. For dissolving one
picture into another the iris diaphragm of one objective is opened gradually
while the other is gradually closed. This is accomplished by pulling up or
down on the rod connecting the two iris diaphragms in the objectives.

Some lecture rooms are supplied with double lanterns, not so
much for the dissolving effect, as for the rapid passage from one
slide to another. In most cases the "push -through" carrier with a
single lantern will accomplish this as effectively.

§ 46. Multiple lanterns for "effects." — Formerly certain
"effects" or striking appearances were produced by the use of two
or more lanterns which were in operation and projected their light

36 MAGIC LANTERN WITH DIRECT CURRENT [CH. I

upon the screen at the same time. For example, to show falling
snow, in one of the lanterns is a slide showing a landscape, city
street, etc.. in another is a black band with irregular perforations
of minute size which give the appearance of snow-flakes. If now
the light in the lanterns is properly regulated, and the black
perforated band is moved up over the face of the condenser,
the snow-flakes will appear to fall either gently or rapidly in the
landscape or street as one moves the band slowly or rapidly. One
can give the appearance of a driving storm by tilting the black
band, for this will make the flakes seem to fall obliquely.

For rain effects the black band should have slit-like perforations.

MOVING SLIDES FOR SINGLE LANTERNS

§ 47. "Effects" with single lanterns. — The appearance of
movement may also be produced in a single lantern. For this two
slides must be superposed, and one moved over the other. By
this means various combinations of designs may be made, and also
appearances of relative movement. Here, naturally, the two
slides must be close together, or one will be too much out of focus.
Special slide carriers are constructed for showing these single-
lantern "effects."

For simple experiments use a single slide-carrier. The slides
should have no cover-glass, but may be varnished. Then one
slide is put in place as for an ordinary exhibition, and another is
inserted over it and pushed by the fingers into different positions
to show various combinations. For this experiment the bellows
between the slide-carrier and the objective should be removed to
give freedom to the hands in making the various changes necessary.

§ 48. "Slip-slides" for optical deceptions. — Slides with lines
at various angles, etc., are used to demonstrate these. The lines
can be shown separately, and then by pushing one slide over the
other one can get various combinations. For suggestions as to
slides the reader is referred to works on physiology and experi-
mental psychology under "optical deceptions."

§ 49. Most of the "effects" produced by the movement of two
slides over each other, and the use of multiple lanterns are so far

CH. I] STEREOSCOPIC SCREEN IMAGES 37

exceeded in every way by the moving picture that it is hardly worth
while to go to the trouble to get together the apparatus and slides
to show these small "effects" when such wonderful ones are shown
daily in every moving picture theater.

The moving picture was originally invented to illustrate scientific
facts; and the indications now are that it is to become a great
factor in education by its striking portrayal of the processes of
nature. (See Ch. XI).

STEREOSCOPIC SCREEN IMAGES

§ 50. For a stereoscopic screen image the same fundamental
law must be observed as for any other stereoscopic effect. That
is, there must be two slightly different images corresponding with
the image seen by the left eye and that seen by the right eye.
These images must be projected on the screen so that they nearly
coincide, then by some means the left eye sees its left-eye image,
but not the right-eye image; and the right eye sees the right-eye
image, but not the left-eye image. The two images are then
combined in the brain and the stereoscopic effect follows as with
ordinary naked eye binocular vision or when using a stereoscope.

With the magic lantern this effect has been produced in three
principal ways:

(1) By the aid of prism spectacles. — Lantern slides of a stereo-
scopic pair are projected on the screen so that they nearly coincide
by the use of two lanterns. When this is done some people can
get the stereoscopic effect by looking at the pictures with the naked
eye, but for most people it is necessary to look through prism
spectacles so that the right eye shall see only one image and the
left eye only one.

(2) By the aid of polarized light and Nicol-prism spectacles. —
According to this method two lanterns are used and two lantern
slides, making a stereoscopic pair. For one lantern there is used a
Nicol-prism or a glass pile and the projection is made with the
ordinary polarized light. A similar prism or pile is used for the
other lantern, but the extraordinary polarized light is used for
projecting its image. These two images are projected so that they

38 STEREOSCOPIC SCREEN IMAGES [Cn. I

nearly coincide upon the screen. The screen is covered with silver
foil to prevent the depolarization of the reflected light. Now to
look at the screen image and to make it possible for each eye to see
only its own image, the observer must wear polarizing or analyzing
spectacles with the prisms or piles corresponding, with the one
supplying the light for its own image. For example, if the right
eye image is made by extraordinary polarized light, then the right
eye of the observer must have its prism spectacle so that it trans-
mits the extraordinary polarized light, but extinguishes theordinary
polarized light which produces the left eye image. And the left eye
must have its prism so that it will receive the polarized light from
its image, but extinguishes that from the right eye image. Each
eye then sees its own image, but not the one for the other eye, and
the conditions for stereoscopic vision are fulfilled.

(3) The two-color method. — For this method two complementary
colors are selected — usually red and green.

(A) With two lanterns there are projected the two images of a
stereoscopic pair so that they nearly coincide. There is put some-
where in the path of the beam of one lantern a plate of red glass and
in that of the other lantern a plate of green glass. The observer
must have spectacles or viewing glasses of corresponding colors.
Then with one eye he sees the red image and with the other the
green image. The combination of these colored images by the
brain gives a stereoscopic image in black and white.

(B) With a single lantern the two-color stereoscopic effect can
be produced as follows: The two pictures of a stereoscopic pair
are printed by one of the color processes so that one is a red picture
and one a green picture. These two are placed together so that
they nearly coincide, then they are projected by one lantern.
With the naked eye the pictures look like any two-color picture
where the colors do not register, and such a screen picture is any-
thing but satisfactory; but now if spectacles or viewing glasses of
corresponding colors arc held before the eyes, one eye sees the green
picture and one eye the red picture and the stereoscopic effect
comes out very strikingly.

The simplest way to determine which color to put in front of the
right and which in front of the left eye is to try first one color then

CH. I] CENTERING THE MAGIC LANTERN 39

the other. In general it will be found that if the red parts are at
the right then the red glass must be over the right eye and similarly
for the green. Presumably if one used the wrong color then there
should be a pseudoscopic effect, convex objects looking concave,
etc.; but this effect is difficult to obtain.

It is seen that in all these methods the observer must be supplied
with some means by which only one of the projected images is seen
by one eye, the other by the other eye. Stereoscopic projection is
necessarily, therefore, expensive.

For most people any good lantern slide shows perspective and
relief sufficiently.

CENTERING THE VARIOUS PARTS OF THE LANTERN AND
SEPARATING THEM THE PROPER DISTANCE

§ 51. Centering. — By this is meant the arrangement of the
source of light, the condenser and the projection objective so that
the source of light, and the principal optic axis of the condenser and
of the objective shall be in one straight line, and each lens be
perpendicular to that straight line (fig. 1-4).

When the different elements are once centered along one straight
line the objective and the condenser should be fixed in position so
that they cannot be raised or lowered or turned sidewise. If the
source of light gets slightly out of center by the burning of the
carbons, it may be recentered by bringing the carbons nearer
together or by regulating the position by the fine adjustments of
the lamp.

In the right-angled arc lamp the upper carbon, which furnishes
the light, is constantly in the optic axis. With oblique carbons
(fig. 39) the source of light constantly shifts with the burning away
of the carbons ; and with the direct current lamp the source of light
gradually rises above the axis. With the alternating current and
V-arranged carbons one source shifts above and one below the axis,
or one to the right and one to the left depending upon the arrange-
ment of the V. In centering the lamp one should start with the
carbons in contact and take the point of contact to center from.

40 CENTERING THE MAGIC LANTERN [Cn. I

Remember that one should never change the position of the
condenser or of the objective to compensate for the lack of center-
ing of the source of light.

§ 52. Mechanical method of centering. — This is the method
most satisfactory for both manufacturer and user in getting the
various parts properly aligned.

Generally some form of track (optical bench) is used on which
the various parts are placed and along which they can slide. The
straight line or axis to which all parts are to be centered is at a
selected, definite position above the base-board or table supporting
the track (fig. 3 , 40) .

The first thing, then, is to decide upon the distance the axis is to
be above the base-board or table.

For all work upon centering, the bellows between the condenser
and the objective should be removed so that the faces of all parts
can be seen.

The position of the common axis may be determined by some
part of the apparatus, such as the condenser. Or one can decide
upon some convenient level which will give sufficient room for the
arc lamp and its carbons, and then adjust all parts to this level. A
good way to get all at the proper height is to make a measure or
gauge of wood just the height of the axis. If this is a board which
just fits between the tracks, and has a peg indicating the middle
point between the tracks it will help to get the parts perpendicular
to the axis as well as at the right level. If the wooden gauge is
carefully made it will enable one to center the parts to within one
or two mm. (Vi« to y24 inch). Very slight variations from perfect
mechanical centering can be compensated for by using the fine
adjustment screws of the arc lamp.

§ 53. Getting the center of the lens faces. — This can be done
by using a rule in millimeters or Meth's inch. Or it can be done by
pressing some white paper against the lens face and creasing it
around the edges with the finger. The center of this circle of paper
can then be found as shown in fig. 18. If the center is marked and
the paper then put over the lens face one will have a guide to center
by.

CH. I]

CENTERING THE MAGIC LANTERN

FIG. 1 8. FIGURE SHOWING HOW TO
FIND THE CENTER OF A CIRCLE.

Draw two chords (ch ch) and erect perpendiculars at their middle points.
Where these perpendiculars cross is the center of the circle (C).

As stated above, when once centered, the objective and con-
denser should be fixed in position.

§ 54. Avoidance of obliquity. — Not only must all the parts be
at the same level and in one straight line, but the lenses must be
perpendicular to that straight line and not oblique. Then the
straight line or common axis passing from the crater of the upper
carbon to the screen will coincide with the principal axis of the
condenser and the projection objective, and the arrangement for
perfect projection will be realized (fig. 1-4, 26).

One can usually tell when the parts are in line and not oblique by
sighting along them with the eye, or by the use of a straight edge
like a T-square. To make sure by measurement one can put the
optical bench or base-board (fig. 158, 1 59) , on a level table and next
a smooth wall. Then by measuring horizontally the central points
can be determined exactly as their height was determined (§ 52).

CORRECT DISTANCE APART OF THE DIFFERENT ELEMENTS

§ 55. Radiant and condenser. — With the three-lens condenser
the radiant is at the right distance when it is at the principal focus
of the first element of the condenser (fig. 2). This will give a

CENTERING THE MAGIC LANTERN

[CH. I

FIG. 19. CONCENTRIC CIRCLES ON THE

FACE OF THE CONDENSER, SHOWING

THE SIZE OF THE CIRCLE OF LIGHT

WITH VARIOUS POSITIONS OF

THE RADIANT.

When the radiant is at the proper distance, the entire face of the condenser
is illuminated (/).

As the radiant and condenser are separated the part illuminated becomes
smaller and smaller (2-4). (See also fig. 20).

cylinder of approximately parallel rays between the two elements
of the condenser, and will fully light the face of the second element.
One can determine this easily by putting a sheet of white paper
over the face of the condenser which is toward the objective. If
the radiant is in the right place the entire face will be light. If the
radiant is too far off, only a part of the face will be illuminated
(fig. 19). If the radiant is too close the face will be lighted,
but the light will be diverging between the condenser lenses.
In this case a part of the light falls outside the second element and
is lost. There is liable also to be a defective screen image (fig. 28).
One can get the condenser at the right distance from the lamp by
first separating the lamp and condenser a considerable distance and
then gradually bringing them closer and closer together until the
condenser face is just filled with light. Sometimes the radiant is
put nearer than the principal focal distance on purpose, so as to
correct in part for the lack of proper proportion between the con-
denser and the objective (§ 56).

CH. I] CENTERING THE MAGIC LANTERN 43

With the two-lens condenser used for lantern slides the lamp is
usually closer than the principal focal distance of the first lens, this
makes the beam between the lenses diverging, hence it is best to
have the two lenses as close together as possible to avoid loss of
light (fig. i).

With this condenser and diverging light between the lenses the
only rule that can be given is to adjust the distance between the
lamp and the condenser until the best light is obtained on the
screen. If this brings the crater of the arc lamp within 8 to 10 cm.
(4 in.) of the first lens, then it will be necessary to substitute longer
focus lenses for either the first or the second condenser lens or for
both. In general, the first lens should be of about 15 cm. (6 in.)
focus and the second lens should have a somewhat shorter focal
length than the projection objective. For example, if the projec-
tion objective is of 38 cm. (15 in.) focus, the second lens of the con-
denser in the two-lens form should be of about 2 5-30 cm. (10-1 2 in.)
focus. This will bring the diverging cone to a focus near the center
of the objective.

§ 56. Condenser and projection objective. — If the projection
objective and the condenser are properly proportioned the conden-
ser will focus the light near the center of the projection objective
when the lantern slide is in focus on the screen (fig. 1,2).

If the condenser is of so short a focus that the light from the
condenser comes to a focus before reaching the objective the field is
restricted and bordered by a red margin (fig. 29).

If, on the other hand, the condenser is of too long a focus for the
objective the light will not come to a focus by the time it reaches
the center of the objective (fig. 28). In this case the field will be
restricted and bordered by blue.

OPTICAL TEST FOR CENTERING

§ 57. Optical test for centering the radiant and the condenser.

— If these are properly centered along one line, and the two are
separated a considerable distance when the lamp is burning, the
light spot on the face of the condenser looking toward the objective

CENTERING THE MAGIC LANTERN

[CH. I

FIG. 20 A. CONDENSER FACE WITH THE SPOT OF LIGHT IN THE MIDDLE,
SHOWING THAT THE LAMP AND CONDENSER ARE ON THE SAME Axis.

FIG. 20 B. CONDENSER FACE WITH THE SPOT OF LIGHT OUT OF THE

CENTER. THIS SHOWS THAT THE CONDENSER AND LAMP ARE NOT ON

ONE Axis.

To get the appearance here shown the lamp must be pulled back considerably
beyond the principal focus of the first element of the condenser.

will appear in the center (fig. 20) . This can be easily seen by hold-
ing a piece of paper against the condenser face. In case the two
are not properly aligned, the white spot on the paper will appear
outside the center, at the right or left, above or below. On account
of the inverting effect of lenses the arc light will be too far from the
center in just the opposite direction from the spot of light. For
example, in figure 2oB the light spot is too far to the left,
consequently the crater of the positive carbon must be too far to
the right. One should change it to the left by the adjusting screws
until the circle of light appears exactly in the middle (fig. 20 A).

§ 58. Optical test for centering the condenser and the objec-
tive.— After the condenser and the radiant are properly centered,
and the radiant put at the principal focus of the condenser one can
tell whether the objective is on the same axis by looking at both
ends of the objective when it is at the proper distance from the
condenser (fig. 1-2).

If the objective is in line with the lam]) and the condenser the
spot of light from the condenser can be seen in the middle of the

CH. I] CENTERING THE MAGIC LANTERN 45

first lens of the objective. The light should strike the middle of
the first lens and leave through the middle of the last lens of the
objective (fig. i).

If it is not centered the cone of light will strike at one side of the
center and leave at one side. If it is greatly out of center the cone
of light may fall wholly outside the objective; this frequently
occurs in micro-projection.

To center the objective it should be moved up or down, to the
right or to the left, until the cone of light strikes it exactly in the
center and leaves the center. No change of the lamp or the con-
denser should be made, for that would spoil the centering of those
two elements. After the objective is centered, it should be fixed
firmly in position. Any slight variation from the center by the
irregular burning of the carbons, can be corrected by the fine
adjusting screws of the lamp (fig. 3, L. A.; V. A.).

CENTERING THE OBJECTIVE IN A VERTICAL POSITION

§ 59. When the objective must be made vertical in projecting
horizontal objects, the radiant and the condenser should first be
centered as described above (§ 55). Then the second element of
the condenser should be removed and placed in a horizontal posi-
tion with the convex face downward, and the flat face upward
toward the objective. A plane mirror at 45 degrees is placed in
the path of the beam of light from the first element of the con-
denser. The light will be directed vertically upward. The hori-
zontal condenser lens must be moved until it receives this vertical
cylinder of light and continues the central or axial ray in a vertical
direction. One can tell when this is the condition by pulling the
arc lamp back from the condenser until a small circle of light
appears on the horizontal condenser lens (fig. 2oA, B.). If it is
centered the spot of light will be in the middle. If it is not in the
middle move the upper lens until it is, but do not change the posi-
tion of the lamp. When the horizontal lens is centered, move the
arc lamp up toward the condenser until the horizontal lens is filled
with light (§ 55).

TROUBLES WITH THE MAGIC LANTERN

[CH. I

§ 60. Centering the vertical objective. — After the horizontally
placed condenser lens is centered the objective is placed in a vertical
position over it and moved sidewise until the cone of light enters
the middle of the first face and leaves the middle of the last face of
the objective. One proceeds exactly as for centering it in the
horizontal position (§ 55, 58). Just over the objective is placed a
45 degree mirror silvered on the face, or a right-angled prism, to
direct the vertical rays horizontally to the screen (fig. 16). The
lower mirror may be an ordinary glass mirror silvered on the back,
but the mirror over the objective must be silvered on the face to
avoid a duplication of the image.

TROUBLES: HOW TO AVOID AND HOW TO
OVERCOME THEM

THE LAMP CANNOT BE STARTED

§ 61. This may be because there is no voltage in the main line.
The presence of current is easily determined by using the testing
incandescent lamp (fig. 21). An incandescent lamp in the circuit
as shown in fig. 2 or 4 will show whether the current extends to the
lamp switch.

FIG. 21. TESTING INCANDESCENT LAMP.

Wl W? The two supply wires for the lamp.

For this testing lamp a socket without key switch is best. It is also wise to
have the lamp protected by a wire guard. The wires at W-, W2 should be
exposed only a short distance as shown.

To test with the lamp put the naked ends of the wires Wt W., upon metallic
parts of the circuit to lie tested being sure to make contact with both conduc-
tors of the circuit. For example, the two wires or the two blades of a knife
switch, etc. If there is voltage in the line at that point the lamp will light up.

§ 62. The connections in the arc lamp may not be good, that is,
the set screws holding the connecting wires may have become

CH. I] TROUBLES WITH THE MAGIC LANTERN 47

loosened, or a wire may have become wholly separated from its
connections.

§ 63. A fuse may have burned out somewhere along the line.
Commencing with the fuse nearest the lamp, take each fuse out and
examine it. Use the testing incandescent lamp also.

§ 64. A fuse plug may not be screwed in tightly enough to make
good contact. Occasionally some one puts a piece of paper or wood
in the fuse socket, thus preventing metallic contact. Such
obstructions should be looked for and removed; then the fuse plug
can be made to produce metallic contact.

§ 65. The switches may not be properly closed, and hence the
circuit is not complete.

§ 66. The carbons may be so short that they cannot be brought
in contact, and thus the circuit cannot be completed. Put in new
ones.

§ 67. The range of the lamp movement may be at its limit, so
that the carbons cannot be approximated. This must be corrected
by turning the screws back and then setting the carbons by hand,
if long enough, or by putting in new carbons.

§ 68. If one uses an automatic arc lamp, it may be that the
mechanism does not work. Before looking elsewhere for the
trouble, one should try the hand-feed device present in all auto-
matic lamps and make sure that the carbons are brought in con-
tact and then slightly separated to establish the arc.

§ 69. Of course, if one uses a hand-feed lamp it will not start
until one brings the carbons in contact by the proper device for the
purpose. As soon as the carbons touch there will be a flash of
light; then the carbons should be slightly separated.

§ 70. There may be a short circuit in the lamp itself due to a
burning out of the insulation. This may be detected by opening
the double-pole knife switch slowly. If there is a big spark when
the switch finally opens, a short circuit in the lamp is strongly
indicated.

48 TROUBLES WITH THE MAGIC LANTERN lCn. I

Unless one has considerable knowledge of arc lamps it is advis-
able to get an electrician to repair the lamp.

Short circuiting in the lamp is a rare trouble and less liable to
occur than almost anything else.

GOING OUT OF THE LAMP

§ 71. This may be due to the stopping of the dynamo.

§ 72. A fuse may burn out somewhere along the line.

§ 73. Some connection may burn out or one or both wires may
be disconnected.

§ 74. The carbons may have burned off so that the interval
between the ends is too great for the current to pass. This is a
very common cause, and is, of course, easily remedied by the use
of the feeding screws of the lamp to bring them closer together.
If the carbons are so short that they cannot be brought together,
new carbons must be inserted. Always open the table switch
before putting in new carbons.

Sometimes the screw holding the lower carbon is not set up
enough and the carbon falls down. If this is the trouble open the
table switch and replace the lower carbon in its proper position and
tighten well the set screw holding it.

Always look at the carbons first in case the lamp goes out
unexpectedly (see also above § 66-67, 7° and an the causes for no
current § 61-70).

NOT ENOUGH CURRENT

§ 75. There may not be enough in the line.

§ 76. The line may be grounded. Test for this with the testing
incandescent by touching one of the terminal wires of the incan-
descent to some metal object connected with the ground, like the
metal tube enclosing the wires, a water or gas pipe or radiator, and
the other to one of the exposed metal parts of the conductors, first
on one side and then on the other. If there is a connection of
either wire with the ground the testing lamp will light when its two
wires are connected, one with the radiator, etc., and the other with

CH. I]

TROUBLES WITH THE MAGIC LANTERN

the line wire which is not grounded. In some cases one wire is
purposely grounded. In such cases great care must be taken not
to ground the other wire (see also fig, 266-267 § 689).

§ 77. There may be too much resistance in the circuit. Open
the rheostat wider, if it is adjustable (fig. 281), keeping an eye on
the ammeter to see when the current is of the desired amperage.

FIG. 22. INCLINED AND VERTICAL CARBONS
IN THE CORRECT RELATIVE
POSITION.

The upper carbon is positive and supplies the light in both cases.

FIG. 23. CARBONS IN THE CORRECT RELATIVE POSITION FOR BOTH
DIRECT AND ALTERNATING CURRENTS.

A Inclined carbons in the correct position for alternating current.

B Inclined carbons in the correct position for direct current.

C Carbons at right angles in the correct position for either direct or
alternating current. Direct current is indicated.

D Carbons arranged in a V-shaped position. For this position alternating
current only is employed; and the crater on each carbon contributes to the
light. The V may be either in a vertical or in a horizontal plane. The ver-
tical arrangement is the more common.

TROUBLES WITH THE MAGIC LANTERN lCn. I

FIG. 24. CARBONS IN BAD POSITION; THE UPPER CARBON CUTTING

OFF THE LIGHT FROM THE UPPER PART OF THE CONDENSER, AND

HENCE CASTING A SHADOW ON THE LOWER PART OF THE SCREEN.

A Carbons at an inclination of about 25 degrees, with the upper or positive
carbon too far forward.

B Carbons at right angles, with the upper carbon too far forward.

S Screen image of the condenser face. As the upper carbon is in the way,
the upper part of the condenser is partly in shadow, and hence the screen image
will be shaded on its lower part due to the inverting action of the objective.

FIG. 25. CARBONS IN BAD RELATIVE POSITION, THE LOWER OR NEGATIVE
CARBON EXTENDING UP IN FRONT OF THE POSITIVE CARBON.

A Carbons at right angles, with the lower carbon too high.

B Both carbons vertical, but the lower or negative one standing in front
of the upper one.

.V Screen image of the condenser face. As the condenser is not well lighted
on its lower part due to the shading action by the lower carbon, the screen image
will be shaded correspondingly on its tipper part due to the inverting action of
the objective.

CH. I] TROUBLES WITH THE MAGIC LANTERN 51

IRREGULAR OR INSUFFICIENT LIGHT ON THE SCREEN

§ 78. There may be an insufficient current flowing through the
lamp. Consult the ammeter (§ 7, 75-77).

§ 79. Improper relative position of the carbons. — Look at them
occasionally through the window in the lamp house. They should
be in the relative position shown in fig. 23. If they are in a wrong
position (fig. 24, 25), one cannot expect to get a good screen light.

It sometimes happens that one or both of the carbons has no soft
core, although the hole in the carbon is present. In such a case the
crater is liable to jump around as with a solid carbon. Easily
corrected by substituting a properly cored carbon.

§ 80. Wrong polarity of the supply wires. — As stated above
(§5) the positive supply wire should be connected with the lamp
so that the current passes along the upper carbon and from its tip
over to the lower carbon, whence by means of the negative wire, it
passes back to the generator or dynamo. In case the wires were
reversed in position, the lower carbon would be positive and the
bright crater would be on it. This would give a poor light, for the
crater would not face the condenser, and as this carbon would burn
away more rapidly than the upper carbon the upper one would soon
be in the position shown in fig. 246. There would then be a
double reason for a poor screen image, viz., the crater would not
face the condenser, and the upper carbon would act as a shield to
cut the light off the condenser. To determine whether the wires
are connected to the lamp properly, insert carbons, turn on the
current, and let the lamp burn a minute or two. Then turn off
the lamp and watch the hot ends of the carbons. The positive one
will remain red hot longest. (See also Ch. XIII, § 701-703 for
determining the polarity). In case the lower carbon remains
glowing longer than the upper, the polarity is wrong (fig. 271).

Open the switch and remove both wires from their binding posts
and insert them in the reverse position. Then repeat the experi-
ment and the upper carbon should remain glowing longest.

After one has had some experience it is easy to tell whether or not
the wires are properly connected by watching the carbons through

$2 TROUBLEvS WITH THE MAGIC LANTERN [Cn. I

the lamp-house window when the lamp is burning. The upper
carbon should always be considerably brighter than the lower one.
When one has found the correct polarity it is wise to mark the
positive wire red and the negative wire black. It is also a good
plan to mark the positive switch connections plus with red and the
negative connections minus with black. But one must not forget
that the polarity is liable to be changed by the changing of the
wires in the main line when repairs are made, so one must be on the
alert to detect polarity change.

§ 81. Non-registering of the direct current ammeter. — In first
installing an ammeter if the hand does not register on the dial when
the current is turned on and the arc lamp started, either the
instrument is out of order, or more likely the wires are wrongly con-
nected. Remember that the ammeter must be inserted in one
wire, then if it does not register when the lamp is burning the wires
were inserted wrong. Turn off the current and reverse the wires
in the binding posts of the ammeter. If now the wires are properly
connected both to the ammeter and the arc lamp, the polarity in
both will be changed by a change in polarity in the main line, and
the wires must be changed around in the binding posts in the
ammeter and in the arc lamp to get the polarity correct in both.
As the lamp and the ammeter are wholly independent instruments,
the polarity may be correct in both or wrong in both, or correct in
one and wrong in the other. (See also Ch. XIII, ;o2a for ammeter
which can be used with both alternating and direct current) .

DEFECTIVE OPTICAL RESULTS

§ 82. There may be direct light falling on the screen from some
window or some lighted lamp in the room . This will make the disc
of light, or the lantern picture on that part of the screen receiving
the adventitious light, look faded or gray instead of brilliant. It
will look as if that part of the screen were not so brilliantly illumin-
ated, when, in fact, more light may be falling on it. To be effec-
tive the light must reach the screen from the lantern and from no
other source.

CH. I]

TROUBLES WITH THE MAGIC LANTERN

SHADOWS AND RESTRICTION IN THE Disc OF LIGHT ON THE SCREEN
§ 83. The radiant, i. e., the crater of the upper carbon (fig. 27)
may be outside the main axis (above, below, to the right or to the
left of it). If sufficiently outside the center there will be only an
elliptical light area present. On the side toward which the crater
is displaced there will be a blue crescent or spot, and on the oppo-
site side a dark crescent, bordered, in extreme cases, by red.
Remedy: get the crater back in the axis.

§ 84. The condenser may be out of center. — This will give the
same defective light on the screen as when the light source is off

FIG. 26 (A). DIAGRAM OF A MAGIC LANTERN AND A SCREEN IMAGE WHEN
ALL THE PARTS ARE IN CORRECT PROPORTION AND ON ONE Axis.

Axis The common axis passing from the radiant along the principal axis
of the condenser and the objective to the screen.

C Condenser of three lenses, the first clement (L,) composed of a meniscus
and a plano-convex; the second element (LJ, is a plano-convex. The con-
vex surfaces face each other as usual.

F Principal focal distance of the condenser.

0 Projection objective.

R Radiant giving the light.

S The screen fully and perfectly lighted.

FIG. 27 (B). DIAGRAM SHOWING THE EFFECT OF HAVING THE RADIANT
BELOW THE Axis.

There appears a blue shadow on the lower part of the screen (S).

Whenever the radiant is off the axis the dark blue shadow will be on the
corresponding side of the screen. In this case the radiant would have to be
raised to get rid of the shadow. If the shadow were on the left it would be
necessary to move the radiant to the right and so on.

TROUBLES WITH THE MAGIC LANTERN lCn 1

FIG. 28 (A). DIAGRAM SHOWING THE EFFECT ON THE SCREEN IMAGE
WHEN THE RADIANT is TOO NEAR THE CONDENSER.

In this case the conjugate focus of the condenser (/) is considerably farther
off, i. e., beyond the objective, the screen image is made smaller, and the light
disc on the screen is bordered with blue. With some condensers there is a
dark or blue disc in the center also. (Lettering as in fig. 26).

FIG. 29 (B). DIAGRAM SHOWING THE EFFECT ON THE SCREEN IMAGE

WHEN THE RADIANT is BEYOND THE PRINCIPAL Focus

OF THE CONDENSER.

This brings the conjugate focus of the condenser (/) nearer the condenser,
and in this case just before the light reaches the objective. It narrows the
screen image and the light disc is bordered with red. (Lettering as in fig. 26).

the axis, but the blue spot or disc will be on the side away from
which the condenser is displaced, being just the reverse of the
position when the light source is off the axis (§ 83).

If the condenser is too high the blue spot or disc will be on the
lower part of the screen; and if the condenser is too low the blue
edge will appear on the upper part of the screen ; if to the right the
blue disc will be at the left, etc. That is the condenser inverts the
position (fig. 27).

The condenser should be correctly centered once for all and
firmly fixed in position so that it need never be changed.

§ 85. The projection objective may be off the main axis. — The

effect will be the same as when the source of light is off the axis.
This is due to the fact that while the condenser inverts the rays,

CH. I] TROUBLES WITH THE MAGIC LANTERN 55

they are re-inverted or erected by the objective. If the condenser
and source of light are on one axis and the objective off that axis,
it must be recentered; but as stated above (§ 54) when the objec-
tive and condenser are once centered they should be fixed in posi-
tion, then the only element of the lantern to become decentered is
the crater of the arc lamp, i. e., the source of light. The fine
adjustment screws of the lamp will enable one to center the light.
By limiting the changes to one element, viz., the source of light,
corrections can be made quickly and accurately. If one tries to
get the light centered by changing two or all three of the elements it
leads only to chaos.

§ 86. For the effects of spherical aberration and for a ghost, a
white or black spot in the center of the field, see also § 828.

§ 87. The radiant (i. e., crater of the upper carbon) may be too
close to the condenser. This will give a restricted field with a blue
margin or there may be a blue circle in the center of the disc (fig.
29, 30).

§ 88. The radiant may be too far from the condenser. This
will produce a restricted screen disc with the edge bordered with
red (fig. 29). It is easily corrected by bringing the radiant and
condenser closer together.

§ 89. The condenser may be of too short focus, so that the
light comes to a focus before reaching the objective when the
lantern slide is in focus (see § 56, fig. 29, 30). Correct the defect
by using a lens of longer focus for the second element of the con-
denser. It may be less satisfactorily compensated for by putting
the radiant nearer the condenser.

§ 90. The condenser may be of too long a focus (see § 56,
fig. 28). Correct by using a shorter focus condenser. It may also
be compensated for in part by removing the radiant farther from
the condenser, but this lessens the available light.

§ 91. There may be dirt, mist or opacities on some of the glass
surfaces. This is easily remedied by cleaning the glass.

TROUBLES WITH THE MAGIC LANTERN

[On. I

FIG. 30. ARRANGEMENT AND CENTERING OF THE RADIANT.

(From the catalogue of Fuess).

(/) The Radiant, i. e., the crater is too far to the right.
(2) The crater is too far to the left,
(j) The crater is too high.

(4) The crater is too low.

(5) The crater is too far from the lamp condenser.
(6-7) The crater is too near the condenser.

(8) The crater is in the correct position.

One of the condenser lenses may be cracked. If a new lens can-
not be inserted, but the cracked one must be used at the time,
rotate it around until the crack is least noticeable.

There may be strings or wires hanging down in the path of the
beam of light. They will give sharp shadows on the screen.
Remove them.

§ 92. Defective or too opaque lantern slide. — The lantern
slides may be cracked, producing a dark streak through the picture.
There may be dirt or mist on one or more of the glass surfaces.

The slide may be too opaque. There is a tendency to make
lantern slides so opaque that only the most powerful radiants can
give anything like satisfactory screen images. This is a great
mistake. Lantern slides properly made arc very transparent and
show all the delicate shading, from the densest to pure transparency
(clear glass). Probably 99 slides are too dense where one is not
dense enough. The opacity of the slides made by the autochrome

CH. I] TROUBLES WITH THE MAGIC LANTERN 57

or starch process is one of their great drawbacks. Only powerful
radiants give satisfactory screen images.

§ 93. Shadow on the screen with water-cell. In case the water
in the water cell has evaporated in part there will be a very dis-
agreeable shadow on the lower part of the screen (fig. 31). It is on
the lower part of the screen although it is the upper part of the water
cell that will be empty. This is due to the inverting action of the
objective.

FIG. 31. SHADOW ON THE LOWER PART OF THE SCREEN WHEN THE
WATER-CELL is BUT PARTLY FILLED.

S Screen image with shadow on the lower side. The water is of course
present in the lower part of the water cell, and absent from the upper part;
but, owing to the inversion produced by the objective, the screen image shows
the shadow on the lower part.

Occasionally the water is entirely absent from the water-cell.
Then there will be a very poor screen image, the entire screen being
affected by the obscurities on the dry surfaces of the water-cell.

BREAKING OF CONDENSER LENSES

§ 94. It is usually the lens next the radiant that cracks or
becomes shattered. This is due to the too rapid heating or cooling
of the condenser lens, or to the mounting, which may be too rigid
to allow of free expansion of the lens as it becomes hot.

Condenser lenses are especially liable to break: (i) When too
heavy currents are used; (2) when the lamp-house is not well and
evenly ventilated; (3) when currents of cold air strike the hot
condenser; (4) when the lens mounting is not provided with
ventilating openings for free circulation of air between the lenses;

58 TROUBLES WITH THE MAGIC LANTERN [Cn. I

(5) when the lens next the radiant is of such a focus that the
lamp must be put very close to it.

§ 95. Unequal heating. — Breakage often occurs from unequal
heating of the lens. This is perhaps as common with large flame
sources such as the kerosene flame, the al co-radiant or Welsbach
mantle gas flame as with the electric arc. With the electric arc,
if the crater is too close to the lens the thick central part of the lens
expands rapidly before the edge is heated enough to expand with
the middle part. Separating the lamp and condenser somewhat,
for a few minutes after starting the lamp would give the condenser
a chance to expand uniformly.

§ 96. Mounting of the lenses. — This may not give the lenses
sufficient freedom of expansion. In all forms of condensers as now
constructed there is almost invariably provision for this expansion,
and for free circulation of air between the lenses. The lens next
the radiant is usually held by a few obliquely extending springs,
(fig. 36 B), thus giving the greatest freedom. To prevent break-
age some operators avoid all direct contact of the condenser with
the metal mounting by the use of asbestos paper. Others think
that a heavy metal ring around the edge of the condenser will
lessen breakage by preventing the too rapid cooling.

The final solution of condenser breakage will come when the
glass makers produce heat-resisting, optical glass.

§ 97. Breakage due to reversing the ends of the condenser.—

That is, the condenser lens which should be next the projection
objective is put next the lamp. The lens which should be next the
lamp is specially mounted for expansion (§ 96). Furthermore, the
condenser is not designed optically in most cases so that it will give
equally good results if reversed. In the magic lantern the lens next
the objective has frequently a longer focus than the one next the
radiant, so that a reversal injures the optical effect as well as
endangers the condenser.

If the makers of projection apparatus would so construct their
condenser mountings that they could not be reversed, the)- would
be doing a friendly service to many.

CH. I]

SOME AMERICAN MAGIC LANTERNS

§ 98. If the lantern table is on a concrete floor which is damp
the operator is liable to get a shock unless he stands on a mat or
board or other insulating material, provided some part of the cir-
cuit is grounded (see § 689).

SOME EXAMPLES OF AMERICAN MAGIC LANTERNS FOR THE DIRECT
CURRENT ARC LAMP

§ 99. The following examples of American Magic Lanterns are
introduced to give the reader some notion of the lanterns on the
market which can be obtained at any time and at a very moderate
cost.

In subsequent chapters will be found pictures of lanterns for
the different forms of radiants, and for two or more kinds of
projection (combination apparatus).

In the appendix at the end of the book will be found the addresses
of some of the great manufacturers in all countries with the prices
for the different complete outfits for the various forms of projection.

FIG. 32. MAGIC LANTERN IN OUTLINE TO SHOW THE PARTS.
(Cut loaned by Williams, Brown & Earle).

At the left, the side of the lamp-house is removed to show the hand-feed,
right-angled arc lamp with the supply-wires and the carbons in position.

C D The condenser composed of two plano-convex lenses. In the space
(0) a water-cell may be inserted.

G The oblong opening, just in front of the condenser, into which the slide
carrier is inserted.

A The projection objective fastened to the end piece B, which also holds
the bellows.

E F Set screws serving to fix the apparatus on the guide rods.

SOME AMERICAN MAGIC LANTERNS

[CH. I

FIG. 33. SIMPLE MAGIC LANTERN WITH A TWO-LENS CONDENSER.
(Model C, Balopticon; Cut loaned by the Bausch & Lamb Optical Co.}.

Fn;. 34. MAC, ic LANTERN OF THE LATHE-BED TYPE WITH A THREE-
LENS CONDENSER AND WATER-CELL.

(Model D, Bal optic on; Cut loaned by the Bausch & Lomb Optical Co.).

CH. I]

vSOME AMERICAN MAGIC LANTERNS

PIG. 35. SECTIONAL VIEW OF AN ARC LAMP AND A TRIPLE-LENS
CONDENSER WITH WATER-CELL.

+ W Wire going to the positive carbon.
W Wire from the negative carbon.

He Horizontal or upper carbon ; it is positive.

Vc Vertical or lower carbon ; it is negative.

L The crater of the positive carbon ; it is the source of light.

Cond I The first element of the triple-lens condenser. The meniscus is
always placed with the concavity next the source of light.

Cond 2 The second element of the triple-lens condenser. It is a plano-
convex lens and should be of the same focus as the projection objective. The
different lenses should be in the position shown in this diagram. Between the
two convex lenses in the parallel beam of light is placed the water-cell.

BI B3 Blocks supporting the arc lamp and the condenser.

Base The base-board with the track along which the different parts move
(see fig. 40).

Axis The principal optic axis of the condenser and continuous with that
of the projection objective.

FIG. 36 A. MAGIC LANTERN WITH AN AUTOMATIC LAMP AND INCLINED

CARBONS.
(Cut loaned by P. Keller & Co., successors to the J. B. Coll Co.).

This lantern is very widely used. It has a two-lens condenser (see fig. i).
Its main defect is that every part, lamp, condenser lantern-slide holder and
objective can be separately raised or lowered.

SOME AMERICAN MAGIC LANTERNS

[On. I

FIG. 36 B. CONDENSER LENS NEXT THE
RADIANT IN ITS MOUNT.

This is a'picture of the end of the condenser next the radiant of the lantern
shown in fig. 36 A.

The lens is held in place by four thin metal supports, fastened at one end to
the condenser mount, and hooked over the edge of the condenser at the
other. The lens is considerably smaller than the condenser mount, thus
giving abundant room for expansion.

/, 2, j, 4. The four thin metal strips for holding the lens in position. They
are white where they hook over the edge of the lens.

c End view of the metal tube supporting the condenser.

(The white spots in the condenser face are mirror images of the window's near
where the picture was taken).

FIG. 37. MAGIC LANTERN WITH TWO-LENS
CONDENSER, AND HAND-FEED ARC LAMP.

(Portable Sciopticon. Cut loaned by the Mclutosh Stereopticon Co.).

CH. I]

SOME AMERICAN MAGIC LANTERNS

FIG. 38 A. SIMPLE MAGIC LANTERN WITH TWO-LENS CONDENSER AND
A HAND-FEED ARC LAMP WITH RIGHT-ANGLED CARBONS

AND WATER-CELL.
(Model 2, Delineascope. Cut loaned by the Spencer Lens Co.).

FIG. 38 B. DETAILS OF MODEL 2, DELINEASCOPE.

(Cut loaned by the Spencer Lens Co.).

The entire instrument is in one metal box.

At the left is the right-angled arc lamp with the feeding and fine adjustment
screws.

The condenser is of the two-lens type with a water cell (W C) between the
lenses.

.V P S1 The slide-carrier is a flat frame on which the slides are laid and
turned to a vertical position by the crank L.

S When the crank L turns a slide into position the one already in position
is released and it falls down the curved incline to S where it can be removed.

L 0 The projection objective. Its conical holder is hinged so that it can
be readily turned aside to give place to the projection microscope, which, in
the figure, is turned over on the top of the lantern box.

DO AND DO NOT WITH THE MAGIC LANTERN [Cn. I

§ 99i. Summary of Chapter I :

1 . Connect both supply wires
to the arc lamp as indicated in
fig. 3, i. e., connect the positive
wire with the binding post of
the upper carbon and the nega-
tive wire with the binding post
of the lower carbon.

Make sure of the polarity
(§ 80).

2. Always use a rheostat or
other balancing device with an
arc lamp (§6).

3. Insert the rheostat along
one wire (fig. 1-4).

4. Insert the ammeter along
one wire (fig. 2,4).

5. Always have a double-pole
switch on the lantern table (fig.
i-3).

6. Insert the switch along
both wires, and before the
rheostat, so that all the appara-
tus on the lantern table has no
current when the switch is open

(fig. 3).

7. Always open the switch
before changing any of the
wires.

Do NOT

i. Do not connect the nega-
tive wire to the upper carbon
and thus make the polarity
wrong.

2. Never try to use an arc
lamp without a balancing device
—(rheostat, etc.).

3 . Do not connect both wires
with the binding posts of the
rheostat, but insert it in one
wire.

4. Do not connect the am-
meter with both wires. Insert
it in one wire.

5. Do not try to get along
without a double-pole table
switch.

6. Do not insert the switch
along one wire, but connect it
with both wires. Do not put
the switch after the rheostat,
etc., but before.

7 . Never change wires on the
apparatus until the current is
turned off by opening the
switch.

CH. I] DO AND DO NOT WITH THE MAGIC LANTERN

8. Open the switch before
inserting or changing carbons.

9. Center the parts of the
lantern when it is first installed
(§ 51-60).

10. When the condenser and
objective are once centered they
should be fixed in position

8. Do not try to insert car-
bons when the current is on.
Open the switch.

9-10. After the parts of the
lantern are once centered, never
change the position of the con-
denser or objective for center-
ing:.

1 1 . Use the fine adjustments
on the arc lamp (fig. 3) for cen-
tering the light on the screen
after the first centering. Look
at the carbons through the
lamp-house window occasionally
to make sure that they arc in
the correct relative position
(§ 79).

12. Make sure that the arc
lamp and condenser, the con-
denser and objective are separ-
ated the right distance (§ 55-
56).

13. For the triple condenser
select a condenser lens to go
next the lantern slide which
shall be of approximately the
same focus as the projection
objective, then the light from
the condenser will cross at the
center of the objective (fig. 1-2).

ii. Do not fail to keep the
light centered by the use of the
fine adjustments on the lamp
and by keeping the carbons in
the correct relative position.

12. Do not try to use the
lantern when the arc lamp and
condenser are too near together
or too far apart.

The same for the condenser
and objective.

13. Do not try to use an
objective with a condenser that
does not cross its rays at the
center of the objective. Objec-
tive and condenser should have
the same focal length approxi-
mately.

DO AND DO NOT WITH THE MAGIC LANTERN

[Cn. I

14. Make sure that the con-
denser is arranged with the
proper lens next the radiant.
If a three-lens condenser, the
meniscus should face the source
of light ; if a two-lens condenser,
it is the lens in a special mount-
ing (fig. 36 B), or if there is no
special mounting, it is the one
of shorter focus usually, i. e., of
15 to 19 cm. (6-7^ in.), while
the one next the objective is
often of longer focus.

15. Mark or "spot" the lan-
tern slides so that they may be
inserted in the lantern correctly
(§ 23, fig. 7, 8, 13) and arrange
the slides as desired before the
exhibition (§ 21).

1 6. Make sure that every-
thing is in working order, the
room properly darkened, and
the proper amount of current
available (10 to 15 amperes).

1 7 . Light the arc lamp before
the room lights are turned off
(§ 33)-

1 8. Keep the arc lamp burn-
ing until the room lights are
turned on (§34).

19. After the last slide, show
simply a lighted screen (§ 34).

14. Do not reverse the ends
of the condenser and thus have
the wrong lens next the light
and the wrong one next the
objective.

15. Do not try to exhibit
slides that are not in order and
not marked for insertion in he
carrier.

1 6 . Do not attempt an exhibi-
tion unless the room is properly
darkened, and the apparatus in
working order.

17. Do not let the room get
dark, but turn on the arc lamp
before the room lights are out.

1 8. Do not turn out the arc
lamp until the room lights are
turned on.

19. Do not keep the last
slide in the holder too long, but
show a light screen to indicate
that the last slide has been
shown.

CH. I] DO AND DO NOT WITH THE MAGIC LANTERN

20. Study the "Troubles,"
their causes and remedies (§62-
98).

21. Focus the screen image
sharply, using opera-glasses, if
necessary (§38).

20. Do not fail to study the
"Troubles" and their remedies.

21. Do not let the screen
image appear vague and out of
focus. Do not forget the aid
opera-glasses will give, if the
screen distance is great.

CHAPTER II.

THE MAGIC LANTERN WITH AN ALTERNATING
CURRENT ARC LAMP AND ITS USE

§ 100. Apparatus and Material for Chapter II:

Suitable room with screen (Ch. XII) ; Magic lantern with lan-
tern table (§ 102) ; Arc lamp for alternating current with suitable
carbons (§ 1 08); Alternating current supply ; Rheostat, choke-coil
or other balancing device (§ 105-106); Ammeter for alternating
current (§ in); Incandescent lamp, flash-light, gloves with asbes-
tos patches, testing lamp, fuses, extra condenser lenses, screw
driver, pliers, opera-glasses, lantern slides as in Ch. I (§ i).

§ 101. For the historical development of the alternating cur-
rent arc lamp see the Appendix ; and for the character and advan-
tages and disadvantages of alternating current see § 652-653, and
modern works on the subject.

The same books of reference given in § 2, Ch. I, are available for
this chapter.

COMPARISON OF ALTERNATING AND DIRECT ELECTRIC
CURRENTS AND LANTERNS

§ 102. A magic lantern for alternating current may be pre-
cisely like one for direct current, the only essential difference being
that the arc lamp must be of the hand-feed type and the mechanism
for feeding the carbons gives equal movement to the upper and to
the lower one, both carbons being of the same size.

One would never use an alternating current with the magic
lantern if direct current were available. It frequently happens,
however, that the lighting system of a place is of the alternating
current type, and no direct current is available. In such a case
one must make the best of it, or use a motor-generator set or a
rectifier (see § 682-683).

The objections to an alternating current for the arc lamp in
projection are: (i) The lamp is noisy; (2) It requires about two
and one-half times as much current for the same effective light.

CH. II] ALTERNATING AND DIRECT CURRENT LANTERNS 69

That is, if 10 to 12 amperes of direct current give satisfactory
illumination in a given case, it would require from 25 to 30
amperes of alternating current to give the same brilliancy of
screen image. Naturally also the heating with the larger alter-
nating current is greater than with the smaller direct current
(see also § 768).

§ 103. The difference between direct and alternating current is,
in general terms, this : the direct current has a constant polarity
and one carbon is always positive; while the alternating current
has an alternation of polarity, as the current flows in one direction
for an instant and then in the opposite direction. The result is
that each carbon is positive half the time and negative half the
time, hence both carbons have brilliant craters from which light
for the screen image might be obtained. Sometimes an effort is
made to utilize the light from both craters by the arrangement of
the carbons in the form of a V, the apex of the V pointing toward
the condenser (fig. 230).

INSTALLATION OF A MAGIC LANTERN WITH AN ALTERNATING
CURRENT ARC LIGHT

§ 104. Wiring from the supply to the lantern. — This is pre-
cisely as for the direct current lamp. If the lantern is to be used
for experimental purposes it is advantageous to have an incandes-
cent lamp inserted in the circuit as shown in fig. 2.

§ 105. Rheostat or other regulating device. — There must be
introduced along one of the supply wires to the lantern some form
of balancing device. This may be in the form of a rheostat like
that vised for the direct current (§ 6); an inductor or choke-coil,
a transformer, or a mercury arc rectifier may be used. For the
special advantages and disadvantages of the different balancing
devices (see § 736-738).

§ 106. Wiring the lamp. — For the alternating current it makes
no difference which supply wire is connected with the upper carbon,
as each carbon has an approximately equally brilliant crater.

70 ALTERNATING AND DIRECT CURRENT LANTERNS [Cn. II

But in installing a magic lantern for either current, it must
never be forgotten that the arc lamp must not be connected with the
main line without some form of rheostat or regulating device in the
circuit (fig. 3, 40, and § 744).

FIG. 39. MAGIC LANTERN WITH INCLINED CARBONS.

U C, L C The upper and the lower carbon. Only the carbons of the arc
lamp are shown.

A C Alternating current supply wires.

F Fuses at the outlet box (see fig. 40).

L Incandescent lamp for use in working around the magic lantern.

S Double-pole, knife switch.

R Rheostat in one wire.

A Ammeter for indicating the amount of current.

Condenser A two-lens condenser. The light is shown as a parallel beam
between the lenses. It is usually diverging (see fig. i).

L S Lantern slide next the condenser.

Axis Axis The principal optic axis of the condenser and the projection
objective.

Objective The projection objective for forming the screen image.

c Center of the projection objective. The objective and condenser should
be so related that the light from the condenser crosses at the center when the
image is in focus on the screen.

Screen Image The image of the lantern slide on the screen.

§ 107. Double-pole table switch. — This is especially necessary
when using an alternating current, because with it the current can
be turned completely off the lamp whenever desired. Any changes
in the carbons or in the lamp mechanism can then be made with
safety, as the lamp is completely cut off from the electric supply,
which would not be the case if a single-pole switch were used.
The shock from an alternating current supply of no volts is much

CH. II] ALTERNATING AND DIRECT CURRENT LANTERNS 71

more disagreeable than from a direct current supply of the same
voltage.

FIG. 40.

MAGIC LANTERN SHOWING THE WIRING AND THE RELATION
OF THE PARTS.

Supply Wires Wires from the electric supply to the outlet box.

Outlet box The iron box receiving the supply wires and containing fuses of
the cartridge form, a double-pole knife switch and the wires extending to the
wall receptacle.

P W R Polarized wall receptacle from which is taken the current to supply
the arc lamp of the magic lantern. As this receptacle is polarized the cap can
be put on but one way, and hence the polarity will always be the same if
the current is direct. With alternating current this form of connection is
also good.

Arc Supply The wires extending from the wall receptacle to the table
switch and the arc lamp.

Switch The double-pole, knife switch on the lantern table.

Wf The wire extending from the switch to the upper carbon.

W2 W3 Wire from the table switch through the rheostat to the lower
carbon.

Arc Lamp Hand-feed, right-angle carbon arc lamp.

F S Feeding screws for the carbons.

V A Fine adjustment for moving the source of light vertically.

L A Fine adjustment for moving the source of light laterally.

in in Insulation between the carbon holder and the rest of the arc lamp so
that the current will keep to the carbons instead of short circuiting through the
lamp.

5 5 Set screws for holding the carbons in place, etc.

Lamp-House The metal box enclosing the arc lamp. The feeding and fine
adjustment screws project through the back end of the lamp-house.

V Ventilator of the lamp-house.

Condenser The three-lens condenser.

Water Cell The vessel of water in the path of the beam.

72 MAGIC LANTERN WITH ALTERNATING CURRENT [Cn. II

/ The first element of the condenser consisting in a meniscus lens next the
arc lamp and a plano-convex lens.

2 Plano-convex lens toward the lantern slide. The lenses of this condenser
should be arranged as here shown.
Objective The projection objective.

c The optic center where the rays from the condenser should cross when the
objective is in focus.

Base Board The board bearing the track and the blocks for supporting the
different parts.

Block i, Block 2, Block j The blocks supporting the arc lamp, condenser
and objective.

Rods The rods or tubes on the base-board and serving as a track for the
blocks to move upon.

§ 108. Arc lamps for alternating current. — These are almost
invariably of the hand-feed type. Lamps are made to hold the
carbons: (i) at right angles (fig. 1-3); (2) inclined backward 30
degrees (fig. 23, 39); (3) converging in the form of a V (fig. 23
D); or (4) even in a vertical position (fig. 22). Each form is
best adapted to some special purpose.

With carbons of the same size and composition both carbons
burn away at the same rate, and therefore must be fed forward at
the same rate. If the carbons are of different size or material, then
the mechanism must be adjusted to move the two at a rate which
shall hold the ends at the same level.

§ 109. Fine adjustments for the lamp. — As indicated for the
direct current arc lamp (§ 10), there should be some means of
moving one or both carbons separately to compensate for any
unequal burning. There must also be some means of raising and
lowering the lamp and moving it sidewisc so that any slight varia-
tions of the source of light from the axis may be corrected
(§ 10, fig. 3).

§ 110. Lamp-House. — There should be a well ventilated metal
lamp-house of good size and with large doors, so that all the
apparatus within can be easily got at. There should also be a good
sized window (say 5 cm., 2 in. square) glazed with smoky mica
or a combination of green and red glass or some smoked glass of
sufficient depth of tint for the protection of the eyes. This window

CH. II] MAGIC LANTERN WITH ALTERNATING CURRENT 73

should be opposite the craters of the electrodes, so that the position
of the carbons can be readily seen (fig. 133, 145).

§ 111. Ammeter for alternating current. — The ammeter serves
the same purpose for the alternating as for the direct current;
that is, it indicates the amount of current (§7). The construction
for the alternating current is somewhat different, so that the one
for direct current cannot be used for alternating. On the other
hand excellent ammeters are now constructed which can be used
for both alternating and direct currents (§ 664, 7o2a).

§ 112. Mechanical centering in a horizontal axis. — This is done
precisely as for the direct current lantern (§ 51, fig. i, 2 and 40) .

§113. Amount of current necessary. — In genera) it requires
from two and one-half to three times as many amperes of alter-
nating current to get the same brilliancy of image as of the direct
current (see § 755-768). Then for a screen distance of 10 meters
(30 feet) one should have a current of about 25-30 amperes; and
for a distance of 15 to 25 meters (50-75 ft.) one should use from 30
to 45 amperes. If one can be satisfied with less brilliant screen
images, of course the amount of current may be somewhat less.

For a further discussion of the comparative merits of direct and
alternating currents, and means of changing alternating to direct
current see Ch. XIII, § 755-756, 682-683.

USE OF THE MAGIC LANTERN WITH ALTERNATING CURRENT
FOR EXHIBITIONS AND LECTURE DEMONSTRATIONS

§ 114. The suggestions for the lecturer arc as in Chapter I
(§ 21-40).

§ 115. Suggestions for the operator. — These arc the same as
when using the direct current arc lamp (§ 26-42), except that in
using the alternating current arc lamp more care is required to get
good results.

(i) The carbons must be properly proportioned to each other.
If they arc of the same composition they should be of the same size.
If one is solid and the other cored, the solid one is smaller (§ 753a).

74 MAGIC LANTERN WITH ALTERNATING CURRENT [Cn. II

(2) As there are two sources of light it is necessary to take
special pains to focus the lantern slide very sharply on the screen,
or, when the carbons burn away so that the sources of light are
relatively far apart, the image on the screen will appear partly
double like print that has slipped on the press, or like color printing
when the impressions do not register, thus giving two partly super-
imposed images, especially if the carbons are arranged like a V.

If the image is sharply focused and the carbons kept close
together this trouble will be avoided.

(3) The carbons must not be allowed to burn away too far
before they are fed up, or the lantern will become very noisy. The
carbons should be kept about three mm. (Y% in.) apart. This will
involve feeding them toward each other every five minutes (see
also § 131, 753a).

A pair of gloves with asbestos patches (fig. 5) should be at hand
when working about the alternating current lamp.

Practically all of the magic lanterns found in the open market
may be used with an alternating lighting system, provided a lamp
designed for the alternating current is used (§ 102, fig. 3).

TROUBLES WITH A MAGIC LANTERN WITH ALTERNATING
CURRENT ARC LAMP

§ 116. Noisy arc. — There is no way of entirely obviating the
noise in an arc lamp with alternating current. It may be kept at a
minimum by using carbons of the proper size for the amperage used
(§ 753a) and by keeping them relatively close together. As the
carbons burn away, increasing the length of the arc, the noise
increases. If a heavy current (much amperage) is used the noise
becomes very loud and disagreeable.

The noise is also increased if there is any loose part around the
rheostat or lamp which can vibrate in unison with the alternations
of the current.

§ 117. Managing the arc lamp. — Practically all of the arc

lamps used for the magic lantern with alternating current are of the
hand-feed type, hence besides all the other things the operator

CH. II] MAGIC LANTERN WITH ALTERNATING CURRENT 75

must see to it that the carbons are brought toward each other
occasionally by turning the proper screws. With moderate cur-
rents the lamp will run from five to ten minutes without feeding,
but the greater the amount of current the oftener must the carbons
be fed together. As stated above, the noise increases with the
length of the arc ; therefore the carbons should be brought nearer
together every two to four minutes.

§ 1 18. Shadows on the screen. — All the defects indicated under
"troubles" in chapter i (§83) for the direct current light are liable
to appear when using alternating current. This is somewhat
complicated by the presence of an equally brilliant crater on both
the upper and the lower carbons. As with direct current, there is
less trouble with right-angled carbons than with vertical or inclined
ones. With right-angled carbons the defect is greatest when the
lower carbon is too high, thus shading the upper carbon, as in fig.
25 A (for the shadows see fig. 24-25, 27-29). As with the direct
current, the greater the aperture of the projection objective, the
less marked is the screen defect of a slight mal-position of the car-
bons. (See also Ch. Ill, § 127, Ch. IX, § 417, and Ch. X, § 488 for
the arc lamp with small carbons to be used on the house lighting
system).

76 SUMMARY FOR ALTERNATING CURRENT LANTERNS [Cn. II

§ 119. Summary of Chapter II:
Do

i. Connect both supply wires
with the lamp; and remember
that with the alternating cur-
rent lamp it makes no difference
which supply wire goes to the
binding post of the upper and
which to the post for the lower
carbon (§ 106).

2. Insert a rheostat or other
balancing device along one of
the supply wires (fig. 3).

3. Insert the ammeter along
one wire (fig. 2).

4. Install a double-pole switch
before the rheostat (fig. 3).

5. If the lantern table is on a
concrete floor, use a board or
insulating mat to stand on and
thus avoid possibility of a shock
if the metal part of the lantern
is touched (§ 98, 689).

6. Feed the carbons nearer
together every three to five
minutes so that the lam]) will
not be noisy or go out or give
double screen images.

Do NOT

i . Do not fail to connect both
supply wires to the arc lamp.

2. Never try to use an arc
lamp without a rheostat or
balance. Do not connect the
rheostat with both, but with a
single wire.

3. Do not connect the am-
meter with both supply wires,'
but with one.

4. Do not install a lantern
without a double-pole, table
switch which will cut off the
current from all the apparatus
on the lantern table (fig. 40).

5. Do not stand directly on a
moist concrete floor when oper-
ating a magic lantern with an
alternating current lamp.

6. Do not let the lamp go too
long before feeding up the car-
bons.

CH. II] SUMMARY FOR ALTERNATING CURRENT LANTERNS 77

7. Focus the screen image
with special care when using
alternating current lest the two
sources of light produce a doub-
ling of the screen image.

8. Use opera-glasses, if neces-
sary, for focusing sharply a
distant screen image (§38).

9. Look out for shadows on
the screen. Center carefully
and remove all causes for shad-
ows (§83-93).

7. Do not forget the greater
need for accurate focusing with
an alternating current lamp, on
account of the double source of
light.

8. Do not forget the advan-
tage in using opera-glasses for
focusing if the screen distance
is great.

9. Do not permit any defect
in the management of the lan-
tern, suspended strings, etc., to
give shadows on the screen.

10. Study the "Troubles" in
§ 116-118, and 62-98.

10. Do not neglect any of the
causes for "Troubles."

CHAPTER III.

MAGIC LANTERN TO BE USED ON THE HOUSE
ELECTRIC LIGHTING SYSTEM

§ 120. Apparatus and Material for Chapter III :

Suitable room and screen (Ch. XII) ; Magic lantern with lamp-
house and lantern table; Arc lamp for small carbons (§ 127);
Rheostat (§ 129); Flexible cable for connecting the lamp and
rheostat with the house lighting system (fig. 40) ; Separable plugs
and extension plugs (fig. 49-50); Polarized plugs (fig. 48-49);
Nernst lamps (fig. 54-55); Objective shield (fig. 14); Concen-
trated filament, Mazda lamps (§ 136) ; Flash-light; testing lamp,
screw drivers and pliers; lantern slides, etc., as in Ch. I.

§ 121. For the historical summary of the use of the house,
electric lighting system for the magic lantern, see the Appendix.

For works of reference see § 2. Consult also the Microscopical
Journals, and the catalogues of manufacturers of projection
apparatus.

MAGIC LANTERN WITH SMALL CURRENT ELECTRIC LIGHTS FOR
LABORATORY AND HOME USE

§ 122. For public exhibitions and large lecture rooms special
electric wiring and large current arc lamps are necessary, as
described in Ch. I, II and XIII. For small audiences as in labora-
tories and for home use, where less than 100 people are usually
present, very satisfactory results may be obtained by means of
lighting apparatus drawing current from the ordinary house light-
ing system ; and the electric current may be direct or alternating.

§ 123. Kinds of lamps to be used with small currents. — There
are three forms of lamps which have been successful for use with
the magic lantern drawing current from an ordinary lighting
system :

(i) An arc lamp of small size using small carbons, i. e. carbons
of 6 to 8 mm. (% to r/i<; in.) in diameter. A large arc lamp
is equally available if it has long clamping screws, bushings or

CH. Ill] MAGIC LANTERN WITH SMALL CURRENTS

FIG. 41. THE LILIPUT ARC LAMP OF LEITZ.

This lamp was designed to use with the Edinger drawing apparatus and with
the condenser for dark ground illumination, etc. Both carbons are moved
equally by means of the rack and pinion movement. For direct current the
horizontal or positive carbon is larger than the vertical or negative carbon in
the proportion of 8 to 6.

The condensing lens in the tube is mounted in a telescoping sleeve. When
the sleeve is in, the lens is at its principal focal distance from the crater, and
gives a parallel beam of light. When the sleeve is pulled out more or less the
condenser gives a converging beam of light.

For use with the magic lantern the tube and special condenser are removed,
as shown in fig. 47.

adapters for the small carbons. Such carbons require from three
to six amperes of current for the best effect (fig. 41-44).

(2) A Nernst lamp with one or more filaments (fig. 54-55).

(3) A Mazda lamp with concentrated filament (fig. 52).

The arc lamp is permanent. One has simply to renew the
carbons when they are burned out.

If alternating current is used, carbons 150 mm. (6 in.) long and
8 mm. (5/i<5 in.) in diameter last about three hours.

If direct current is used the upper carbon is 8 mm. (5/ir, in.)
and the lower carbon 6 mm. (^4 in.) in diameter. Both are 150
rrm. (6 in.) long, and they last about three hours (§ 753a).

The Nernst and Mazda lamps are fragile and must be handled
carefully. They have a working life of 500 hours, more or less,
then a new lamp must be obtained.

So MAGIC LANTERN WITH SMALL CURRENTS [Cn. Ill

§ 124. Room for projection. — Any room may be used at night,
and this makes these magic lanterns especially adapted for the
home.

In the daytime, of course, the room where they are used must
have shutters or curtains so that it can be darkened.

§ 125. Screen for the image. — The screen need not be over
three or four meters square (9-1 2 feet) . For many purposes a large
sheet of cardboard, 72x120 cm. (28x44 in-) makes the best
possible screen (see Ch. XII).

For home use a white wall or a well stretched sheet will serve.
If the screen is to be used frequently in the same place in the
laboratory or home it is desirable to use a white wall or a regularly
painted screen (see § 621-630).

§ 126. The magic lantern and its support. — Any of the good
modern forms of magic lantern can be used. Special small and
compact lanterns have been constructed for this purpose, and they
are excellent and cheap (see prices in the appendix) (fig. 51-52).

For a lantern support any table of sufficient height may be used.
A pile of books or an empty box on an ordinary table will serve
to raise the lantern sufficiently.

ARC LAMPS FOR THE HOUSE CIRCUIT

§ 127. Small arc lamps, using small carbons only, are con-
venient ; but the ordinary large arc lamp can be used if the screws
for clamping the carbons are long enough, or by means of bushings
or adapters for the small carbons (for wiring and rheostat, see
§ 128-129).

The small carbon arc lamps are easily managed, and the amount
of light they give (see § 756) much more than offsets the attention
they require over the other lamps used on the house circuit.

If a lamp must be purchased for use on the house circuit, one of
small size is preferable. They arc designed for the small carbons
only. They arc nearly always of the hand-feed type, but when
direct current is available there arc automatic lamps to be had.
The Thompson automatic arc lamp, and the Bausch & Lomb

CH. Ill] MAGIC LANTERN WITH SMALL CURRENTS 81

FIG. 42. THE SMALL ARC LAMP OF THE SPENCER LENS Co.

With this small arc lamp the two carbons may be moved separately or
together, as the carbon movement is like that of the larger lamps, i. e., one shaft
within the other, and the corresponding milled heads are placed close together,
so that either can be turned separately or both together.

It is arranged for giving parallel or converging light. When used with the
magic lantern the special condenser and its tube are removed (fig. 47).

automatic lamp are so adjusted, or may be so adjusted if desired,
that they will work with currents ranging from 5 to 25 amperes

(fig. 41-44)-

The small lamps (from their size, called "Liliput or baby" arc
lamps) are largely used for darkground illumination and ultra-
microscopy and for drawing. For these purposes they have a tube
attached with a condensing lens (fig. 41). For use with the magic
lantern the tube and condensing lens are removed (fig. i).

ARC LAMPS WITH SMALL CURRENTS [Cn. Ill

FIG. 43. THE SMALL ARC LAMP OF REICHERT.

This is arranged in the figure for giving a parallel beam of light from the
small condenser; and the mechanism for feeding the carbons can he actuated
at a distance by means of a Hooke's joint and rod.
The horizontal or positive carbon.

Clamp for holding the lamp to the upright at any desired height.
Milled head of the feeding mechanism for the carbons.
Rod extending from the Hooke's joint.
e- ?,••> f- S- Holders and clamping screws for the carbons.
/ Terminal points of the carbons where the arc is formed.
ra The tube holding the condensing lens. It is cut away on one side to
show the carbons.

k' ' n The condensing lens in the end of the tube. It is at the principal focal
distance from the crater and the diverging beam is made parallel; by pulling
it to the right the beam will be converging.

WIRING AND CONNECTING THE ARC LAMP WITH THE HOUSE

CIRCUIT

§ 128. Wiring. — The wiring is in principle exactly as for the
large current arc lamp (fig. 1,2, 45).

One end of a double, flexible cable of sufficient length (2 meters,
6 ft. at least) is connected with a separable attachment plug (fig.
49). The two wires near the other end of the cable arc separated
for a short distance, and one wire is cut. The cut ends of this wire

CH. Ill]

ARC LAMPS WITH SMALL CURRENTS

are then inserted into the binding posts of the rheostat (fig. 45).
This puts the rheostat along one supply wire (in series) .

The cut ends of the cable are then connected with the binding
posts of the arc lamp (fig. 45). For polarity see § 701.

§ 129. Rheostat or other balancing device. — As with the arc
lamp for heavy currents, those to be used on the house circuit must
also have a balancing device of some sort like a rheostat. It must
be in one wire (fig. 45).

Never try to use an arc lamp on any circuit without a rheostat
or other balancing device. If one is not used the fuses will be
burned out.

FIG. 44. REICHERT'S AUTOMATIC ARC LAMP FOR USE ox THE HOUSE
LIGHTING SYSTEM IF DIRECT CURRENT is AVAILABLE.

At the bottom are screws for fine adjustment, laterally or vertically.

§ 128a. In modern wiring for incandescent lamps each group of not over
1 6, or in special cases not over 32, lamp sockets must be protected by a fuse
or cut-out. The wire must be equivalent to a copper wire No. 14 or No. 18
B. & S. gauge, and the fuse or cut-out must be for not over 10 amperes (usually
6 amperes) for a 1 10 volt circuit. This is sufficient for the small arc lamp.

In the older constructions where only one to three lamps were on a single
line, very weak fuses were used which would melt if over two or three amperes
were drawn from the line. Naturally, on a house circuit thus wired and fused,
the fuses would be burned out if one tried to use the small arc lamp upon it, for
that rarely draws less than four amperes and often as many as six.

In using the arc lamp on the house circuit it is therefore necessary to make
sure that the wiring and fuses arc of sufficient capacity for the current needed.

ARC LAMPS WITH SMALL CURRENTS

[CH. Ill

The rheostat needed for the small-current, arc lamp is small and
inexpensive. It need not be adjustable. One has only to be cer-
tain that it will not deliver a current above five or six amperes.

In purchasing a rheostat for the house circuit, tell the manufac-
turer the kind of current (direct or alternating) and the voltage
(no or 220). If one does not know the character and voltage of
his house circuit the information can be obtained at the office of
the company furnishing the current.

Lamp Socket s P

FIG. 45. WIRING AND CONNECTIONS OF THE ARC LAMP USED ON THE
HOUSE LIGHTING SYSTEM.

§ 130. Polarity with the arc lamp. — With alternating current
both wires are the same (see § 103 and 653), but with direct current
one of the wires is positive and one negative, and the positive wire
must be connected with the binding post for the upper carbon.
The most practical ways of determining the polarity are described in
Ch. I, § 80; Ch. XIII, § 702.

In case the lower carbon shows the brightest crater it is positive
and hence the polarity wrong. If the separable attachment plug is
of the polarized form, separate the two parts thus turning off the
current. Then reverse the position of the wires in the binding
posts of the lamp. This will connect the positive wire with the
upper carbon as it should be. A simple way, if non-polarized plugs

CH. Ill]

ARC LAMPS WITH SMALL CURRENTS

are used (fig. 496), is to leave the wires as they are in thelamp.but
pull the separable plug apart and turn it half way round. This will
reverse the position of the connections so that the polarity will be
found correct on lighting the lamp again.

When the correct polarity has been obtained at one particular
lamp socket it is well to make a straight line with a glass pencil, a
pen or a brush across the socket, and the two parts of the separable
plug, then the correct connections can be made with that socket at
any time without trouble.

Arc Lamp

FIG. 46. THE MAGIC LANTERN FOR USE ON THE HOUSE LIGHTING

SYSTEM.

SW Supply wires to the lamp socket (So).

So, K The lamp socket with the key switch.

S — P Separable attachment plug. The cap has been removed to show
the metal prongs serving to make the contact.

L W Wires connecting the cap of the separable plug with the knife switch.
(K S). As shown in fig. 45, 47, the knife switch is more frequently omitted.

K S Double-pole knife switch for opening and closing the circuit.

Rheostat For controlling the current. It is in one wire.

Arc Lamp This is one of the small forms.

5 5 Set screws for holding the carbons in place.

h c Horizontal or upper carbon.

v c Vertical or lower carbon.

In In Insulation between the carbon holders and the rest of the lamp to
compel the current to follow the carbons, and not to short circuit.

fs Feeding screws for moving the carbons.

cl Clamp to fix the lamp at any desired position on the vertical rod.

Condenser The two-lens condenser for illuminating the lantern slide.

i 2 The two plano-convex lenses with their curved surfaces facing each
other.

L S Lantern slide close to the condenser.

Axis Axis The principal optic axis of the condenser and the objective.

Objective The projection objective for giving the screen image.

Image Screen The white screen on which the image is projected.

ARC LAMPS WITH SMALL CURRENTS

[CH. Ill

Condenser

FIG. 47. THE MAGIC LANTERN WITH A THREE-LENS CONDENSER AND A
WATER-CELL FOR USE ON THE HOUSE LIGHTING SYSTEM.

This is the same as fig. 46 except that no double-pole knife switch is used, and
there is a triple-lens condenser and water-cell in place of a double-lens condenser.

It is well also, when one has the lamp properly connected, to turn
off the current by opening the separable plug, and then paint the
positive wire red where it is inserted into the binding post for the
upper carbon. The negative wire can be painted black also. If

HUBBEuOf

FIG. 48. WALL RECEPTACLES WITH SEPARABLE CAP.
(Cuts loaned by II. Ilubbell, Inc.).

A Wall receptacle with the connecting prongs polarized so that the cap
can be put on only one way, thus avoiding change of polarity with direct cur-
rent.

B Wall receptacle in which the cap can be put in place either way around.

Either form can be used with both direct and alternating current.

CH. Ill]

ARC LAMPS WITH SMALL CURRENTS

these precautions are taken, it will be very simple to connect up
the lamp correctly at any time.

"Polarized attachment and extension plugs" are made (fig. 48A,
4pA). These can only be put together one way. They are very
convenient for direct current connections; they are also equally
adapted for alternating current.

FIG. 49. SEPARABLE ATTACHMENT PLUGS.
(Cuts loaned by II. Ilubbell, Inc.}.

A Polarized, separable plug for a lamp socket. The metal prongs are in
planes at right angles and hence can be inserted in only one way, thus avoiding
change of polarity with direct current.

B Non-polarized attachment plug. The connection can be made either
way around as the prongs are in the same plane.

FIG. 50. SEPARABLE EXTENSION CONNECTOR.
(Cut loaned by II. Ilubbell, Inc.).

This is to enable one to extend the line by joining separate cables. These
extension connectors can be had with polarized or non-polarized prongs to the
cap.

§ 131. Carbons for small currents; feeding the carbons. — For

the small currents used with the house circuit, the carbons should
be small. For alternating current of five to six amperes, 8 mm.
carbons answer well. For three to four amperes the carbons
should not be over 6 mm. in diameter.

ARC LAMPS WITH SMALL CURRENTS [Cn. Ill

For direct current the two carbons must be of different size if the
feeding mechanism of the lamp moves the carbons equally. With
an equal feeding mechanism, the upper or positive carbon can be
7 mm., the lower one 5 mm., or the upper 8 mm. and the lower
one 6 mm.

One could use carbons of the same diameter for direct current,
but it would be necessary to feed the upper or positive one more
rapidly than the lower one on account of the unequal rate of burn-
ing, otherwise the correct relative position of the carbons would
not be maintained (fig. 24-25). On a no volt, direct current cir-
cuit, the lamp will burn about six minutes without going out.
The carbons should be fed up every three to five minutes.

For alternating current of no volts, the small lamps will burn
from eight to ten minutes, sometimes longer. It is well to feed the
carbons every five to seven minutes.

In case a choke-coil is used (Ch. XIII, § 736), the lamp burns
more quietly and will burn longer without being fed. If a step-
down transformer is used, then the right-angled lamp will not burn
so long — only one to two minutes — while a lamp with inclined
carbons will burn three minutes, because it takes a higher voltage
to maintain the right-angled than the inclined carbon arc (see Ch.
XIII, § 753, 768).

TURNING THE ARC LAMP ON AND OFF

§ 132. Lighting the small arc lamp. — For this, make sure that
the carbons arc not in contact. Now turn the switch for the room
lights and the snap switch in the socket where the separable attach-
ment plug for the lamp wiring is screwed in. Feed the carbons
together until they touch. There should be a flash of light.
Separate the carbons two or three millimeters as soon as the flash
is seen and the arc will be established and the light will be at full
brilliance. Sometimes it is necessary to keep the carbons almost
in contact for a half minute or so, until the tips arc well heated,
before the arc will bum. If on separating the carbons the light
goes out, they must be brought together again as at first.

CH. Ill] ARC LAMPS WITH SMALL CURRENTS 89

§ 133. Turning off the small arc lamp. — The snap or key switch
in the usual incandescent lamp socket is designed to break the cir-
cuit where, at most, two amperes are used. These key switches, if
used to interrupt a relatively large current, like that used for the
small arc lamp, are liable to start an arc within the socket. If such
an arc is started, the socket will be short circuited, resulting either
in the burning out of a fuse, the burning out of the socket or some-
thing more serious.

The liability of a socket to arc is much greater with direct than
with alternating current. The liability to arc is also much greater
if the key switch is turned slowly than when it is turned quickly.

By observing the following directions the current may be turned
off with perfect safety :

(1) Turn off the current by separating the carbons until the
lamp goes out, then the key switch may be used, or a plug or exten-
sion pulled apart.

(2) Turn off the current by pulling the separable plug or the
separable extension apart (fig. 49-50).

(3) Make use of a knife- or snap-switch (fig. 1,2, 40).

(4) Do not turn off the current by the key switch in the bulb
socket. When the lamp is out, it is safe to turn the key switch in
the socket.

(5) Do not unscrew a plug to turn off the light, for the
break in the circuit is so slow that an arc will almost certainly
be formed.

§ 134. What to do in case the key switch is used and an arc
is formed in the socket:

(1) Turn the key on again as quickly as possible.

(2) If the arc lamp is still burning after turning on the key
switch, turn the lamp off by method i to 3 (§ 133).

(3) Go to the nearest room switch and turn off the current.

In case a fuse is blown out — which is almost sure to occur if an
arc is formed in the socket — or if the lamp socket is burned out, it
is wise to call in an electrician to make the necessary repairs.
This, of course, assumes that the user has not the technical knowl-
edge necessary to make the corrections himself. It is further

QO MAGIC LANTERNS WITH MAZDA LAMPS [Cn. Ill

assumed that if he had possessed the technical knowledge no mis-
takes, and hence no accident would have happened.

§ 135. Use of the small arc lamp for demonstrations and
exhibitions. — The centering of the apparatus to one axis, and
using the correctly proportioned condenser and projection objec-
tive, the lighting and putting out the lamp, arrangement and
insertion of lantern slides, etc., are all exactly as described in Ch.
I, II (§ 26-41, 52, 112).

FIG. 51. MAGIC LANTERN WITH SMALL ARC LAMP.
(Balopticon B.; Cut loaned by the Bausch & Lomb Optical Co.).

MAGIC LANTERN WITH A MAZDA, CONCENTRATED FILAMENT
INCANDESCENT LAMP

§ 136. Next to the arc lamp the Mazda concentrated filament
lamp is perhaps the best electric light at present available. They
arc as simple to use as an ordinary incandescent bulb. No rheo-
stat is necessary. The lamp is on a stand by which it may be
raised and lowered and brought the proper distance from the con-
denser (fig. 52-53).

§ 137. Connections with the house circuit. — This is made by a
double flexible cable, one end of which is connected with a separable
plug, and the other with the lam]) socket of the Mazda lamp. As
no rheostat is used, and as the light is turned on and off exactly as
for any incandescent bulb, this light is absolutely simple in use.
It gives a light sufficient for a small room, where not over 50 to
IDC people are to watch the exhibition.

CH. Ill] MAGIC LANTERNS WITH MAZDA LAMPS 91

FIG. 52. SIMPLE MAGIC LANTERN WITH INCANDESCENT
LAMP AS RADIANT.

(Cut loaned by Williams, Brown & Earle).

This is known as the "Society Incandescent Lantern No. 3 G." It is
especially designed for use with permanently mounted lantern slides (fig. 15).

§ 138. Centering and distance from the condenser. — The

centering along one axis is as with the arc lamp (§ 51).

In general the concentrated filament should be at the principal
focal distance from the condenser. One can determine the best
position by the use of a good lantern slide and changing the dis-
tance of light and condenser until the best position is found. It is
well to mark that position for future use.

FIG. 53. MAGIC LANTERN WITH MAZDA LAMP.
(Balopticon B.; Cut loaned by the Bausch 6" Lornb Optical Co.}.

This lantern can be used with the small arc lamp on the house lighting sys-
tem, with the Mazda incandescent lamp or with acetylene.

92 MAGIC LANTERN WITH NERNST LIGHT [Cn. Ill

§^139. Management of an exhibition with the Mazda lamp.—

The exhibition should be managed as for the arc light (§ 26-41).

One must remember that with this relatively weak light only a
small screen image should be attempted, and that the room must
be relatively darker than for the arc light. In brilliancy the screen
images will be more like that of the old lanternists with their weak
lights. Clear lantern slides are especially desirable. The very
opaque lantern slides sometimes met with can only be well shown
by a large arc lamp.

MAGIC LANTERN WITH A NERNST AUTOMATIC LAMP

§ 140. This is also an excellent lamp to use with a magic lantern
in a small room. Some forms are automatic in starting when the
current is turned on, and some have to be specially heated. The
automatic form is to be preferred, for it is no more trouble to run
than an ordinary incandescent lamp. It takes some time, usually
one to three minutes, for the glowers to come to full brilliancy after
the current is turned on. They are made for the lantern with one,
two, three and four filaments or glowers. The single glower
approximates most closely to the arc lamp in the smallness of the
source of light. Of course, with the multiple glower lamps a greater
amount of light is given off, but they make an extended source.
Whether the lamp has one or more filaments it can be attached
directly to the house lighting system through any incandescent
bulb socket as described for the Mazda lamp (§ 137).

§ 141. Rheostat or ballast for the Nernst lamp. — This lamp
like the arc lamp is always used with a balancing device, but unlike
the arc lam]), the ballast is an integral part of the lamp as pur-
chased, and not a separate apparatus as with the arc light (fig. 54,
55 and i).

The glowers and the ballast must be adapted to each other
and both must be adapted to the line voltage.

The ballasts, which are enclosed in a vacuum glass, as with an
incandescent bulb, sometimes burn out. The filaments will not
then glow when the current is turned on. If a ballast burns out it
must be replaced by a perfect one.

CH. Ill] MAGIC LANTERN WITH NERNST LIGHT

FIG. 54. NERNST LAMP FOR THE MAGIC LANTERN (REICHERT).

This is on a support and has a rack and pinion for raising and lowering the
lamp. It is automatic.

Gl The three filaments or glowers with this lamp.
S 3^ The two supply wires from the house circuit.
Tt Pinion and milled head of the rack work.
WIt W2, W3 The three ballast tubes.

§ 142. Centering and distance from the condenser with the
Nernst lamp. — The lamp must be centered with the condenser and
objective as described in Ch. I (§ 51+). It must have some form
of support with means of raising and lowering the lamp. The
distance of the lamp from the condenser which gives the best
illumination can be determined as follows: Light the lamp, put a
good lantern slide in the lantern, then move the lamp up toward
the condenser, shifting it back and forth until the best screen image
is produced. In general, it will be found that this results when the
glower is at about the principal focal distance from the condenser.
When the best position is found the place should be clearly marked,
then the lamp can be put in this position quickly at any time.

§ 143. Connecting the lamp with the house circuit ; alternating
current. — This is done by means of a flexible conductor connected

MAGIC LANTERN WITH NERNST LIGHT

[On. Ill

with the lamp at one end, and a separable plug at the other (fig.
49 A). The plug is of standard size and can be screwed into the
socket or receptacle of any incandescent lamp.

To light the lamp turn the key switch of the socket as for an
incandescent lamp, and in a minute or longer the glower or glowers
will attain their full brilliancy, and one can use the lamp as long
as desired without further attention.

FIG. 55. NERXST LAMP OR SCHWANN-LIGHT.

(Cut loaned by the Chas. Beslcr Co.}.

If one uses a three or four glower lamp drawing about four
amperes of current there might be a short circuit in the incandes-
cent lamp socket if the snap switch were turned off slowly. If that
is used, turn it as quickly as possible (see § 133). Pulling the
separable attachment plug apart will avoid all danger (§ 133).

§ 144. Nernst lamp and direct current. — If one has a direct
current lighting system, then the Nernst lam]) must be adapted to
that, and must be connected properly as with the direct current arc
lam]). The two connections with the lam]) are marked, plus (-{-)
and minus ( — ); or positive (P) and negative (N); and the corre-

CH. Ill] MAGIC LANTERN WITH NERNST LIGHT 95

spending wires must be attached to these lamp binding posts or
connections, or the lamp will soon burn out. Unfortunately, one
cannot tell by simple observation when the wires are connected
properly, as for the arc lamp ; but he must determine the polarity
of the wires before connecting them with the lamp (see Ch. XIII,
§ 701-703)-

§ 145. Marking the wires and attachments after determining
the polarity with the direct current system. — When the polarity of
the wires is determined, if one is to use the same place for current
repeatedly, it is a good plan to mark the position of the socket and
the two parts of the separable plug by a straight line of colored
paint when all are in position. Then it will be easy to connect up
the parts correctly at any future time. Then if the positive wire
has its insulation material colored red, at least at the lamp end, it
will enable one to connect up with that particular socket correctly
at any future time. It is also a convenience to have polarized
separable plugs (fig. 4pA) , then the two parts of the plug cannot be
reversed if they should become separated. On the other hand, if
the attached part is left in place and the cap or removable part
pulled off, one can make the connection correctly at any time with-
out trouble, as it cannot be put together wrong.

Unfortunately, one cannot be sure that a separable plug and the
lead wires connected to the lamp properly for one incandescent
socket will be so for any other, and one must determine the polarity
for each socket.

An alternating current is more satisfactory for the Nernst lamp
than a direct current, for with the alternating current one does not
have to trouble about the polarity of the two wires, since both are
alike.

§ 146. Management of an exhibition with the Nernst light. —

This is precisely as for any other magic lantern radiant except that
the lamp must be started three to four minutes before it is needed,
for it may take that time to get good illumination. Furthermore,
it is better to leave the light burning during the entire lecture, so
that there will be no delays. The light can be shut off the screen
during the intervals with the objective shield (fig. 14).

96 TROUBLES WITH SMALL CURRENT LANTERNS [Cn. Ill

§ 147. Troubles with the magic lantern on the house lighting
system. — With the arc lamp these are the same as those indicated
in Ch. I, § 62-98; Ch. II, § 116-118. See also § i28a for fuses.
There is also the danger of starting an arc in the incandescent
socket from which the current is drawn unless the precautions given
in § 133 for turning out the light are observed.

For all the lights the management of the exhibition, centering of
apparatus, etc., are the same as for the lanterns in Ch. I, II.

The most striking difficulty will probably be the comparatively
dim screen pictures as compared with the brilliant screen images
given by the large current arc lamp.

The room must be darker and the screen picture smaller with
these lights.

The Mazda lamp may go out on account of the breakage of some
of the connections within the bulb. If this happens the only thing
to do is to use a new lamp. It is wise to have several on hand.

With the Nernst lamp also some of the connections are liable to
break, or the ballast may burn out, or the glower be broken.
Usually only the defective parts must be renewed, and not an
entirely new lamp obtained.

CH. Ill] SUMMARY FOR SMALL CURRENT LIGHTS

§ 148. Summary of Chapter III:

1. Find out the kind of cur-
rent used in the house lighting
system and the voltage (alter-
nating or direct current; vol-
tage no or 220).

2. Wire the small arc lamp
exactly as the large arc lamp is
wired (fig. 40).

3. Always use a rheostat or
some other balancing device
with the arc lamp (§ 129).

4. Use small carbons for the
arc lamp on the house circuit
(§131)-

5. Make sure of the polarity
if direct current is used (§701-
7°3)-

6. Follow carefully the direc-
tions for lighting the arc lamp
(§ J32).

7. Be very careful to turn off
the arc lamp by one of the safe
methods (§ 133).

8. Make the room darker for
the small arc lamp than for the
large one, and have a smaller
screen picture (§ 139).

Do NOT

1. Do not try to use an arc
lamp on the house circuit with-
out knowing the kind of current
and the voltage.

2 . Do not wire the small lamp
differently from the large lamp
except that smaller wire can be
used.

3. Never use the arc lamp
without a proper balancing
device.

4. Do not use large carbons
for the lamp on the house cir-
cuit, they would not heat
enough to give a good light.

5. Do not worry about the
polarity if alternating current
is used. If direct current is
used the polarity must be
attended to so that the upper
carbon is positive.

6. Do not have the carbons
in contact when turning on the
current.

7. Do not turn off the arc
lamp by the socket switch.

8. Do not expect so much of
the small as of the large current
arc lamp. Do not have the
room too light for the small
lamp.

SUMMARY FOR SMALL CURRENT LIGHTS [Cn. Ill

1. Wire for the Mazda lamp
exactly as for any incandescent
bulb lamp.

2. Turn the lamp on and off
by the key switch as for any
incandescent lamp.

3. As this light is relatively
dim, make the room dark and
project a small picture.

It is also wise to have one or
more extra bulbs in case one
burns out.

Do NOT

1 . Do not use a rheostat with
a Mazda lamp.

2. Do not take any more
trouble with the concentrated
filament Mazda than for any
bulb lamp.

3. Do not try to make too
large a screen picture; and do
not have the room as light as
for the arc lamp.

4. Turn the lamp on and off
whenever desired as it gives full
brilliancy in an instant.

4. Do not let the lamp burn
all the time during an inter-
mittent exhibition any more
than with the arc lamp.

1. Find out the kind of cur-
rent and the voltage wherever a
Nernst lamp is to be used.

2. Purchase a Nernst lamp
adapted to the current with
which it must be used.

Do NOT

1. Do not use a Nernst lamp
with a current and voltage for
which it was not constructed.

2. Do not purchase a Nernst
lamp for direct current if it
must be used on an alternating
current line.

3. A special rheostat or bal-
last forms a part of ever)7
Nernst lamp for projection.

3. Do not insert a separate
rheostat in the wiring for a
Nernst lamp.

CH. Ill] SUMMARY FOR SMALL CURRENT LIGHTS

4. Wire the Nernst lamp just
as the arc lamp is wired except
that no separate rheostat is
inserted. Wire the Nernst
lamp for alternating current
just as a Mazda incandescent
lamp is wired.

5. Wire the Nernst lamp for
direct current with the positive
wire in the binding post marked
+ or P, i. e., the same as a
direct current arc lamp is wired,
except that no separate rheostat
is included (§ 141).

6. Determine the polarity of
the supply wires with precision
and care (§ 701-703).

7. Let the Nernst lamp burn
during the entire exhibition, as
it takes from one to three
minutes for the light to reach
full brilliancy.

8. Shut the light off the
screen when not needed, by the
objective shield (fig. 14).

9. Handle the Nernst lamp
carefully, as it is easily injured.

10. Manage the exhibition
with a Nernst lamp as with any
other light, remembering the
need of a dark room and a screen
picture of moderate size for this
relativelv weak light.

4. Do not worry about polar-
ity in wiring the Nernst lamp
for an alternating current sys-
tem.

5. For a direct current cir-
cuit, do not put the positive
wire in the negative binding
post of the Nernst lamp.

6. Do not neglect the polarity
of the two wires on a direct
current circuit.

7. Do not turn the Nernst
lamp out during an exhibition
for it takes too long to light it.

8. Do not forget to use the
objective shield for shutting the
light off the screen when it is
not needed.

9. Do not handle the Nernst
lamp roughly. It is delicate.

10. Do not expect too much
of a Nernst lamp with the magic
lantern. One cannot have the
room so light, nor project such
large screen pictures, nor use
such dark lantern slides as with
the arc lamp.

CHAPTER IV

THE MAGIC LANTERN WITH THE LIME LIGHT
AND ITS USE

§ 150. Apparatus and material for Chapter IV :

Suitable room with screen (Ch. XII) ; Magic lantern with a suit-
able lamp-house and a lime-light burner (fig. 56-59) ; Cylinders
of compressed Oxygen and Hydrogen (§ 154-155); Lime or other
refractory substance for giving the light (§ 153, 157); Oxygen
generator and ether saturator (§ 177-179); Objective shield (fig.
14, 62, § 169); Tubes for making the connections (§ 159, i59a);
Flash-light, screw drivers and pliers, asbestos-patch gloves (fig. 61) ;
lantern slides, etc.; Matches or gas lighters (§ 160).

§ 151. For the discovery that oxygen and hydrogen burning
together give a very hot flame, and that dazzling light is produced
by directing the flame against lime, etc., and the application to the
magic lantern, see the Appendix.

For works of reference see Chapter I, § 2.

THE LIME LIGHT FOR THE MAGIC LANTERN

§ 152. The Magic Lantern used with the lime light is in every
way like the standard magic lantern with the direct current arc
lamp with the single difference of the source of light.

§ 153. The lime light. — This is one of the most brilliant avail-
able lights for projection purposes. It is produced by directing
the exceedingly hot flame of hydrogen burning in oxygen against a
piece of unslaked lime. The oxy -hydrogen flame in itself is not
brilliant, but the heated lime gives a light of dazzling brilliancy
from a very small area ; hence it is especially well adapted for pro-
jection with the magic lantern and the projection microscope.

If the candle-power of a lime light is compared with the other
lights used for projection it will be seen that it stands third, sun-
light being first and the arc light second.

Hydrogen is not always used, but illuminating gas, the vapor of
alcohol, ether or gasoline sometimes takes its place.

Unslaked lime is not the only refractory substance which gives
great incandescence. Zirconium discs and discs made of the mix-

CH. IV] MAGIC LANTERN WITH THE LIME LIGHT 101

ture of thorium and cereum such as is used in Welsbach mantles
have been employed. Nothing gives a more brilliant incandescence
than the unslaked lime, but it deteriorates rapidly by absorbing
moisture when exposed to the air. This is not the case with 7/ircon
and thorium ; the discs of these may be used over and over, some-
times hundreds of times, while with the limes one usually has to put
a new one in place every time the lantern is used (§ 153 a).

FIG. 56. MAGIC LANTERN WITH THE
LIME LIGHT.

(From the Catalogue of the Enterprise Opt. Mfg. Co.).

The door of the lamp-house is open, showing the burner with the lime in
position.

H The hydrogen supply tube, extending to the burner.
0 The oxygen supply tube, extending to the burner.

§ 154. Oxygen gas in steel cylinders. — This is now a great
article of commerce. Nearly every large drug store keeps one or
more of them in stock for the use of physicians. The steel cylin-
ders for containing oxygen were formerly large and contained
oxygen under a pressure of about 17 atmospheres (250 pounds per
square inch). Such cylinders are still used; but at the present

§ 153a. There has lately been introduced a substitute for limes, known as
Guil Pastils. These are rather soft white cylinders of a substance giving
great brilliancy when used in place of lime. The Guil pastil is put into the
holder so that the end is heated, hence the lamp should be in the form shown
in fig._ 57 K, not as in fig. 56 or 59 L. The Guil pastil serves for 10 to 20
exhibitions. It is composed mostly of a zirconium compound and is not hurt
by exposure to the air. It should be heated up gradually as directed for the
limes (§ 162). — Moving Picture World, June 13, 1914, p. 1539.

IO2

MAGIC LANTERN WITH THE LIME LIGHT [Cn. IV

time smaller cylinders with the gas at a much higher pressure (100
to 120 atmospheres) are employed (see also § 156). In using the
gas it is drawn off through a reducing valve by which it can be
delivered at any pressure desired, and of course in any volume
desired.

One should never try to use the gas without drawing it through
the reducing valve. The cylinders have special junctions for the
reducing valve, so that it is easy to make the connections.

FIG. 57. OXYGEN CYLINDER
WITH COMPRESSED OXYGEN,
THE PRESSURE GAUGES AND
THE MIXED JET OR BURNER.

(Catalogue of Schmidt and

Haensch).

B Tip of the nozzle of
the mixed jet.

K Holder for the lime.
The end of the lime is used,
not the side as in fig. 56, 59.
G Handle of the stop-cock
for hydrogen in the tube of
the burner.

S Stop-cock for oxygen.
// The tube conveying
hydrogen to the burner, steel
cylinder not shown.
O Tube conveying oxygen from the steel cylinder to the burner.
/ The high pressure gauge giving the number of atmospheres under which
the gas in the cylinder is compressed.

M The low pressure gauge to show the pressure of the gas after it has
passed the pressure reducing valve (St.).

St The handle of the valve serving to open the pressure reducing apparatus.
V The valve of the cylinder. This must be opened to allow the compressed
gas to escape into the tube passing to the reducing valve and to the high pres-
sure gauge. It must be closed after every exhibition.

CH. IV] MAGIC LANTERN WITH THE LIME LIGHT 103

In Great Britain and on the Continent oxygen cylinders are, by
common usage, painted black, and the screw threads are right-
handed.

Hydrogen cylinders are painted red, and their screw threads
are left-handed.

In the United States of America this uniformity of color and dis-
tinction of screw threads is not always found.

§ 155. Hydrogen in steel cylinders. — Hydrogen gas is also
compressed in steel cylinders, and forms an article of commerce.
It must also be drawn off through a pressure reducing valve.

Every precaution should be taken to avoid mixing the two gases
in large quantities. Safety lies in mixing the gases only at the
moment of exit from the two tubes of the blow-through jet or in
the small mixing chamber of the mixed jet.

§ 156. Pressure gauges for gas cylinders. — While a pressure
reducing valve is a practical necessity, the pressure gauges are
highly desirable.

The one beyond the pressure reducing valve is a low pressure
gauge and may indicate the pressure in millimeters of mercury or
in centimeters of water (or, of course, in inches of water or mercury).
This shows the pressure under which the gas is actually being used.

The gauge next the cylinder registers the full pressure within.
The figures on the dial usually represent atmospheres of pressure,
one atmosphere being 760 mm. of mercury. The special purpose
of this gauge is to enable one to determine the amount of gas in the
cylinder at any given time, hence it is sometimes called a "capacity
meter" or a "finimeter."

If the pressure gauge does not indicate directly the atmospheric
pressure, it may give the number of pounds per square inch or the
number of kilograms per square centimeter. To change these to
atmospheres one can use the approximate values: 15 Ibs. per
square inch = i atmosphere; or i kilo per square centimeter = i
atmosphere (§ is6a).

§ 156a. The exact values are:

One atmosphere equals 14.73 pounds per square inch.

One atmosphere equals 1.033 kilograms per square centimeter.

104 MAGIC LANTERN WITH THE LIME LIGHT [Cn. IV

For example, suppose the pressure gauge indicates 1800 Ibs. per
square inch, then it would be under a pressure of 1800 -r- 15 = 120
atmospheres.

If the pressure gauge should read 100 kilograms per square
centimeter, then it would be under approximately 100 atmospheres
pressure.

From Boyle's law of the relation between the volume of a gas
and the pressure to which it is subjected it is known that if one
starts with a cylinder holding five liters of oxygen or hydrogen, or
indeed of any other gas, under one atmosphere pressure, it will hold
twice as much under two atmospheres, etc., so that a cylinder of
five liters capacity at one atmosphere, will hold 500 liters at 100
atmospheres pressure. Now to determine the amount of gas
present in a given cylinder with the high pressure gauge one must
know the capacity of the cylinder under the ordinary atmospheric
pressure, and multiply this amount by the number of atmospheres
of pressure indicated on the gauge. For example, if the capacity of
the cylinder is i o liters at one atmosphere (often called no pressure)
and the high pressure gauge indicates that the gas in the cylinder
is under a pressure of 25 atmospheres, then the amount of gas is
10 X 25 = 250 liters of gas; and so in like manner with any other
pressure. For example, in England, the cylinders are filled under
120 atmospheres pressure; this would give in the above case
10 X 120 = 1200 liters to the full cylinder.

On the Continent, the filling pressure is often 100 atmospheres
and the cylinder of the same capacity would then contain 10 X 100
= 1000 liters of the gas.

The practical application of this knowledge is to determine in a
given case whether there is sufficient of the gases present for the
exhibition. Authors differ somewhat in estimating the amount
of gas used per hour with the lime light lantern. A conservative
estimate would be, for oxygen, 85 liters (about three cubic feet)
and, for hydrogen, something over twice that volume, as, in prac-
tice, there is an excess of hydrogen (§ 161).

§ 157. Limes. — The masses of unslaked lime (calcium oxid)
used for the lime light are usuallv cylindrical in form. For some

CH. IV] MAGIC LANTERN WITH THE LIME LIGHT 105

burners they are placed on a pin or axle, and then must have a
corresponding central hole. With other burners they are pressed
into place between surrounding springs, somewhat as a lamp
chimney is put on its burner (fig. 57, 59).

The limes are sealed hermetically in glass tubes, or are packed in
powdered unslaked lime, in air-tight tin cans, to prevent the access
of moisture.

If moisture reaches the limes they will slake and become powdery
and useless for the light. To avoid any moisture reaching them
they should not be removed from their protective covering until a
few minutes before they are to be used.

§ 158. Lamp for the lime light. — This consists of a burner or jet
for conducting the two gases, oxygen and hydrogen, to a point
where they can be mixed and burned ; and a device for holding the
lime in a proper position, and raising, lowering, rotating and adjust-
ing the lime with reference to the burner.

There are two principal forms of burner or jet :

(1) The blow-through jet. — In this a stream of oxygen is blown
into a flame of hydrogen on the principle of the gas or alcohol blow-
pipe (fig. 58).

(2) Mixed jet. — In this form the two gases (oxygen and hydro-
gen) meet and mix in a common chamber just before the nozzle

H O H O HO

FIG. 58. FORMS OF BLOW-THROUGH JETS (Lewis Wright).

The form c shows best that the principle is that of a blowpipe.

The form d approaches the mixed jet somewhat.

With all of them the hydrogen, or hydrogen substitute (illuminating gas,
ether or gasoline vapor) passes out from the supply through the tube marked
H at the left. The oxygen is then blowrn through the flame from the tube at
the right marked 0. Not so much light can be got with these jets as with the
mixed jet, but for illuminating gas or ether vapor, etc., this form, especially
a, b, c is safer in the hands of amateurs than the mixed jet.

106 MANAGEMENT OF THE LIME LIGHT [CH. IV

opens; then the mixed gases burn on emergence from the nozzle
(fig. 59). This form of jet gives the greater amount of light but
the two gases should be under considerable pressure. The tip of
the nozzle (fig. 59 N) makes an angle of 40 or 45 degrees with the
lime. This gives a source of light above the tip of the nozzle, and
hence there is free passage for the light to the condenser.

The blow-through jet is usually 10 to 15 mm. (y2 inch) from the
lime while for the mixed jet the nozzle is within about 3 mm. (y%
inch) of the lime.

MANAGEMENT OF THE LIME LIGHT

§ 159. Connecting the gases with the burner. — This is accom-
plished by means of rubber tubes of thick walls, and the ends of
the tubes should be tied or wired to the supply pipes and to the
burner (§ i59a).

It is a great advantage to have the two parts of conducting tubes
of the burner of the same color as the gas tanks, viz., red for hydro-
gen and black for oxygen, then there will be less liability to error
in connecting the gas supply.

It is only while using the gas that the cylinder valve (fig. 57 V)
should be opened. And in opening it care should be taken to open
slowly so that the sudden rush of the compressed gas may not injure
the pressure gauges or the reducing valve.

When through with the cylinder at any time the cylinder valve V
should be closed.

The pressure of the two gases should be about equal. This can
be arranged by the pressure reducing valve. Set this to give the
desired pressure, which ordinarily is equal to a column of water
about 28 to 50 cm. high (n to 22 inches) or 2 to 4 cm. of mercury
(^4 to 1^2 in. Hg) a pressure of .03 to .06 kilos per sq. cm. (.4 to .8
Ibs. per sq. in.).

§ 160. Lighting the jet. — Turn on the hydrogen slightly and
light it with a match or a cerium-iron gas lighter, then continue to

§ 159a. Flexible metallic tubes. — There is now available flexible metallic
tubing with rubber connections at the ends to use in place of rubber tubes for
conducting gases (fig. 60).

CH. IV] MANAGEMENT OF THE LIME LIGHT

107

FIG. 59. MIXED BURNER OR JET FOR THE LIME LIGHT.
(From the Catalogue of Williams Brown & Earle).

H H The metal tube of the burner conveying the hydrogen to the mixing
chamber (M}. It should be painted red to correspond with the color of the
hydrogen cylinder of compressed gas.

0 0 The metal tube conveying oxygen to the mixing chamber (M). It
should be painted black to correspond with the color of the oxygen cylinder of
compressed gas.

M The common chamber into which open the oxygen and hydrogen tubes.
Here the gases mix before passing out through the nozzle (N).

N The nozzle or outlet tube from the mixing chamber. It is at an inclina-
tion of about 40 to 45 degrees with the vertically standing lime face; and
when the burner is in action the nozzle and lime are about 3 mm. (J/gth in-)
apart.

L The support and springs for holding the lime.

5 The milled heads of the pinions by which the lime is rotated or raised and
lowered. The lime support slides back and forth on the supply tubes O and //
so that the lime may be withdrawn from or made to approach the tip of the
nozzle (N).

open the stop-cock until the flair.e is from 8 to 15 cm. (3-6 in.) long.
Then turn on the oxygen slowly until the flame just commences to
hiss. After the lamp has been going some minutes the operator
can slightly increase or decrease the oxygen until the most brilliant
light is obtained. One must learn by experience. The flame will
become very small as the oxygen is turned on, and this small,
intensely hot flame heats a very small part of the lime, hence the
source of light is very small, something like the crater of the posi-
tive carbon in the direct current arc lamp.

Caution. — Always turn on the hydrogen first and light it before
turning on the oxygen.

Never turn on the oxygen first, and never until the hydrogen
has been lighted, and then turn it on slowly.

If both were turned on before lighting the hydrogen, there would
be a greater or less explosion. This might not be very dangerous,

io8 MANAGEMENT OF THE LIME LIGHT [Cn. IV

but it has a dangerous sound; and the purpose of the exhibition is
to instruct or entertain, not to scare the audience. To insure the
correct use of the gases it is a good plan to have the stop-cock
handles of the two gases so different that one can tell by feeling
which one is being turned on.

FIG. 60. FLEXIBLE METALLIC TUBING WITH RUBBER CONNECTORS AT

THE ENDS.

(Cut loaned by the Pennsylvania Metallic Tubing Co.).

§ 161. Regulating the flame. — Theoretically the proportion of
the two gases should be their combining quantities (H2 O) ; but
experience has shown that better results are gained when the
hydrogen is in excess. When the oxygen is in exactly the combin-
ing proportion there is liable to be a snap and the light goes out.
If there is an excess of hydrogen this docs not happen. As stated
above, the oxygen should be added until the flame just begins to
hiss.

§ 162. Putting a lime in position. — A fresh lime from the box
should be put in position in the burner (fig. 59 L) before lighting
the hydrogen, but the lime should at first be 3 to 5 cm. (i to 2 in.)
distant from the tip of the nozzle (fig. 59 N), and it should be
rotated, raised and lowered until it is warmed. If the full heat of
the O-H flame were directed against one point of the cold lime for
too long a time the lime would be liable to break. After it is well

CH. IV]

MANAGEMENT OF THE LIME LIGHT

109

warmed up the lime is not liable to break. Some operators warm
the lime by means of the hydrogen flame only. When the lime is
warm the oxygen is turned on slowly until the most brilliant light
is obtained.

§ 163. Arranging the lime and the burner ; rotating the lime.—

After warming the lime for half a minute or so it should be grad-
ually brought toward the nozzle until it is only about 5 mm. (%
inch) distant. If now one watches the disc of light on the screen
and slowly moves the lime slightly closer to and farther from the
tip of the nozzle it is easy to tell when one gets the most light. It
is to be remembered that the best light is not practically instan-
taneous, as with the arc lamp, but is produced after the lime has
been half a minute or so in one position.

§ 164. Changing the position of the lime. — The intense heat of
the oxy-hydrogen jet makes a little pit in the surface of the lime.
In about two minutes this pit gets so deep that the light is greatly

FIG. 61. GLOVES WITH ASBESTOS PATCHES ON THE THUMB, INDEX AND
MIDDLE FINGERS FOR USE IN WORKING ABOUT THE HOT LIME-
LIGHT LANTERN.

Right hand, palm up: p, Pollex or thumb; i, Index or fore finger; m,
Medius or middle finger; c, the index and medius used as pincers to grasp a
hot lime or a hot carbon.

Left hand, palm up: i, 2, j The first, second and third digits, but num-
bered instead of named.

no MANAGEMENT OF THE LIME LIGHT [Cn. IV

lessened, and one must move the lime a little so that a new surface
may be acted upon.

In practically all the modern burners there is a screw mechanism
for rotating the limes and for raising and lowering them (fig. 59 S).
With a little experience one learns by the looks of the screen
light when to turn the lime. If the limes must be handled, use
tongs or asbestos-patch gloves (fig. 61).

§ 165. Turning out the light. — Always turn off the oxygen first,
then the hydrogen. Never turn off the hydrogen until after the
oxygen is turned off.

Perhaps it will help to remember the order by keeping in mind
that (i) the Hydrogen is the first to come and the last to go. (2)
And the Oxygen, like the best in human nature, is last to come and
first to go.

MANAGEMENT OF THE LIME LIGHT MAGIC LANTERN FOR AN
EXHIBITION OR DEMONSTRATION

§ 166. Preparation for an Exhibition. — Before the exhibition
the operator should see that everything is in perfect order and
readiness. The gas cylinders should be connected with the burner,
and a perfect, fresh lime should be in position in the burner. The
box of limes should also be at hand in case anything goes wrong
with the one in the burner.

§ 167. To start the light. — It takes much longer than for the
arc lamp. It is usually about half a minute before the brightest
light possible is produced, and one must not forget the precaution
to warm the lime before subjecting one spot to the full power of the
O-H jet.

Light up as directed above (§ 160).

If there is a snap and the light goes out, turn off the oxygen, and
relight the hydrogen. Turn on the oxygen slowly until the best
light is obtained (§ 160).

§ 168. To put out the light. — Turn off the oxygen first, then the
hydrogen (§ 165).

CH. IV]

MANAGEMENT OF THE LIME LIGHT

§ 169. Shield for cutting off the light from the screen. — As it

takes considerable time to start the lamp after it has been put out
it is not so easy to use the lime light intermittently as the arc lamp,
hence in a lecture or demonstration in which the lantern slides are
to be shown at several different times, it is best to leave the lamp
burning all the time. But the screen should not be lighted all the
time, and to avoid this the objective shield (fig. 62) may be used.

FIG. 62. SHIELD FOR THE OBJECTIVE IN INTERMITTENT PROJECTION,
WHEN SLOW-LIGHTING RADIANTS ARE USED.

Sl Shield up to allow the light to pass from the objective to the screen.

51 Shield down to cut the light off from the screen. This shield is especially
desirable when slides are to be shown at intervals, as in a demonstration lecture
with the lime light, a Nernst light, a kerosene light, or an alcohol light (§ 169).

Sometimes also to avoid using so much gas and burning out the
lime too quickly there are regulating valves, by which only a small
amount of the two gases is allowed to pass, without changing the
relative proportions. When these valves are opened again the full
amount needed and in the original proportions is allowed to flow
again. Even in this case there should be a shield before the objec-
tive to avoid lighting the screen.

§ 170. Proper lighting for the screen. — The light on the screen
should be uniformly brilliant. This can be attained by following
the directions for centering and getting the proper distance of the
lamp from the condenser exactly as with the direct current arc
lamp (§ 51-57).

If there arc shadows on the screen make the proper change in the
position of the lamp, etc., as indicated in fig. 27-30, § 83-93.

H2 LIME LIGHT WITH OXYGEN AND GAS [Cn. IV

If everything has been put in perfect order before the exhibition
the changes required during the exhibition should be very slight.

§ 171. Arrangement of lantern slides, their insertion and
focusing. — Follow the directions in Ch. I, § 21-23; 35~4I-

§ 172. Lighting the room. — As the lime light gives only about
Vs to VG as much light as the arc lamp the room must be darker
if the same brilliant contrast is desired. One can determine by a
little experiment with the set of slides to be exhibited at any time
how dark to have the room. The more transparent the lantern
slides, the lighter can the room be. Many lantern slides are
altogether too opaque, and require a dark room, no matter what
light is used in the lantern.

§ 173. Avoidance of intervals of total darkness in the room.—

This can be accomplished by leaving the lantern on all the time and
by using the objective shield (fig. 62). If that device is not used,
then the operator should not turn out the lime light until the room
lights are turned on. And whenever the lantern is to be used, the
lecturer must give two or three minutes warning to the operator
before turning off the room lights.

THE LIME LIGHT WITH OXYGEN AND ILLUMINATING GAS

§ 174. Frequently the lime light is produced with illuminating
gas drawn from the house supply, and with oxygen gas in a steel
cylinder (§ 154).

If illuminating gas is used instead of hydrogen it is to be remem-
bered that the pressure as drawn from the house supply is very
slight, i. e., about equal to a column of water from 5 to 12 cm.
high (2 to 5 in.) or only about Vr, the pressure of the hydrogen
and oxygen when these gases are drawn from steel cylinders (§ 1 54).

The oxygen is used at a much higher pressure than the house gas,
and many operators use for this combination the "blow-through
jet" (fig. 58). Mixed jets arc also constructed for this combina-
tion, but the "blow-through" is considered safer. The user of this
form of apparatus would do well to get the combination found best
by the manufacturers of his apparatus.

CH. IV] OXYGEN GENERATOR AND ETHER SATURATOR 113

§ 175. For lighting the lamp. — Whatever form of burner is used
turn on the illuminating gas first and light it; then turn on the
oxygen until the flame is made much smaller, as with hydrogen.
For warming and arranging the lime and its distance from the
nozzle of the jet see § 158, 162-164.

§ 176. Putting out the lamp. — Turn off the oxygen first, then
the illuminating gas.

Remember that oxygen is always on last, and off first.

LIME LIGHT WITH OXYGEN GENERATOR AND ETHER SATURATOR

§ 177. Oxygen generator. — There has recently been perfected
a method of preparing sodium peroxide so that it gives off oxygen
gas when water is added, somewhat as
calcium carbide gives off acetylene gas
when put in water. This substance gives
about 300 times its volume of oxygen,
and serves very well for an oxygen sup-
ply when used in a proper generator.

§ 178. Hydrogen substitute. — The

substitute for hydrogen with this outfit
is sulfuric ether or gasoline. But ether
and gasoline should never be mixed.

§ 179. Use of the apparatus. — There
must be a burner and lime holder as for
the oxy-hydrogen lime light. The sodi-
um peroxide (Oxone, oxodium, oxylithe
are trade names) is put into the generator
and the oxygen gas conducted over to the
ether saturator. In the saturator, the
stream of oxygen from the generator is
divided, one stream of the oxygen going

directly to the burner through one tube, F]G 63. PORTABLE OXYGEN
and another part going through the ether
chamber of the saturator and becoming
loaded with ether vapor. This oxygcn-

GENERATOR AND ETHER
SATURATOR.

H4 TROUBLES WITH THE LIME LIGHT [Cn. IV

ether vapor is inflammable and takes the place of hydrogen or
coal gas. The pure oxygen mixed with it just before it emerges
from the burner gives the necessary intensity to the flame.

In using this outfit it is necessary to follow very precisely the
directions of the manufacturers to avoid accidents. In particular,
one must be sure to turn on the oxygen-ether first and light it;
then turn on the pure oxygen until the light is best. In turning
the light out: Turn off the oxygen first, then after a moment,
turn off the oxygen-ether supply.

The oxygen produced from one charge of 3^ pounds of the
sodium peroxide (oxone) gives about 6.6 cubic feet of oxygen gas,
enough to last from two to three hours for the magic lantern. One
filling of the ether saturator requires about one pound of sul-
furic ether and will supply the ether vapor for the charge of
oxone. It is said by the manufacturers that if used economic-
ally the single charge of oxone and ether will supply a double lan-
tern for an entertainment lasting an hour or an hour and a half.

TROUBLES WITH THE LIME LIGHT

§ 180. Snapping out of the light. — This is usually due to an
excess of oxygen. The oxygen should always be less than the
hydrogen or any of its substitutes, i. e., illuminating gas, ether or
gasoline vapor, acetylene gas. To invert the statement, the
hydrogen or its substitutes, i. e., the inflammable gas or vapor
should be in excess of the actual combining proportions. If the
lime is too close to the burner tip the light will snap out.

In case the light snaps out, at once turn off the oxygen. Light
the hydrogen and slowly turn on the oxygen again until a satis-
factory flame is obtained. Be sure the lime is not too close to the
burner tip.

§ 181. Going out of the light. — This may be due (i) to a lack of
one or of both the gases used, that is, the supply may be exhausted.
Look at the capacity meter.

(2) Some of the valves may be clogged.

(3) A rubber tube may have split or come off at the connection.

CH. IV] TROUBLES WITH THE LIME LIGHT 115

(4) A lime may have broken so that there is nothing for the
hot flame to make incandescent.

Remedy. — Turn off the gases the first thing; oxygen first then
the hydrogen or other gas. One can then investigate each of the
possible causes for the going out of the lamp. The broken lime
and the split or separated rubber tube can be most easily detected
and corrected, and consequently should be looked for first.

§ 182. Irregular light or shadows on the screen. — The fault

may lie in any of the following, to name which is to suggest a
remedy :

(1) The lime may be too deeply pitted where the flame strikes
it. Change the position of the lime (§ 164).

(2) The lime may be in bad position, too high or too low, too
far from or too close to the burner tip.

(3) The incandescent spot may not be centered on the axis, i. e.,
be too high or too low; too far to the right or to the left with the
resulting shadows as with the crater of the arc lamp (fig. 27-30).

(4) The light may be too close to or too far from the condenser.

(5) The nozzle of the burner may be in the way and cast a
shadow. If so, it must be lowered or the distance from the lime
or the angle changed (see also § 82-91).

§ 183. Roaring or hissing of the burner. — A slight hissing sound
is usually heard when the right amount of oxygen is being used.
But when the roaring becomes annoying its cause must be found
and remedied. It may be due to: (i) The inside of the nozzle
tube may be rough.

(2) The lime may not be the right distance from the tip of the
nozzle.

(3) The pitting of the lime may be too great.

(4) There may be too great a supply of the gases for the bore
of the nozzle.

§ 184. Cracking of the lime. — This is usually due to a sudden
heating of the lime. If it is warmed gradually by rotating it at
first at some distance and then closer to the flame the breaking is
usually avoided. If broken, the lime should be removed from the

Ii6 TROUBLES WITH THE LIME LIGHT [Cn. IV

holder and a new one put in place. This should then be grad-
ually warmed (§ 162).

SPECIAL PRECAUTIONS IN USING THE LIME LIGHT

§ 185. Remember that hydrogen and all the substitutes used
for it, illuminating gas, ether and gasoline, are very inflammable.

Oxygen with hydrogen and also with the other substances forms
an explosive compound. Hence, the greatest care must be taken
to avoid mixing these gases except in the mixer of the burner
(fig. 59). Hence also in filling any part of the apparatus and in
working about it there should be no open flames or glowing parts
to ignite any accidentally escaping hydrogen, gasoline, ether, etc.
Fill the apparatus by daylight, or use an electric light or an elec-
tric flash-light if the work must be done in a dark place. In this
way no chance for igniting the gases will occur. Naturally one
should not smoke when filling the apparatus.

It is economical to buy the best apparatus throughout. The
makers adapt the burners and all other parts to give the best
results in the safest manner, therefore, unless one is an expert in
such matters it is safer to take the outfit assembled and recom-
mended by some reliable manufacturer.

The makers send out with their apparatus very precise directions
for using it with safety, and it is the height of wisdom to follow
their directions faithfully.

CH. IV]

DO AND DO NOT WITH THE LIME LIGHT

117

§ 186. Summary of Chapter IV:

1. Use gas cylinders which
are plainly marked Oxygen
and Hydrogen, and have right-
handed screws for the oxygen
and left-handed screws for the
hydrogen (§ 154). Be sure that
there is plenty of gas in each

(§ 156)-

2 . Connect the cylinders with
the burner by means of rubber
or metallic tubing, colored to
correspond with the cylinders
(OorH) (§ 154, 159, i59a).

3. In starting the burner,
turn on the hydrogen or its
substitute first and light it,
then turn on the oxygen slowly
(§ 160).

4. Heat up the lime slowly by
having it at some distance from
the flame (§ 162).

5. Turn the lime occasionally
so that the pit will not get too
deep (§ 164).

6. In putting out the lamp,
turn off the oxygen first, then
the hydrogen after a moment.

7. If the light snaps out, turn
off the oxygen then the hydro-
gen. Turn on the hydrogen,
light it and then turn on the
oxygen slowly as in (3).

Do NOT

i. Do not use gas cylinders
which are not plainly marked.
Do not start an exhibition
unless there is plenty of gas.

2. Do not be careless in con-
necting the cylinders with the
gas burner.

3 . Do not turn on the oxygen
first. Oxygen is last on, first off.

4. Do not turn the full heat
of the O-H flame against a cold
lime which is close up to it.

5. Do not let the lime stay
too long in one position. Ro-
tate it occasionally.

6. Do not turn off the hydro-
gen first, but turn off the
oxygen first. Oxygen is on last,
off first.

7. Do not leave the gases
turned on if the light snaps out.
Oxygen off first, then Hydrogen.

DO AND DO NOT WITH THE LIME LIGHT [Cn. IV

8. After the exhibition is over
remove the lime or it will
slake in the holder.

8. Do not leave the lime in
the holder to slake after the
lecture.

9. Conduct the exhibition
exactly as with an electric
lantern (Ch. I, § 21-40).

10. As the hydrogen or its
substitute is inflammable, and
the oxygen is a perfect supporter
of combustion, follow the direc-
tions given by the manufac-
turers of a special apparatus
intelligently and exactly.

9. Do not spare any pains in
conducting an exhibition with
the lime-light magic lantern.
More care and skill are neces-
sary than with the electric light
lantern.

10. Do not take any chances
when dealing with the oxy-
hydrogen lantern. Do things
in the right order, and do not
neglect the directions of the
manufacturers.

CHAPTER V

MAGIC LANTERN WITH PETROLEUM LAMP; VERTICAL

AND REFLEX MANTLE GAS LAMPS; ACETYLENE

LAMP; ALCOHOL LAMP WITH MANTLE

§ 190. Apparatus and Material for Chapter V:

Suitable projection room with screen ; Magic lantern with lamp
and chimney for petroleum (fig. 65-67); High grade petroleum for
burning in the lamp; Gas burners for vertical and reflex mantles
(fig. 68-69); Illuminating gas supply; Acetylene burner and
reflector, (fig. 70) ; Acetylene gas supply (house supply, prestolite
tank of compressed acetylene in acetone or an acetylene generator) ;
Special alcohol lamp with mantle (fig. 72-73); Strong alcohol
(95%) ethyl, methyl or denatured. The magic lantern for all but
the oil lamp must have a lamp-house into which the burner can be
placed. There must be lantern slides, screw drivers, pliers and
matches or safety lighters (§ 160), for all of them.

§ 191. Historical development and references to literature.—

For the history see the Appendix, and for general works of reference
see the list of books in the first chapter (§2).

The directions sent out by the manufacturers of these light
sources should be studied carefully and followed exactly unless one
has technical knowledge on the subject.

OIL AND GAS LAMPS

§ 192. Early sources of light. — For a long time after the inven-
tion of projection apparatus there were but two sources of light
known :

(1) The sun, which has ever remained the most brilliant source
of light available, and

(2) Some form of torch, candle, or oil lamp.

The first oil lamps burned animal or vegetable oil and had no
lamp chimney.

After the discovery and proper refinement of petroleum, that
became and has remained the oil most used for illumination.

If one reads the early works on projection it seems astonishing
that the workers of those times were able to produce screen images

119

120

MAGIC LANTERN WITH OIL AND GAS LAMPS [Cn. V

which showed general form and details with anything like satisfac-
tion to large audiences. But screens as large as four meters square
(12 ft. sq.) were used with the petroleum light.

When the feeble lights discussed in this chapter are compared
with the powerful electric arc light giving from 1,000 to 5,000 candle-
power it would seem that the results of earlier times must have been
very unsatisfactory.

But the older lanternists gave very successful exhibitions. They
did this by observing with scrupulous care the requirements for
projection with their appliances.

Condenser

FIG. 64. MAGIC LANTERN WITH LARGE LIGHT SOURCE.

Lamp Illuminating gas lamp with Welsbach mantle.
Condenser Triple-lens condenser without water-cell.
S Lantern slide.

Objective Projection objective with inverted image of the luminous mantle
between the lenses.

Screen Image The image of the lantern slide on the white screen.

§ 193. Requirements for projection with a feeble light:

(A) The lantern slides must be very transparent ; and the old,
hand-painted slides were very transparent.

(B) The room must be very dark. There must be no stray
light from the windows or from the apparatus ; the only light must
be that issuing from the lantern objective and reflected from the
screen .

(C) The management of the lantern must be the best possible,
so that all the available light may be utilized for producing the
screen image.

(D) The projection objective must be of large aperture so that
as much as possible of the light issuing from the large source (lamp

CH. V] MAGIC LANTERN WITH PETROLEUM LAMP 121

flame or incandescent mantle) , may be utilized in making the screen
image. This is of fundamental importance (fig. 64, 90).

(E) Use of twilight vision. — It is astonishing how dim a picture
can be clearly seen after one's twilight vision has become fully
established. According to careful investigations the sensitiveness
of the eye may be increased from 35 to 2500 times by the adapta-
tion to dim light (§ 281).

The old lanternists used to advise that the exhibition should not
begin until the audience had been in the darkened room for half an
hour "to get," as they said, "the sunlight out of their eyes." We
would say to "get the twilight vision well established."

§ 194. Time required for lighting up. — The gas light and the
acetylene light are quickly established, but the petroleum and
the alcohol lights require several minutes to get up the best illumi-
nation. These two should then burn during the entire time of an
exhibition. If the lecturer cannot arrange to have all the slides
continuously, but must have them at intervals during the lecture,
the operator should make use of an objective shield (fig. 14, 62),
and leave the lights on all the time.

§ 195. Rehearsals. — As these lights are more difficult to man-
age and the results are less satisfactory than with the more power-
ful radiants, so much the more should the operator rehearse before
the lecture and make sure that everything is in as nearly perfect
order as human skill can make it.

THE MAGIC LANTERN WITH A PETROLEUM LAMP

§ 196. The petroleum lamps now used as radiants for projec-
tion have two, three or four wicks. The wicks are wide (about five
cm., two in.) and are placed edgewise to the condenser. If more
than two wicks are used the two outer ones are inclined inward
(%. 66).

Sometimes instead of being ranked side by side, the different
wicks are arranged like the lines forming the letter W, but there is
no advantage in this.

MAGIC LANTERN WITH PETROLEUM LAMP [Cn. V

FIG. 65. MARCY'S MAGIC LANTERN OR "SCIOPTICON" WITH A
MULTIPLE-WICK, PETROLEUM LAMP.

(From Dolbear's Art of Projecting).

a-b. c-d The lenses of the projection objective.

p-q The condenser lenses.

Z S The oil reservoir of the lamp.

E The flames of the lamp with their edges toward the condenser.

G-G Two glass plates at opposite ends of the lamp-house to allow the light
to pass to the condenser, and so that the reflector II can return the backward
extending light.

C I J The chimney and ventilator of the lamp-house.

W W At the right, the milled heads for turning the lamp-wicks up or down.

There is a common reservoir and a common chimney, but each
wick has a separate burner and a separate mechanism for raising
and lowering the wick.

§ 197. Chimney and reflector. — There is a common chimney.
This is usually of metal with a window on opposite sides, and with
either a telescoping extension or a segment which can be put on top

CH. V] MAGIC LANTERN WITH PETROLEUM LAMP 123

for getting the best draught when the lamp is turned up full
height.

The reflector is a concave mirror placed with its center of curva-
ture coinciding with the flame. This serves to reflect the backward
extending light to a focus on the flame again, and from thence it
passes onward to the condenser with the rays passing directly from
the flame to the condenser.

FIG. 66. MULTIPLE-WICK, PETROLEUM LAMP FOR THE MAGIC LANTERN.
(From the Catalogue of the Mclntosh Battery and Optical Company, 7889).

This figure shows that there is a single oil reservoir but four separate wicks,
each with a mechanism for turning the wick up or down. It also shows
clearly the inclination toward each other of the separate wick holders, and
finally that the lamp has a single chimney.

The openings in the metal chimney for the reflector and the con-
denser must be covered with glass or with clear mica or the lamp
will smoke.

§ 198. Management of the lamp. — Before an exhibition the
reservoir is filled nearly full with good petroleum (kerosene oil).
The wicks must be carefully trimmed until the flame burns without
tails. One must be careful in preparing the lamp not to get any
oil on the outside, for when the lamp gets hot this oil is sure to smell
badly.

124

MAGIC LANTERN WITH PETROLEUM LAMP [Cn. V

Light the wicks and turn them up moderately and allow them to
burn for five or ten minutes before the exhibition. This is to get
the apparatus warmed up. One cannot get the best light from a
petroleum lamp instantly, but only after it has become warm.
Finally turn up each wick as high as possible without having it
smoke. The central wicks can usually be turned higher than the
marginal ones. When the wicks are at their full height the chim-
ney, if adjustable, must also be at its full height to give the best
draught.

After the exhibition is over the lamp-wicks are turned down, the
small flames blown out, and then the unused oil poured into a con-
tainer, the wicks taken out and carefully dried between blotting

FIG. 67. NEWTON'S FOUR-WICKED, PETROLEUM LAMP KOR THE MAGIC

LANTERN.

(From Catalogue No. 4 of Newton £r Co.}.

The chimney is in two segments. For the maximum light after the lamp is
warmed up, the top segment is added.

CH. V] MAGIC LANTERN WITH MANTLE GAS LAMP 125

papers. If the lamp is kept perfectly clean, and no oil is allowed to
remain on the outside, the disagreeable smell of partly oxidized oil
will be avoided.

§ 199. Amount of oil used. — It takes about half a liter (one
pint) of kerosene per hour for the best lamps.

§ 200. Candle-power and size of screen. — The candle-power of
the best petroleum lamps is not much above 100. While the older
lanternists used large screens (4 meters, 12 ft. square) it is better
to use, with this light, screens of small size, 2 to 3 meters square
(6-9 ft.), and to keep in mind the requirements for good images
with these feeble lights (§ 193).

§ 201. Relative position of lamp and condenser. — In general,
the middle of the flame should be in the axis of the condenser and
it should be at about the principal focal distance of the first ele-
ment of the condenser from it (fig. 64). One must get the best
possible position at any one time by experiment, i. e., by moving
the light a little closer or farther away than the focus of the con-
denser. For the two-lens condenser the lamp must be closer than
for the three-lens condenser (§ 17, 55).

§ 202. The management of an exhibition is as described in
Chapter I, § 21-41, and above, § 193-194.

MAGIC LANTERN WITH A MANTLE GAS LAMP

§ 203. Gas and gas lamps. — The illuminating gas may be
drawn from the house lighting supply.

The lamps are of two kinds, the vertical and the inverted or
reflex form (fig. 68-69). The burner is of the Bunsen type. It
heats the mantle to incandescence. While there is a very brilliant
light and a great deal of it, the source is very large, and cannot be
utilized so completely as the small source of the electric arc lamp
(see fig. i, 64).

§ 204. Position of the incandescent mantle. — As this is the
source of illumination, the middle of the face next the condenser
should be on the horizontal axis (fig. 64).

126

MAGIC LANTERN WITH MANTLE GAS LAMP [CH. V

FIG. 68. UPRIGHT GAS BURNER WITH WELSBACH MANTLE AND
CONCAVE REFLECTOR FOR THE MAGIC LANTERN.

(From Max Kohl, A. G., Price List No. 50, Vol. I).

The distance from the condenser giving the best light must be
determined by experiment, as with
other extended sources. But, in gen-
eral, it will be found to be at about
the principal focal distance from the
first element of the condenser, as
with the arc lamp, but closer for the
two-lens than for the three-lens con-
denser § (55).

§ 205. Reflector.— As with the pe-
troleum light, a concave reflector is
sometimes used behind the mantle to
reflect back to the mantle and thence
to the condenser the light which passes
backward from the mantle. This is
not always used, but it would increase
the light somewhat (§ 210).

§ 206. Connecting the gas supply
with the lamp. — Use for this a perfect
rubber tube or one of the flexible me-
tallic tubes (fig. 60), and secure the

FIG. 69. INVERTED GAS BIR-
NER WITH WELSBACH MAN-
TLE FOR THE MAGIC
LANTERN.

(From Schmidt und llamch'a Cata-
logue, A'o. IV, Projektions — Apparte,

CH. V] MAGIC LANTERN WITH ACETYLENE LAMP 127

ends to their connections by tying a string tightly around
them, if rubber tubes are used. If the supply is at a con-
siderable distance there should be a stop-cock at the lamp to
regulate the amount of gas, and to turn it off completely if desired.
At the end of the exhibition the gas must be turned off at the source
of supply.

§ 207. The management of the exhibition is simple, and should
follow th3 general lines laid down in Chapter I (§ 21-41). It is
not wise to try to use a screen more than two to three meters square
(6-9 ft.), and one must keep in mind the requirements for feeble
lights (§ 193).

THE MAGIC LANTERN WITH AN ACETYLENE LAMP

§ 208. Source of acetylene. — This may be from a house supply,
a special generator, or from a tank or cylinder of acetylene dis-
solved in acetone under pressure (prestolite tank).

§ 209. Acetylene lamp. — The burners now used are in pairs.
Two jets set at an angle give a fused, flat flame. For the magic
lantern the lamp has from one to four of these twin burners in
a line. Behind the burner is a concave reflector returning the
backward reflected light to the burner and from thence on to the
condenser, so that as much of the light as possible is utilized for the

screen image (fig. 70).

§ 210. Position of the concave mir-
ror. — If a concave mirror is used to save
the light extending away from the screen,
its center of curvature should coincide
with the flame of a single burner, or its
center should be at the middle flame, if
there are several burners in a row.
FIG. 70. DOUBLE-JET The acetylene flame is very transpar-
R^K™oRLTo'R "HE «it, so that a mirror behind the burner
MAGIC LANTERN. will increase the light nearly the theo-

retical amount (75%)- while with nearly

128

MAGIC LANTERN WITH ACETYLENE LAMP

[CH. V

opaque sources, such as the incandescent mantle light or the
petroleum flame, a mirror placed behind the light does not in-
crease the brilliancy so much.

§ 211. Position of the acetylene lamp. — This should be so that
the middle point of the flame is on the axis (fig. 64) and it should
be at a distance from the condenser of about the principal focal
length of the first element of the condenser and the middle flame
of the burner. For the best position in practice one must experi-
ment while looking at the screen image or disc of light, and arrange
the lamp to give the best effect (§ 17, 55).

FIG. 71. UPPER AND LOWER ENDS OF A PRESTOLITE TANK USED \VITH

THE MAGIC LANTERN.
FIG. 7iA. UPPER END OF THE PRESTOLITE TANK.

V Outlet valve. It is opened and closed by a special wrench.

Connector The metal connector for joining the gas supply and the acetylene
burner.

Rt Rubber or flexible metal tube extending from the connector to the
burner.

N Nut for holding the conical part of the connector in gas-tight union with
the hollow cone of the tank-valve. This valve must be set gas-tight before
opening the outlet valve (V).

FIG. 71 B. LOWER END OF THE PRESTOLITE TANK SHOWING THE

PRESSURE GAUGE.

P G Pressure gauge indicating the pressure of the gas within the tank.
The pressure is given in atmospheres or in pounds per square inch or in both.

CH. V] MAGIC LANTERN WITH ACETYLENE LAMP 129

§ 212. Connecting the burner to the gas supply. — For this a
heavy and perfect rubber tube or a flexible metallic tube (fig. 60)
should be used and the connections with the supply and with the
burner should be tied unless special fittings are present.

As with illuminating gas, the best light is obtained when the
correct amount of gas is delivered at the tip of the burner. If too
much gas is flowing the jets will blow, and if too little, there will
not be light enough.

If a tank of compressed acetylene in acetone is used (fig. 7 1 A) ,
the adjustments must be made at the valve on the cylinder. If one
turned this on full head and tried to regulate by the stop-cock at
the burner the pressure accumulating in the rubber tube would
probably blow the tube from its connections or burst it (§ 2 1 2 a) .

§ 212a. Prestolite tanks supplying acetylene for the Magic Lantern. — A

steel cylinder is packed with asbestos and this is saturated with acetone.
Acetylene gas is then pumped into the cylinder and is dissolved by the acetone.

The tanks are charged under a pressure of approximately 15 atmospheres at
i8J-^ degrees centigrade (65° F.) this is 15.82 kilos per square centimeter or 225
Ibs. to the square inch.

The tanks are of various sizes, and their holding capacities, under 15 atmos-
pheres pressure, are as follows:

"A" contains 70 cubic feet of gas, (1982 liters), cost $25.00

"B" contains 40 cubic feet of gas, (1132.6 liters), cost $18.00

"E" contains 30 cubic feet of gas, (849.5 liters), cost $15.00

Motor-cycle tank contains 10 cubic feet of gas, (283 liters), cost $ 8.00

The burner for a magic lantern requires from one to two cubic feet of acety-
lene gas per hour. The motor-cycle tank full of gas will then supply light, for
from five to ten hours. It costs less than $1.00 to have the tank recharged,
hence, the cost of gas per hour is from 10 to 20 cents.

It is of importance to know at any given time whether there is gas enough
to last for an exhibition or for a number of exhibitions. As shown with the
lime light the cylinders are supplied with a gauge showing the pressure of the
gas within the cylinder, and from Boyle's law that the amount of a gas in a
given space depends on the pressure, it is easy to determine at any time the
amount of gas available. It is only necessary to know the capacity of the
cylinder under ordinary atmospheric pressure and to multiply that volume by
the number of atmospheres indicated on the pressure gauge (see also § 156).

For example, the gauge of a motor-cycle tank (fig. 71 B), shows that the
pressure is 12 atmospheres, how many cubic feet of acetylene gas arc avail-
able?

As the tank under 15 atmospheres holds 10 cubic feet of gas its capacity at
atmospheric pressure must be 10 -=- 15 = % of a cubic foot. If it holds % of
a cubic foot under one atmosphere, under 12 atmospheres pressure it will hold
% multiplied by 12 = 8 cubic feet.

The tank will then supply gas for four or for eight hours of continuous light
depending upon the capacity of the burner.

130

LANTERN WITH ALCOHOL LAMP AND MANTLE [Cn. V

§ 213. The management of an exhibition is as for the direct
current arc lamp, keeping in mind the general statements in this
chapter (Ch. I, § 21-40; § 193).

THE MAGIC LANTERN WITH ALCOHOL LAMP AND MANTLE

§ 214. An alcohol flame burning in the air, is very hot. This
has been taken advantage of to heat a mantle to incandescence in
the same way that illuminating gas with a Bunsen burner heats a
mantle to incandescence.

FIG. 72. MAGIC LANTERN WITH THE ALCO-RADIAXT.
(Cut loaned by Williams, Brown & Earle}.
For the details see fig. 32 and 73.

For the best results the alcohol is vaporized, and the vapor burn-
ing in a special burner gives the Bunsen flame necessary to heat the
mantle.

The light is as intense or more intense than gas light with a
mantle.

§ 215. Alcohol supply and burner. — There must be a reservoir

for alcohol (95% ethyl, methyl, or denatured). This is connected
with the burner by means of a metal tube with a stop-cock. In
use the reservoir is filled over half full, but must always have an
air space above. Connected with this air space is a force-pump
by which the alcohol is put under pressure.

CH. V] LANTERN WITH ALCOHOL LAMP AND MANTLE 131

§ 216. Lighting the lamp. — (i) Place the lamp in a metal tray;
put a mantle in position over the burner, and burn it off as for a new
gas mantle.

(2) Place the heater or torch in position under the burner
(fig. 73 L) . Wet the torch well with strong alcohol, using a pipette.
Sometimes the torch is saturated with alcohol by pouring the
alcohol upon it from a bottle before it is put in place under the
burner. This is usually wasteful, as some alcohol is almost sure
to be spilled.

FIG. 73. ALCO-RADIANT, SHOWING THE PARTS.
(Cut loaned by Williams, Brown & Earle).

BM The mantle.

BS The gas burner for the volatilized alcohol.

L H The heater to start the volatilization.

S The handle for opening and closing the air valve of the burner.

R Valve for turning on and off the alcohol supply.

W The tank holding the alcohol supply.

Connection for the pressure tube.

Rubber bull) for forcing air into the alcohol reservoir.

The round

object in the course of the rubber tube is an air reservoir to make the pressure
steady.

132 LANTERN WITH ALCOHOL LAMP AND MANTLE ICn. V

(3) When the torch is in place and wet with alcohol, open the
stop-cock from the supply tank (fig. 73 R), and then light the
torch. The alcohol flame will heat the burner and stand-pipe, and
the alcohol in the stand-pipe will be vaporized and pass over
through the small pipe to the burner where it will catch fire and
burn. Open the air intake partly. In using the lamp this air
intake must be regulated as for a Bunsen burner, the more pressure
the more the valve must be opened.

Soon the mantle should begin to glow brightly from the burning
vapor in the burner. When this occurs commence to put pressure
on the alcohol tank (fig. 73 W). This is done by connecting the
pressure apparatus by means of the rubber tube to the alcohol
tank, at T, (fig. 73), and squeezing the bulb.

In case the first burning off of the torch does not start the lamp
one must burn it off again, but do not add the alcohol until the
torch or heater is out, and then use a pipette. Relight the heater
and it will almost surely start the lamp.

Do not connect the pressure apparatus until the mantle com-
mences to glow. If pressure were on the alcohol tank at first the
liquid alcohol would be forced over from the stand-pipe into the
burner and would run down on the torch and upon the table.
Remember that alcohol is very inflammable and also very unman-
ageable \vhen it is on fire, so be exceedingly careful.

(4) As soon as the mantle begins to glow brilliantly consider-
able pressure can be put on the alcohol tank. The greater the
pressure the wider must the air-intake at the burner be opened and
the more brilliant will be the light; but as the pressure increases
the lamp roars more loudly until, when the pressure is considerable,
it roars like a young blast furnace. By watching the results one
can avoid the excessive noise, and still get a brilliant light.

§ 217. Management of the exhibition. — This is in general like
any other magic lantern, but as the light depends largely on the
pressure regulation, one must be careful to keep up the proper
amount of pressure during the entire time. Do not expect too
much of this light. It gives fairly good lantern-slide images for a
screen from two to three meters (six to nine ft.) square. As the

CH. V] TROUBLES IN CHAPTER V 133

source is large, one needs a good projection objective of large aper-
ture (see §855).

§ 218. Putting out the lamp. — As this lamp is difficult to light
it should be kept burning during the entire exhibition. One can
shut the light from the screen by the objective shield (fig. 62).

At the close of the exhibition, take the lamp from the lamp-house,
remove the rubber tube from the pressure apparatus to the tank
to relieve all pressure on the alcohol. Close the supply valve so
that no more alcohol can pass over to the stand-pipe. Close the
air-intake of the burner. Use a sponge well wet with water and
apply it to the burner as near the mantle as possible without touch-
ing the mantle. The sponge will naturally rest against the small
conducting pipe and the stand-pipe in this operation. This cools
the burner and the stand-pipe and stops the vaporization of the
alcohol. The flame then goes out as with any gas burner when the
supply of gas is cut off.

§ 219. Precautions. — Remember that alcohol is very inflam-
mable, therefore special care should be exercised that none of it
overflows from the reservoir or leaks from poor joints. It is per-
fectly safe in burning through the burner, but any alcohol outside
the lamp is dangerous, for if it catches fire it cannot be extinguished
unless one has plenty of sand or non-inflammable dust to throw on
it and smother the flame, or one of the modern chemical fire extin-
guishers.

TROUBLES IN CHAPTER V

§ 220. The prime difficulty with these relatively weak lights
is the dim screen pictures. That is, they will be dim in comparison
with the bright pictures obtainable with the direct current arc
light.

Remember the conditions requisite for screen images with weak
lights (§ 193).

§ 221. Smoking of the petroleum lamp or of the acetylene
burner. — This shows that the wicks are not properly trimmed or
that they are turned up too high for the height of the chimney.

134 TROUBLES IN CHAPTER V iCn. V

With the acetylene flame if too much gas is turned on the flame
will smoke and roar.

§ 222. The image of the lamp flame may show on the screen.
This is because the objective is too far from the condenser or the
lamp flame is not in the proper position with reference to the con-
denser. Try removing the lamp farther from the condenser or
bringing it nearer. When it is in the correct position its image
will not appear on the screen.

§ 223. Roaring of the alco-radiant lamp. If the roaring is
excessive it shows that the pressure on the alcohol reservoir is too
great. This can be remedied by ceasing to pump the air in
till the noise is within reasonable bounds.

CH. V]

DO AND DO NOT IN CHAPTER V

135

§ 224. Summary of Chapter V:
Do

i. For these relatively weak

sources of light use a good
screen, and make the room dark

(

2. Use
slides.

transparent lantern

3. The objective to select is
one of large aperture for these
large sources (§ 217, 855).

4. Have perfect containers
for liquids and gases so that
none can escape into the room.

5. For the petroleum light
and the alco-radiant use the
objective shield (fig. 62) as it
takes so long to get a good light.

Do NOT

1. Do not try to give an ex-
hibition with these weak lights
in a room with much stray light,
and do not use a dirty screen.

2. Do not try to use opaque
lantern slides.

3. Do not use an objective of
small aperture with these large
sources.

4. Do not use leaky con-
tainers for the gases or liquids
used in this chapter. They are
all dangerous when out of their
proper containers.

5. Do not turn off the alco-
radiant or the petroleum light
during the exhibition; it takes
too long to start them.

6. Follow carefully the direc-
tions sent with the apparatus
by the manufacturers.

6. Do not fail to read care-
fully and follow strictly the
directions sent out by the manu-
facturers.

7. Do your part with great
care and even these weak lights
will give good projection within
their range of possibility, i. e.,
for a screen two to three meters
(six to nine feet) square.

7. Do not expect too much of
these weak sources, but give
them a chance to do their best.

136

DO AND DO NOT IN CHAPTER V

[CH. V

1. Use a good quality of
petroleum (kerosene) .

2. Keep the lamp clean, and
the wicks properly trimmed.

3. Use a chimney of the
proper height for the flame.

4. Turn the flame up as high
as possible without having it
smoke.

5. The edge of the flames
should face the condenser, the
middle flame being in the axis.

1. For gas use the best kind
of mantles.

2 . Make the connections with
rubber tubing of good thickness
and quality or flexible metallic
tubing (fig. 60).

1 . For acetylene use a proper
burner and reflector, that is,
one which is made by a reliable
house that has proved its safety
and excellence.

2. Use a safe gas supply,
such as a house supply or a
prcstolite tank is best.

Do NOT

1. Do not use poor oil, it will
not give a good light, and may
explode.

2. Do not let the lamp get
dirty or the wicks burn with
tails. Clean and trim.

3. Do not use a low chimney
for a large, high flame.

4. Do not turn the wicks up
till they smoke. Stop just
before that.

5. Do not have the face of the
flame, but the edge toward the
condenser.

Do NOT

1 . Do not use mantles of poor
quality, or that are broken.

2. Do not make connections
with thin or used up rubber
tubing.

Do NOT

1. Do not use an untried
lamp and general outfit for the
acetylene light. Acetylene is a
good servant but a cruel master.

2. Do not try to use a make-
shift gas generator. The smell
will be disagreeable and the
escaping gas possibly dangerous

CH. V]

DO AND DO NOT IN CHAPTER V

137

3 . Use thick and good quality
rubber tubing or flexible metal-
lic tubing (fig. 60) to make the
connections.

4. Study carefully the direc-
tions for the use of the acetylene
outfit with the magic lantern
sent out by the manufacturers.

5. Use perfect burners with
the gas turned on sufficiently,
but not enough to blow.

6. Keep all naked lights away
from an acetylene supply. Use
an electric torch light if a light
must be used.

3. Do not use poor rubber
tubing for connections.

4. Do not neglect the careful
study of the directions for using
the acetylene outfit with the
magic lantern.

5. Do not try to use broken
burners, and do not turn the gas
on until it blows.

6. Never let any naked lights
come near an acetylene gas sup-
ply.

i . For the alcohol light , follow
with care the directions accom-
panying your alco-radiant lamp.
Alcohol is dangerous stuff and
should not be trifled with.

Do NOT

i. Do not fail to follow with
scrupulous care the directions of
the manufacturers of the lamp
you use.

CHAPTER VI
THE MAGIC LANTERN WITH SUNLIGHT: HELIOSTATS

§ 230. Apparatus and material for Chapter VI:

Suitable room for projection, preferably one with southern
exposure ; Screen of proper size ; Porte- Lumiere or hand-regulated
heliostat; Heliostat with clock-work for regulation; Condenser
for bringing the parallel rays of sunlight to a focus (plano-convex
or achromatic combination) ; Slide-carrier and projection objective.

See also Ch. I, § i.

§ 231. Historical.

For the history of the magic lantern and all other projection
apparatus with sunlight, see the Appendix.

For Foucault's clock-driven heliostat see his: Recueil des
Travaux Scientifiques, 1878, pp. 427-433.

For the Heliostat of Mayer, using a lens and prisms, see Amer.
Journal of Science, IV Ser. Vol. IV, (1897), pp. 306-308.

For the Heliostats of Fuess, see C. Leiss, Die Optischen Instru-
mente der Firma R. Fuess, 1899, pp. 284-305. For Heliostats
like fig. 82, see Ambronn's Handbuch Astron. Instr. p. 649, fig. 637.

Dolbear. — Art of Projecting.

LIGHT FROM THE SUN

§ 232. The limitless supply of light from the sun would be used
in preference to any artificial source if it were only always avail-
able. In many regions it is available during most of the year, and
will no doubt be much more utilized as time goes on. Its use is
strongly recommended in sunny regions.

The sun is the brightest known source of light. Its intrinsic
brilliancy is, in round numbers, 421,000 candle-power per square
centimeter (2,720,000 candle-power per sq. inch). (See § 23 2a).

Sttnlight also serves as the standard for color values.

§ 232a. The intrinsic brilliancy of the sun. — The intrinsic brilliancy of a
source can be determined if its area and its candle-power are known. With the
sun it is in convenient to make the reckoning in these terms as both the candle-
power and distance are so enormous. The light from the sun near the zenith
in clear weather amounts to 288,000 meter candles, that is, the sunlight is as
powerful as the illumination due to 288,000 standard candles at a distance of
one meter. (A. Arrhenius — Lehrbuch der kosmischen Physik).

138

CH. VI]

HELIOSTATS FOR THE MAGIC LANTERN

139

§ 233. Heliostat. — From the rotation of the earth on its axis
from west to east the sun seems to move over the face of the sky

FIG. 74. MAGIC LANTERN WITH SUNLIGHT.

5 Sunlight.

Mirror The plane mirror serving to direct the sunlight horizontally into
the condenser.

Condenser The single plano-convex lens serving to converge the parallel
beam of sunlight. (Compare the second element of the condenser in fig. 2).

Ls Lantern slide.

Objective The projection objective for projecting an image upon the white
screen. The projection objective and the condenser should be of approxi-
mately the same focus.

c Center of the projection objective where the rays from the condenser
should cross.

Axis Axis The principal optic axis of the condenser and of the projection
objective.

Screen Image The image of the lantern slide upcn the white screen.

The apparent diameter of the sun's disc is 32'36" in midwinter and 31' 32" in
midsummer, or it averages 32' 04" (Abbot, The Sun, p. 3; Ball's Astronomy,
p. 127).

The apparent area of the sun's disc at a distance of one meter is determined
as follows: Its diameter is 32'o4" or .5343°. One centimeter at a distance of
one meter subtends an angle of .573°, hence at one meter the sun's disc would

appear to have a diameter of i = .933 centimeters. The area of such a

•573

circle is : ' \ IT = .684 square centimeters.
4

. . .,.,,. Candle-power

1 he intrinsic brilliancy is then, -

Area

288,000

- = in round num-

.684

bcrs, 421,000 candle-power per square centimeter or 2,720,000 candle-power per
square inch.

140 HELIOSTATS FOR THE MAGIC LANTERN [Cn, VI

from east to west. In order that the sun's rays may shine in one
place continuously it is necessary to counterbalance in some way
the apparent motion of the sun.

If one holds a plane mirror in the hands, it is possible to keep a
spot of sunlight on one place indefinitely by making slight changes
in the position of the mirror to correspond with the changes in
apparent position of the sun. This is possible from the law of
reflection: "The angle of incidence and the angle of reflection are
equal." (See fig. 80 and Chap. XIV, § 794).

A heliostat is then simply a mechanism for holding the mirror
so that the sun's rays may be reflected in a constant direction. As
this seems to make the sun stand still, the name is appropriate. It
was given by the original inventor, s'Gravesande, 1742 (fig. 77).

There are three principal forms of heliostats :

(1) The hand-regulated heliostat or porte-lumiere with one
mirror and a double movement up and down, and on the axis, so
that it may be made to follow accurately the sun's apparent motion

(fig. 75). '

(2) A heliostat with one mirror, in which the movements of the
mirror are brought about by clock-work (fig. 77-79).

(3) A Heliostat with two mirrors. One mirror is attached to
the end or side of the clock-shaft. The other mirror is not con-
nected with the clock-work. The second mirror serves to reflect
the beam from the movable mirror in the desired direction, and
is set by hand, once for all, at the beginning of the experiment.

The clock-shaft rotates once in 24 hours with the single mirror
heliostats (fig. 77-79), and also with the two mirror heliostat
with the mirror at the end of the clock-shaft (fig. Si).

When the mirror is attached parallel to the clock-shaft (fig. 82),
the clock-shaft rotates once in 48 hours (fig. 82, A, B, C).

INSTALLATION AND USE OF A HAND-REGULATED HELIOSTAT OR

PORTE-LUMIKRE

§ 234. The hand-regulated heliostat or portc-lumiere consists
of a plane mirror so mounted that it can move on two axes. The
mirror should be about 15 x 30 cm. (6 x 12 in.) in size and sup-

CH. VI]

HELIOSTATS FOR THE MAGIC LANTERN

141

ported by a framework. This frame should be hinged so that it
can be moved from the horizontal up to the vertical position. It
must also be so mounted that it can be rotated around at right
angles to the hinge motion.

FlG. 75. PORTE-LUMIKRE WITH PLANO-CONVEX LENS AND PROJECTION

OBJECTIVE.
(Cut loaned by the C. II. Stoelting Co.}.

The mirror for the reflection of the sunlight into the condenser is moved as
necessary by the milled head just above the condenser.

The two movements arc made by hand as often as needed while
one is using the apparatus. In the original forms of Cuff and
Adams (1744-1746, fig. /5a), and in some modern forms, there are
two handles or milled heads extending into the projection room;
with one of them the operator can raise or lower the mirror on its
hinges, and by the other he can rotate it on the other axis. In the
form here shown there is but one handle. This serves as a crank

142

HELIOSTATS FOR THE MAGIC LANTERN

[CH. VI

to turn the mirror around in a circle on its axis, and as a screw by
means of which it is raised or lowered on its hinges (fig. 75).

§ 235. Setting up the hand-regulated heliostat. — The appara-
tus must be so placed that it receives the full sunshine on the
mirror.

In the forenoon an eastern exposure can be used, and in the
afternoon a western one; or a southern one nearly all day. In
practice a person will naturally use the window best adapted to his
particular needs if he has a choice.

SOLAR, PROJECTION MICROSCOPE OF ADAMS, WITH
PORTE-LUMIERE.

(From Adams' Essays, 1771, PI. VI).

Fig. 4 shows the movable mirror (K-L) placed outside the shutter in the sun,
0-P, screws in the square plate to fasten the instrument in the shutter; M-N;
thumb screws by which the mirror is turned to hold the sun's rays in the right
direction. The large tube, A-C-D, contains the condenser and receives the
shorter tube, fig. 5. Fig. 5 shows the tube into which the objectives are fixed.
If for large objects the lens (fig. 6) is screwed into the end, g, for smaller objects,
the objectives are arranged in a piece (fig. 8) sliding into the opening at q.
Notches along the objective slider indicate when the lens is centered. The
specimen to be examined is inserted at h. For high powers the substage con-
denser shown at fig. 7 is put in the tube between d-h. At b is a rack and pinion
for focusing the object.

CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 143

§ 236. Darkening the room. — The room is darkened in the
usual way with curtains or shutters. The window where the
apparatus is to be placed must be darkened by a shutter or a cur-
tain with a hole in it, through which the instrument may be ex-
tended out into the sunshine and through which the sunshine
can be reflected into the room.

The window frame must either be raised entire or one of the
panes must be hinged so that it can be opened when desired. One
can use the heliostat within the room utilizing the sunlight passing
through the window glass, but this is far less satisfactory than hav-
ing the heliostat out in the free air where the sun shines directly
upon it.

Finally it must be possible to close the openings completely so
that the room may be made as dark as desired.

§ 237. Operation of the apparatus. — In starting work at any
time the mirror is inclined on its hinges and rotated until the sun
shines upon it, and then until the light is reflected into the con-
denser. Finally some further slight changes may be necessary to
get the light accurately centered so that it will pass from the
condenser along the common axis to the objective and thence to
the screen (fig. 74). By changing the position of the mirror
slightly every three to five minutes to compensate for the apparent
motion of the sun, the light will continue to pass through the magic
lantern to the screen.

§ 238. Adjustments necessary for the different windows.—

(A) For a southern exposure — For this exposure it is desirable
to have the entire outfit in a north and south direction with the
objective pointing toward the north. In the morning the mirror
is turned on its hinges to about 45° and then rotated toward the
east until it receives the light of the sun (fig. 76). It must then
be turned slightly by one or both of its possible movements until
the light is reflected in the desired direction. As the sun continues
to rise in the sky the mirror must be rotated on the axis from the
east toward the west to follow the apparent movement of the sun.
As the sun gets higher and higher the mirror must be turned on its

144

HELIOSTATS FOR THE MAGIC LANTERN

[Cn. VI

hinges more and more until at noon it will be nearly horizontal
(fig. 76). In the afternoon, as the sun moves toward the west, the
mirror must be rotated to follow it. At the same time it must be
turned more and more on its hinges until late in the afternoon, it
will be at the same angle as in the morning, and rotated as far
toward the west as it was toward the east in the earlier part of the
day (fig. 76).

(B) For an eastern exposure — In this position, the axis of the
entire instrument is preferably east and west with the objective
pointing westward. The earlier the time the more nearly hori-

North

FIG. 76. DIAGRAM SHOWING THE POSITION OF THE MIRROR NECESSARY
TO RELFECT THE SUNLIGHT DIRECTLY NORTH AT THREE DIFFERENT
PERIODS OF THE DAY — (6A.M.; 12 M.; 6 p. M.).

The diagram is for the latitude of Ithaca and at the season of the equinox
when the sun seems to rise directly in the east and set directly in the west. In
the morning thu mirror is turned toward the east at an angle of 45° and inclined
about 10° toward the south. In the evening it is turned similarly toward the
west and south.

At noon the mirror is raised on its hinges about 28° above the horizontal. At
all intermediate points the mirror must be set accordingly: that is, so that it
will reflect the sun directly north.

The diagram also shows the apparent course of the sun from sunrise to
sunset.

Cn. VI] HELIOSTATS FOR THE MAGIC LANTERN 145

zontal must the mirror be. As the sun gets higher and higher the
mirror must be raised more and more on its hinges ; and as the sun
seems to move toward the south as well as upward, the mirror must
be rotated on its axis toward the south.

(C) For a western exposure — If a western exposure is used,
the entire instrument should be placed pointing east and west if
possible. The mirror will be raised on its hinges and turned south-
ward early in the afternoon. As the sun sinks toward the west the
mirror will be made more and more nearly horizontal, and as the
sun seems to move toward the north as well as toward the west,
the mirror will finally be nearly horizontal on its hinges and rotated
somewhat northward.

These movements of the mirror become intelligible if one
observes the position of the sun in the different periods of the day.
By consulting fig 86, 87, it is also clear that the mirror must have
different positions owing to the declination or position of the sun
with reference to the horizon at different times of the year.

HELIOSTATS DRIVEN BY CLOCK-WORK

§ 239. Types of clock-driven heliostats. — A fundamental
character of all heliostats is that the clock-work rotates a shaft
corresponding with the post carrying the hour hand of an ordinary
clock, and that it is this shaft which directly or indirectly gives
motion to the mirror.

This shaft must be made parallel with the earths axis wherever
the instrument is used.

(A) Single-mirror type. — This is so constructed that the clock-
work gives a double motion to the mirror something as one can
give a double motion to a mirror held in the hands, i. e., an up and
down motion and a motion of rotation on the axis (fig. 77-79).

(B) Double-mirror type. — In this type one mirror is fixed at
the end, or the side of the clock-shaft. The second mirror is not
moved by the clock-work, but is set by hand at the beginning of
each experiment (fig. 81-84).

As one might conclude, the second or two-mirror type is of
simpler construction and therefore correspondingly inexpensive.

146

HELIOSTATS FOR THE MAGIC LANTERN [Ca. VI

FIG. 77. ONE-MIRROR HELIOSTAT OF S'GRAVESANDE.
(From his: Elementa Mathematica Phy sices, Tomus II, Tabula LXXXIII).

This picture is a facsimile except that the clock-shaft has been extended
above and below.

B B Base of the mirror support with leveling screws.

P Hollow cylinder in which the mirror support C can rotate.

C The pillar with mirror fork at the upper end.

5 Plane mirror.

D K Shaft at right angles to the mirror and extending to the fork at the
end of the clock-hand (N 0).

L L M Foot of the support for the clock-work.

Ill The leveling screws of the support.

CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 147

G F Column supporting the clock-work.

N P L° The clock-shaft. It is parallel with the earth's axis and hence
points toward the celestial north pole. The angle L° at the lower end is equal
to the latitude of the place where the heliostat is used.

T R Fork where the movement of the clock-hand (N 0) is transferred to
the shaft actuating the mirror (D E S).

f g Plate bearing the clock-work. It must be elevated sufficiently to make
the angle of the clock-shaft equal to the latitude of the place (see fig. 85).

It answers very well for all the work required by the photographer
and the projectionist.

§ 240. How to make the clock-shaft parallel with the earth's
axis at any given place. — For this it is necessary to know two
things :

(1) One must know the north and south direction.

(2) One must know the latitude of the place.

The first information can be gained by referring to the pole star.
Buildings are often set due north and south, and thus serve as
guides; or one might use a compass. If a magnetic needle is used
it must not be forgotten that there is a certain variation from the
true north and south line assumed by the compass needle, and for
accurate observations it is necessary to know the magnetic varia-
tion at any given place and to correct for it.

For the latitude, a good map like that issued by the U. S. geologi-
cal survey will give the information. The geological survey maps
also give the magnetic variation.

Making the clock-shaft parallel with the earth's axis is easily
accomplished if one knows the latitude and the north and south
direction. As a general statement all that is necessary is to make
the clock-shaft point toward the north star or more accurately,
toward the celestial north pole.

By referring to fig. 85 it is evident that this is brought about by
putting the instrument due north and south and then elevating the
clock-shaft above the level or horizontal line an amount equal to
the latitude of the place.

For example, if an experiment with the heliostat is to be made
in one of the buildings of Cornell University at Ithaca with, in
round numbers, a latitude of 42.5 degrees, the instrument is set on
a level place and due north and south, then the free end of the

148

HELIOSTATS FOR THE MAGIC LANTERN [CH. VI

FIG. 78. ONE-MIRROR HELIOSTAT OF FOUCAULT.
(From his Recueil des Travaux Scientifiques).

Modified by extending the clock-shaft above and below.

B Clock-work.

T The clock-shaft.

C The upper end of the clock-shaft.

N P Continuation of the clock-shaft above the mirror (M).

L° The angle above the horizontal to make the clock-shaft parallel with the
earth's axis. It equals the latitude of the place where the heliostat is used (see
fig- 85).

D Divided semicircle to enable one to set the instrument according to the
sun's declination.

N1 N, N Cl Connections between the clock-work and the mirror.

clock-shaft is raised above the level 42.5 degrees. If now one were
to sight along the clock-shaft it would be found pointing directly
toward the north star.

As seen from the diagram, fig. 85, it will then also be parallel
with the earth's axis.

CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 149

Sometimes the co-latitude and the vertical line are used instead
of the horizontal line and the latitude. This brings about the same
result, for if the clock-shaft is vertical to start with, it must be
tipped over toward the north from the vertical an amount equal to
the co-latitude. That is, in Ithaca, it must be inclined 47.5 degrees
from the vertical.

In general the clock-shaft must be inclined upward from the
horizontal, a number of degrees corresponding with the latitude at
the place of observation or it must be inclined downward toward
the north from the vertical position a number of degrees correspond-
ing with the co-latitude. The sum of the latitude and the co-lati-
tude in every case equals 90 degrees. (See fig. 85 and its explana-
tion) .

INSTALLATION OF A SINGLE-MIRROR HELIOSTAT
§ 241. Setting up the heliostat. —

1. In the first place the instrument must be leveled and

arranged accurately in a north and south direction.

2 . The clock-shaft must next be elevated to an angle correspond-

ing with the latitude of the place where it is to be used
(§ 240) so that it will point toward the north star. It will
then be parallel with the earth's axis (fig. 85).

3. To give the proper angle to the mirror, depending on the

declination of the sun, and to get also the correct local
time, loosen the clamp holding the clock-arm (fig. 79 c)
and turn the clock-arm toward the sun until the light
shines through both sights along the line q-p. Then
clamp the set screw at c.

4. To get the spot of light in the desired place, loosen the clamp-

screws in the position arm F-B and below H in the rotating
collar and then raise or lower the shaft o, fig. 79 and rotate
the position arm around the column A till the light is
reflected where it is wanted, then tighten the clamping
screws and the clock-work should cause the mirror to move
so that it will reflect the beam of light in the same place so
long as the sun shines on the mirror.

HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI

FIG. 79. UNIVERSAL, ONE-MIRROR HELIOSTAT.
(From the Catalogue of R. Fuess).

Modified by extending the clock-shaft above and below and by adding the
abbreviations in and rf for the incident and reflected ray on the mirror (M).

This heliostat is called universal for it is adjustable so that it can be used in
any latitude and at any season of the year. See fig. 80 and § 241 for further
explanation.

The dials showing the time and declination may be used for
setting the heliostat, but one can get the apparatus set accurately
by trial as just described. If the time and declination scales are
to be used one must consult a nautical almanac for the sun's
declination for the given date, and an accurate clock for the time
of day.

§ 242. For centering the magic lantern when a heliostat is used
the same general principles must be followed as with the arc light
magic lantern (Ch. I, § 51-57, fig. i, 74).

To center the light one must be able to adjust the mirror by hand
after it has been set to follow the sun. This is provided for in all

CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 151

forms of single-mirror heliostats. In fig. 79, for example, the
position arm B-F can be raised or lowered and the entire arm can
be rotated around the column A. When the light is accurately
directed, all the clamps can be tightened and the clock-work should
cause the mirror to hold the light constantly in position. It will
be found much easier to center the light on one axis if the heliostat
is at about the same level as the condenser and objective. This
position can be secured by raising the heliostat or the lantern,
whichever is more convenient, provided the two are not on the
same level to start with.

o
FIG. 80. PRINCIPLE OF THE UNIVERSAL HELIOSTAT SHOWN IN FIG. 79.

O A The clock arm pointing directly towards the sun.

0 B The position arm, pointing in the direction in which it is desired to
reflect the light.

in The incident light parallel to 0 A.

rf The reflected light.

A B The mirror. The mirror is perpendicular to the plane passing through
A , 0 and B.

0 N Perpendicular to the mirror A B.

In order to prove that incident light parallel to A 0 will be reflected from
the mirror parallel to 0 B it is necessary to prove that A O, 0 B and 0 N are
in the same plane and that O N bisects the angle A 0 B. The mirror being
perpendicular to the plane containing A , 0 and B and the line O N perpendicu-
lar to A B must also be in this same plane. The triangle A O B is isosceles by
construction, as A O and 0 B are made equal, hence the perpendicular to the
base must bisect the vertex angle.

15.2 HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI

INSTALLATION AND USE OF TWO-MIRROR HELIOSTATS

§ 243. Heliostat with the mirror at the end of the clock-shaft. —

Place the heliostat in a position either inside a room or outside a
window where the full light of the sun can fall upon the movable
mirror. The stand supporting the clock-work, etc., must be made
level, and set in a north and south direction (fig. 81).

Elevate the clock-shaft above the level to an angle equal to the
latitude of the place where it is to be used. One can use a good
protractor for this. The clock-shaft will then point toward the
north star, and be parallel with the earth's axis (fig. 85).

This form of heliostat often has the clock-shaft in a fixed position
for cheapness of construction (fig. 81). If such a heliostat is
purchased, the manufacturer must know the latitude of the place
where it is to be used, then he will give the proper inclination to the
clock-shaft so that when the instrument is arranged in a north and
south line the shaft will point toward the north star.

§ 244. Arranging the movable mirror. — The mirror is fixed to
the end of the shaft by a collar which permits it to rotate around
the shaft. It is also held in a kind of fork, which permits the
mirror to be raised and lowered in a way similar to the hinge
movement of the porte-lumiere (fig. 75).

For setting this mirror so that the clock-work will cause it to
throw a beam of light in one direction continuously, it is necessary
first of all to set the mirror for the local time. This is done by the
use of a perforated screen admitting a narrow pencil of light from
the sun. This screen is so placed that the pencil of light falls upon
the mirror. The mirror is then turned by loosening the clamp
(fig. 8 1 c) and rotating it on the shaft, and by tipping it in the fork
until the pencil of light is reflected back along its path through the
hole again.

Then the clamp is tightened and the screen removed. The
mirror is now tipped in the fork until the light is reflected from it
directly in line with the clock-shaft, i. c., directly toward the north
star (fig. 81 N. P.). The easiest way to do this is to take a piece
of white cardboard with parallel black lines on it and place it

CH. VI]

HELIOSTATS FOR THE MAGIC LANTERN

153

FIG. 81. TWO-MIRROR HELIOSTAT WITH THE MOVABLE MIRROR AT THE

END OF THE CLOCK-SHAFT.
(From the Catalogue of Max Kohl).

The figure has been modified by extending the clock-shaft and by adding the
second mirror and the light rays. This heliostat is usually fixed for a given
latitude, hence in ordering one, the latitude of the place should be given. It
could be made adjustable for latitude, but that would naturally increase the
cost.

N-P, L° The clock-shaft pointing to the celestial north pole above, and
indicating the angle corresponding with the latitude below.

c Clamp to hold the mirror (M) to the revolving clock-shaft.

M Movable mirror. It is adjusted in the fork and on the clock-shaft until
the reflected rays proceed parallel with the earth's axis, hence also parallel with
the clock-shaft.

M2 The fixed mirror to be set by hand in the beginning.

S and the Arrows. The sun's rays.

154 HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI

parallel with the clock-shaft. When the beam of light from the
mirror extends out parallel with these lines, as indicated by the
streak of light, the mirror will be in the correct position.

§ 245. Arranging the second mirror. — For getting the light in a
desired direction, a second mirror is used in the path of the beam
extending directly northward, from the first mirror, and so arranged
that the light is reflected as desired (fig. 81 M2).

§ 246. Heliostat with the mirror parallel with the clock-shaft.—

With the other heliostats described in this chapter, the clock-work
rotates the shaft once in 24 hours, but with this form, the rotation
is once in 48 hours, i. e., half the rate of rotation of the earth. The
clock-shaft is somewhat extended and the mirror is fixed directly to
the shaft and parallel with it. The mirror is therefore in a plane
which if extended would cut the celestial north pole (fig. 82).

Light reflected from this mirror may be made to take any direc-
tion in a circle.

§ 247. Setting up the heliostat with the mirror parallel with the
clock-shaft. — The heliostat is placed in a proper position for
receiving the sunlight. The support is made level, and the instru-
ment set north and south. The clock-shaft is then elevated from
the horizontal until it is at an angle equal to the latitude of the
place where it is to be used. As the mirror in this form may be set
to reflect the light anywhere in a circle, it is best to loosen the clamp
of the clock-shaft and rotate the mirror until it receives the full
light of the sun and reflects it in a convenient direction. Then
clamp the shaft to the clock-work and the mirror will follow the
sun.

§ 248. Arranging the second mirror. — The second mirror is now
placed so that it will receive the beam from the movable mirror,
and then it is turned, raised, or lowered on its stand, until the light
extends in the desired direction. It should continue to hold the
light in one place so long as the sun shines on the movable mirror
(fig. 82). One must make sure that the position of the second
mirror is such that it will not shade the heliostat mirror as the
sun moves toward the west.

CH. VI]

HELIOSTATS FOR THE MAGIC LANTERN

FIG. 82.

TWO-MIRROR HELIOSTAT WITH THE MOVABLE MIRROR ATTACHED
PARALLEL TO THE CLOCK-SHAFT.

This heliostat is adjustable for latitude and can be used anywhere in the
northern hemisphere, and by reversing the motion, in the southern hemis-
phere (§ 253).

C Clock-work mounted on a hinged plate.

M1 Rotating mirror attached to the side of the clock-shaft. From this
arrangement its plane would pass through the celestial north pole if extended.

*To get this picture, the heliostat was set in the west window of Stimson Hall
at 2:30 P. M., May 20, 1912, and the mirrors arranged to receive and reflect a
small beam of sunlight as indicated. A black cord was extended from the
small hole in the shutter to the point on the first mirror receiving the sunbeam,
and from thence to the second mirror along the path of the sunbeam ; and from
the second mirror to a point on the screen receiving the sunbeam. The
apparatus was then photographed. To make the course of the sunbeam very
sharp for this cut its course was traced on the photograph by a right line pen.
The clock-shaft was also extended above and below and an arc of a circle added
between the clock-shaft and the horizon to indicate the angle of elevation of
the clock-shaft, corresponding with the latitude of Ithaca (42.5° North Lati-
tude).

156

HELIOSTATS FOR THE MAGIC LANTERN

[CH. VI

M2 The fixed mirror. This is adjusted at the beginning of the experiment
to reflect the light in the desired place and usually needs no attention during
the experiment.

N P Continuation of the clock-shaft pointing toward the north pole.

L 42.5° The angle made by the clock-shaft, and the horizon at Ithaca,
New York, U. S. A. It indicates the latitude of that place, and the elevation
of the clock-shaft to make it point toward the celestial north pole.

S B Sunbeam admitted through a hole in the shutter. It strikes the first
mirror and is reflected to the second mirror, and from it in any desired direction.

\2 M

FIG. 82 A, B, C. DIAGRAMS SHOWING THE POSITION OF THE FIRST
MIRROR OF THE HELIOSTAT (Fig. 82) AT DIFFERENT TIMES OF THE DAY
TO REFLECT THE SUNLIGHT CONSTANTLY IN THE SAME DIRECTION.

The eye is supposed to be looking along the axis of the clock-shaft. It is to
be noted that between 6 A. M. and 6 P. M. (12 hours) the mirror has turned
through an angle of 90°, and at this rate it takes 48 hours for the mirror to
make a complete revolution of 360°.

The arrows indicate the direction of the light before and after reflection from
the mirror.

A Position of the mirror at 6 A. M.

B Position of the mirror at 12 M.

C Position of the mirror at 6 P. M.

At intermediate periods the mirror will be in correspondingly intermediate
positions to reflect the sun constantly in the same direction, that is, the mirror
must follow the sun.

This is one of the easiest heliostats to manage, as one needs to
know only the latitude and the north and south direction. The
arrangement of the two mirrors can be easily made at any time
and in any place by trial.

HELIOSTATS ix THE SOUTHERN HEMISPHERE

§ 249. U]) to the present, the discussion has been with reference
to heliostats in the northern hemisphere. For those to be used in
the southern hemisphere certain modifications are necessary as seen
from the following considerations :

CH. VI] HELIOSTATS FOR THE MAGIC LANTERN

157

FIG. 83. LENS AND PRISM HELIOSTAT OF ALFRED M. MAYER.
(From the American Journal of Science, Vol. 154, iSg1^).

This heliostat is in principle like the two-mirror heliostat with the movable
mirror attached to the end of the clock-shaft (fig. 81).

J Biconvex lens about 10 cm. (4 in.) in diameter to receive the sun's rays
and render them convergent.

K Concave lens to render the converging beam parallel.

g Rack and pinion movement to change the position of the concave lens
and thus increase or diminish the size of the beam.

/ Right-angled prism receiving the parallel bundle from K and reflecting it
to a fixed prism (L) or to a mirror, by which it is reflected in any desired
direction.

The two lenses J K and the prism /, are all on one common axis and are
rotated by the clock-shaft G, and thus made to follow the sun like the mirror
on the end of the clock-shaft in figure 81. The clock-shaft G must be at an
elevation corresponding to the latitude of the place (see also fig. 84).

158 HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI

§ 250. If one were looking at the north pole of the earth from
a position along the earth's axis, the direction of the earth's rota-
tion would appear in a direction opposite to the hands of a clock or
watch. To compensate for this, a mirror to hold a spot of sunlight
in one position would need to be rotated around an axis parallel
with that of the earth, but in an opposite direction to the earth's
rotation, that is in the clockwise direction.

§ 251. At the equator, the clock-shaft must be horizontal in
order to be parallel with the earth's axis. The clock-shaft must be
turned from east to west. This can be accomplished either by a
clock-work located at the southern end of the shaft turning in the
clockwise direction as in fig. 77-79, or by a clock-work located at
the northern end of the shaft turning in a counter-clockwise direc-
tion.

§ 252. At the north pole of the earth, the axis of rotation of the
shaft would be vertical and the direction of rotation as seen from
above, would be clockwise.

At the south pole the axis would also be vertical and the direction
of rotation would be clockwise as seen from below — i. e., from the
north — or counter-clockwise as seen from above.

§ 253. A heliostat constructed for the southern hemisphere
would be exactly similar to one for the northern hemisphere except
that the clock-shaft must rotate in the counter-clockwise direction,
that is, from right to left.

§ 254. Setting up a heliostat in the southern hemisphere. — If

a heliostat is properly constructed for the southern hemisphere it
is set up at any given south latitude by arranging the instrument
due north and south with the free end of the clock-shaft pointing
south. Then the clock-shaft would be elevated above the horizon
a number of degrees corresponding with the south latitude. This
would make the clock-shaft parallel with the earth's axis and it
would point toward the celestial south pole (fig. 85). Indeed, the
entire procedure for getting the light in the desired direction, the
use of the condenser and projection objective, etc., is exactly
as for the northern hemisphere.

CH. VI]

HELIOSTATS FOR THE MAGIC LANTERN

159

FIG. 84. LENS AND PRISM HELIOSTAT OF ALFRED M. MAYER.
(From the Catalogue of Optical Instruments by R. Fuess),

The figure has been modified by extending the clock-shaft above and below.
As here shown the instrument is suitable for any latitude. It uses a mirror
instead of a second prism as in the original of Mayer (fig. 83).

U Clock-work.

N P, L° The clock-shaft extended to indicate the direction of the celestial
north pole above, and below the angle of elevation corresponding to the lati-
tude of the place where the instrument is used.

P D Z Three divided scales; P for the latitude, D for the Sun's declination,
Z for the time of day.

S, k and Pr. The convex and the concave lens, and the prism as shown in
fig- 83.

Sp Mirror to take the place of the prism (L) in fig. 83.

i6o

HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI

FIG. 85. DIAGRAM SHOWING THAT THE ELEVATION OF THE CLOCK-SHAFT
AT AN ANGLE EQUAL TO THE LATITUDE OF A PLACE WILL MAKE THE
CLOCK-SHAFT PARALLEL WITH THE EARTH'S Axis.

EQ Equator of the earth.

Axis Axis The earth's axis with the north pole of the earth above and the
south pole below.

N P The earth's north pole.

S P The earth's south pole.

42.5° Latitude of Ithaca, New York, U. S. A.

h h Horizontal lines, that is, tangents to the earth's surface at the two
latitudes shown (42.5° north, 30° south).

Z Zenith.

A A Clock-shaft elevated from the horizon an amount equal to the latitude.
If continued toward the equator the clock-shaft would meet the plane of the
equator at right angles, hence it is parallel with the earth's axis and points
toward the celestial poles.

A h Latitude (42.5° north and 30° south).

A Z Co-latitude (47.5° north, 60° south).

§ 255. Finally, a hcliostat constructed for the northern hemi-
sphere would work equally well for the southern hemisphere if it
were attached to the ceiling (i.e. wrong side up) instead of being on
a table or window-sill right side up, for this change in position would
make the clock-shaft rotate in the counter-clockwise direction, as.
seen from above.

CH. VI] CONDENSER FOR SUNLIGHT 161

CONDENSER FOR SUNLIGHT

§ 256. As sunlight is composed of practically parallel rays, the
condenser consists of a single plano-convex lens with the convexity
receiving the light (fig. 74); or one may use an achromatic com-
bination (fig.324).

The condition is practically like the ordinary condenser after the
light has been rendered parallel by the first element of the condenser
(fig. 3). Having parallel rays to start with, only the second ele-
ment of the condenser is needed.

§ 257. Increasing the illumination. — The greatest difference
between the use of sunlight and the arc light for projection appears
when one wishes to increase the illumination. With the arc lamp
one simply uses more current, and this increases the candle-power
and makes the screen image more brilliant. With the same size
condenser and picture the illumination of the screen with the arc
light is directly proportional to the illumination of the condenser
face.

With sunlight, the illumination of the condenser face is a con-
stant quantity except for haze, etc. As all the light which strikes
the screen must pass through the condenser, the screen illumina-
tion can be increased with sunlight only by using a condenser of
larger diameter and correspondingly greater focal length. And for
this one must have heliostat mirrors of sufficient size to fill the
condenser with light.

§ 258. The water-cell with sunlight. — This light is accom-
panied by so much radiant heat that it is desirable to use a water-
cell with the apparatus, and thus reduce the liability of over-heating
lantern slides or other specimens used for projection (see § 848 for
the discussion of the need of a water-cell) .

CONDUCT OF AN EXHIBITION WITH SUNLIGHT

§ 259. The general principles given in Ch. I, § 21-41 are
applicable.

§ 260. Lighting of the room. — Sunlight is sufficiently powerful
so that the room used need not be very dark for showing lantern

162 USE OF SUNLIGHT FOR PROJECTION lCH. VI

slides. Care must be taken to have no direct light fall on the screen
except that from the lantern, but the room can have sufficient
diffused light to take notes comfortably (see also Ch. XII,
§ 605-608).

§ 261. Size of the room and the screen. — By using a condenser
of proper size and of a focal length adapted to the projection
objective, there is no practical limit to the possibilities of projection
with sunlight.

§ 262. Turning on and off the light. — For shutting out the sun-
light one can use a metal shield between the mirror and the con-
denser or one can use the objective shield (fig. 14 and 62). The
first method is preferable, for there will be less heating of the
apparatus.

TROUBLES
§ 263. The troubles with sunlight are:

1 . The difficulty of keeping the beam of sunlight in a constant

direction. With the porte-lumiere one must be con-
stantly on the alert to make the slight adjustments of the
mirror necessary.

2. The clock-driven heliostats, if well made and regulated

accurately, should give no trouble when they are prop-
erly setup.

If a person is fortunate enough to live near an astronomical
observatory and can get the help of the astronomer in charge he can
learn to overcome difficulties that seem to be insurmountable when
working alone. The apparatus of an observatory is also of first
rate quality, and it helps any worker to know what good apparatus
looks like.

§ 264. Lack of sunlight. — This is the one great trouble. Of
course it is not available at night anywhere. And in the most
thickly populated regions where projection apparatus is used there
is liable to be so much cloudy weather that sunlight is not available
even in the daytime during much of the year. Smoke also obscures
the sun when clouds are absent.

CH. VI]

TROUBLES WITH SUNLIGHT

163

Fortunately, in many parts of America the sun can be counted
on in the daytime ; and for those parts the use of sunlight for
projection of all kinds is strongly recommended.

North Pole

South Pole

FIG. 86. DIAGRAM OF THE CELESTIAL SPHERE WITH THE PLANES OF THE

CELESTIAL EQUATOR AND OF THE ECLIPTIC; AND WITH THE APPARENT

POSITION OF THE SUN AT DIFFERENT SEASONS.

Earth This is shown as a small black sphere at the center.

North Pole, South Pole The two poles of the celestial sphere. They are at
an infinite distance from the earth.

West, East East and west points of the celestial sphere. The plane of the
celestial equator touches these points.

Equator The plane of the celestial equator (shaded in lines) dividing the
celestial sphere into a northern and a southern hemisphere. A plane at right
angles to this traversing the north and south poles would divide it into an
eastern and western hemisphere.

Ecliptic The plane (shaded in dots) around the outer edge of which the
sun seems to move during the year. It is inclined to the equator at an angle
of 23° 27.'

Equinox When the sun appears at the equator the days and nights are of
equal length (March 21, Vernal or Spring Equinox, and Sept. 23, Autumnal or
Fall Equinox).

Solstice The point on the Ecliptic the farthest north or south of the Equa-
tor. (Summer Solstice, when north of the equator, June 22 ; Winter Solstice,
when south of the equator, Dec. 22).

(See also fig. 87).

164

DO AND DO NOT WITH SUNLIGHT

[Cn. VI

§ 265. Summary of Chapter VI:

Do Do NOT

i . Utilize sunlight when that i . Do not use artificial light
is available, for it is the bright- in a region where bright sun-
est light to be had on our planet light is constantly available.
(§ 232).

2. For sunlight some sort of
a heliostat is necessary to
counterbalance the rotation of
the earth, and make the sun
shine in one place continuously
(§ 233).

3 . Two motions to the mirror
are necessary, an up and down
motion and a rotary motion at
right angles to this (§ 233).

4. If a clock-driven heliostat
is used, the instrument must be
set up so that the shaft of the
clock shall point toward the
celestial pole and thus be
parallel with the earth's axis
(§ 239-241).

5. To make the shaft parallel
to the earth's axis raise it from
the horizontal an amount equal
to the latitude of the place
where it is to be used (§ 240).

6. The two-mirror heliostat
is simplest and least expensive
(§ 239).

2. If a porte-lumiere is used
to keep the sun shining in one
place, do not forget to adjust
the mirror frequently. Remem-
ber that the earth never stops
rotating.

3-4. For a clock-driven helio-
stat do not forget that the shaft
moving the mirror must point
toward the north pole (or
south pole, if south of the
equator) .

5. Do not forget to elevate
the clock-shaft an amount equal
to the latitude of the place.

6-7. Do not put the second
mirror of the heliostat so that
the sun cannot shine on the
first mirror.

CH. VI]

DO AND DO NOT WITH SUNLIGHT

165

7. One mirror is attached to
the shaft and is driven by the
clock-work. The other mirror
is set by hand at the beginning
of the experiment (§ 239, 248).

8. As the rays of sunlight are
practically parallel, only one
element of the condenser is
needed, viz., the one next the
lantern slide (fig. 74, § 256).

9. To increase the illumina-
tion use a larger mirror and
condenser (§ 257).

10. To turn the light on and
off, use a metal shield (§262).

n. Use a heliostat designed
for the hemisphere where you
are to work (§249-255).

8. Do not use a condenser
with two or three lenses for sun-
light as for a near light source,
use only one lens or an achroma-
tic combination designed for
parallel light (fig. 74).

9. One cannot increase the
illumination without increasing
the size of the mirror and con-
denser.

10. Do not use inflammable
shields to block the light. Use
a metal shield between the
mirror and condenser.

1 1 . Do not try to use a helio-
stat in the southern hemisphere
which was constructed for use
in the northern hemisphere.

§ 270. Apparatus and material for Chapter VII:

Suitable projection room with screen (§ 286); Lantern with
projection objective of large aperture and with suitable radiant
and condenser (§ 275, 277, 279, 294-296); Suitable objects for
projection (§ 285).

FIG. 88. CAMERA FOR DRAWING LANDSCAPES.
(From the Catalogue of Queen & Co., 1880).

The dark room is made of opaque cloth over a tripod. The 45° mirror at
the top rotates to take in any desired part of the surrounding country and an
objective projects the image down upon the horizontal drawing shelf.

If there is to be combined projection, a tinted glass to make the
lantern-slide image as dim as the opaque image (§ 282).
See also the outfit given in § i, Ch. i.

1 66

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 167

§ 271. Historical development. See appendix.

References to literature : See the books referred to in Ch. i, § 2,
also the special catalogues and directions furnished by the manu-
facturers of Opaque Lanterns and combined projection apparatus.

PROJECTION OF IMAGES OF OPAQUE OBJECTS
§ 272. All of the images seen on a white screen within a dark
room were originally of opaque objects. These objects were
brilliantly illuminated by the sun, and the light reflected from them

FIG. 89. CAMERA FOR EXHIBITING SURROUNDING LANDSCAPES.
(From the Catalogue of McAllister).

In a kind of cupola at the top is situated a plane mirror and beneath that a
projection objective. The cupola rotates, thus enabling the operator to bring
any desired scene upon the horizontal screen within the room. Such cameras
were once common at fairs and in parks.

1 68 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII

10 11 12 13 14 15

FIG. 90. DIAGRAM SHOWING OPAQUE PROJECTION.

In both these diagrams (fig. 90-91) the same amount of light illuminates the
object, and the objects are of the same size, and the objectives have the
same aperture.

Fig. go. Opaque projection In. L. Incident light of parallel rays imping-
ing upon a picture in white and black.

Object The opaque object in black and white the size of a lantern slide.

1-15 The beams of light illuminating the object. The light must of
course fall upon the surface facing the projection objective.

R L Reflected light. From each point on the surface of the opaque object
the light falling upon it is reflected nearly equally throughout the entire hemi-
sphere.

Ax Axial beam on the principal optic axis of the objective.

Objective The projection objective. Its aperture is such that it receives
and transmits about 20° of the 180° reflected from each point.

From the formula given in § 857:1 such an objective transmits to form the
screen image approximately 3% of the light reflected from the opaque object.

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 169

FIG. 91. TRANSPARENCY PROJECTION.

In L Incident light. This is supposed to be exactly the same as that
striking the face of the opaque object. In this case it traverses the condenser
lens, passes through the transparency, and the objective, and passes on to
the screen with very little loss.

1-15 Parallel beams of light reaching the condenser and passing onward.

Condenser A plano-convex lens to render parallel rays converging.

L S Transparent lantern slide.

Ax The principal optic axis.

Objective The projection objective. Its aperture is the same as in fig. 90,
but is much larger than necessary for the transparency.

I yo PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. .VII

passing through a hole, or later a lens, in the wall of a dark room
sufficed to produce the picture on the white wall or screen.

Later it was found that it was possible to illuminate objects
sufficiently with artificial light to get screen pictures; and still
later transparencies were used (§ 272a).

Every one who looks at the picture of a landscape, etc., depicted
on the ground glass of a photographic camera sees inverted images
like those originally observed in darkened rooms on translucent
screens.

CONDITIONS FOR OPAQUE PROJECTION: COMPARISON OF PROJEC-
TION WITH OPAQUE AND TRANSPARENT OBJECTS

§ 273. In order to deal intelligently and successfully with
opaque projection it is necessary to comprehend in the very begin-
ning the difference in the conditions for obtaining a screen image
of an opaque object, and for a screen image of a transparency
(lantern slide, moving picture film or microscopic specimen).

With a transparent or semi-transparent object, the light comes
from behind and traverses the object, and goes on with practically
no variation in direction to the projection objective. As the light
reaching the lantern slide or transparency is directed by the con-
denser (fig. 91), the light which illuminates the transparency passes
on and enters the projection objective and therefore serves for the
production of the screen image (fig. 1-2).

With the opaque object, on the other hand, all the light which
produces the screen image must be reflected from the surface of the
object, and the light which illuminates the object must strike its

§ 272a. In the early days of opaque projection with artificial light the whole
face of a man was sometimes shown; this, of course, required very large lenses.

This is what Hepworth says concerning these exhibitions: "At one time a
large instrument of this type was made for casting the image of a human face
on the screen, the lenses being of immense size. . . It was, of course, fitted
with a reversing (erecting) lens (fig. 208), so that the face should appear right
way up. The owner of this face, by the way, suffered tortures during the short
time of exhibition, for the powerful lime lights close to and on each side of his
head, were so hot that they blistered his skin. He was made to smile at the
audience, and then to drink their good health in a glass of wine, a refreshment
which the poor man really needed after his grilling." (P. 246).

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 171

face instead of traversing it, — that is, it must extend in the opposite
direction from that used with the transparency.

The light falling upon the face of the opaque object must then
be reflected from each point. But unlike the tranteparent object,
in which practically all of the light illuminating each point of the
object goes directly to the projection objective (fig. 91), with the
opaque object, each point reflects the light irregularly and in all
directions within the entire hemisphere ( 1 80 degrees, fig. 90) . This
being the case, only a part of the light reflected from each point can
get into the projection objective, all the rest falling outside the
objective. Of course, the larger and closer the objective, the more
of the light will be received; hence, in selecting an objective for
opaque projection, keep in mind that the greater the diameter of
the lenses the more light from each point can be received, and con-
sequently the more brilliant will be the screen picture.

It is assumed in this discussion, and in the accompanying dia-
grams (fig. 90-91), that the opaque object is black and white and
that it and the transparent lantern slide are of the same size;
that both are lighted by a similar beam of parallel light rays, and
that none of the light is lost by absorption.

§ 274. Relative amount of light for the images with trans-
parencies and opaque objects. — If, for example, as in the diagram,
the projection objective can receive but 20 degrees of the hem-
isphere of light from each point, then 160 degrees will fall outside the
objective and not aid at all in the formation of the screen image.
If the objective could take in all of the light from each point, the
opaque object would give as brilliant a screen image as the lantern
slide, but the actual proportion of light represented by the angle of
twenty degrees is only three per cent, of that represented by 180
degrees. As only three per cent, of the light from each point helps
in the formation of the screen image of the opaque object, the
opaque object can give a screen picture only three per cent, as
bright as the transparency where practically all of the light helps
to form the screen image (fig. 90-91).

In practice, how great a proportion of light serves for the screen
image and how much is absorbed or lost depends upon the opacity

172 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII

of the lantern slide and the reflecting qualities of the opaque object
(see § 274a).

§ 275. Aperture of the projection objective for transparencies
and for opaque objects. — By comparing figures 90-91 it will be
seen that for a transparency, relatively small aperture for the
projection objective is sufficient. This also shows that if one were
to use the same objective for both transparencies and for opaque
objects, that the difference in brightness would be enormously
exaggerated, if one used only the necessary aperture for the trans-
parencies. If one used the proper objective for the opaque object,
it would answer well for the transparency, but only a part of the
aperture would be utilized. As the large aperture makes the
objective very expensive, one wastes money by having the large
aperture for transparencies In the best practice, an objective of
moderate aperture is used for transparencies, and one of relatively
very large aperture for opaque projection.

§ 276. As will be shown later (Ch. XIV, § 8 5 ya), with a given
object and a given illumination, the brilliancy of the screen image
depends upon the aperture of the objective and its distance from
the screen. The larger the diameter of the lenses of an objective

§ 274a. Light flux getting through the objective with opaque projection. — •

It will be shown in § 857a that the light received from a perfectly white, per-
fectly diffusing surface is

vSin 26 d20 _ _ T B

— (i— cos 20)

(i — cos 20) lumens per square centimeter of the white reflecting

surface, where I is the intensity of illumination of the surface measured in
meter candles, and 6 is the half angle of light subtended by the objective, or 26
is the angle of light subtended by the objective. The light received by the
surface is I/io,ooo lumens and the proportion of light received by the surface

i — cos 2B
which strikes the objective is then

In this problem the angle of light subtended by the objective is 20°, i. e.
26 = 20°. The proportion of light received by the objective is then (i — cos
20°)/2 = (i — -9397)/2 = .0603/2 = .0302 or about T,r"(.

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 173

of given focus, the greater will be the brightness. With the same
objective, the greater the distance of the objective from the screen,
the less will be the brightness

FIG. 92. CHADBURN'S OPAQUE

LANTERN WITH ONE SOURCE

OF LIGHT.

(From Chadwick, Hepworth and
Wright).

L Source of light shining directly
upon the opaque object.

M Beam of light from the opaque
object to the objective and to the
screen.

FIG. 93. CHADBURN'S OPAQUE LAN-
TERN WITH Two SOURCES OF LIGHT, i
(From Chadwick, Hepworth and Wright).

This form requires two sources of
light and two condensers. The light
is projected directly upon the object
and from the object it extends out
through the objective to the screen.
This method is still often employed.

The same lantern, connected in the
usual way, was employed for trans-
parency projection (fig. i).
L-L Source of light and condenser arranged to send the light directly to
the opaque object.

D-D Hinged door for the support of the book, picture or other object.
When the door is closed, the light from both sources shines directly upon the
opaque object.

B Beam of light from the object to the objective.

A Objective of large aperture for projecting the image of the opaque object
upon the screen.

174 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII

§ 277. Brilliant screen images of opaque objects. — It is intel-
ligible from the above discussion and the diagrams that to produce
a brilliant screen image of an opaque object five things are neces-
sary:

1. The light for illuminating the opaque object must be very

brilliant, like sunlight or the electric arc light.

2. The opaque object must be capable of reflecting most of the

light illuminating it, or must be on a white background.

3. The projection objective must have lenses of large diameter.

4. The distance of the objective from the screen must not be

too great.

5. Besides the above, the projection room must be dark or the

screen image will not have sufficient contrast.

FIG. 94. DOLBEAR'S OPAQUE PROJECTOR WITH SUNLIGHT.

(From Dolbear's Art of Projecting).

H Heliostat, porte-lumiere or simply a plane mirror to direct the sunlight
through the bi-convex condenser.

r Movable mirror to reflect the sunlight upon the opaque object at d.
The handle for changing the inclination of the mirror is seen at the right.

d Opaque object with the light from the mirror (r) illuminating it.

o Projection objective.

S Screen for the image.

§ 278. Position of the radiant. — The radiant or source of light
for illuminating opaque objects for projection may have either of
two positions:

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 175

1. It may be in front of the object so that the light emitted

shines directly on it. This is the original device and gives
the greatest amount of light (fig. 92-93); or the radiant
may be tilted (fig. 105, in).

2. The second method is to have the light not in front, but a

mirror reflects the light from the radiant upon the opaque
object (fig. 94, 95). This is usually a more convenient
arrangement than the above, but a certain amount of the
light (between 10% and 25%) is lost when reflected
from a mirror.

§ 279. Use of a condenser or concave reflector with opaque
projection. — This is frequently employed for the object is often at a
considerable distance from the radiant, and too small a part of the
light from the radiant would be available but for the help of the
condenser.

In most cases only the first element of the condenser is used.
This projects upon the object or the mirror a cylinder of parallel
rays (fig. 90,103). Sometimes also a converging lens of long focus
is put in the path of the parallel cylinder to concentrate it more or
less, depending upon the size of the object to be shown. Instead
of a condenser, there is sometimes used a reflector (fig. 95, 96)
behind the radiant.

§ 280. Darkness of the projection room. — Owing to the diffi-
culty of obtaining a sufficiently brilliant screen image it is necessary
to have the projection room very dark.

COMBINATION LANTERN SLIDE AND OPAQUE PROJECTION
§ 281. Daylight and twilight vision. — Nearly all modern
apparatus giving opaque projection also gives transparency pro-
jection with a slight change. These two kinds of projection are
mutually antagonistic for the adjustments of the eyes of the specta-
tors. For transparency projection the image is so brilliant that
the eyes are adjusted for daylight vision in large part, while for
the opaque projection the image is so dim that the eyes should be
adjusted for twilight or night vision.

1 76 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII

ZEISS EPIDIASCOPE FOR OPAQUE AND FOR TRANSPARENT OBJECTS
IN A HORIZONTAL POSITION.

(Zeiss' Special Catalogue).

As shown in this figure the apparatus is set up for opaque objects. For
transparent objects M2 (mirror 2) is removed when the light striking M3 is
reflected to M4 and thence up through the object to M1 and to the screen.

Commencing at the right : R Parabolic reflector, which projects the light
from the crater through (W) the water-cell to M1 the mirror which is at the
proper angle for reflecting the light down upon the opaque object. From the
opaque object the light is irregularly reflected through the objective to M1.
M1 serves to reflect the rays from the objective to the screen.

V Ventilator. M3 and M4 are mirrors for use in reflecting the light through
horizontal transparent objects.

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 177

This apparatus is designed to project opaque objects as large as 22 centi-
meters in diameter, at a magnification of five to ten with a 30 ampere current.
For a smaller object one may magnify as high as 25 diameters. With a 50
ampere current and a larger reflector the magnification may be from 14 up to
37 diameters.

In this instrument the carbons are horizontal and in the optic axis. The
parabolic reflector (R) serves to direct the light in a parallel beam along the
line of the optic axis.

It takes considerable time for the eyes to adjust themselves,
hence, if one passes quickly from opaque projection to lantern
slides the screen images are dazzling. On the other hand in passing
from lantern-slide images to opaque images, the eyes being adjusted
for daylight vision, the screen images seem exceedingly dim at
first, although the screen image may be as brilliant as it is possible
to obtain with the best apparatus. After the eyes gain their
twilight vision the images on the screen appear much brighter, as
if the light had been greatly increased. As old observers put it:
"It is necessary to get the brilliant sunshine out of the eyes before
the relatively dim screen images are satisfactory."

§ 282. Dim and brilliant light in combined projection. — This
difficulty can be avoided in two ways :

1. In showing lantern slides, the current may be lessened until

the light forming the image of the transparency is of about
the same intensity as is that of the opaque object with the
full current.

2. A neutral tinted glass of the proper shade can be put in the

path of the beam going to the lantern slide, to tone down
the brilliancy (§ 282a).

§ 282a. In 1908-1909 this difficulty was in part overcome by Mr. A. O.
Potter by putting a tinted glass of the proper light reducing power in the path
of the beam going to the lantern slide. This reduces the image of the trans-
parency to the same dimness as the opaque object, hence one can pass from
one to the other without any adjustment of the eyes.

If only lantern slides are to be shown, the tinted glass can be removed and
the full light employed.

Some combined lanterns, as those of the Bausch & Lomb Optical Co., and
perhaps others, are now regularly supplied with the light reducing glass for the
magic lantern part.

178 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII

; ' ^ I V' *fct

FIG. 96. UNIVERSAL PROJECTION APPARATUS WITH THE PROJECTION
MICROSCOPE IN POSITION.
(Cut loaned by E. Leitz).

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 179

This apparatus is designed for all kinds of projection, and with the objects
either in a vertical or in a horizontal position. When the object is in a vertical
position the illuminating device (arc lamp with parabolic reflector) sends the
light horizontally through the specimen, apparatus and to the screen as would
be the case in the figure here shown.

If the object is in a horizontal position the lamp and reflector remain in a
horizontal position and the light is reflected by a mirror upon the opaque
object; or for vertical opaque objects the radiant is turned sidewise.

For transparencies in a horizontal position the lamp and reflector are lowered
to the level of one of the mirrors below, and this mirror reflects the horizontal
beam up through the transparent object whence it passes to the projector and
the screen.

The entire apparatus is covered by a dark curtain (compare fig. 95).

USE OF OPAQUE PROJECTION FOR EXHIBITIONS AND FOR
DEMONSTRATIONS

§ 283. Testing the lantern. — The directions given in Chapter I,
§ 26 are applicable here.

§ 284. Size of objects for opaque projection. — The size of
object which can be shown with an opaque projector varies greatly.
The smallest size is usually larger than a lantern slide. The lan-
tern-slide opening is rarely greater than 6.5 x 7.5 cm. (2.6 x 3 in.),
while the smallest picture usually shown in the opaque lantern is
rarely less than postal card size (8 x 12.5 cm., 3x5 in.). From
this minimum the size ranges all the way up to 50 cm. (20 in.)
square.

Of course the radiant and condenser must vary accordingly
(see fig. 107).

§ 285. Objects for opaque projection. — The best of all are dull
white objects, like marble figures, or black print on white paper,
pictures in black and white. Colored pictures in which the bright
colors of the spectrum like red, yellow and green, are predominant,
give good images. Metallic objects with polished surfaces give
good images. Among these the works of a watch or small clock
show well; also coins and medals. Bright metallic objects show
best on a dark ground.

Objects and pictures which are very light-absorbing naturally
will not give good screen images, no matter how brilliant the light
or good the apparatus. If the outlines of such objects are what is

i8o PROJECTION OF IMAGES OF OPAQUE OBJECTS ICn. VII

FIG. 97. THOMPSON'S REFLECTOSCOPE, MODEL G-2, 1913.
(Cut loaned by A. T. Thompson & Co.).

As here shown the instrument is ready for opaque and for transparency
projection.

There are additional attachments by which microscopic projection can be
done with either a horizontal or a vertical microscope. There is also an
arrangement for placing the magic lantern objective in a vertical position,
and thus projecting horizontal objects.

Commencing at the right : The lamp-house with arc lamp and condenser.
This is at an angle so that opaque objects in a vertical position are lighted
directly as in Chadburn's opaque lantern (fig. 92). In this case the screen
picture has the rights and lefts reversed.

Above is the magic lantern objective for transparencies.

Below is the large aperture, long focus projection objective for opaque ob-
jects. The objective is inserted in the dark chamber containing mirrors for
reflecting the light upward for transparency projection, or downward for the
opaque objects in a horizontal position.

Above is shown a lantern slide in the carrier and below a book in a horizontal
and a picture in a vertical position.

With the opaqt:e object in a horizontal position the light is reflected from a
mirror down upon the object, the light from the opaque object is then reflected,
in part, back to the same mirror and from the mirror out through the projec-
tion objective to the screen. The screen image in this case will be erect in
every way if properly placed on the holder.

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 181

wanted very good results can be obtained by using a white back-
ground. They will appear like silhouettes, but almost no details
will show.

§ 286. Screens for opaque projection. — On the whole no screen
is so satisfactory as a white one of the best quality (see § 621).

If the room is narrow, so that all the spectators are included
in about 30 degrees, the metallic screen answers fairly well. If the
room is wide, those on the sides near the screen will get only a very
dim screen image from the metallic screen. With the white screen
it is practically as good in one place as in another, for the reflection
is about equal throughout the entire 180 degrees (§ 622, 630).

For darkening the room see § 280 and § 608.

§ 287. Magnification of the picture and size of screen image. —

For lantern slides the magnification can be 30 to 60, with resulting
brilliant pictures; but with opaque projection one can rarely
magnify more than six to ten times and get good results.

If the area to be shown is relatively small and the illuminating
beam is made converging and a powerful radiant (50 amperes) is
used, the magnification may be carried up to 25 or 37 diameters
(Zeiss, p. 6) or perhaps more.

The screen image should not exceed 2 x 2, or 3 x 3 meters (8 x 10
feet), (Zeiss, p. 6).

§ 288. Screen distance. — In opaque projection, the screen
images are usually not magnified so much as lantern-slide images
and the screen distance is usually from three to ten meters. The
correct magnification (six to ten) is obtained by using an objective
of the proper focal length, i. e., for a magnification of six and a
screen distance of three meters there should be an objective of 50
cm. or 20 in. If the magnification is to be 10 and the screen dis-
tance three meters then the objective should have a focus of 30 cm.
or 12 inches. For the discussion relating to magnification, screen
distance, and focus of the objective see § 3Q2a.

Sometimes it is necessary to project at a screen distance of 1 5 to
20 meters (50 to 70 feet) . As the magnification of the screen image
must not usually exceed six to ten, a very long focus projection

182 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII

FIG. 98. THE NEW REFLECTING LANTERN OF WILLIAMS BROWN & EARLE
(No. 3 BR 15).

(Cut loaned by Williams Brown & Earle).

This is a combination projector for lantern slides and for opaque objects.

Commencing at the right:

N Arc lamp in the lamp-house with the feeding screws extending beyond
the lamp-house.

M Lamp-house of metal with the ventilator at the top.

C First element of the condenser for giving approximately parallel rays.

D The opaque object in position. The light from the lamp shines directly
upon it and is reflected outward toward the projection objective (£).

E Projection objective for opaque objects.

F Mirror for reflecting the image of the opaque object to the screen and for
correcting the right to left inversion.

B Water-cell and second element of the condenser for transparency pro-
jection.

A Opening for the lantern-slide carrier.

L Projection objective for lantern slides.

For lantern-slide projection a mirror at C is brought into position to reflect
the light out along the optic axis of B and L.

objective must be used for such a screen distance. (For a magnifi-
cation of six and a 15 meter screen distance, an objective of 250 cm.
(100 inches) is necessary).

§ 289. Arc lamp and amount of current. — If one wishes to use

more than 25 amperes, the arc lamp should be hand-feed. Up to
25 amperes, the right-angled carbons work well. Beyond that
amount the inclined or vertical carbons are more satisfactory for

CH.VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 183

the right-angled arc goes out easily from the magnetic blow when
the current is above 25 to 30 amperes. Of course, in opaque pro-
jection, where the most powerful light available is demanded,
alternating current is far less satisfactory than direct current ; still
with skillful application of the light available even alternating

FIG. 99. THE INDEPENDENCE POST-CARD PROJECTOR.
(Cut loaned by Williams Brown & Earle).

This is in principle exactly like Chadburn's opaque lantern with two lamps
(fig- 93)- In this projector the lamps are usually of the incandescent form, and
connection is made with the house-electric lighting system.

current radiants give fairly good opaque projection (see Ch. XIII,
§ 7 53 a for size of carbons with different currents, etc.).

For favorable objects and good conditions one must use not less
than 20 to 25 amperes of direct current for successful screen pic-
tures of opaque objects. Those with most experience in the work
use 40 to 50 amperes.

For alternating current satisfactory results can hardly be
obtained with less than 40 amperes, and 60 to 80 are better.

1 84 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Ca. VII

FIG. 100. HOME BALOPTICON FOR OPAQUE OBJECTS.

(Cut loaned, by the Bausch & Lomb Optical Co.).

In this instrument there is used a small arc light for attachment to the house
lighting system. The rheostat is shown at the left.

The object is horizontal and the lamp shines in part directly upon the object
and in part the light is reflected upon the object by a mirror. From the object
light is reflected to a mirror above the arc light, and from the mirror directed
out through the objective to the screen. The projected mirror image appears
erect on the screen.

FIG. 101. HOME BALOPTICON FOR LANTERN SLIDES AND
OPAQUE OBJECTS.

(Cut loaned by the Bausch & Lomb Optical Co.).

opaque projection is precisely as in fig. 100. For lantern-slide projec-
tion the mirror in front of the arc lamp is turned up out of the way and the
light passes on to the condenser, lantern slide and objective as in ordinary
lantern-slide projection (fig. i).

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 185

FIG. 1 02. UNIVERSAL PROJECTOSCOPK.
(Cut loaned by C. II. Stoelting Company).

This instrument as shown in the picture is designed to project:

(1) Lantern slides and other transparencies in the usual vertical position
or in a horizontal position.

(2) Opaque objects.

(3) Microscopic objects. For this the lantern-slide objective is turned
back and the microscope turned up in place.

1 86 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII

§ 290. Precaution for heavy currents. — The lamps for heavy
currents are mostly of the hand-feed type and burn large carbons.
When starting the lamp it is much safer to make sure that the car-
bons are separated before closing the knife switch. Then one can
use the feeding screws and bring the carbons together to strike the
arc, and separate them a short distance immediately. If the

FIG. 103. DIAGRAM OF THE PARTS AND COURSE OF THE RAYS IN THE

UNIVERSAL PROJECTOSCOPE FOR OPAQUE AND LANTERN-SLIDE

PROJECTION.

(Cut loaned by the C. II. Sloelting Company).

The instrument is here arranged for the projection of opaque objects. The
mirror, Af,, reflects the parallel beam from the first element of the condenser
(C), down on the horizontally placed object. The large aperture projection
objective directly above, and the 45° mirror beyond, project the image upon
the screen.

Ordinary lantern-slide projection is shown by the broken lines, (for a de-
tailed description of all the parts see fig. 16).

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 187

carbons are in contact after striking the arc, so much current flows
that there is danger of blowing the fuses or burning out some con-
nection. Be sure that the fuses and wiring are adapted to the
current (fig. 3, § 691).

§ 291 . Illuminating the entire opaque object. — For illuminating
opaque objects, Zeiss uses the principle of the search-light. That
is, the twro carbons are horizontal, the positive one has its crater
facing the concave mirror (fig. 95, 96). This mirror then reflects
the light toward the object. Depending upon its position, it can

FIG. 104. NEW MODEL CONVERTIBLE BALOPTICON IN POSITION FOR

OPAQUE PROJECTION.
(Cut loaned by the Bausch & Lomb Optical Co.).

In the new (1913) models of projectors by the Bausch & Lomb Optical Com-
pany provision is made in each case to place the object in a horizontal position
and then to illuminate it either by a mirror (fig. iO5a) or preferably by tilting
the radiant and first element of the condenser (fig. 105), so that the light from
the lamp is projected directly upon the object. From the object a part of the
light extends out through the vertically placed projection objective to the
mirror and from the mirror to the screen. The mirror gives correct images on
the screen.

1 88 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII

direct a parallel beam, a converging or a diverging beam (see also
Ch. XIII-XIV on radiants and lighting).

If a condenser is used, its size must be adapted to the size of the
object, that is, the diameter of the cylinder of light must be some-

FIG. 105. DIAGRAM SHOWING THE OPTICAL PARTS AND THE COURSE OF
THE RAYS IN THE CONVERTIBLE BALOPTICON IN OPAQUE PROJECTION.

(Cut loaned by the Bausch & Lomb Optical Co.).

The lamp-house, radiant and first element of the condenser are so inclined
upward that the light from the condenser falls directly upon the opaque object.

A Upper carbon of the arc lamp furnishing the light.

B First element of the condenser to render the diverging light parallel.
The lens beyond the meniscus is double-convex instead of plano-convex as in
fig. 3-

D Position of the opaque object. Objects as large as 20 x 20 cm. (8x8
inches) can be illuminated and projected.

E Large aperture projection objective in a vertical position.

F Mirror beyond the objective to reflect the image to the screen and correct
the inversion.

C Mirror. It serves to increase the illumination of the opaque object by
reflecting back upon it some of the scattered light.

6" Second element of the condenser for lantern-slide projection (fig. 3).

// Projection objective for lantern slides.

0 Bellows.

M Lathe bed on which slide the objective, etc.

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS i!

FIG. losa. DIAGRAM SHOWING THE COURSE OF THE LIGHT RAYS FOR
TRANSPARENCY AND OPAQUE PROJECTION WITH THE RADIANT HORIZONTAL.

(Cut loaned by the Bausch Of Lomb Optical Co.).

A Upper carbon of the arc lamp.
B The first element of the condenser (fig. 3).

C C Mirror horizontal when using lantern slides and inclined for opaque
projection.

D Horizontal surface for opaque objects (20 x 20 cm., 8x8 in.).

E Projection objective for opaque objects.

F Mirror for reflecting the light to the screen and correcting the inversion.

G Second element of the condenser for lantern slides.

// Projection objective for lantern slides.

N Support for condenser and bellows.

0 Bellows.

M Lathe bed on which move the various supports.

what greater than the diagonal measuring the size of the picture, as
for lantern slides (see § 314, fig. 114). A diverging beam could be
used by pushing the radiant within the focal distance, and a con-
verging by separating farther than the focal distance. Sometimes
there is no condenser but the radiant shines directly upon the
object (fig. 99, 100, 107).

190 PROJECTION OF IMAGES OF OPAQUE OBJECTS [CH. VII

§ 292. Avoidance of shadows. — With solid objects there will be
very heavy shadows unless the light is evenly distributed. With a
single lamp this is not easily accomplished, and if no mirror is used
practically impossible. It is better to use two lamps, one on each
side, as in the original apparatus of Chadburn (fig. 93). The two
lamps have the further advantage of doubling the light. Two arc
lamps are used in the large opaque lantern of the Bausch & Lomb
Opt. Co. (fig. 107).

In the Spencer Lens Co.'s opaque lantern, plane mirrors line a
part of the projection chamber where the object is placed, and much
of the light lost by absorption without this arrangement is reflected
back upon the object. This also helps to obviate the shadows
when one lamp is used (fig. 1 1 1).

ERECT IMAGES WITH OPAQUE OBJECTS

§ 293. Inversion of the image with an opaque object. — Besides
being upside down the image of an opaque object on an ordinary
white screen has the rights and lefts reversed.

§ 294. How to get an erect image with the object in a vertical
position. — Put the opaque object in the vertical position upside
down. Point the objective at right angles to the screen, use a
mirror at 45 degrees, or use a 45 degree prism to direct the image-
forming rays upon the vertical opaque screen (fig. 95, in). If
the inversion of the rights and lefts is unimportant, put the object
upside down in the vertical holder and point the objective directly
toward the screen (fig. 97, 109).

If a translucent screen like ground glass is used the image will be
erect in every way if it is put upside down in the holder and the
objective pointed directly toward the screen.

§ 295. How to get an erect image of an opaque object in a
horizontal position. — Place the opaque object with its upper edge
away from the screen. The objective is usually in a vertical
position so that the image would appear on the ceiling above the
instrument. The mirror or prism used to direct the image forming
rays upon the vertical screen corrects also the mirror image, and

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 191

FIG. 106. NEW MODEL UNIVERSAL BALOPTICON IN POSITION FOR
OPAQUE PROJECTION.

(Cut loaned by the Bausch & Lomb Optical Co.).

Opaque objects are placed in a horizontal position and the lamp-house, lamp
and first element of the condenser are inclined as in fig. 105. The light from
the opaque object is reflected upward to the right face of an inclined mirror
and from the mirror reflected out through the projection objective, giving an
erect screen image.

When used for lantern slides the lamp-house is horizontal and the horizontal
light is reflected upward by the left face of the mirror to the mirror at the left
of the lantern-slide attachment. This second mirror reflects the light hori-
zontally through the lantern slide.

the object will be erect in every way (fig. 95-111). (See also the
discussion of the reflecting lantern of Thompson in which a mirror
image is projected, and hence appears erect on the screen (fig. 97,
100). If a translucent screen is used with the object in a hori-

1 92 PROJECTION OF IMAGES OF OPAQUE OBJECTS lCn. VII

FIG. 107. BAI.OPTICON FOR THE PROJECTION OF LARGE OPAQUE OBJECTS.

(Cut loaned by the Bausch & Lamb Optical Co.).

This opaque projector is especially designed to show large objects and large
surfaces (20 inches, 50 cm. square). To avoid shadows in projecting machines
and other solid objects, and to supply the needed illumination there are two
25 ampere lamps tilted to throw their light directly upon the two opposite
sides of the object. Each lamp has its own rheostat and table switch.

CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 193

The projection objective is of the Tessar Ic series of very large aperture
(114 mm., 4T3 in., in diameter and 50 cm. 19^ in. equivalent focus). The
focusing is accomplished by a screw which raises or lowers the table supporting
the object.

This instrument enables one to demonstrate to an audience the workings of
a machine like a cash register, or a quarto size page of illustrations or print.
With the vertical objective and a mirror to reflect the light to the screen the
image will be erect. The reflecting mirror is silvered on the front to avoid
the doubling of the image.

FIG. 108. MODEL 5 DELINEASCOPE FOR OPAQUE AND LANTERN-SLIDE

PROJECTION.
(Cut loaned by the Spencer Lens Co.),

With the arc lamp and the first element of the condenser in a horizortal
position the light extends directly to the right through the lantern slide or
other object and the projection objective, or projection microscope, or it ir.ay
be reflected upward through the vertical projection microscope (fig. 175).

For opaque projection, the arc lamp and first element of the condenser are,
by means of the crank, rotated within the lamp-house to the right position to
direct the light upon an opaque object in a vertical or in a horizontal position
as desired.

If the object is in a horizontal position the light from it is reflected to a mirror
and from the mirror out through the large projection objective. It will appear
correct in the screen image. The vertical object will have the rights and lefts
inverted. Objects or surfaces 15x23 cm. (6x9 in.) can be projected with
this instrument.

1 94 PROJECTION OF IMAGES OF OPAQUE OBJECTS [CH. VII

zontal position the image will only be erect with the screen at right
angles to the axis of the objective, no mirror or prism being used.
If a mirror or prism is used to project upon a vertical screen then a
translucent screen will give a mirror image, but an opaque screen
an erect image.

FIG. 109. DIAGRAM FHOWING THE PARTS AND COURSE OF THE RAYS IN*

MODEL 4-5 DELINEASCOPE.
(Cut loaned by the Spencer Lens Co.).

This diagram shows the arc lamp and first element of the condenser in posi-
tion to illuminate a vertical object in opaque projection. Above is shown in
outline the course of the rays for the projection microscope or the magic lantern.

Commencing at the left :

OH The object holder for objects 15 x 23 cm., 6x9 in.

II Handle for operating the object holder.

X The horizontal axis on which rotates the arc lamp and the first element
of the condenser.

OO The large projection objective for opaque objects.

WC Water-cell for removing the radiant heat.

ID Large iris diaphragm.

L Lantern slide, and crank for turning the slide up in the vertical position
in front of the condenser behind the objective.

P Platform on which is laid the lantern slide.

LO Lantern-slide objective turned to one side to allow the microscope to
get to the horizontal position.

M Mirror to reflect the horizontal beam of light through the vertical
microscope.

CH. VII] TROUBLES IN OPAQUE PROJECTION 195

§ 296. Troubles:

i . The one great trouble will be a dim screen image. This can-
not be wholly avoided. It can be made tolerably good:
( i ) by having the room very dark ; ( 2 ) by using a powerful
radiant; (3) by having a projection objective of large
aperture; (4) by magnifying the screen image very
moderately (5 to 10 diameters).

FIG. no. MODEL 8 DELINEASCOPE FOR ALL KINDS OF PROJECTION.
(Cut loaned by the Spencer Lens Co.).

In this instrument there is provision for lantern-slide projection with the
slides or other objeets in a vertical or in a horizontal position.

It provides for opaque objects in a horizontal position and lighted directly
by the radiant (fig. in), and for objects in museum jars in a vertical or hori-
zontal position.

Finally it provides for micro-projection with the objects in a vertical position
or in a horizontal position, and for the drawing of objects on a horizontal or on
a vertical surface.

TROUBLES IN OPAQUE PROJECTION

[CH. VII

FIG. in. DIAGRAM OF MODEL 8 DELIKEASCOPE SHOWING THE POSITION
OF THE RADIANT AND THE COURSE OF THE LIGHT RAYS FOR OPAQUE
PROJECTION WITH THE OBJECT IN A HORIZONTAL POSITION.
(Cut loaned by the Spencer Lens Co.).

T Table for opaque objects.

W Wheel by which the table is raised and lowered.

D Diaphragm above the table for flattening out the page of a book.

B Incandescent bulb which always gives light for the interior of the
machine.

C Condensing lenses in front of the arc.

O Large objective for opaque projection.

()1 Smaller objective for vertical projection.

,]/ Mirror for throwing light downward for the lantern-slide compartment
or upward through the vertical attachment.

-If, Mirror for reflecting a perpendicular beam of light out through the
lantern-slide compartment; shown thrown up against the water-cell in this
figure (see fig. 177)

M3 Mirror used in connection with projection of the vertical side of an
object.

M^ Mirror which assumes a position at 45° when the microscope is used
perpendicularly.

P Prism which is thrown into the prism box when the microscope is used
in a perpendicular position.

S Shelf upon which the lantern slide is placed previous to throwing it up
into the optical axis by the handle.

// Handle of the lever for raising the slide into position.

CH. VII] TROUBLES IN OPAQUE PROJECTION 197

2. If the amperage is to exceed 25 or 30, it is better to use an arc

lamp with inclined or vertical carbons, not those at right
angles for the magnetic blow puts the right-angled arc out
too easily.

3. Do not have the carbons in contact with a hand-feed lamp

when the current is turned on. Feed them together after
the current is on, then they can be separated properly
immediately after the arc is struck.

4. Inverted screen image. The object not properly placed on
{ the support, or no erecting mirror or prism is used.

5. No detail in the screen image. The object may be too light-

absorbing, or the light may not be sufficient.
(See Troubles in Ch. I.).

DO AND DO NOT IN OPAQUE PROJECTION tCn. VII

§ 297. Summary of Chapter VII:

1. Select an objective of large
aperture for opaque projection

(§ 275)-

2. Use a light of great brill-
iance like sunlight or the arc
light (§ 274, 277).

3. Make the screen image
only six to ten times as large as
the object (§ 287).

4. Make the projection room
very dark (§ 280).

5 . Use a very white screen or
under some conditions a metal-
lic screen (§ 286, 621).

6. From 25 to 50 amperes of
direct current are needed to
give good opaque projection

(§ 289).

7. If lantern slides and
opaque objects are projected at
the same exhibition, use a
neutral tint (smoky) glass to
make the lantern-slide image as
dim as the image of the opaque
object (§ 282).

8. Use a condenser for opaque
objects somewhat larger than
the object (see fig. 114).

Do NOT

1. Do not undertake opaque
projection with an objective of
small aperture.

2. Do not expect good
opaque projection unless from
20 to 50 amperes of direct cur-
rent, or greater amperages of
alternating current are avail-
able.

3. Do not try to magnify the
object too much.

4. Do not try to project in a
light room. It must be dark.

5. Do not be satisfied with a
dirty, non-reflecting screen. It
must be white.

6. Do not expect brilliant
screen images with a weak light.

7. Do not pass quickly from
the dim pictures of opaque
objects to the brilliant pictures
of transparencies. Dim the
transparencies down to the
opaque images.

8. Do not use a small con-
denser for a large object.

CH. VII] DO AND DO NOT IN OPAQUE PROJECTION

199

9. Use two radiants or mir-
rors for avoiding shadows with
solid objects (§ 292).

10. Select objects which re-
flect well for opaque projection
(§ 285).

11. If very light-absorbing
objects must be projected, use a
white background (§ 285).

1 2 . Use a hand-feed arc lamp
for opaque projection (§ 289,
290).

13. Make sure that the wir-
ing is adapted to the heavy
currents needed for opaque pro-
jection (§ 290).

14. Use carbons of the proper
size for the current drawn

(§ 290, 7S3a).

1 5 . Make the images erect by
placing the object up-side down
for the vertical position, or with
the upper edge away from the
screen for the horizontally
placed objects (§ 293-294).

1 6. Use a mirror or prism to
avoid a mirror image on a ver-
tical, opaque screen (§ 293-
295)-

9. Do not light solid objects
so that there will be deep
shadows. Use two radiants, or
mirrors, or arrange so that the
light strikes the object directly,
not obliquely.

10. Do not select badly re-
flecting objects for opaque pro-
jection.

1 1 . Do not use a black back-
ground on which to place dark
objects.

12. Do not use an automatic
right-angle carbon arc lamp for
the heavy currents needed for
opaque projection.

13. Do not nm any risks by
using the heavy currents on
wiring not adapted to it.

14. Do not use small carbons
for big currents.

15. Do not get the images
wrong side up on the screen.

1 6. And do not expect too
much in opaque projection.
Know the principles involved;
study fig. 90-91.

CHAPTER VIII
PREPARATION OF LANTERN SLIDES

§ 310. Apparatus and Material for Chapter VIII:

A photographic dark room; Camera with suitable objectives and
plate holders (fig. 116-119); Lantern-slide plates, negative plates
of various kinds; Chemicals for developing, etc.; Colors and
brushes for tinting the slides; A retouching frame (fig. 113);
Cover-glasses and binding strips and mats for the slides ; Markers
and labels for the slides; Cabinet for the slides (fig. 120).

§ 311. For the historical development of lantern slides see the
works referred to in Ch. I, § 2, and for photographic lantern slides,
The Journal of the Royal Society of Arts, Vol. LIX (191 1), pp.

255-257-

For making and coloring lantern slides see the works inCh. I, § 2,
and Lambert, Lantern-slide making and coloring.

The Photo-Mineature series No. 9, Lantern Slides, and No. 83,
Coloring Lantern Slides.

§ 312. Modern lantern slides are of several standard sizes as
follows: (See§3i2a).

A. American slides. — These are oblong plates 82.5 x 102 mm.
(3^ x 4 inches). They are designed to go into the lantern -slide
carrier with the long side horizontal (§ 35).

B. British slides. — These are square, being 82.5 x 82.5 mm.
(3 Y* x Z% inches) (§37).

C. French slides. — These are, following the recommendations
of the French Congress of Photography for 1889, 85x100 mm.
(3n/32 x 3lrri6 inches). That is, the standard is practically like the
American, and French slides can be used in American lantern-slide
carriers.

D. German slides. — In Germanic countries, slides of 85 x 100
mm. are much used, but the German standard is often given as
90 x 120 mm. (3%; x \y± inches). Those of 130 x 180 mm. are
likewise employed.

CH. VIII]

PREPARATION OF LANTERN SLIDES

201

E. Italian slides. — In Italy the sizes are 85 x 85 mm., 85 x 100
mm. and 90 x 120 mm., that is, the British (B), the French and
American (A, C) and German (D) sizes.

In all countries those of larger and smaller sizes than the above
standards are used for special purposes; and provision is made

FIG. 112. AN AMERICAN LANTERN SLIDE, FULL SIZE, WITH INSTRUCTIONS

FOR MAKING LANTERN SLIDES DIRECT. THE SLIDE is PROPERLY

"SPOTTED."

everywhere for the square British slides of82_5x82.5 mm. and also
for the oblong form of 82 or 85 x 100 mm. of the French and Ameri-
can manufacturers.

Any oblong form has the advantage that it is always put into
the carrier with its long side horizontal and therefore requires only
one mark or spot to indicate how it shall be inserted for an erect

202 PREPARATION OF LANTERN SLIDES [Cn. VIII

image (fig. 6-8, 112). For a square form two marks are needed
(fig. 13, 113).

§ 3 13. Actual size of the free opening with lantern slides. — The

sizes given above are the measurements from the extreme edges
of the plates. The actual size of the picture to be projected is
always less, as part of the slide is covered when inserted in the
carrier. The mat between the slide and its cover, and the binding
around the edge lessen the size a variable amount. It requires
from 5 to 10 mm. all around the edge for the binding and the part
covered by the slide-carrier. This leaves a clear opening in the
lantern slide of that much less. The smaller the slide to start with
the less will be the proportionate amount of clear space left after
the mounting of the slide.

The free opening of the American slides is rarely greater than
70 x 75 mm. and much more frequently the free opening is con-
siderably less.

§ 314. Diameter of the condenser required for different sized
lantern slides. — The final element of the condenser next the lantern
slide (fig. i, 2, 114) must be somewhat greater in diameter than
the diagonal of the free opening of the lantern slide to be projected.

The accompanying figures show the British, French and Ameri-
can, and German standard sizes of lantern slides with the minimum
diameter of the condenser which should be used with them (fig.
114).

§ 312a. There is some confusion as to the exact outside measurement of
lantern slides. For example, the exact size of the British square slides is
3>4*3/4 inches (82.5x82.5 mm.) In the two French works consulted
(Trutat, p. 311, and Fourtier, tomeii, p. 18) the British size is given as 80 x 80
mm.

In Italy the size is given as 85 x 85 mm. In the German work of Wimmer
the exact size is given (82.5 x 82.5 mm.). Neuhauss speaks of slides 85 x 85
mm. (p. 27).

The standard French slides are given as 85 x 100 mm. This is one of the
standard sizes in Germany and Italy. Hence, it is concluded that the standard
British slide is meant whenever 80 x 80, 82.5 x 82.5, or 85. x 85. mm. slides are
mentioned. Also that the standard French and American slide of 3^x4
inches (82.5 x 100 mm.) is meant whenever slides of 85 x 100 mm. are men-
tioned.

CH. VIII]

PREPARATION OF LANTERN SLIDES

203

§ 315. Making lantern slides. — In the use of the lantern at the
present day, one will find occasion to make lantern slides by all of
the different ways that have ever been devised. That is, they may
be drawn or painted wholly by hand ; made partly by photography

FIG. 113. BRITISH LANTERN SLIDE OF FULL SIZE WITH TWO "SPOTS."

The "spots" are on the upper corners in the English slides.

The picture shown on the slide is of a retouching stand suitable for use in
coloring slides.

5 The slide.

R A reflector to throw the light up through the slide. This may be a
mirror or simply white paper.

and then hand-colored; made wholly by photography, or trans-
parent natural objects may be used.

Natural objects of the right transparency may be mounted on
glass slides and used in the lantern. For example, seaweeds, thin
leaves, skeletonized leaves, large wings of insects; crystals on

2O4

PREPARATION OF LANTERN SLIDES [Cn. VIII

glass, thin sections of wood or animal organs mounted on glass,
fibers of wood, thin cloth, spiders' webs, etc., etc.

FIG. 114. STANDARD BRITISH, FRENCH, AMERICAN AND GERMAN LANTERN-
SLIDES WITH THE CONDENSER NECESSARY TO FULLY ILLUMINATE
THEM. (ABOUT HALF NATURAL SIZE).

§ 316. Hand-made lantern slides. — Practically no one now
makes the beautiful hand-painted lantern slides of former times;
but for outline diagrams, for tables and for short statements, it is
easier and cheaper to make the slides direct than to first make a

CH. VIII] PREPARATION OF LANTERN SLIDES 205

diagram or table, etc., and then have a photographic lantern slide
made.

In preparing these slides direct, a device of the artists of earlier
times who painted lantern slides, is used. That is, the slide is
cleaned carefully and then coated with a thin solution of some hard
varnish or with gelatin (fig. 112, § 317). After the varnish has
thoroughly dried one can use a pen or a brush upon the varnished
surface with the same facility as upon paper. The hand-made slide
is then mounted as usual and can, of course, be used indefinitely.

If they are for a special occasion — as in projecting election
returns, games, etc., — the slides are used without a cover-glass.
They may be easily cleaned off with turpentine or xylene and used
over and over.

§ 317. Coating the lantern-slide glass with varnish. — One of

the best varnishes for this purpose is composed of 5% dry Canada
balsam or gum dammar in xylene or in turpentine; or 10% natural
Canada balsam in xylene or toluene. Or one can take some good,
varnish, especially Valspar, one part and xylene, toluene, gasoline
or turpentine nine parts. All of these thin solutions should be
allowed to stand until they are clear, and only the clear part used.
If one is in haste it is possible to filter the thin varnish through filter
paper.

For coating the glass, the best way is to hold the clean glass flat
by grasping the edges with the thumb and fingers. Then varnish
is poured on, and the glass tilted slightly until the whole surface
is covered. The excess is poured off one corner back into the
bottle. Then the glass is stood on edge to dry. In a warm dry
room 15-20 minutes will suffice for varnish in xylene or toluene.
If turpentine is used it may require half a day or more. When the
varnish is once dry the glass can be used at any time.

As it is not easy to tell which side has been varnished, a slight
mark in one corner of the varnished surface with a glass pencil or
pen will enable one to tell quickly and with certainty.

§ 318. Coating the lantern-slide glass with 10% gelatin. — For

this, some clear gelatin is made into a 10% solution in hot water,

206 PREPARATION OF LANTERN SLIDES [CH. VIII

and filtered through filter paper. The slides are coated with the
gelatin as described for the varnish. When the gelatin is dry the
surface receives a pen or brush well. Gelatin slides are not so
satisfactory as the varnished slides.

FIG. 115. AMERICAN LANTERN SLIDE OF FULL SIZE WITH GUIDE LINES
FOR MAKING SLIDES DIRECT.

The thumb tacks at the four corners are to hold the slide firmly in position
while writing or drawing upon it. The lined area represents about the maxi-
mum size of opening projected in ordinary work (65 x 75 mm.), (2>i X2 %in.).

§ 319. Inks and pens. — One can use any ink and any pen on
the varnished or gelatinized slides.

For making tables, etc., it is best to use water-proof India ink
and a fine pen, a crow-quill, steel pen is excellent.

§ 320. Drawing diagrams on varnished slides. —One can draw
freehand on these varnished slides as well as upon paper. For

CH. VIII] PREPARATION OF LANTERN SLIDES 207

those not especially skillful, it is probably better to draw the sketch
first and then trace the sketch on glass as follows: Place the
lp,ntern-slide glass on the drawing, varnish side up, and arrange it
as desired. Select very thin glass for this, so that the drawing sur-
face will be near the picture to be traced. Now with a pen or
brush trace the outlines. One can also use colored inks if desired.

§ 321. Guide for table making and for writing. — For making
lantern-slide tables or written matter direct on the slide it is best
for most workers to have a guide which shall show the maximum
size which can be projected (fig. 115). If one has no special guide,
cross-section paper or catalogue cards will serve well.

To hold the glass in position while writing or making diagrams,
thumb tacks at the corners are efficient (fig. 115).

§ 322. Ink and pen to use on unvarnished glass. — For tem-
porary use, as in reporting games, etc., the glass is cleaned and
then the fingers rubbed over it. Now with a ball-pointed pen one
can write upon the glass. The lines will be coarse, but that will
not matter. One can write with an ordinary pen also, but not so
surely as with a ball-pointed pen (§ 322a).

The ink can be of almost any kind. The black India ink gives
the sharpest images.

A special ink called "glassine" has recently been put on the
market. It is in six colors, white, black, red, green, blue and violet.
The ink is thick and with it one can write on untreated glass with
any pen, although a ball-pointed pen is here also an advantage
(§ 322b). The ink is easily washed off with water so that the same
glass slide can be used over and over.

§ 322a. The writers are indebted to Dr. E. M. Chamot for the suggestion
to use the ball-pointed pens on the unvarnished glass, also the advantage of
rubbing the fingers or palm over the cleaned glass to prevent the ink from
spreading.

According to Lewis Wright, p. 412, one can write on glass well if the glass is
licked, and the thin coating of saliva so spread upon the glass is allowed to dry.
The ink will not spread, and the saliva-coated glass takes the pen well.

§ 322b. "Glassine announcement slide ink." — This ink is made by the
Thaddeus Davids Co., 127 William St., N. Y., and is supplied in I oz. (30 cc.)
bottles, the full set of six colors costing $1.00. See the Moving Picture World,
March, 1914.

208 PREPARATION OF LANTERN SLIDES ICn. VIII

§ 323. Smoked glass. — For some purposes nothing is better
than smoked glass slides. On these one can write or draw with a
sharp point either before or during the exhibition. If one takes the
precaution to commence writing on the lower edge of the slide and
on the face looking toward the condenser the writing or diagram
will appear right side up on the screen (see § 3 5 for proper position
of lantern slides in the holder) .

Smoked slides must be handled carefully or the surface will be
spoiled.

§ 324. Thin sheets of mica or of gelatin. — On a sheet of mica,
of gelatin or of non-inflammable cellulose one can write or draw
with a pen or brush, using any colored ink. India ink is best for
outlines and for written words, letters, or numerals.

As these sheets are very thin it is best to put a slide made upon
one of them between two glasses, so that the sheet will be held flat
and be protected. (For other methods of hand-made slides see
Dolbcar, pp. 29-32).

PHOTOGRAPHIC LANTERN SLIDES

§ 325. Nearly all of the lantern slides now used arc made
wholly or in part by photography.

Negative. — First, there is made a negative of the object to be
represented in the lantern slide. This negative may be on any
size of plate, but the picture should be, if convenient, of the proper
size for a lantern slide. That is, its outside dimensions must not
exceed 75 x 70 mm. (3 x 2.8 in.).

This negative should be very sharp and free from defects. Any
lack of sharpness or any defects will come out with distressing
prominence when the picture is magnified by the lantern. One
must then use a good objective in making the picture, or if the
objective is not particularly good a very small diaphragm is used.
If it is desired that print shall be read easily by all in the room, the
lantern slide should not have the letters smaller than six point type
(see fig. 216 for sizes of type).

CH. VIII]

PREPARATION OF LANTERN SLIDES

209

§ 326. Printing the lantern slide from the negative. — If the

picture on the negative is of the proper size for a lantern slide, it is
put into a printing frame exactly as for printing with paper. Then
in the dark room a lantern-slide plate is put with its sensitive side
next the negative and arranged so that the picture will be straight
on.the lantern slide. The cover of the printing frame is put on and
held in place by the hands or by the springs. The exposure may
be in diffused daylight, or about 30 cm. from any good artificial
light (incandescent bulb, Welsbach gas light, kerosene lamp).

Base

FIG. i I 6.

CAMERA FOR MAKING LANTERN SLIDES BY MEANS OF AN
OBJECTIVE.

Base The base of the camera resting on the table.

Objective The photographic objective in the middle segment of the camera.
The objective is shown as if the enclosing bellows were transparent.

Front The front of the camera where the negative is placed.

Reflector A white sheet of paper or cardboard placed on a shelf at 45°.
This reflector serves to illuminate the negative.

By varying the relative distances of ground glass, objective and negative,
the lantern slide can be larger or smaller or of the same size as the corresponding
part of the negative.

The exposure required varies with the negative, but it is less than
for most developing papers.

§ 327. Developing the lantern slide. — Any good developer may
be used, but as a rule the directions given in the box of plates are
the best to use with that brand of plate. One should develop until
the picture appears clearly. The temptation is to develop too
much and thus make the slide too opaque. Black, like printed
letters, should be opaque in the correct lantern slide, but there
should be all gradations from that to clear glass in the whites.

PREPARATION OF LANTERN SLIDES

[Cn. VIII

Any one who can make a good negative and a good paper print
from it can make a good lantern slide. The lantern slide is a
positive and the lights and shades should appear as in the object
when one looks through the slide toward the light. These lantern
slides are small transparencies, and some of them make beautiful
ornaments when used as transparencies in a window.

There is more danger of getting the slides too opaque than not
opaque enough. The beginner should try each lantern slide with

FIG. 117. COPYING, ENLARGING OR REDUCING CAMERA.

(From the Catalogue of Anthony & Co.).

0 The objective. The bellows have been cut away to show it.

/ Front of the camera with frames or "kits" for negatives of various sizes.
For making enlargements with this camera the objective can be placed in the
front .

a moderate light in the lantern. If the picture on the screen is
brilliant and shows all the details with the moderate light, it will,
of course, give a more brilliant picture with the electric light of
3000 to 4000 candle-power. If the slide is too opaque, it will not
come out well with the moderate light and, while the powerful
electric light may show it fairly well, so much radiation will be
absorbed and transformed into heat that the slide is liable to break
if left in the lantern a considerable time. The more transparent
slides allow the radiant energy to pass through them and naturally
they are not so greatly heated.

CH. VIII]

PREPARATION OF LANTERN SLIDES

§ 328. Negatives as lantern slides. — Many objects appear
equally well and equally clearly when projected from a negative as
from a positive or transparency. That is, there will be white lines
and white letters, etc., on a black background. This was a
favorite method of illustrating in the older works on physics and
projection. For examples, look at the pictures in Dolbear's Art of

FIG. 1 1 8. PHOTOGRAPHIC CAMERA UPON A BASEBOARD HINGED TO A

TABLE.
(From The Microscope).

This is one of the copying, enlarging and reducing cameras. The objective
may be at the end, in a cone, or in the middle segment. For lantern-slide
making it is in the middle segment and the negative at the end, the whole
camera being directed upward toward the sky.

By reversing the position of the camera, and placing the hinged board in a
vertical position, objects in liquids and any object in a horizontal position can
be photographed.

NOTE. — The arrangement shown in fig. 1 18 with a baseboard hinged to the
table, and with a camera which could be placed pointing upward or downward
was devised by the senior author in 1878 especially for photographing objects
in liquids or objects which must remain in an inclined or horizontal position.
The baseboard carrying the camera can be fixed in any position from the
horizontal to the vertical. (Proc. Amer. Assoc. Adv. Sc. Vol. XXVIII (1879),
p. 489; Science, Vol. Ill, p. 443, and Vol. IV, p. 5 (1884).

212

PREPARATION OF LANTERN SLIDES

[Cn. VIII

Projecting, Deschanel's Physics, etc.,
and fig. 141, 190, 211, 214.

There is one serious drawback to such
lantern slides. The background being
nearly opaque stops the light and other
radiant energy from the lamp, and the
great heat developed is
liable to crack the slides
(see § 18, 845).

FIG. 1 19. FOLMER & SCHWING'S TILTING CAMERA AND ADJUSTABLE BACK.

(From the Catalogue of Folmer & Sch-wing. Cut loaned by the Eastman

Kodak Co.).

A Tilting camera for making lantern slides or other transparencies with an
objective, or for photographing objects in a horizontal or inclined position.

B Adjustable back for the tilting camera. The adjustments are to the
right or left, up or down and enable one to center accurately any desired part
of the negative or other object to be photographed. The rotary motion of the
back enables one to get the lines on the negative or object exactly parallel with
the edge of the lantern slide.

CH. VIII] PREPARATION OF LANTERN SLIDES 213

§ 329. Printing lantern slides by the aid of a camera. — Unless
the negatives from which lantern slides are to be made have the
part to be shown of exactly the size of a lantern slide, the trans-
parency or positive cannot be printed by contact. Then one can
use a photographic camera and print the transparency as follows :
The negative is put in a suitable opening or in the proper "kit" or
frame in the end of a copying camera (fig. 116-119), and the
objective in the second segment. The picture or film side of the
negative must face the objective. Then the end of the camera
holding the negative is elevated sufficiently to get a sky background
through the window ; or the camera is left level and a large piece of
cardboard or white blotting paper is set at an angle of about 45
degrees out of a window and the camera pointed toward it. In
either case the entire lantern slide will be evenly illuminated and a
good print can be obtained.

Now focus the picture of the negative sharply on the ground
glass of the camera and get it of the proper size by pulling out or
closing up the bellows.

Print the positive by putting a lantern-slide plate in the plate
holder in the usual manner and exposing it. Then develop as
usual.

It is to be noted that the film surface of the negative and the
sensitive surface of the lantern-slide plate face each other by this
method exactly as for contact printing (§ 32ga).

§ 330. Camera for lantern slides. — If one is to make many
lantern slides it is a great convenience to have available a special

§ 329a. White prints on a black ground. — By using an ordinary negative
giving black lines on a white ground one can get white lines or a white picture
on a black ground by applying the method just given for printing lantern slides
by means of a camera and an objective. Place the negative in position, but
with the film side facing away from, not toward the objective as for an ordinary
lantern slide. Use a lantern slide or any other kind of plate and make the
picture just as for the lantern slide. The glass picture thus produced will be a
positive like a lantern slide but it will have all the parts reversed exactly like a
negative. If now this picture is used as a negative and printed with cyco,
velox, argo, haloid or any other printing paper the picture will appear white on
a dark ground.

Of course, any lantern slide can be used for making prints, but the picture
will be reversed in every way, the lights and darks, the printing, etc. To pre-
vent the inversion of the printing one can use an objective and camera as
described in Ch. X, § 512.

214 PREPARATION OF LANTERN SLIDES [Cn. VIII

camera known as a "copying, enlarging, and reducing camera"
(fig. 116-119). As seen from the picture, the objective is placed
in the middle segment if lantern slides are to be made from nega-
tives, and the negative is placed in the proper sized frame or "kit"
at the end of the camera. No light then reaches the negative
except on the face looking toward the light, hence there will be no
trouble from reflections.

In the best form of these cameras there is a "back with revolving,
rising and vertical sliding lantern-slide attachment" for printing
and for making the negatives (fig. 119). The picture can be got
on the plate in the exact position desired, i. e., lines of print, etc.,
exactly parallel with the edge of the plate. By means of a camera
one can print lantern slides from the negatives before they are dry.
This is sometimes a great convenience.

§331. Printing lantern slides by artificial light. — With contact
printing one can use daylight or any convenient artificial light —
petroleum, gas, acetylene or electric. For printing with the
camera, however, it is not so easy to get the negative evenly
illuminated. A good way to evenly illuminate the negative is to
use a 45 degree cardboard reflector illuminated with one or two
incandescent lights, preferably with frosted bulbs in a horizontal
position. Mantle gas lights serve well for illuminating the card-
board. The negative is set vertically some distance from the card-
board.

The time for printing lantern slides by contact or by the aid of a
camera will vary with the negative as for paper prints; much
depends on the intensity of the light and on the rapidity of the
plates used.

To give an example of the time required in a given case the
following table is added :

The same objective with a diaphragm opening of F/8 was used
for all, and the same negative was used in each case. All the plates
were from the same box and the same developer was used for all,
so that the only variable was the light.

1. Sky background, diffused light 10 seconds.

2. Cardboard at 45 degrees, under the sky 15 seconds.

CH. VIII] PREPARATION OF LANTERN SLIDES 215

3. Cardboard at 45 degrees, lighted by a 40 watt mazda lamp
above the cardboard 30 seconds.

4. Cardboard at 45 degrees with a 16 candle-power frosted bulb
above the cardboard 120 seconds.

For contact printing with the same negative, 30 cm. (12 in.) from
the light, if artificial, the following times sufficed: Diffused day-
light, 2 sec.; Mazda, 40 watt lamp, i sec.; Frosted bulb, 16 c.p.
lamp, 10 sec.; Petroleum lamp, 10 sec.; Gas mantle, 5 sec.

§ 332. Rapid preparation of lantern slides. — It occasionally
happens that one needs a lantern slide at very short notice. In
such a case, the negative can be taken and fixed in the hypo,
rinsed in water, and put into the camera and a lantern slide
exposed (§ 329). Then the negative can be washed as usual.
The lantern slide is then developed and fixed, and washed a few
minutes in water. It is then placed a few moments in 95% alcohol
or denatured alcohol for dehydration. After removal from the
alcohol it is dried in a draught or in the current of an electric fan.
Negatives can be quickly dried in the same way. One can then
make contact prints.

§ 333. Type written lantern slides. — It frequently happens that
one desires to project some statement or some table. This can be
written as stated above (§ 316, 321), or the statement or table
can be made neatly with a typewriter, using a black ribbon.
Then this can be used just as any other printed matter and a
photographic lantern slide made from it.

If in a great hurry one can use the negative form of lantern slide
and dry quickly (§ 332). This will give white letters on a black
ground (§ 329a). (For film slides see § 333a).

§ 333a. Film lantern slides. — There has been recently introduced by the
Eastman Kodak Co., a method of producing lantern slides on celluloid
films, comparable to film negatives. The celluloid film is quite thick. There
must be a negative as for glass lantern slides. The film is used in place of a
lantern-slide plate. The printing is like printing cyco, velox or other paper.
When the lantern-slide film is dry, after being developed and washed like a film
negative, it is varnished and placed between two pieces of paper with the
proper opening for the picture.

Naturally, these film slides are very light and are not fragile. Unfortunately
the substance of which the film is composed is inflammable, and therefore the

216 PREPARATION OF LANTERN SLIDES [Cn. VIII

§ 334. Mounting lantern slides. — In the original method,
which is still followed to a certain extent, each slide was mounted in
a wrooden frame — that is, each slide had its own carrier which was
put in place when it was to be shown (fig. 15).

For teaching and for many other purposes glass lantern slides
arc not now put in separate wooden frames, but are covered with a
clear glass (cover-glass) of the same size and the two bound
together by adhesive paper. They are far less bulky in this way of
mounting, although they are not as well protected as in the earlier
form.

In mounting them the slides are thoroughly dried, then some
form of opaque mat or mask is put over the picture on the picture
side of the transparency or negative. There are on the market
masks or mats of various shapes and sizes of opening. These may
be used or masks may be made by using strips of black paper.

When the mat is in place a cover-glass of exactly the same size
as the lantern slide is thoroughly cleaned and placed over the
picture surface of the slide. Then a narrow strip of adhesive paper
is put all around the edge. This holds the slide and the cover in
position, and prevents the sharp edges of the glass from cutting the
fingers when handling the slides. The mat not only cuts out any
part which is not to be shown, but it separates the cover-glass
slightly from the picture and prevents rubbing or other injury to it.
The size and shape of the opening in the mat to give the best effect
depends upon the picture or other matter on the lantern slide. The
mat is a kind of frame and like any other frame it should be suited
in form and size to the object to be shown.

§ 335. Marking or "spotting" the mounted slides. — As

pointed out in Chapter I (§ 23) each slide should have some kind of

Kodak Company recommend that the film slides be used only with a magic
lantern having a water-cell (fig. 2, 3).

Furthermore, even if non-inflammable film were used, it would not do to
leave those slides in a lantern without a water-cell too long for the heat would
make the celluloid buckle and get out of shape or char it, although of course
it would not be set on fire.

The lightness rind small space1 required for such slides arc of great advantage,
but their limitations are so great that for the general, and rough usage of ordi-
narv lantern slides they are not so well adapted as glass slides.

CH. VIII] PREPARATION OF LANTERN SLIDES 217

mark on it so that the operator can put it into the lantern correctly
without closely inspecting each slide.

Unfortunately there is no general system of marking slides. The
method recommended by the British Photographic Club
(Bayley, p. 78) is to put two white spots on the upper edge of the
slide (fig. 113). Two spots are necessary for the square slides, but
for oblong slides one "spot" or mark is sufficient (fig. 112).

In America it is common to have the mark or spot on the lower
left hand corner of the slide (§ 112), then when the slides are in a
pile for inserting in the lantern the spot will be turned upward
(fig. 8) as it must be to give an erect screen image. In the British
method of "spotting" the slides would have the spots on the lower
edge when piled up ready for insertion in the lantern.

§ 336. Coloring lantern slides. — Photographic lantern slides
have been colored from their first production. To do this in the
best manner possible requires considerable practise and natural
artistic ability, but any one can color lantern slides sufficiently well
to add to clearness in teaching — for example, veins blue, arteries
red, etc. All that is needed is a small artist's brush and some of
the desired color.

Transparent colors in sets are on the market (see Appendix), or
one can employ the aqueous stains used in histology. It takes
some experience to get the right dilution of the color and to put it on
neatly with the brush. The slide should be held over some white
paper in a light place so that it is possible to see exactly what is being
done. The frame for holding slides is a convenience (fig. 113).

If one wishes to become expert it will be a great help to study the
works of reference given at the head of this chapter, for they give
many valuable hints.

One very important thing for the beginner to do is to test every
slide that is colored in the lantern to make sure that the colors look
right in the screen image. Sometimes a slide that looks well to the
naked eye in daylight will not look well when projected on the
screen. It is, of course, the screen image that must be satisfactory.

The early lantern slides were mostly colored with transparent
oil colors, and then when entirely dry, the slide was mounted in
Canada balsam, and a cover-glass put on exactly as microscopic

218 PREPARATION OF LANTERN SLIDES [Cn. VIII

specimens are now mounted. This gave a very transparent and
vivid picture.

§ 337. Labeling lantern slides. — Besides the mark or spot as
guide to inserting the slides in the carrier, every lantern slide should
have a label stating what it is, and if copied from some book or
periodical it should give the name of the publication from which
derived and the number of the figure.

Slides are also numbered for convenience in arrangement at the
time of an exhibition. Some workers simply number the slides
and have no label. This is, of course, feasible for a small collection
to be used by one individual, but the slides are practically useless
for any one else unless they are labeled.

Sometimes slides are numbered, and a catalogue kept with cor-
responding numbers and a description of the slide. For one
unfamiliar with the collection the numbers and the cards are not
easy to put together. Then one is liable to have more than one
series, and the series are liable to get mixed. With a label on each
slide, the collection can be made use of by any one.

§ 338. Storing lantern slides. — The problem of storing a large
collection of lantern slides is a serious one. A still more serious
problem is to find the slides needed for a given lecture or demon-
stration.

A common method of storing is to have a cabinet like that used
for the card catalogue of libraries, and to put the slides in the draw-
ers as the catalogue cards are filed.

One can use name cards to designate groups of slides as they are
used to group catalogue cards.

In order to store and make them most easily available for use, Pro-
fessor George S. Molcr of the department of Physics in Cornell
University has devised a cabinet which holds the slides in a single
vertical layer, so that when any holder is pulled out the slides are
all exhibited, and one can see exactly what the slides are and select
those desired.

This seems to the writers of this book, by all odds, the most prac-
tical cabinet yet devised for safely storing slides and making them
available with the least trouble and the least waste of time (fig. 120).

CH. VIII] PREPARATION OF LANTERN SLIDES 219

§ 339. Troubles in making lantern slides. — These are the
troubles liable to be met in photography. They must be over-
come by following intelligently the directions for photographic
work in general and for lantern-slide making in particular. Study
the directions coming with the lantern-slide plates used.

In making written slides or diagrams on varnished slides the pen
will not work well, and the ink will crawl if the varnish is not dry.

FIG. 120. THE MOLER SECTIONAL LANTERN-SLIDE CABINET.

(Cut loaned by G. S. Moler).

This cabinet holds 1200 lantern slides. It consists of a box with twenty
vertical, sliding frames, each frame holding 60 slides.

In the picture the cabinet is shown on a table. One of the frames is entirely
removed and leans against the table leg. One frame is pulled out for examin-
ing the slides stored in it.

In coloring lantern slides one must learn to use colors which give
the correct effect with the artificial light used in projection. A tint
which does not seem right by daylight may give exactly the desired
effect by lamp-light. This is why the advice is given to test the
work frequently in the lantern.

Remember that there is more danger of getting the lantern
slides too opaque than not opaque enough.

Sometimes when being exhibited a lantern slide shows a mist or
fog spreading over it. This may partly or wholly disappear.
This is a real fog, and comes from the moisture in the slide, or its
mounting. If the slides are thoroughly dried before they arc put
into the lantern this fog does not appear.

220

PREPARATION OF LANTERN SLIDES

[Cn. VIII

340. Summary of Chapter VIII:

1. Use the standard size of
lantern slides in the country
where you live (§ 312).

2. Make the lantern slides
with moderate intensity, then
they can be used with all lan-
terns, no matter what the source
of light (§327).

3. Make the picture small
enough so that all desired parts
can be projected (§334).

4. Take pains in mounting
the slides so that the frame will
appear suited to the subject
(§ 334).

5. In making slides direct on
the varnished glass, write finely,
neatly and clearly (§ 316).

6. Printed or written matter
on the slide should be large
enough to be read by all in the
room (§325).

7. Mark or spot the lantern
slides so that they can be in-
serted in the holder without
hesitation (§ 335).

8. Label every lantern slide
so that any one can tell what it
is (§ 337)-

9. Store the lantern slides so
that they can be found quickly
(§338).

Do NOT

1. Do not use odd sized
pieces of glass to make lantern
slides on.

2. Do not make the lantern
slides so opaque that only the
best electric lanterns can ex-
hibit them.

3. Do not make the picture
on the slide too large to be
exhibited.

4. Do not mount the slides in
a slovenly, inartistic manner.

5. Do not use nourishes in
writing on the varnished slides.

6. Do not reduce the written
or printed matter so that it
cannot be read in the screen
image.

7. Do not leave the slides
unmarked and expect every
chance operator to insert them
properly at railroad speed.

8. Do not leave the lantern
slides unlabelcd, for no one else
can make the best use of them.

9. Do not store the slides in a
miscellaneous heap.

CHAPTER IX
THE PROJECTION MICROSCOPE AND ITS USE

§ 350. Apparatus and Material for Chapter IX :

Suitable room with screen, for projection; Projection Micro-
scope; Sunlight or the electric arc light; Specimens suitable for
projection (§ 399) ; Tools etc., as for Ch. I.

REFERENCES AND HISTORY

§ 351. For the history of the origin and development of the
projection microscope, refer to the appendix at the end of the book.
In this history will be given many references to the original sources
of information upon the subject.

For works dealing with modern micro-projection, the reader is
advised to consult the works given in § 2 of Ch. I. He is especially
advised to consult the catalogues of Zeiss and the other modern
makers of projection apparatus, for in them he will find directions
and suggestions for making the best use of the most modern instru-
ments. His attention is also especially called to the Journal of
the Royal Microscopical Society and to the Zeitschrift fiir wis-
sentschaftliche Mikroskopie. See also the Zeitschrift fur Instru-
mentenkunde, the English Mechanic and the Scientific American
with its Supplement. In every volume of these periodicals there
are almost always articles bearing directly on the problems in-
volved in Projection.

GENERAL CONSIDERATION OF THE PROJECTION MICROSCOPE
§ 352. Similarity of all projection apparatus. — All devices for
projection are fundamentally alike in giving images of brilliantly
lighted objects. These images are projected upon some reflecting
surface or screen in a dark room. The projection microscope simply
gives images of greater enlargement than the other forms of
apparatus. It imperceptibly merges into the magic lantern, as
the magic lantern merges into the camera obscura. (Compare
fig. 121-122).

221

222

THE PROJECTION7 MICROSCOPE

[CH. IX

FIG. 121. PROJECTION MICROSCOPE.

/, 2 Feeding screws of the arc lamp,
j Set screw for the upper carbon.

4 Set screw for holding the stem of the arc
lamp in the socket on block i.

5 Set screw for the lower carbon.

Hc-\r The horizontal, upper carbon. It
must be made positive (+).

L The source of light, i. e., the crater of
the upper carbon.

Vc — The vertical or lower carbon. It is
negative ( — ).

Axis, Axis, Axis The principal optic axis
from the positive crater of the arc lamp
extending through the condenser, the stage
water-cell, and the microscope to the screen.

i Condenser 2 The triple condenser for
receiving and concentrating the light from
the crater of the arc lamp.

1 The first element of the condenser which
renders the diverging light parallel. It con-
sists of a meniscus next the light and a
plano-convex lens (compare fig. 105, in).

2 The second element of the condenser
which concentrates the parallel beam.

W Water-cell between the two plano-con-
vex lenses in the parallel beam of light.

As a projection microscope uses ob-
jectives of shorter focus and smaller
diameter than the magic lantern,
greater care must be exercised in get-
ting all the elements, radiant, con-
denser and projection objective, cen-
tered along one continuous line or
axis, and in having the different ele-
ments the right distance apart.

CH. IX]

THE PROJECTION MICROSCOPE

223

Micro-projection is simply a refinement of ordinary magic
lantern projection. If one understands the principles, and has
mechanical skill to apply them, there is no great difficulty in micro-
projection. But if ordinary magic lantern projection is unsatisfac-
tory7 in untrained hands, micro-projection in such hands is in-
tolerable.

This is, however, such a powerful aid to the teacher and the
lecturer that the time necessary to learn to use it properly is not to
be counted. With micro-projection the beauties of structure and

Condenser

H C

FIG. 122. MAGIC LANTERN FOR COMPARISON WITH THE PROJECTION
MICROSCOPE (See fig. 2).

form are made visible to an entire audience with all their color,
delicacy and exquisite perfection.

Furthermore, the teacher or lecturer can indicate on the screen
the special points to be noted, and feel confident that his auditors
see the special features and do not get confused by the mass of
details, as when looking into a microscope. Often too, the most
interesting and important structures in a specimen are not so
striking as some less important detail, and the important points are
likely to be missed unless pointed out.

§ 353. Limitation of the Projection Microscope. — Perfect and
useful as the projection microscope is, it is limited in its powers.
One can show with full satisfaction to a large audience (200 to
1000) only those details which an experienced observer can see by

224 MICRO-PROJECTION FOR LARGE CLASSES [Ca. IX

looking directly into a compound microscope supplied with a low
ocular and a 16 mm. objective. For a small audience near the
screen higher powers are satisfactory (see § 401).

§ 354. Size of specimens for projection. — To meet the require-
ments of teaching and demonstration the modern scientific man
and public lecturer should be able to commence with the projection
microscope where the magic lantern leaves off, and carry the pro-
jection to the smallest size adapted to micro-projection; that is,
from a specimen 60 mm. in diameter to one of half a millimeter or
less in size. This requires an opening in the stage slightly larger
than the largest specimen, that is, at least 65 mm. in diameter.

CHARACTER AND RANGE OF PROJECTION OBJECTIVES FOR DEMON-
STRATION TO LARGE CLASSES

§ 355. Objectives from 125 mm. to 4 mm. equivalent focus are
especially useful in micro-projection. The powers of 125, 100, 75,
50, and 25 mm. equivalent focus, and in some cases those of 20 and
1 6 mm., are constructed on the plan of photographic objectives
(fig. 123). These are always to be used without an ocular, and
their iris diaphragms are wide open.

At the present time the low objectives used in ordinary micro-
scopic observation are also used in projection. The field is not
flat, as with the micro-planar and other forms of photo-micro-
graphic objectives, but they are much cheaper and the screen
images are very brilliant. Formerly many of the objectives used
in projection were made especially for that purpose. They gave
very brilliant, flat fields over a narrow angle, but they were neither
satisfactory for ordinary microscopic observation nor for
photography.

Most of the projection with the microscope is, however, accom-
plished with objectives of about the following range: 50 mm.,
1 6 mm., and 8 mm. With these in a triple nose-piece or revolver,
the projection microscope can accomplish great things, especially
if assisted occasionally by amplifiers. For an audience of 2 50 to 500
and a screen distance from 7.5 to 10 meters (25 to 33 ft.) the mag-
nifications will range from about i 50 to 3000 diameters (§ 391).

CH. IX] MICRO-PROJECTION FOR LARGE CLASSES

225

For a larger audience and a correspondingly larger room the
screen distance might be made 15 to 20 meters (50 to 65 ft.), and
the magnification raised from 250 at the lower limit up to about
5,000 diameters at the upper limit. The smaller room enables
one to get more brilliant screen images, and to use a wider range of
objects (see table of magnifications § 391). In the smaller room
the screen should be at least 4 meters (12-13 feet) square, and in
the larger room 5-6 meters (15-20 feet) square.

FIG. 123. DIAGRAMS SHOWING THE CONSTRUCTION OF OBJECTIVES FOR
MICRO-PROJECTION AND FOR PHOTOGRAPHY.

(From the Catalogues of Zciss, Leitz, and the Bausch & Lomb Optical Co.).

A Microsummar of Leitz.

B Microplanar of Zciss.

C Microtessar of the Bausch & Lomb Optical Co.

When used for micro-projection the diaphragm is wide open and no ocular is
employed.

In the diagram of the Microtessar, F represents the front lens, d the dia-
phragm, and B the back combination of the objective. The arrow indicates
the direction of the light.

In articles and books upon projection, it is advocated sometimes,
that oil or water immersion objectives as high as i .5 or 2 mm. should
be used for class demonstration.

There is no doubt that brilliant images with short screen dis-
tances can be obtained with high power objectives, but such pro-
jection is only applicable for small numbers ; and if fine details are
to be seen, the observer must be very close to the screen. Further-
more, no screen image in its finest details is equal to that which
one gets in looking directly into a compound microscope. (For
high power projection sec § 401).

If it is high magnification that is desired, it is vastly better to use
lower objectives with an amplifier (§ 356, fig. 126). The lower

226

MICRO-PROJECTION FOR LARGE CLASSES [Cn. IX

objective with larger lenses admits much more light, hence the
screen image will be brighter. For example, suppose it were
desired to obtain the magnification which is given by a 2 mm. objec-
tive, it would be much better to use a 4 mm. objective and an
amplifier doubling the size of the real image. This would make the
screen image of the same magnification as the 2 mm. would give,
and it would be far brighter and show a larger field. In like
manner and for the same reason, it is better to use an 8 mm. objec-
tive and an amplifier, than a 4 mm. objective without the amplifier
(but see § 401).

A B C D

FIG. 124. FIGURES SHOWING THE GENERAL CONSTRUCTION OF MICROSCOPE

OBJECTIVES.

A Low power objective of a single combination (50-30 mm. equivalent
focus).

B, C Medium power objectives with two combinations (25-12 mm.
equivalent focus). Sometimes the front combination is composed of two and
sometimes of three lenses as shown.

D High power objective (8 to 2 mm. equivalent focus). Usually the front
combination is of a single lens, the others of two or three lenses as shown.
Many high power objectives have but three combinations.
(D is from Voigtlander's Catalogue).

The writers have found that in projection for actual class demon-
strations, objectives of higher power than 4 mm. arc unsatisfactory.
We believe also that the purpose of class-room projection is not
the demonstration and study of minute details which require that
the observer should be close to the screen image, but the general
outlines and broad features which can be seen clearly at a distance
when suitably magnified.

CH. IX] MICRO-PROJECTION FOR LARGE CLASSES 227

The fresh blood corpuscles of man, for example, are about 7.5/4
in diameter. To see these as discs on a screen at a distance of 10
meters would require a magnification of 4,000 and preferably of
8,000 diameters. With such a high magnification the sharpness of
the outline, and the distinction between the corpuscles and the
medium in which they float is almost lost, and there is nothing but
a vague haze with shadowy outlines. If one goes up closer to the
screen to see the images well, one will be sorely disappointed, for
they are vague in outline and wholly unsatisfactory as compared
with the appearance gained by looking directly into a microscope
(§ 355a).

§ 355a. Visibility of objects or their magnified images. — It has been found
by careful observation and experiment that the most sensitive part of the eye
is in the fovea centralis or yellow spot; and that in order to see two points, by
the fovea, as separate, they must be far enough apart so that the visual angle
is one minute. If the visual angle is less than one minute, two points appear to
most eyes as one.

The question now is, how far separated must the parts of an object be in
millimeters or inches in order that the form of the object can be distinguished.
To answer this it is necessary to know the actual length of the one minute of arc
when the eye is at different distances.

To determine the length of one minute of arc in any case, the eye is con-
sidered to be at the center of a circle and the object at the circumference, and
no matter how great the visual distance, the object must subtend one minute
of the arc of the circle of which the visual distance is the radius in order to have
its parts distinguishable.

To determine the actual length in millimeters or inches of one minute of arc
in any circle, it is only necessary to remember that the circumference of a circle
is 6.2832 times its radius and that it is divided into 360 degrees or 21,600
minutes (fig. 125).

If, now, the radius of the circle, or the distance of the eye from the object is
I meter, the circumference of the circle will be 6.2832 meters or 6,283.2 milli-
meters. As there are 21,600 minutes in the circumference, the length of one
minute is 6,283.2 mm. --.- 21,600 = .2908 mm. or approximately .3 mm. That
is, with the eye at one meter distance, the parts of an object should be separated
.3 mm. to be seen as distinct points.

For the standard distance of distinct vision (25 cm.), used in microscopic
magnification, the object must be Kth this size or .075 mm.; and for a dis-
tance of i o meters it must be 10 times as great or 3 millimeters, and for 6 meters,
the distance used for testing vision, it must be .3x6 = 1.8 mm.

A greater separation of the points is desirable for the most accurate deter-
mination, but those given above are the minimum for most observers.

Now to apply the above to the magnification necessary for a screen image of
the human blood corpuscle which has a size of 7.5," (.0075 millimeters; .000295
inch). To give the necessary sized screen image of .3 mm.; .075 mm. and 3
mm. at distances of i meter, ^th meter, and 10 meters, it is only necessary to
divide the size of the screen image in each case by the size of the object (7.5^
or .0075 mm.).

228 MICRO-PROJECTION FOR LARGE CLASSES [Cn. IX

FIG. 125. DIAGRAM SHOWING VISUAL ANGLE.

The nodal point or optic center of the eye is placed at the center of the circle,
and the rays from the extremities of the object which cross at this nodal point
show the visual angle.

It is clearly seen from the diagram that the object must increase in length
in direct proportion to its distance from the eye if the visual angle remains
constant.

Visual Angle The angle between the lines extending from the extremities
of the visible object and crossing at the nodal point (n) of the eye.

Axis The straight line extending along the principal optic axis of the eye
to the visible object on one side and to the retina on the other side of the nodal
point (n).

n Nodal point or optic center of the eye.

ri Retinal image. The size of the retinal image of a given object depends
upon the visual angle and the visual angle depends upon the distance of the
object from the nodal point.

For I meter (.3 mm. -=- .0075 = 400 diameters magnification).
For YJ, meter (.075 -: .0075 = 100 diameters magnification).
For 10 meters (3 -:• .0075 = 4,000 diameters magnification).
For anything like a satisfactory view of the corpuscles, it would be desirable
to double these magnifications.

CH. IX]

MICRO-PROJECTION WITH AMPLIFIERS

229

§ 356. Amplifiers. — An amplifier is a concave lens or combina-
tion producing divergence instead of convergence of light rays,
hence placing an amplifier in the path of the image-forming rays
from the objective produces a larger image (fig. 126), and there is
little loss in light. It should be made as great in diameter as the
large tube (fig. 121) of the microscope will receive to avoid cut-
ting down the field, and should be mounted in a short tube which
can be easily slipped into a cloth-lined collar screwed into the
end of the microscope tube (fig. 133).

The amplifiers most generally useful are of -5 and -10 diopters.
The average increase in magnification given by the -5 diopter
amplifier is 1.7 and that given by the -10 diopter is 2.5 (see § 356a).

Object

Objective

Microscooe Tube 122 x 46 mm. ***•*, ~~~~^--

FIG. 126. AMPLIFIER FOR PROJECTION.

Object The object to be projected.

Objective The projection objective.

Axis Optic axis of the apparatus.

A mplifier The concave lens diverging the rays from the objective and thus
increasing the screen image.

Images The ones with broken lines show the images with a - s diopter and a
-10 diopter lens. The full lines show the image which the objective alone
would give.

The microscope tube is 122 mm. (4.8 in.) long and 48 mm. (1.9 in) in
diameter.

§ 356a, 403a. Diopter, Dioptre, Dioptry.— For spectacle lenses especiallv
this is the unit of strength. It is the strength of a lens of i meter principal
focus.

As the focal length of a lens varies inversely as its power, the focal length of a
lens of 2 diopters is one-half as great as the standard, hence it has a focal length
of y2 meter; and one of 10 diopters has a focal length of i/io meter and so on

tor lenses having a strength less than the standard of i meter the focal
length will also be inversely as the power, and hence a Y* diopter lens will have
a focus of 2 meters and a i/ioth diopter lens has a focus of 10 meters. In
general, the less the dioptry or strength the longer is the focus, and the greater
the dioptry or strength the shorter is the principal focus.

Convex lenses with a real principal focus arc indicated by the plus sign ( + ).

230

MICRO-PROJECTION WITH OCULARS [Cn. IX

FIG. 127. HUYGENIAN OCULAR IN SECTION.
(From The Microscope).

F. L. Field lens. This aids the objective in forming a real image.

D Diaphragm in the ocular. It is at this level that the real image is
formed in ordinary microscopic observation.

E. L. The eye lens. In projection this acts like an objective and projects
upon the screen an image of the real image (see fig. 207).

A xis The optic axis of the microscope.

E. P. Eye-point or Ramsden's circle.

§ 357. Projection oculars. — Any ocular may be used for pro-
jection. The lower powers, x 2, X3, x 4, x 6, (§ 357a) are
better than the higher powers, for they cut down the field less, there
is less loss of light, and there is not an inordinate magnification.

Concave lenses having a virtual focus are indicated by the minus sign ( — ) .

If the dioptry of a lens is given, to find the principal focus: divide i meter
by the dioptry. For example, the dioptry of the amplifiers mentioned above
(§ 356) is — 5 for one and — 10 for the other. Their foci are then i meter,

~5
i meter. That is, they are concave lenses of 1/5 and i/io of a meter focus.

-10

On the other hand, to find the dioptry of a lens whose principal focus is known,
divide i meter by the principal focus and the result will represent the dioptry
of the given lens. Taking the same case as before where the amplifiers have

principal foci of 1/5 and i/io meter,

As the lenses are

known to be concave, the minus sign is placed before the dioptry: • — 5, — 10
diopters.

The increase in magnification given by the amplifiers, — 5, — 10 was found
to average 1.7 for the — 5 and 2.5 for the — 10. The average was obtained by
considering all the screen distances and all the different objectives shown in the
table, § 391. See also § 392a.

CH. IX] MICRO- PROJECTION WITH OCULARS 231

In using the ordinary oculars a small tube must be screwed into
the large microscope tube as for ordinary observation (fig. 147,

197).

Special oculars have been designed for projection. Some, like
those of Zeiss (fig. 128) give sharp brilliant images, but the field
is very small. Williams, Brown and Earle have a very large pro-
No. 2.

No. 4.

FIG. 128. PROJECTION OCULARS OF ZEISS.
(From Zeiss' Catalogue, No. jo).

A section has been removed to show the construction. Both are of the
negative form.

The eye lens is in a smaller tube with spiral movement to enable the operator
to focus the image of the diaphragm of the ocular sharply on the screen.
Below are shown in face view the upper ends of the oculars with their graduated
circles. By noting the position in any experiment it is easy to set the position
exactly the same if the experiment is to be repeated.

No. 2, No. 4 These numbers indicate that the ocular magnifies the image
two or four times (see § 391).

jection ocular of the Huygenian form which magnifies about twice.
On account of the loss of light and the restriction of the field of
view, the writers of this book do not advocate the use of oculars for
ordinary micro-projection, but sec § 401.

§ 357a. Designation of oculars. — At the present time an ocular is usually
designated by the increase in magnification it gives a microscopic image when
the microscope is used in the ordinary way. For example, if the objective alone
would give an image 10 times as long as the object, then an ocular x 2 should
double that size, thus giving an image magnified 20 times, and an ocular x 4,
an image magnified 40 times and so on.

232

MICRO-PROJECTION WITH OCULARS

ICH. IX

§ 358. Micrometer ocular for demonstration. — It is so difficult
for most students to understand the workings of the ocular micro-
meter, that it is of great help to them to use a micrometer ocular
like fig. 130 to 131 on the projection microscope, then the object
and micrometer lines can be projected together by suitably adjust-
ing the eye-lens of the ocular. A stage micrometer might also be
used as object and the students shown, all together, how to deter-
mine the ocular micrometer valuation (see Gage, The Microscope).

Oculir lo 2

FIG. 129. COMPENSATION OCULARS.

(From Zeiss' Catalogue, No. jo).

A section has been removed to show the construction.
The numbers 2, 4, 8, /2, 18, indicate the magnification of each
ocular (see § 357a, 391 a).

§ 359. Substage condensers. — The writers believe, from their
experience and experiments in photometry under the different
conditions, that it is better to use for illumination only the large
condenser (fig. 121).

The use of a substage condenser is for either one of two purposes :
(i) to enable the position of the object and the projection objective

The average increase in magnification given by the different oculars with the
different objectives and screen distances shown in the table (§ 377) is as follows:
Projection ocular X2 gives a magnification of ................... 1.99

*4 " ................... 3-^9

Compensation " X2 " ................... 2.05

Huygenian x4 " ................... 4.2 1

From these figures it is seen that the increase in magnification for projection
work can be closely enough approximated by multiplying the image given by
the objective alone by the number designating the ocular, i. e., 2 or 4.

If very precise results are desired, one must use a stage micrometer and pro-
ceed as described in § 391 a.

CH. IX] PROJECTION WITH SUBSTAGE CONDENSER

233

•to be different from what it would be with the main condenser only ;
or (2) to make the aperture of the illuminating cone correspond
with that of the objective.

The positional reason (i) can only have weight when combined
apparatus is used, that is, when a magic lantern objective as well
as microscopic objectives are used without changing the distance
between the main condenser of the microscope or the magic lantern
objective.

FIG.

130. OCULAR MICROMETER WITH MOVABLE SCALE.
(Cut loaned by the Spencer Lens Co.).

This is a Huygcnian ocular with a 5 mm. scale divided into twenty K" mm.
intervals. The pitch of the screw moving the scale is Y\ mm., therefore one
complete revolution of the drum moves the scale one interval or '--4 mm. The
drum is divided into 100 graduations thus enabling one to measure looth of
an interval on the micrometer scale. This ocular micrometer combines the
advantages of the ocular micrometer with fixed scale and the filar micrometer.
To complete the measurement of an object not exactly between any two
micrometer lines the drum need be revolved only partly around.

With reference to the aperture (2) it is one of the fundamental
laws of microscopic vision that the brilliancy and clearness of
details depend largely upon the aperture of the light which illumin-
ates the object, and which passes through the objective to form the
retinal or the screen image. As the numerical aperture of objec-
tives varies greatly it is necessary, if the clearest and most brilliant
images are to be produced, to light the object with a numerical
aperture equal to that of the objective. Where substage con-
densers are used arrangements must be made for this.

234

PROJECTION WITH SUBSTAGE CONDENSER [Cn. IX

FIG. 131. FILAR MICROMETER OCULAR.
(Cut loaned by the Bausch & Lomb Optical Co.}.

This filar micrometer ocular is of the Ramsden type and consists of a positive-
ocular with a movable hair line and two reference lines at right angles to each
other as shown in A. The movable line must be carried over the entire length
of the object to be measured by rotating the drum.

A Field of the filar micrometer showing the movable and the cross lines,
and the comb. The teeth serve to measure the total revolutions of the'drum.

FIG. 132. ILLUMINATING OBJECTS OF VARIOUS SIZES ix MICRO-PROJEC

TION WITH THE MAIN CoNDEXSER OxLY.

The object must be put in the cone of light at a point where it will be fully
illuminated.

For high powers it will be at or very near the focus (/). For larger objects
and low powers the object is at 2 or j, or even closer to the condenser face.

Arc Supply The right-angled carbons of the arc lamp.

L1 L2 The first and second elements of the triple condenser.

Water- Cell The water-cell for absorbing radiant heat. It is in the parallel
beam between the first and second elements of the condenser.

Axis The principal optic axis on which all the parts are centered.

If only the main condenser is used (fig. 121), the cone of light
from the condenser must be sufficient to fill the aperture of the
projection objective. This requires that the second clement of the

CH. IX] PROJECTION WITH SUBSTAGE CONDENSER 235

main condenser (fig. 132 La) have a focus of 150 to 200 mm. (6 to 8
inches). With such a main condenser one can do successful pro-
jection with objectives from 125 to 4 mm. focus. The aperture
will not be completely filled in the 8, 6 and 4 mm. objectives, but
brilliant screen images are obtained even with them for a 7 . 5 meter
(25 ft.) screen and 12 amperes of direct current. One can also use
a -5 diopter amplifier when good specimens are projected. (For
the position of the objective and specimen see § 376).

With a substage condenser there is a great loss of light from
reflection and absorption so that the increased aperture hardly
compensates for it, and the increased detail is lost for the observers
are too far from the screen to see them (see § 35ga).

For special demonstrations and for drawing where the observers
are very close to the screen, the substage condenser and also an
ocular are advantageous, and for fine details, necessary (see § 401,
477).

SUITABLE ROOM AND SCREEN FOR MICRO-PROJECTION

§ 360. From the small size of the objective for micro-projection
the image on the screen cannot be made as bright as with the magic
lantern, hence it is necessary in micro-projection to have a room
that can be made very dark ; and the devices for cutting out stray
light, — bellows, objective hood and shield — must be efficient (fig.
133. 139)-

§ 359a. i. Wright, p. 212, says: "The iris of the substage condenser is
opened or closed until the best effect is produced." This can mean only that
not the whole cone of light is used in some cases.

2. To determine the amount of aperture of the objective used in projection,
take a thick piece of smoked mica or combine brown and blue, or deep red and
blue, or red and green glass and put them over the front of the objective to
soften the light. Or one might hold one of these light softeners just in front
of the eye. Then in any given case look along the microscope tube directly
toward the light, and the aperture of the objective actually filled by the enter-
ing cone of light can be seen. If the entire aperture is used, the back lens of
the objective will be filled with light; if only a part of the aperture, then there
will be a central brilliant circle and a dark zone of glass surrounding it (fig. 151).

It must be remembered too that the large specimen cooler (fig. 121, 134)
cannot be used with a substage condenser; and in our opinion this overbalances
any advantage that the substage condenser might yield for demonstrations to
large classes.

236 MICRO-PROJECTION WITH DIRECT CURRENT [Cn. IX

The screen must be as reflecting as possible. Nothing has ever
yet exceeded in satisfactory quality a smooth, dull, white, wall.
For a full discussion of screens see Ch. XII, § 621.

MICRO-PROJECTION WITH THE DIRECT CURRENT ARC LAMP AS THE

LIGHT SOURCE

§ 361. Arc lamp and wiring for the same. — The direct current
arc light is the only fully satisfactory artificial light known at
present for micro-projection. Hence it will be taken as the
standard, as with the magic lantern (Ch. I). Furthermore, as the
upper carbon is always made positive and hence is the source of
light, this carbon is made horizontal and the crater faces the con-
denser and is in the optic axis. That is, for micro-projection
we take the right-angled arc lamp as the standard (fig. 3, 121).

The wiring, rheostat and ammeter are as with the direct current
magic lantern radiant, (figs. 2, 3, 133). The rheostat should be an
adjustable one. The ammeter can be omitted, but it is more
important than with the magic lantern, for the conditions of
micro-projection must be made as nearly perfect as possible. With
the ammeter one can tell instantly whether the proper amount of
current is flowing. If there is sufficient current the light should be
satisfactory, or if it is not satisfactory it will be due to some fault
in optical adjustment. The ammeter is urged upon all users of the
projection microscope because the tendency is to run in more and
more current if the projection is unsatisfactory, hoping by pure
brute strength, so to speak, to overcome difficulties due to improper
adjustment. In case one cannot afford an ammeter, then the next
best thing is, when installing the apparatus, to measure the current
flowing through the arc with the different settings of the adjustable
rheostat, and to mark these values on the rheostat dial. One can
then set the rheostat at the proper amperage for the given projec-
tion; but as the voltage on the line is subject to variation, one
cannot be sure that the proper current is flowing at any given
moment unless an ammeter is present to indicate the amount.
With many lighting circuits, the fluctuations in voltage are very
small, and one can be reasonably sure of getting the current indi-

CH. IX] MICRO-PROJECTION WITH DIRECT CURRENT 237

cated on the dial of the rheostat. When current is drawn from an
overloaded power line, however, the voltage fluctuations are often
so great that an ammeter, as well as an adjustable rheostat,
is a necessity.

§ 362. Fine adjustment for the arc lamp. — For micro-projec-
tion it is absolutely necessary to have fine adjustments on the arc
lamp so that the position of the crater can be changed slightly
during an exhibition. In the burning of the carbons there is a
slight shift in position of the crater even with soft-cored carbons.
The crater may be in perfect alignment to start with, and by the
shifting as the carbon burns away it may get far enough outside
the longitudinal axis on which the apparatus is placed to spoil the
light on the screen. This is emphatically true for high powers (16
mm. and higher). If now there are fine adjustments on the lamp
(fig. 3, 146), by which the crater can be slightly raised or lowered
or turned toward the right or left, compensation for this shifting
can be made, and the most brilliant part of the crater kept strictly
in the axis where it must be to give satisfactory illumination.
Furthermore, it is necessary to have an independent adjustment
for one or both of the carbons, so that one or both carbons may be
moved independently. This is because the carbons are liable to
wear away somewhat unequally, and some one of the mal-positions
shown in fig. 24, 25 would occur if the carbons were not adjustable.

§ 363. Condenser. — The triple form with a meniscus next the
radiant (fig. 121, 132) is very satisfactory for micro-projection,
although many use the double form (fig. 146) with success. As
the objectives used for projection with the microscope are of short
focus and rather large aperture the final element of the condenser
used to bring the light to a focus should not be of too great focal
length. A focus of 150-200 mm. (6-8 in.) is a good average for
the condenser with the objectives usually employed (125 to 4 mm.,
§ 355)- See § 401 for condenser with substagc condenser.

§ 364. Water-Cell to prevent overheating. — For micro-projec-
tion a water-cell in connection with the large condenser is a neces-
sity. It absorbs most of the radiant energy in the infra-red part

238 MICRO-PROJECTION WITH DIRECT CURRENT [Cn. IX

FIG. 133. PROJECTION MICROSCOPE WITH AMPLIFIER.

This picture shows the projection microscope arranged for use in a lecture
room.

Commencing at the left :

The supply wires to the table switch.

A The ammeter to indicate the amount of current. It is along one wire
(in series).

R The adjustable rheostat. It is along one wire.

10-20 These figures indicate that the rheostat is adjustable; the lowest
current allowed to flow being 10 amperes and the highest 20 amperes. The
arrow indicates the direction to turn the knob to increase the current.

The arc lamp in the lamp-house. This is the three- wire, automatic arc lamp
of the Bausch & Lomb Optical Co.

The wiring is shown to be:

One wire from the negative pole of the switch to the pole for the lower carbon.

One wire passes from the positive pole of the switch to the middle binding
post of the motor mechanism of the automatic lamp. The current for the
motor does not traverse the rheostat.

One wire passes from the positive pole of the switch to the ammeter, to the
rheostat and from the rheostat to the positive ( + ) binding screw of the art-
lamp.

CH. IX] MICRO-PROJECTION WITH DIRECT CURRENT 239

The metal lamp-house is semi-transparent as it was in position during only
a part of the exposure for the photograph.

The condenser and water-cell are connected to the stage by a bellows to
exclude stray light.

The microscope shows the objectives on a revolving nose-piece and behind
them a metal shield to keep stray light from the screen.

An amplifier is shown in place, at the end of the large tube of the microscope.

The arc lamp, condenser, stage and microscope are each on an independent
block which moves along the optical bench on the single baseboard. The
vertical white lines on the baseboard indicate the position of the various blocks
for the optical combination here shown.

On the front legs of the table is the adjustable drawing shelf upon which are
demonstration preparations.

The scale of this picture is shown by the 10 centimeter rule just above the
table drawer at the right.

of the spectrum and thus helps to avoid the overheating which
would result if all of this energy remained. The best position of
the water-cell is between the first and second elements of the con-
denser, where the rays are practically parallel (fig. 121). For
further discussion of the avoidance of heating the specimens, see
§ 852.

§ 365. Stage for specimens. — The stage should be of ample
size, and should have an opening sufficiently large for the largest
specimens to be used in micro-projection, that is, not less than 65
mm. (2l/2 in.) square.

§ 366. Mechanical stage. — If serial sections are to be used
with the apparatus then the stage should be supplied with a
mechanical stage of great range, that is about 50 x 65 mm. This
is about the maximum range for the sections mounted on slides
50 x 75 mm. (2x3 in.) (fig. 135, 136). ,

§ 367. Stage cooling device. — While the large water-cell in
connection with the condenser absorbs practically all the long
waves of radiant energy that can be absorbed by water, it is very
desirable, and for many specimens necessary, to have some device
for carrying off the heat developed in the specimen itself by the
absorbed light. The most practical stage cooling device is a stage
water-cell. The one found very efficient and satisfactory in every
way is shown in fig. 121, 134. The specimen rests directly against
the glass side of the water-cell and is cooled bv conduction. Manv

240

MICRO-PROJECTION WITH DIRECT CURRENT [Cn. IX

FIG. 134. FACE VIEW OF THE
STAGE OF THE PROJECTION
MICROSCOPE AND SECTION-
AL VIEW OF THE STAGE
WATER-CELL.

(About half size) .

(From The Microscope, Ninth
Edition, 1904).

A Sectional view of the
stage with the stage water-
cell.

S Metal part of the stage
in section.

S w. Stage water-cell.

gsf Glass front of the stage
water-cell. The microscopic
specimen rests directly upon
the glass front, and heat from
the specimen is conveyed
away by conduction.

B Face view of the metal
part of the stage of the projec-
tion microscope (fig. 121),
and the optic bench. In this
case the base (E) with V's is
of cast iron as is also the
block (D). Both were pre-
pared on a lathe (Compare

fig- 158, 159).

E End view of the guide
piece with V's.

D Apparatus block.

C Post of the stage in the
block socket. Two set screws
hold the post in place. It is
better to use but a single
screw for this.

The stage proper has a very
large opening, and the water-
cell inserted in this opening
permits of the demonstration
of specimens up to 65 mm. in
diameter.

specimens like those of
the nervous system
stained with Wei^ert's
hcn~.atoxylin, or by the
Gol^i ir.cthod absorb a
<rreat deal of the li<rht

CH. IX] MICRO-PROJECTION WITH DIRECT CURRENT

241

falling upon them, and hence, following the law of the conserva-
tion of energy, all this absorbed light is transformed into heat.
The darker the specimen the more light is absorbed, and the quicker
it will be spoiled by overheating. The stage water-cell against
which the specimen rests conducts this heat away, in part, and
makes it possible to exhibit the specimen a longer time (see § 852).

FIG. 135. MECHANICAL STACK OF GREAT RANGE.

(Cut loaned by the Spencer Lens Co.).

This can be clamped to any rectangular microscope stage and as no part of
the clamp extends above the stage the full range of 85 by 65 mm. is available
and slides 50 x 75 mm. (2 x 3 inches) can be examined to the edges. This is
of the greatest convenience in examining serial sections, and also in projecting
them on the screen.

§ 368. Microscope-tube, and focusing device. — If a tube
for receiving the objective is used it should be a large one, (fig.
121, 145). The small tubes used on most microscopes, and on all
when using an ocular, cut down the field too greatly (fig. 137, 147).
The tube should be short, that is, about 9 to 10 cm. (4 in.) long, and
4 to 5 cm. (2 in.) in diameter. There should be coarse and fine
adjustments as for the ordinary microscope (fig. 121).

§ 369. Mounting of objectives of low power. — For the lowest
powers (125 to 75 mm. equivalent focus) it is better to have no
tube at all, but to have a black shield about 15 cm. (6 in.) in diam-

242

BLACK APPARATUS FOR MICRO-PROJECTION [Cn. IX

eter into the center of which is screwed the objective (fig. 138), then
the field is not at all restricted. The low power objectives can be
focused easily by moving their supports back and forth along the
optical bench by hand (fig. 158-159).

FIG. 136. MECHANICAL STAGE OF WIDE RANGE.

(Cut loaned by the Bausch c? Lomb Optical Co.).

This mechanical stage can be attached to any microscope with square stage,
and it permits the use of large slides. The right to left scale is 80 mm. and the
front to back one 58 mm. The actual range available depends on the size of
the stage of the microscope.

BLACKENED APPARATUS

§ 370. The light necessary for micro-projection is so dazzling
that it should IDC kept strictly within the projection apparatus by
means of a proper lamp-house and bellows, so that the only light
which finally reaches the screen is that which passes through the
projection objective. But this ideal condition cannot be wholly
realized in practice, hence the necessity of making the outside of
the entire apparatus, from lamp-house to microscope tube, dull
black. Then any escaping light will not be reflected from polished
surfaces and scatter light into the room, on the one hand, or blind
the eyes of the operator and annoy the auditors on the other.
Projection apparatus found in institutions, in many cases, have a
finish of polished brass or nickel. If the operator cannot focus

CH. IX] BLACK APPARATUS FOR MICRO-PROJECTION

243

properly and has ill success in general, there is no wonder, as he has
blinding reflections constantly in his eyes. With a dull black finish
for all outside surfaces, if the apparatus is properly built, this
defect will be abolished. Polished black finish will not answer, for
it reflects almost perfectly. The finish must be dull, dead, or luster-
less, then the light will be mostly absorbed, and so small a part
reflected that no inconvenience is produced.

§ 371. Blackening the interior of projection apparatus. — As

with the exterior of projection apparatus, so the interior of all the
parts should be dull black to avoid internal reflections and conse-
quent confusion. This is especially true of the objective mount-
ings, the tube of the microscope and the amplifier tube. Lewis

FIG. 137. DIAGRAM TO SHOW THE SIZE OF IMAGE WITH THE SAME OBJEC-
TIVE AND DIFFERENT LENGTH AND DIAMETER OF MICROSCOPE TUBE.

Objects The different lengths of object shown.

Objective The projection objective.

Microscope Tube Microscope tubes with diameters of 48, 30 and 23 milli-
meters.

i, 2 Rays which are stopped by the largest tube.

Ray 3 The marginal ray allowed to pass by the largest (48 mm.) tube.

Ray 4 The extreme marginal ray allowed to pass by the 30 mm. tube.

Ray 5 The marginal ray allowed to pass the 23 mm. tube.

Axis The optic axis.

Images The one in full lines is for the smallest tube. The others in
broken lines for the tubes of larger size.

By tracing back to the specimen it is seen that the larger tubes show corre-
spondingly more of the object, the projection objective remaining the same.

244 BLACK APPARATUS FOR MICRO-PROJECTION [Cn. IX

FIG. 138. PROJECTION WITH PHOTOGRAPHIC
OBJECTIVES OF 75 TO 125 MM. Focus.

Commencing at the left:

The supply in this case is from the house circuit for a current of five amperes.

There is first a separable attachment plug in the lamp socket. On the table
is a separable extension. This is to serve as a safe switch for turning the cur-
rent on and off.

R Small rheostat for five ampere currents. It is in scries, along one wire.
In this case it is the positive wire, if direct current is used, and goes to the bind-
ing post of the upper or horizontal carbon.

The other wire extends between the binding post of the arc lamp and the
separable extension.

The arc lamp with small carbons, in the metal lamp-house. The lamp-
house appears transparent as it was in place during only a part of the exposure.

Following the lamp-house is the triple condenser and water-cell (fig. 122).

The stage with the stage water-cell and the mechanical stage of great range
(fig. 121, 135).

Support for the photographic projection objective.

All the parts are supported by posts and blocks and all move independently
on the baseboard with track. The vertical white lines on the baseboard
indicate the proper relative positions of the different blocks.

At the extreme right is shown the adjustable drawing shelf attached to the
legs of the table. On this shelf is the projection microscope with three objec-
tives in the revolving nose-piece.

The shield behind the objectives is to prevent stray light from reaching the
screen. Demonstration preparations are also shown in the slide box on the
shelf.

The projection table with the drawer for holding apparatus is shown with
the legs partly removed. The entire table drawn to scale is shown in fig. 182.
In this picture the scale is shown by the 10 centimeter rule just above the
drawer at the right.

CH. IX] BLACK APPARATUS FOR MICRO-PROJECTION 245

Wright (p. 194) in speaking of the necessity of a dull finish in the
interior of objectives says : "I may add here that some really good
lenses [objectives] when used with brilliant light such as projection
demands, give a "mist" over the image purely from flare, or reflec-
tion in the lens mount, and which is removed by careful blacken-

ing."

Finally, there may be a bright spot or "ghost" in the screen image
from the internal reflections of a shiny microscope tube, especially
if the tube is small. If an ocular is used this ghost usually dis-
appears. It can also be avoided by having the interior of the
microscope tube a dull black (§ 37ia).

Objective Hood

FIG. 139. PROJECTION OBJECTIVE WITH
BLACK METAL HOOD.

§ 372. Hoods for projection objectives. — Usually the ends of
objectives are tapering and finished in polished nickel, making them
veritable mirrors. As the image of the source of light spreads more
or less beyond the opening of the front lens upon this mirror surface
the dazzling light is reflected into the face of the operator, and also
more or less around the room. The operator is likely to be so
blinded by the reflections that he cannot see to focus properly.

§ 371a. When necessary, a person can give polished surfaces a dull finish
himself. A camel's hair artist's brush should be employed for the finer work.
For the dull finish, dead-black japalac thinned somewhat with xylene (xylol of
the Germans) toluene or turpentine answers well.

Dull black may be prepared by adding to thin shellac varnish plenty of good,
dry lamp-black. After thorough shaking, this should be filtered through
gauze to take out any coarse particles. If the shellac is too thick the resulting
finish is more or less shiny, but if the proper mixture is used the surface will
be very dull, but not so smooth as the japalac.

As the black surface wears off by use, the bright surfaces underneath are
exposed, and occasionally one should go over the apparatus and reblacken all
bright spots.

246 BLACK APPARATUS FOR MICRO-PROJECTION [Cn. IX

The light scattered in the room is liable also, if the room is finished
in a light tint, to diminish the brilliancy of the screen image by
lessening the contrast.

To avoid the troubles just considered, the objective should have
a perforated hood over its front. The perforation should be of the
diameter of the front lens. The free surface of the hood over the
front of the objective should be perfectly flat, and should be finshed
in dull black (fig. 139-140). Such a hood is also of the greatest
use in enabling one to center the light (§ 375, 372a).

A B

FIG. 140. END VIEW OF A HOODED OBJECTIVE SHOWING THE LIGHT
CENTERED AND OFF CENTER.

In A the image of the crater is directly over the opening in the hood and
therefore gives the greatest light for projection.

In B the crater image is at the right and only a small amount of light enters
the objective.

In both A and B the negative or lower carbon is shown by cross lines. It is
above owing to the inverting action of the condenser.

§ 373. Light shield beyond the objective. — There should be a
flat or concave shield beyond the objective to prevent any stray
light reaching the screen from the apparatus except what passes
through the objective (see fig. 133, 138).

CENTERING THE PARTS OF THE PROJECTION MICROSCOPE ON ONE
LONGITUDINAL Axis

§ 374. For micro-projection it is absolutely necessary that all
the parts or elements should be on one straight longitudinal axis
like beads on a rod. With the large lenses used in magic lantern

§ 372a. If one docs not have the metal-hooded objectives (fig. 139),
ordinary, nickel-plated objectives can be greatly improved by painting the
bright surfaces with dull black (§ 3/ia). The objectionable reflections can
also be prevented by tying black velvet or blackened asbestos paper around the
objectives.

CH. IX] CENTERING FOR MICRO-PROJECTION 247

objectives a slight variation from perfect alignment would do no
particular harm, but the lenses arc so small in micro-projection
objectives that a very slight displacement from the axis would
throw the light outside the objective and spoil the projection.

The fundamental principles and precise directions for centering
projection apparatus are given in Ch. I. § 51-58.

§ 375. Final centering of the projection objective. — After the
lamp and condenser are centered as nearly as possible and are at the
right distance apart (§ 55, 56, 376), move the stage up toward the
condenser so that there is plenty of room between it and the objec-
tive. Use some dust or smoke to find where the cone of light from
the condenser comes to a focus (fig. 132, 323).

Now move the microscope on its mounting toward the condenser.
If the objective is centered, then the point of light at the focus will
enter the front lens through the hole in the objective hood (fig. 140).
If it is not centered then it will appear at one side or even entirely
outside the objective. Use the fine adjusting screws of the arc
lamp and change the position of the image of the crater sufficiently
to direct the cone of light into the front lens of the objective. In
case the objective is greatly out of center it may be found necessary
to change the position of the entire microscope.

§ 376. Distance of the objective from the condenser. — The

objective should be at a distance which will bring the crossing point
of the rays in the cone from the condenser within the objective, as
for the magic lantern objective (fig. 122). As the center of the
objective is but slightly beyond the front lens, the following method
has been found to give excellent results. The objective is drawn
up toward the condenser until the image of the crater is shown
within the opening upon the black hood in front of the objective
(fig. 140). As the image is inverted the lower or negative carbon
will appear above in the image. If now the stage with a specimen
is moved up toward the objective until the microscopic object on
the stage is in focus, the image on the screen will be very brilliant.
One should make slight adjustments toward and away from the
condenser to get the most brilliant image. It will be found that

248 CENTERING FOR MICRO-PROJECTION [Cn. IX

the greatest brilliancy is when there is a slight yellowish tinge to the
light. It will be pure white if one moves the stage and objective
slightly nearer the condenser, but it will not be so brilliant. Guid-
ing marks should be made on the apparatus at the best position
for the different objectives used (fig. 133, 138).

§ 377. Table of Can die-Power and Current with Direct Current
Arc and Right-Angled Carbons :

Size of Carbons Amperes Candle-Power

6 mm. 2 200

6 mm. 3 400

6 mm. 4 650

6 mm. 5 goo

8 mm. 7.5 1.500

ii mm. 10 2,200

1 1 mm. 12.5 2,900

ii mm. 15 3 ,700

1 1 mm. 17.5 4>5°o

13 mm. 20 5.400

13 mm. 25 7.500

15 mm. 30 9.500

§ 378. Increase in size of the crater with increase of amperage.

— As the size of the crater and hence its image increases with the
increased amperage, the gain for actual micro-projection is not so
great as would appear, for the larger crater image will be larger
than the lenses of objectives of high power, hence, much light is
wasted (fig. 141).

The heating is also much increased by the higher amperage.
It has been found by experience with everything in the best possible
condition that 12 amperes is sufficient for most micro-projection.
A current above 20 amperes is a pure waste, as well as a source of
danger to the specimens and apparatus by overheating. The light
given by 10 amperes properly utilized yields far better results than
that from 20 amperes only partly utilized. For the candle-power
with different amperages see § 377.

CH. IXl

USE OF PROJECTION MICROSCOPE

249

USE OF THE PROJECTION MICROSCOPE

§ 379. Objectives in a revolving nose-piece. — For most projec-
tion a battery of three objectives would be sufficient. These
should be: (i) a low power objective to show entire specimens
(one of 40 to 50 mm. focus is good) ; (2) an intermediate objective
of 16 to 18 mm. focus; and (3) a high power, that is, one of 10 to 4
mm. equivalent focus (§ 355).

FIG. 141. SIDE AND FRONT VIEWS OF THE CRATER AND CARBONS

BURNING WITH 10 AND WITH 20 AMPERES OF DIRECT

CURRENT (Natural size).

This picture is to show the increase in size of the crater with the larger cur-
rent. (See also fig. 292-293).

(In making the photographs, the lamp was burning with the amperage indi-
cated, and an instantaneous exposure was made with a diaphragm of F/32.
The current was then turned off and the carbons exposed 90 seconds with a
diaphragm of F/8. This brought out the carbons, and gives the appearance
gained by the eye when suitably screened and looking at the burning lamp.)

250 USE OF PROJECTION MICROSCOPE [Cn. IX

The three objectives selected should be in a revolving nose-piece
(fig. 142) so that one can pass quickly from one power to another.
The lecturer and operator must always keep in mind that for an
audience giving their entire attention, a delay of even a quarter of
a minute seems a very long time, hence every precaution should be
taken to avoid delays.

§ 380. Preparation of the carbons for an exhibition. — The

carbons supplied for projection are soft-cored, and sharpened
somewhat like a lead pencil. This end form is unlike that assumed

FIG. 142. TRIPLE NOSE-PIECE OR REVOLVER FOR QUICKLY CHANGING

OBJECTIVES.

(From the Catalogue of Viogtldnder und Sohn).

in the actual use of the carbons (fig. 141), and until the carbons
have burned for some time, one will not get the best light from
them. Hence it is wise to get the carbons formed by burning them
in the lamp for five minutes or so before using them for a lecture or
an exhibition.

Soft-cored carbons arc a necessity for micro-projection, for the
crater remains more uniform and it does not wander around the
end of the carbons and thus get out of line of the general axis so
frequently as would be the case with solid carbons (§ 38oa).

§ 380a. Cored and solid carbons. — Some workers with the projection
microscope use a large, cored carbon above (i.e., for the positive) and a solid
carbon for the negative one. For example, in a projection outfit from Zciss
the upper carbon was 19 mm. in diameter and soft-cored. The lower omega-

CH. IX] USE OF PROJECTION MICROSCOPE 251

§ 381. Screen image of the carbons. — One of the good ways of
learning to get the carbons in the correct relative position is to
study their image on the screen. For this use an objective of
50 or 100 mm. focus. By moving the objective somewhat
beyond the focus of the condenser an image of the burning car-
bons will be projected on the screen and one can tell the exact
appearance of the crater and the relative position of the car-
bons. The glowing upper carbon ought to show the crater
well and appear to face directly toward the observer. As this is
an image of the real image of the carbons formed by the condenser
the screen image will appear right side up. If the negative or lower
carbon is not in the correct position it will shade the image (see
fig- 24, 25).

§ 382. Centering the light and getting the objective at the
correct distance from the condenser for an exhibition. — In using
any of the objectives on the revolving nose-piece it is always to be
kept in mind that the centering is most easily accomplished by
drawing the objective toward the condenser until the image of the
crater and the tip of the negative carbon appear in the opening and
upon the objective hood (fig. 140).

Now if this image is not so that the brightest part is over the
opening in the objective hood, use the fine adjustment of the arc
lamp and get the image of the crater directly in the opening. The
screen image will then be evenly and brilliantly lighted. In case
one side is more brilliantly illuminated than the other, one can
make the illumination even by the fine adjustments of the arc lamp
(fig. 3, 146).

One can sometimes improve the illumination slightly by looking
at the screen image and moving the microscope slightly nearer or
farther from the condenser, but as a rule, when the image of the

tivc carbon was 13 mm. in diameter and solid. In Ewon's lamp the upper or
positive carbon is eored and 18 mm. in diameter; the lower carbon is 12 mm. in
diameter and solid.

Experience leads us to recommend cored carbons below as well as above.
For the size of carbons for different amperages see § 377, 753a.

For alternating current both carbons are of the same size, and most workers
recommend that thev be cored.

252 USE OF PROJECTION MICROSCOPE [Cn. IX

crater and the negative carbon arc most sharply defined on the
objective hood the light on the screen will be the best attainable.
Occasionally, during an exhibition, it will be necessary to use the
fine adjustments on the arc lamp (fig. 146) to get the crater back in
exact alignment as the crater changes position slightly on the wear-
ing away of the carbons. As the carbons sometimes wear away
unevenly it is necessary to have a mechanism by which one carbon
can be moved without affecting the other, otherwise there would
result some one of the malpositions shown in fig. 24, 25.

§ 383. Specimens for projection. — The specimens giving the
best images with the projection microscope are those which are best
for ordinary observation, that is those with the most definite out-
lines and sharpest details. They must, of course, be more or less
transparent. For staining, any color which gives definite details
can be used, but one must remember that the red colors are trans-
parent to the longer, visible waves of light and hence red-colored
objects can remain on exhibition much longer than hematoxylin,
osmic acid or other dark stained objects which are more opaque to
the long waves in the red end of the spectrum (fig. 307).

No matter how large the water-cell or the cooling stage, a thick,
darkly stained specimen will be spoiled after a time by the trans-
formation of the absorbed light into heat (§ 852).

§ 384. Masks for microscopic slides. — The light used in pro-
jection is of necessity so brilliant that the scattered light from the
microscopic glass slide is very liable to dazzle the eyes of the
operator when he looks at the slide in arranging it for the projection
of the object or objects thereon. If one has a series for example, it
is very difficult to select with ease and certainty just the sections
that are to be shown with this scattered light in the eyes. It must
always be remembered, too, that a very short time seems long to a
waiting audience; and that it lessens their confidence in the lec-
turer to have too much blundering in showing the specimens he
wishes them to see.

All this difficult}- can be easily avoided by properly masking the
preparations to be shown (fig. 143, 148).

CH. IX]

USE OF PROJECTION MICROSCOPE

253

§ 385. Kind and color of paper for the masks. — The best paper
to use is one that allows only a moderate amount of light to pass,
and that cuts out the green-blue end of the spectrum.

The color found best for this is an aqueous solution of the
microscopic stain known as "Orange G." For the quality of
paper, a white linen bond paper of moderate weight is used. It is
stained by soaking it a few minutes (10-30) in a saturated aqueous
solution of the "Orange G." It is then hung up to dry.

FIG. 143. SLIDE OF SERIAL SECTIONS WITH MASK.

The sections to be demonstrated are left uncovered.
Sus (Sus scrofa, the pig).

Ser. ii This shows that the slide is from the 1 1 th series of pig embryos.
si 60. The 6oth slide of series 1 1 .

Sec ij/J- This indicates that the sections of this embryo were cut 15 microns

(.015 mm., .00058 in.) thick. ^i"

i goo The year in which the series was prepared. fit*

ii 60 At the left; series 1 1, slide 60. K*

&
Paper thus colored allows a moderate amount of light to pass,

and allows practically all of the long waves of reel and infra-red to
pass, so that it will not burn very quickly in the focus of the con-
denser. If black paper were used it would burn almost instantly
in the focus. Of the many yellows and oranges tried for masks the
"Orange G" proved most satisfactory.

§ 386. How to employ the masks. — The paper is cut of the
right size for the slide and then square or round holes arc made in
it to give a clear field for the different objects to be shown on the
slide, then it is pasted on the cover-glass (fig. 143). It is put on

254 L"SE OF PROJECTION MICROSCOPE [Cn. IX

the cover-glass and not on the slide for the reason that if it were put
on the slide it would almost entirely overcome the good effect of the
stage cooling cell, as it would hold the slide away from the glass
surface, so that the heat could not be carried off by conduction.
If it is on the cover-glass, the slide can then rest directly against
the stage water-cell.

If one ever wishes to remove the mask it is easily done by putting
a piece of wet blotting paper upon it till thoroughly softened. It
can then be peeled off, and the cover-glass cleaned with a wet cloth.

§ 387. Field of view in the screen image. — Except with objec-
tives corrected in the manner of photographic objectives the screen
image will not be equally sharp over the entire field where the large
tube and where no tube is used (fig. 138, 145). To obviate this,
oculars may be used, or iris diaphragms to cut off the outer margin
which is not sharp. This margin also shows color from the
chromatic aberration of the condenser. But demonstrations in
histology and embryology, at least, depend largely in their effec-
tiveness upon the relations of parts shown in a large field. The
part to be shown with greatest distinctness is brought into the
middle of the field as with ordinary microscopic observation.

The importance of a large field in which the relation of parts can
be shown, can be illustrated by a simple experiment. For example,
let a well known friend cover his face with a mask having only eye-
holes, or with a hole to show a part of the cheek or forehead. It
would be hard to recognize him from that limited view alone.

§ 388. Objectives needed for different sizes of field. — In fig.
1 44 there is given a graphic representation of different sizes of field
or object which one might wish to project, and the objective or
objectives with which it can be done. It will be seen that the
larger the field the longer must be the focus of the projection
objective. In this figure it is assumed that no ocular is used and
that the field is not restricted by the tube of the microscope, hence
for the largest fields the objective must be mounted in a shield
without tube (fig. 138). In fig. 137 is shown how the field may be
cut down by using microscope tubes of different diameter. See
also the table of magnification and field (§ 391).

CH. IX]

USE OF PROJECTION MICROSCOPE

255

1 mm. Field
Objectives {JSS:

2-1.5 mm. Field
Objectives i!::;

2.5 mm. Ffeld
Objectives {'?05mT

5 mm. Field
Objectives { IS mT

25-20 mm. Field
Objectives {35

FIG. 144. SIZES OF FIELD AND OBJECTIVES NECESSARY TO PROJECT
OBJECTS OF THESE SIZES.

256

USE OF PROJECTION MICROSCOPE

[CH. IX

§ 389. Sharpness of the screen image. — It is a mistake to think
that it is necessary that the screen image should be photograph-
ically sharp. As well said by Lewis Wright, p. 191: "A certain
breadth or coarseness of line is a positive advantage in the image to
be viewed many feet [meters] away." Of course, the image should
be focused as sharply as possible, but a line or structure that
appears perfectly distinct at a considerable distance may appear
indistinct when the observer is close to the screen. If the operator
is at a considerable distance (15 to 20 meters, 50 to 65 ft.) from the
screen, he will find good opera-glasses a help in getting the screen
images properly focused.

FIG. 144:1. DIAGRAM TO Snow THE POSITIONS OF THE .SAME OBJECT AND

THE SIZE OF THE SCREEN IMAGE FOR OBJECTIVES OF =JO, 2O AND IO MM.

Focus.

In this figure it is assumed that the object in each case is practically at the
principal focal distance from the objective and that, the screen distance is the
same for all. As the size of the image varies inversely with the distance of the
object from the objective it is seen that the screen must be larger for an
objective of short than for one of long focus in accordance with the general
law of the relative size of the object and image (§ 392a).

CH. IX]

MAGNIFICATION IN MICRO-PROJECTION

257

Any one can get a pretty correct idea of the screen image and the
details visible at different distances by putting the first page of a
newspaper up in a well lighted place and then moving back from
it. Close up, the ordinary print can be read, farther away the
ordinary headlines, and still farther the title of the newspaper or
some gigantic headline. Meantime the ordinary print and the
ordinary headlines have merged into a gray haze.

§ 390. Position of the object on the stage. — For many micro-
scopic specimens it makes no difference how the specimen is placed
upon the stage, except that for high powers the cover-glass must
be next the objective. If a specimen must have a given part, end
or border at the top in the screen image, then with an objective only
or an objective and an amplifier, the object must be put on the
stage so that the part is down which is to appear at the top in the
screen image. With an objective and ocular the object should be
placed on the stage as the image is to appear on the screen. For
getting the screen image exactly like the object see § 36, 512.

§ 391. Magnification and Screen Image of Various Objectives
as Found by Actual Measurement (see § 39 ia).

SM. (i6ft.)

7.5 M. (25 ft.)

10 M. (33ft.)

Screen Distance

Objective

Field of
Objective

Magni-
fication

Screen
Image of
Field

Magni-
fication

Screen
Image of
Field

Magni-
fication

Screen
Image of
Field

No TUBE (Fig. 138)

125 mm.

55 mm.

2.25 M. 61

3.56 M.

80.6

4.50 M.

IOO

5<>

2.70 74

3.16

98.4

4.20

7°

5«

3-50

107

5.10

143

7.10

3-15

123

3-7"

1 68

5-20

101

3-30

147

4-50

202

6.10

142

3-50

2IO

5.10

285

7.60

3«

167

3-30

250

4.90

330

6.70

253

2.85 370

4.20

495

5-40

258 MAGNIFICATION IN MICRO-PROJECTION

LARGE TUBE (Fig. 121)

[CH. IX

5 M. (i6ft.)

7.5 M. (25 ft.)

10 M. (33ft.)

Screen D stance

Screen distance

Scieen Distance

Objective

Field of
Objective

Magni-
fication

Screen
Image of
Field

Screen
Magni- Irnage of
fication Field

Magni-
fication

Sc:een
Image of
Field

No TUBE (Fig. 138)

70 mm.

31 mm.

72 1.85 M.

107

3-34 M.

143

4.26 M.

60 '

25 "

2.IO

127

3-oo "

: 1 68

4.10 "

50 "

101

2-30 " 155

3-50 "

205

4-50 "

35 '

142

2-35 "

218

3.00 "

285

4-45 "

30 '

167

2.OO "

250

3-35 "

1 330

4.10 "

20 '

253

2.10 "

380

3.00 "

1 500

4.10 "

16 "

5-75 "

322

i-75

488

2.85 "

! 650

3-73 "

12.5"

4-5

385

i. 80 "

590

2.65 "

750

3-38 "

10 "

3-7

454

•75 "

7OO

2-59 "

900

3-33 "

8 "

2-5

640

.80 "

940

2-35 "

1280

3-20 "

6 "

2.18 '

760

.80 "

I 1 2O

2.44 "

1460

3.18 "

4 '

1.42 '

1080

.70 "

I6OO

2.27 "

!2i8o

3.10 "

2 '

0.42 '

2600

.10 "

3820

1.70 "

5080 2.OO "

MAGNIFICATION AND SCREEN IMAGE OF VARIOUS OBJECTIVES AS FOUND
BY ACTUAL MEASUREMENT.

First is given the magnification of the objective only, using the large tube
of the micioscope (fig. 121) ; then are given the magnification, etc., with am-
plifiers (fig. 126) and wuh oculars. With the latter the draw-tube is in place
(fig. 147, 172, §3913).

5 M.

d6ft.)

7.5 M. (25 ft.)

10 M. (33 ft.)

Screen

Distance

Screen Distance

Objective

Ampli-
fier

Micro-
Ocular scope
Field

Magni-
fication

Screen
Image of
Field

Magni-
fication

Screen
Image of
Field

Magni-
fication

Screen
Image of
Field

1 6 mm.

5.75 mm.

322

1-75 M.

488

2.85M.

650

3-78 M.

" " - 5d

4-2

550

2.30 "

820

3.56 "

I IOO

4.85 "

" " -loci

4-2

800

3-20 "

I 1 6()

5.00 "

1650

6.89 "

11 ii

Proj. X2 1.45 '

640

•95 "

950

1.40 "

1320

I.9I "

ii ii

X4 1.32 '

I 170

1.70 "

1900

2.65 "

2610

3-50 "

ii ii

Comp. x2 2.35 '

650

i-55 "

IOOO

2.35 "

1350 3-10 "

ii ii

" X4 i. 60 "

1320

2-35 "

2000

3-40 "

2880

4.60 "

ii 11

Huyg. X4 i. 60 "

1310

2.25 " i 2250

3-35 "

2700 14.45 "

12.5 mm.

4.5 mm.

385

i.8oM.

590 [2.67M.

750 3.40 M.

11 ii

- 5<1 3-5

650

2.30 "

990

3.60 "

1280 4.85 "

ii ii

-iod 3.5 910

3-3<> "

1430

5-20 "

1900 6.50 "

ii 11

Proj. X2 1.25 ' 730

•95 "

1075

1.35 "

1450 1.86 "

ii 11

X4 1.25 ' 1300

.70 "

2OOO

2-45 "

2700 3.30 "

ii ii

Comp. x2 2.00 " 750

•5° "

1150

2.30 "

1550 3.05 "

ii 11

" X4 1.50 " ! 1520

2-35 "

2350

3-50 "

3200 4.75 "

ii 11

Huvg. x4 i .40 " , 1520

2.2.S "

1 2350

3-37 "

3150 4-45 "

CH. IX]

MAGNIFICATION IN MICRO-PROJECTION

259

5 M. (i6ft.)

7.5 M. (25 ft.

10 M. (33ft.)

Screen Distance

Screen Distanca

Objective

Ampli-
fier

Ocular

Micro-
scope
Field

Magni-
fication

Sci een
Image of
Field

Magni-
fication

Screen
Image of
Field

Magni-
fication

Screen
Image of
Field

10 mm.

3.70 mm.

454

1.75 M.

7OO

2.52 M.

9OO

3.65 M.

" "

- 5d

2.70 "

766

2. 2O "

HIS

3-40 "

1540

4.70 '

" "

-xod

2.70 "

IIOO

3.20 "

1630

4-50 "

2225

6.50 '

" "

Proj. X2

1.05 '

870

•95 "

1300

1.40 "

1750

1.90 '

" "

" X4

1.05 '

1600

1.70 "

2330

2.40 "

3100

3-30 '

" "

Comp. x2

1.70 '

870

1.50 "

1330

2.25 "

1780

3-05 '

" "

" M

1-25 '

1820

2-35 "

2720

3-50 "

3680

4.70 '

Huyg. X4

1-25 '

1800

2.25 "

2700

3-30 "

3680

4-55 '

8 mm.

2.5 mm.

640

i.SoM.

940

2.55 M.

1280

3-31 M.

" "

- 5d

1. 80 "

II2O

2.30 "

1650

3-24 "

2250

4-36 "

" "

-lod

1. 80 '

I6OO

3.20 "

2430

4-35 "

3250

6.00 "

" "

Proj. x2

0.7

1280

.90 "

1960

1.40 "

2610

1.89 "

" "

" X4

0.7

2360

1.70 "

3521

2-35 "

4820

3-35 "

" "

Comp. x2

I.IO '

1380

1.50 "

2030

2.25 "

2770

3.18 "

" "

0.82 "

2750

2.30 "

4I2O

3.60 "

5560

4.90 "

Huyg. X4

0.78 "

2750

2.25 "

4050

3-35 "

5560

4-50 "

6 mm.

2.18 mm.

760

i.SoM.

1 1 20

2.50 M.

1460

3.50 M.

" "

- 5d

1.7 "

I27O

2.25 "

1950

3-30 "

2570

4-5° "

" "

-lod

i-7

I850

3-25 "

2760

4-95 "

3700

6.50 "

" "

Proj. x2

0.60 '

1500

•95 "

225O

1.41 "

3100

1.91 "

" "

" *4

0.60 "

2720

1.65 "

42OO

2-55 "

i 5700

3-37 "

" "

Comp. x2

0-93 "

1550

1.50 "

24OO

2.30 "

3200

3-05 "

ii 11

\4 0.70 "

3120

2-35 "

4800

3-55 "

6500

4-75 "

Huyg. X4

0.67

3100

2.25 "

4700

3-37 "

6500

4-50 "

4 mm.

1.42 mm.

IO8O

1.70 M.

I6OO

2.40 M.

2180

3.ioM.

11 11

- 5d

1.05 "

I9IO

2.30 "

272O

3.20 "

3800

4-30 "

ii ii

-lod

1.05 '

2750

3-25 "

4l6O

4-50 "

5650

6.50 "

ti ii

Proj. x2

0.40 '

2250

.92 "

3460

1.40 "

4500

1.90 "

ii ii

" *4

0.40 "

4I2O

1.65 "

6800

2-75 "

9OOO

3.60 "

ii it

Comp. x2

0.62 "

2350

1.50 "

3500

2.28 "

4830

3.10 "

ii ii

" *4

0.47 "

4820

2-37 "

72OO

3.60 "

9800

4.80 "

Huyg. X4

0-45 '

4770

2.25 "

7IOO

3-37 "

9820

4-50 "

2 mm.

0.42 mm.

26OO

i.ioM.

3820

1.70 M.

5080

2.OO M.

" "

- 5d

0.42 "

4440

i-95 "

6560

2.85 "

8900

3.65 "

ii ii

-lod

0.42 "

722O

2.60 "

9480

4-50 "

12550

7.00 "

ii ii

Proj. x2

0.185 "

4940

o-93 "

7400

1.40 "

10500

2.37 "

" "

" *4

0.18 "

9160

1.67 "

13800

2.50 "

18750

3.70 "

Comp. x2

0.28 "

5120

1.50 "

6666

2.28 "

IO6OO

3.00 "

ii u

0.215 "

10500

2. 2O "

16150

3-35 "

2I25O

5.00 "

Huyg. X4

0.20 "

10750

2. 2O "

15500

3-45 "

2IOOO

5.10 "

260

MAGNIFICATION IN MICRO-PROJECTION

[CH. IX

2.5 M. (8ft.)

Screen Distance

_, . .. i Ampli-
Objective j fier

Ocular

Microscope
Field

Magni-
fication

Screen
Image of

Field

2 mm.

0.42 mm.

1300

0.56 M.

11 11

- 5d

0.42 '

2130

0.97 "

" "

-lod

0.42 "

3080

1.38 "

U 11

Proj. x2 0.185 "

2350

0.44 "

" "

X4 0.18 "

4400

0.78 "

11 11

Comp. x2 0.28 "

2440

0.71 "

X4 0.215 "

5000

I.IO "

Huyg. X4 0.20 '

4960 i i.io "

§ 391a. In preparing this table the apparatus shown in fig. 121, 138 was
used. The second element of the condenser giving the cone of light, had a focus
of 30.3 cm. (8 in.), and the stage was moved up in the light cone (fig. 132) to
give the largest and brightest field possible for the given objective. No sub-
stage condenser was used except for the 2 mm. oil immersion.

A stage micrometer in millimeters, tenths and one-hundredths was used as
object. The screen image of one or more of the micrometer divisions was
measured with a metric rule and the magnification obtained by dividing the
size of the image by the known size of the object. For example: if the
micrometeris in one-tenth millimeters (o.i mm.) and the screen image of two
spaces (0.2 mm.) measures 20 centimeters or 200 mm. the magnification of the
screen image must be 200 divided by 0.2 = 1000. That is, the image is one
thousand times the size of the object, therefore, the magnification of the pro-
jection apparatus in that case is 1000. The size of the field of the projection
apparatus is found by the use of the micrometer as follows: The micrometer
is arranged on the stage so that the image shows one of the lines on one edge
of the field (the circle of light). Then one simply counts the spaces to the
other edge of the field. For example, suppose that it requires 14 of the o.i
mm. spaces, then the size of the field is 1.4 mm. and an object larger than this
cannot be projected entire with this objective.

To get the size of the screen image of this field a tape measure or meter stick
is used and the diameter of the circle of light on the screen is measured.

This method of finding the size of the field of the projection apparatus, the
magnification and the size of the screen image, depends upon direct observation
anil is applicable to any projection outfit whether an objective only or an objec-
tive and an amplifier or an objective and an ocular are used (see also § 392a).
The amplifiers used had a free opening of 36 mm. (i y^ in.), and were placed at
the end of the large tube (fig. 133) at a distance of about 1 1 cm. (4^4 in.) from
the objective.

CH. IX]

MAGNIFICATION IN MICRO-PROJECTION

261

§ 392. Magnification and Screen Image of Various Objectives
as Found by Calculation (see § 392a).

5 M. (i6ft.)

7.5 M. (25ft.)

10 M. (33ft.)

Screen Distance

Objective

Field of
Obiective

Magni-
cation

Screen
Image of
Field

Magni-
fication

Screen
Image of
Field

Magni-
fication

Screen
Image of
Field

No TUBE (Fig. 138)

125 mm.

90 mm.

40.0

3.6oM. 60

5.40 M.

7.2oM.

55 "

2. 2O "

3-30 "

4.40 "

IOO "

75 "

50.0

3-75 "

5.62 "

IOO

7-50 "

50 "

2.50 "

3-75 "

5.00 "

70 " 50 '

7i-5

3-57

107

5-35 "

138 6.90 "

60 ' 42 '

834

3-50 "

125

5-25 "

166 7.00 "

50 " 38 '

IOO.O

3.80 " 150

5-70 "

200 7.60 "

35 ' 26 '

143-0

3.72 " 214

5.56 "

286 7.43 "

30 '

20 '

166.6

3-34 "

250

5.00 "

333 6.66 "

20 ' ii '

250.0 , 2.75 "

375

4-13 "

500 5.50 "

LARGE TUBE (Fig. 121)

70 mm.

31 mm.

7i-5

2.22 M.

107

3-34 M.

138

4.27 M.

60 "

25 " 83.4

2.08 "

125

3-12 "

1 66

4.16 "

IOO.O

2.20 "

150

3-30 "

200

4.40 "

143-0

2.OO "

214 3.OO "

286

4.00 "

166.6

2.0O "

250 3.00 "

333

4.00 "

250.0

2.OO "

375

3-00 "

500

4.00 "

5-75 "

312.5

1.79 "

468

2.69 "

625

3-59 "

12.5 "

4-50"

400.0

1. 80 "

600

2.70 "

800

3.60 "

3-70"

500.0

1.85 "

750

2.78 "

IOOO

3-70 "

2.50"

625.0

I.56 "

937

2-34 "

1250

3-12 "

6 "

2.18"

«33-3

1.82 "

1250

2.72 " 1666

3-63 "

1.42 "

1250.0

1.77 "

1875

2.66 "

2500

3-55 "

2 "

0.42 " i 2500.0

1.05 "

3750

i-57 "

5000 2.69 "

§ 392a. This table was derived by calculation from the optical law that:
The size of the image is to the size of the object as the distance of the image
is to the distance of the object from the center of the projecting lens or objec-
tive (fig. 209). In each case the objective's principal focus is marked upon it
by the maker, and the distance of the screen from the objective is known.
R~ef erring to the diagram (fig. 121) it is seen that the focus of the objective
represents approximately the distance of the object from the center of the
objective when the screen distance is relatively great. The focus of the objec-
tive and the screen distance being known their ratio is easily found. For
example, with the 20 mm. objective and a 5 meter screen distance, the object
will be 20 mm. from the center of the objective (fig. 209) and the screen image
is 5 meters (5000 mm.) distant, then the ratio is 250 to i (5000/20) and it
follows from the optical law given above, that the magnification in this case
is 250.

The field in each case was determined by the use of a stage micrometer as
\vith§39ia. From fig. 209 it is evident that the screen image of the entire

262 ORDINARY MICROSCOPE FOR PROJECTION [Cn. IX

MICRO-PROJECTION WITH AN ORDINARY MICROSCOPE

§ 393. Magic lantern with optical bench and ordinary micro-
scope.— If one has a magic lantern with an optical bench, the
bellows and lantern-slide objective may be removed and an ordinary
microscope put in place. The microscope is made horizontal and
firmly clamped to a suitable block (fig. 145, 187). This block
should be furnished with cleats or grooved so that it will slide on
the rods or guides of the magic lantern, and be of sufficient height
to put the objective and tube of the microscope in the optic axis.
The mirror and the substagc condenser may be removed or turned
aside and the object lighted by the cone directly from the large
condenser as in fig. 145 or the condenser and ocular may be left
in place (fig. 187).

field is magnified, hence to get the size of the screen image, the size of the field
is multiplied by the magnification of the apparatus in any given case. In the
case of the 20 mm. objective the entire field measures 8 mm., hence its screen
image, with a magnification of 250, should be 8 x 250 = 2000 mm. or 2 M.

If one compares the tables obtained by actual measurement and that
obtained by calculation it will be seen that they do not exactly agree. This is
due to two things: first, the rated focus of the objective is only an approxima-
tion, and second, the measurement of the diameter of the screen image is not
very exact from the difficulty of deciding just where to begin and where to
leave off in measuring to get the magnification and for determining the size of
the field or the screen image of the field.

The table of calculated values is only for the objective without the use of
amplifiers or oculars.

If one knows the magnification of the objective for a given screen distance
the magnification obtained when using an amplifier or an ocular with the
objective may be obtained approximately as follows:

For — 5d amplifier multiply the magnification of the objective only, b\

For — lod amplifier multiply the magnification

For x2 projection ocular multiply the magnification

For x4 projection ocular multiply the magnification

For x2 compensation ocular multiply the magnification

For x4 compensation ocular multiply the magnification

For X4 Huygenian ocular multiply the magnification

As the field of the projection apparatus is cut down by the vise of an amplifier
or an ocular one must determine the size of the field by the use of a micrometer
as with the objective alone. The screen image can then be calculated by
multiplying the observed size of the field by the magnification of the combined
objective and ocular or amplifier. It will be seen that the objective with an
ocular x2 or x4 does not give a magnification exactly twice or four times as great
as the objective alone. The oculars are rated for the ordinary distance of
distinct vision (254mm., 10 in.) and the relation does not hold strictly for the
much greater screen distances (§ 357a).

CH. IX] ORDINARY MICROSCOPE FOR PROJECTION 263

FIG. 145. ORDINARY MICROSCOPE FOR PROJECTION.

This figure is to show how an ordinary microscope can be used for projection
if one has an arc lamp and condenser.

Commencing at the left :

The supply wires coming to the table switch.

From the negative pole of the switch one wire proceeds to the negative bind-
ing post of the arc lamp, i. e., to the one for the lower carbon.

From the positive pole of the switch extend two wires for the automatic lamp
of the Bausch & Lomb Optical Co. One wire goes to the binding post of the
automatic mechanism (the middle post). This means that the automatic
mechanism receives current which does not go through the rheostat. The
other wire from the positive pole of the switch goes to the ammeter (A), and
from the ammeter to the rheostat (R), and from the rheostat to the positive
binding post for the arc lamp, i. c., for the upper carbon.

The arc lamp is shown through the metal lamp-house. The lamp-house
appears transparent as it was left in position during only a part of the exposure.

Following the lamp-house is the triple condenser and water-cell.

The microscope is bent over in a horizontal position to bring the axis of the
objective in line.

The microscope is clamped to a block which raises it to the right level.

264 ORDINARY MICROSCOPE FOR PROJECTION [Cn. IX

As here shown the substage condenser and mirror have been removed, and
also the draw-tube and ocular (see fig. 147, 192 for the ordinary microscope
with substage condenser, draw-tube and ocular in position).

The lamp, condenser and microscope are on independent blocks and can be
moved to any desired position on the baseboard.

A The ammeter to indicate the amount of current.

R Adjustable rheostat. This rheostat is adjustable between 10 and 20
amperes. The arrow indicates the direction of increase in current.

5 Adjustable drawing shelf attached to the front legs of the table. In this
picture the shelf supports the stage of the projection microscope (fig. 121), and
a box of demonstration specimens.

The scale of the picture is indicated by the 10 cm. rule just above the table
drawer at the right.

If the tube of the microscope is large it is an advantage, but with
the small tube one can do much. If the ocular is not to be used,
then it is better to remove the draw-tube so that only the main
tube remains. One should be sure that the interior of the tube is
dull black (§ 370).

§ 394. Magic lantern with rods, and an ordinary microscope.—

If the magic lantern has the simple construction with rods and feet
(fig. 32, 33, 36) an ordinary microscope can be used with it as
follows: Remove the rods, bellows and projection objective, and
support the arc lamp and the condenser on a block which will lift
them high enough so that the microscope in a horizontal position
will be in the optic axis. Place all on a baseboard with guides
(fig. 146). Clamp the microscope to a suitable block with grooves
or cleats to enable one to move the block accurately along the
guides. When properly centered this form of apparatus w^orks
well.

§ 394a. For a water-cell one of the plane-sided glass boxes found on the
market can be used, or a cell can be prepared in the laboratory as follows:
Select some good plane and clear glass. For the ends of the box make two
strips about 2^2 cm. (i in.) wide and about 10 cm. (4 in.) long. For the sides
use two sheets about 10 cm. (4 in.) wide and II cm. (4^2 in.) long; and for
the bottom a rather thick sheet or strip about u cm. (4^2 in.) long and 3 cm.
(i '4 in.) wide. The pieces of glass are then put together by placing the bottom
on a level table and the other pieces in position and held in place by a string
or by narrow strips of gummed paper.

The joints are then gone over carefully with an artist's brush dipped in
Ripolin white paint or Valspar varnish. Each coat should be allowed to dry
thoroughly before adding the next, that is, for two to five days. Finally one
can add water to see if the joints are all tight. It" not, dry the glass box am',
then add more of the Ripolin paint or Valspar varnish.

CH. IX] ORDINARY MICROSCOPE FOR PROJECTION

265

FIG. 146. USE OF THE SIMPLE
MAGIC LANTERN CONDENSER
AND LAMP AND AN ORDINARY
MICROSCOPE FOR PROJECTION.

This is a magic lantern with
iron legs and rods for the support
and guidance of the parts (fig.
33). The slide-carrier bellows
and lantern objective with the
guide rods have been removed,
leaving only the condenser, arc
lamp and lamp-house. The short
tubes for the lamp are supported
at the left by the ordinary legs
of the apparatus. In front a
support of wood is used when
necessary . As the whole lamp and
condenser would be too low for
the axis of the microscope it is
raised on a block (BlockJ to the
proper height. There is a base-
board on which all the apparatus
is placed, and at the left there is
a track made of rods or tubes as
in fig. 158, 159 on which the block
supporting the microscope can
be moved back and forth in line
of the axis. For a water-cell, a
glass box made as described in
§ 394a is set on a block in the
path of the cone from the con-
denser.

Commencing at the left :
Arc lamp The hand-feed,
right-angle carbon arc lamp.
5. s Set screws.

This is also an excellent meth-
od of making small glass boxes for
experimental work where water is
the liquid medium. Such boxes
also have been used continuously
for months for observing the
growth of aquatic plants. If one
side is made of cover-glass, then
high powers of the microscope
can be used to study the growth
on the inside face of the cover-
glass.

We are indebted to Prof. Romyn
Hitchcock for the method of mak-
ing water-cells by the aid of
Ripolin paint.

266 ORDINARY MICROSCOPE FOR PROJECTION [Cn. IX

F. S. Feeding screws for the carbons.

V. A. Vertical fine adjustment for centering the crater.

L. A. Lateral fine adjustment for centering the crater.

Wt Supply wire to the upper carbon.

Wa Supply wire from the lower carbon through the rheostat (K).

R Rheostat in the wire from the lower carbon.

Rods The short tubes or rods supporting the lamp.

Z,,, L2 The left and right supports or legs of the lamp-rods.

Block^ The block on the baseboard to elevate the arc lamp and condenser
to the axis of the microscope.

Lamp-House The metal enclosure of the arc lamp.

V Ventilator of the lamp-house.

Condenser The two-lens condenser. It is supported by the front end of the
lamp-house.

/, 2 The two plano-convex lenses forming the condenser.

Water-cell The glass vessel with plane sides filled with water and placed in
the path of the cone of light from the condenser to absorb the radiant heat.

Microscope An ordinary microscope turned in the horizontal position. The
draw-tube and ocular have been removed, also the substage condenser.

Stage The stage of the microscope.

SS The substage condenser sleeve. The condenser has been removed.

Axis, Axis The principal optic axis of the condenser and the microscope.

() Objective.

// Handle for carrying the microscope.

c, f Coarse and fine focusing adjustments.

cl Clamp for holding the microscope to the block.

FM Foot of the microscope.

Block2 The wooden block supporting the water-cell.

Block3 The block to which the microscope is clamped. It moves back and
forth on the track (tr).

tr The rods on the baseboard serving for a track.

Base Board The board on which all the apparatus is placed.

§ 395. Stray light, and a water-cell. — For a water-cell, any glass
vessel with plane sides can be used, and it can be put between the
condenser and the stage of the microscope instead of between the
lenses of the condenser as in fig. 4, 167. For cutting off stray
light one can use a black cardboard shield, or a black disc may be
perforated and hung on the end of the tube of the microscope
beyond the focusing mechanism. For bellows between the con-
denser and stage, use a sheet of asbestos paper.

§ 396. The directions for using the ordinary microscope in
projection arc precisely as for the special microscope shown in fig.
121, and discussed in the first part of this chapter. As there is no
stage-cooling device one must be careful not to overheat the speci-
mens.

CH. IX] ORDINARY MICROSCOPE FOR PROJECTION 267

I h

FIG. 147. PROJECTION WITH THE MICROSCOPE IN A VERTICAL POSITION.

W vSupply wires from the outlet box (fig. 3).

r Rheostat of the theater-dimmer type.

t w Wires to the arc lamp from the switch.

/ a Fine adjustment screws projecting behind the lamp-house.

h f Hand-feed screws for the carbons of the arc lamp.

/ h Lamp-house. It is of sheet iron, but was left in position only a part of
the time, hence it appears transparent.

g Observation window opposite the crater.

C Triple condenser with water-cell (fig. 121).

a a Principal optic axis. The mirror of the microscope reflects the light
vertically along this axis and through the microscope, then the mirror or prism
over the ocular reflects it horizontally again.

m Mirror or prism over the ocular to reflect the light horizontally to the
screen.

sh Shield to cut off stray light.

b Baseboard with track for an optical bench.

a s Adjustable shelf for drawing.

268

PROJECTION OF HORIZONTAL OBJECTS

[CH. IX

PROJECTION OF HORIZONTAL OBJECTS

§ 397. As with the magic lantern, so with the projection micro-
scope some objects must be left in a horizontal position for projec-
tion. This requires that the microscope be in a vertical position.
As the light source is for giving light in a horizontal direction
(fig. 121), it is necessary to use a mirror or prism to reflect the
horizontal light upward through the vertical microscope and then
another mirror or prism above the microscope to reflect the vertical
light horizontally to the screen. This is shown in fig. 147, 175.

The ordinary mirror of the microscope serves very well for mak-
ing the light vertical, but for reflecting it horizontally to the screen
a prism or a plane mirror silvered on the face is best, as it gives a
single image, not a double image as would the ordinary glass mirror
silvered on the back.

§ 398. Avoidance of stray light with a vertical microscope.—

This is easily accomplished by using a vertical piece of blackened
cardboard just beyond the microscope as shown in fig. 147. If
light escapes from the sides one can use pieces of black cardboard
or asbestos to enclose the microscope more completely. Ordi-
narily, however, the single black shield beyond the microscope
will answer.

§ 399. Sample Objects Suitable for Projection with the Differ-
ent Objectives (see also §39ga).

PHOTOGRAPHIC TYPE OF OHJECTIVES (Micro-Planars, etc.)
Xo Tube (fig. 138)

Magnification with: —

Object

Size of Object

Objective

5 Meter
Screen

7.5 Meter
Screen

10 Meter
Screen

Brain Section

55 to 90 mm.
50 to 75 mm.

35 to 50 mm.
25 to 40 turn.

20 to 35 mm.

125 mm.
ioo mm.

70 mm.
60 mm.

50 mm.

35 mm.

30 mm.
20 mm.

39
48

72
lS5

101

142

167
253

6 1

107
123

147

210

250
370

80.6
98.4

143
1 68

202
285

330
4<>.S

Cerebellum and Brain
Stem

Longitudinal Section
of 40 mm. Embryo .
Section of Eve

Section of Injected
Kidnev

,V> I lour Chick Entire
Transaction of Human
Esophagus . .

15 to 25 mm.

10 to 20 mm.
51011 mm.

Appendix ( Homo) . .

CH. IX] OBJECTS FOR MICRO-PROJECTION 269

Large Tube (fig. 121)

Magnification with: —

Object

Size of Object

Objective

5 Meter
Screen

7.5 Meter
Screen

Jo Meter
Screen

Pyloric Stomach ....
Medulla and Olives
Scalp

20 to 30 mm.
15 to 25 mm.
12 to 22 mm.
10 to 14 mm.
8 to 12 mm.
5 to 8 mm.

70 mm.
60 mm.
50 mm.
35 mm.
30 mm.
20 mm.

85
101

142
I67

253

107
127

155
218

250
380

143
168
205
285
330
500

Human Spinal Cord
Thyroid .... ....

Adrenal

ORDINARY MICROSCOPIC OBJECTIVES
Large Tube (fig. 121)

Section of Lung or
Artery

4 to 5 mm.

1 6

mm.

1,22

488

650

Neural Plate of Am-
blystoma

2 to 4 5 mm

5 mm

18=;

SQO

7 CQ

Transection of
Trachea

2 to 3.7 mm.

mm.

-IS-l

700

QOO

Striated Muscle Longi
and Transactions
Nerve Cells in Spinal
Cord

i to 2.5 mm.
i to 2 mm.

8
6

mm.
mm

640

760

940
I I2O

1280
1460

Goblet Cells of Intes-
tine, Mucicarmin
Stain
Silvered Endotheliutn

i to 1.2 mm.
0.2 to 0.4 mm.

4
2

mm.
mm.

1080
2600

I6OO

3820

2180
5080

§ 399a. The preparations listed in the above table are simply examples of
objects which can be shown entire with the different objectives without oculars.
In practice any good microscopic preparation and many living things can be
shown with the projection microscope.

For the complete understanding of any specimen it is necessary to see it as
a whole and then by using higher and still higher powers (§391) to get views of
finer and finer details.

In demonstrating the finer details one can show but a very small specimen or
a small part of a large specimen. For large specimens it is a great advantage to
have objectives of different powers on a revolving nose-piece so that it takes
only a moment to turn from one to the other. If only the large condenser is
used (fig. 121) the objective remains practically stationary, but the specimen
must be on a movable stage so that it can be farther from the objective or
nearer to it depending upon the focal length of the objective (fig. 132).

If one uses substage condensers the stage remains stationary and a long
focus substage condenser is used for low powers and a short one for high powers
and the objective is placed at approximately its focal distance from the object.

It must be remembered that many living things are soon destroyed by the
intense light necessary for projection. While the circulation of the blood seems
an ideal demonstration with the projection microscope it is found in practise to
be a very poor way to demonstrate it. If this is tried the microscope in a ver-
tical position (fig. 147) is convenient. The screen distance should not be very
great (3 to 5 meters, 10 to 16 ft.). In the author's experience the demonstra-
tion of blood circulation under a microscope is vastly superior to anything that
can be done with a projection microscope.

270 EXHIBITION WITH PROJECTION MICROSCOPE [Cn. IX

CONDUCT OF AN EXHIBITION OR DEMONSTRATION WITH THE
PROJECTION MICROSCOPE

§ 400. What is said in Ch. I, § 21-40 is entirely applicable to
the projection microscope by substituting microscopic specimens
for the lantern slides. Only from the greater difficulty and pre-
cision demanded in using the projection microscope, it is impera-
tive that the operator be prepared, hence the greater necessity of
making certain that everything is in absolute order before the
lecture begins.

If any of the projection objectives (i. e., those of 125 to 20 mm.
focus) have iris diaphragms, open these as widely as possible.
Never try to project with the iris of the objective partly closed.

FIG. 148. SI.IDH-TKAV WITH MASKKD PREPARATIONS TO HE USED IN
PROJECTION. (About Y* size).

Three series are here represented on different sized slides.

The seetions to be shown are not eovered with the masking paper. The
numeral on the side give the number of the series (ser. 90, ser. 17, ser. 15). On
eaeh slide is also the number of the slide in the series as ser. 15, slide 57, 63, 67,
etc.

CH. IX] EXHIBITION WITH PROJECTION MICROSCOPE 271

An experiment with the iris partly closed and then wide open will
show the necessity of observing this rule.

The microscopic slides should be in order and properly masked
(§ 384) and marked in some way so that the operator can tell which
edge up they should be placed on the stage.

It is also a great advantage to have marked on the microscopic
specimen the objective or objectives that should be used in pro-
jecting it to bring out the structural details which it is desired to
show.

FIG. 149. SLIDE Box TO HOLD PREPARATIONS FOR DEMONSTRATION.
(Cut loaned by the Spencer Lens Company).

For ease in getting hold of the slides to be exhibited, either a
shallow tray can be used or a slide box (fig. 148, 149). As with
lantern slides, it is advantageous to have the microscopic specimens
so placed that they can be grasped easily, and put on the stage as
desired without hesitation.

Some teachers, including the senior author, have found it
advantageous to manage the projection themselves, giving the
explanations from the position of the lantern.

The best way to point out the parts in the screen image to be
especially noted is to have a slender pointer about two meters (six
feet) long, like the upper two-thirds of a bamboo fishing rod, and
to hold this out in the beam of light. The shadow appears on the
screen sharply, and one can point out details with the same clear-
ness as by using a pointer on the screen. It is easier also, because
the speaker does not get his eyes dazzled by looking into the light
beam, as so often happens when standing near the screen in the
usual lecture position.

SPECIAL DEMONSTRATIONS WITH HIGH POWERS
§ 401. Substage condenser in projection. — As indicated in
§ 359 the authors of this book believe that projection for large
audiences and with low objectives is best accomplished without

272 HIGH POWER MICRO-PROJECTION [Cn. IX

substage condensers, and without oculars; but they realize that
in laboratory work and for some special lectures to small classes it
is of the highest advantage to be able to show pictures of photo-
graphic sharpness in all details. For this it is necessary to use, first
of all, a substage condenser which will give a light cone of sufficient
aperture for the details; and secondly there must be a proper
screen, i. e., the screen must be very white and very smooth, but
not shiny (§ 409, 621). White cardboard answers well. Finally
there must be an ocular used, and the observers must be near
enough the screen to see the fine points.

FIG. i5oA. ACHROMATIC, SUBSTAGE CONDENSER WITH

CENTERING SCREWS.
(From Zeiss' Catalogue).

There has been a segment of the condenser cut away to show the construc-
tion.

The centering screws (c-s, c-x) enable the operator to get the condenser in
the optic axis of the microscope. The iris diaphragm for this condenser is
between the lower and middle combinations, not below the condenser as with
the Abbe form.

This form of condenser is especially desirable for projection and for photo-
micrography.

The substage condenser for micro-projection must either be of a
special form to use with the main condenser of the apparatus or
special means must be employed to utilize the light cone from the
main condenser when the ordinary substage condenser is used.

This is because the substage condenser ordinarily used on micro-
scopes is designed for approximately parallel beams of light, not
for those markedly converging or diverging. By examining the
figures of the light cone from the main condenser it will be seen

CH. IX]

HIGH POWER MICRO-PROJECTION

273

that the cone of light is converging to the focal point and diverging
beyond that point (fig. 122, 132 and 320-323). If the converging
cone is used the substage condenser brings it to a focus too soon
and if the diverging cone, then the substage condenser brings it
to a focus too far beyond it.

§ 402. Methods of rendering converging or diverging light
parallel. — There are two principal ways of utilizing the light cone
from the main condenser.

Object

FIG. 1506. ABBE SUBSTAGE CONDENSER SHOWING PARALLEL AND
CONVERGING INCIDENT LIGHT.

In this form of condenser the iris diaphragm is below both condenser lenses
(compare fig. 150).

With parallel, incident light the condenser focuses the light just above the
condenser, with converging light the focus is within the upper lens and the
light is diverging on leaving the upper lens.

o, o Object.

Objective The front lens of the projection objective.

A. Rendering the converging cone of light approximately
parallel by means of a concave lens. As it is desirable to use all the
light in the cone, the concave lens is put in the cone where its
diameter is slightly less than the diameter of the substage con-
denser, that is about 25 mm. (i in.). The trial glasses used by the
oculist are excellent for the purpose. A fork with stem is desirable,
and this is placed in the socket for the mirror stem. This brings
the fork carrying the spectacle lens near the substage condenser.
Concave spectacle lenses of 10 to 20 diopters (100 to 50 mm., 4 to 2
in. focus) have been found excellent. The microscope for projec-
tion is so placed that the fork carrying the concave lens is about

274 HIGH POWER MICRO-PROJECTION [Cn. IX

2^ to 3 cm. (t to i J/2 in.) from the focus of the converging cone.
The concave lens will render the converging light approximately
parallel, and this cylinder of light is small enough to enter the
substage condenser. By a small manipulation of the screw of the
substage condenser bringing it slightly nearer the specimen or
slightly farther from it the most brilliant screen image can be pro-
duced. A slight change in the position of the substage condenser
often works wonders.

FIG. 151. RELATION OF THE APERTURE OF THE LIGHT FROM THE COX-
DENSER TO THE APERTURE OF THE OBJECTIVE.

(From Nelson, Jour. Roy. Micr. Soc.).

A The cone of light from the condenser just fills the aperture of the objec-
tive (B).

B Back lens of the objective entirely filled with light.

C The cone of light from the condenser is not great enough to fill the aper-
ture of the objective (D).

D Back lens of the objective lighted by the condenser (C).

The dark ring shows the aperture of the objective not lighted by the con-
denser.

B. Rendering the diverging cone of light approximately parallel
by the use of a convex lens. If a convex lens is placed in the path
of the diverging cone at its focal distance from the focus of the main
condenser, the light will be rendered parallel. In order to have a
cylinder of light of the right size to enter the substage condenser a
convex lens of the proper focal length and diameter must be used.
Trial lenses arc excellent. Those of 10 and 20 diopters (100 and
50 mm., 4 to 2 in. focus) arc excellent for the main condenser with
a focus of 150 to 200 mm. (6 to 8 in.). The microscope must be
put in such a position that the trial lens in the fork before the sub-
stage condenser shall be at its focal distance from the focus of the
main condenser. The diverging cone of light will be made approxi-

CH. IX]

HIGH POWER MICRO-PROJECTION

275

mately parallel (fig. 1536), and by slight adjustments of the sub-
stage condenser brilliant images are produced.

FIG. 152. MICROSCOPE FOR PROJECTION AND FOR DRAWING.

W — i The negative supply wire from the outlet box (fig. 3).

W + I The positive supply wire from the outlet box.

S Double-pole, knife switch.

W — 2 Wire from the switch to the binding post of the lower carbon.

W + 2 Wire from the knife switch to the rheostat.

W + 3 Wire from the rheostat to the upper carbon (+ // — C).

ri, r2 The two binding posts of the rheostat.

Rheostat The controlling device for the current.

ILC Incandescent lamp cord.

Inc. Lamp The incandescent lamp with a wire lamp guard.

This lamp is for use in working about the projection apparatus. It is con-
nected to the supply wires at their connection with the switch so that the
incandescent lamp will burn whether the knife switch is open or closed (sec also
fig. 2, 4).

Radiant The arc lamp.

S-\-, S — The set screws for the carbons.

HC, VC The horizontal or upper and the vertical or lower carbons.

Condenser The triple-lens condenser with water-cell in the parallel beam
between the two plano-convex lenses.

Axis, Axis The optic axis of the condenser and the microscope.

Substage Condenser The achromatic condenser under the stage of the
microscope.

P L The concave lens for making parallel the converging light from the
large condenser before it enters the substage condenser.

St Stage of the microscope.

Objective The projection objective.

Ocular The ocular of the microscope used in projection.

Af2 The mirror or prism placed just beyond the ocular when it is desired
to reflect the light downward.

Screen Image The image projected upon the white screen by the projection
microscope.

276

HIGH POWER MICRO-PROJECTION

[CH. IX

Bl. R The block carrying the radiant on the optical bench.

Bl C The block carrying the condenser on the optical bench.

Bl M The block carrying the microscope on the optical bench.

Base Board The board bearing the track made of rods and serving as an
optical bench.

Projection Table The table supporting the apparatus and holding it at the
proper height for use.

The above method refers especially to high powers — objectives
of 2 to 8 mm. equivalent focus. For powers lower than those just
mentioned one can get better results by the use of a main condenser
with a second element of 200 to 150 mm. focus and no substage
condenser, or by adopting the method given below or in § 403 .

Substage
p [_ Condenser

FIG. 153. DIAGRAMS TO SHOW METHODS OF PARALLELIZING THE CONE
OF LIGHT FROM THE MAIN CONDENSER.

A Method of parallelizing the converging cone of light from the main
condenser by means of a concave lens within the focus (/).

B Method of parallelizing the diverging cone of light from the main con-
denser by means of a convex lens beyond the focus (/).

Arc Supply The right-angled carbons of the arc lamp.

L, L2 The first and second elements of the triple, main condenser.

Water Cell This is to remove the radiant heat.

Axis The principal axis on which all the parts are centered.

/ The principal focus of the second element of the main condenser.

P. L. Parallelizing lens. Concave in A, Convex in B.

Substage Condenser This is the first or lowest element of the substage con-
denser of the achromatic form (fig. i5oA). See also fig. 150 B. for the Abbe
form of substage condenser.

CH. IX]

HIGH POWER MICRO-PROJECTION

277

Finally if one uses a main condenser with a focus of 30 or 38 cm.
(12 to 15 in.) excellent results can be obtained with all powers (16
to 2 mm.) by so placing the microscope that the converging cone of
the main condenser shall enter the substage condenser at a point
where the light cone is of about the diameter of the substage con-
denser (fig. I54A-B). It may be necessary to raise or lower the
substage condenser slightly to obtain the most brilliant screen
image.

Fair results can also be obtained in this way by using main con-
densers of 15, 20 and 25 cm. (6, 8, 10 in.) focus, but much more

Substage
Condenser

FIG. 154.

DIAGRAMS TO SHOW THE POSITION OF THE SUBSTAGE CONDENSER
WHEN NO PARALLELIZING LENS is USED.

A The substage condenser is within the focus (/) at a point where the long,
light cone is of about the same diameter as the substage condenser.

B The substage condenser is beyond the focus (f) of the long focus main
condenser, at a point where the diverging cone is of alaout the same diameter as
the substage condenser. This is the better position for the s.ubstage condenser
of the ordinary microscope.

Arc Supply The right-angled carbons of the arc lamp.

Lr L2 The first and the second elements of the main condenser.

Water Cell This is to remove the radiant heat.

Axis The principal axis on which all the parts are centered.

/ The principal focus of the second element of the main condenser. In both
cases the focus is long as compared with fig. 153.

Substage Condenser This is the first or lowest element of the substage con-
denser. It is of the achromatic type (fig. 150 A). See figure 150 B for
the Abbe form of substage condenser with parallel and with converging light.

278 HIGH POWER MICRO-PROJECTION [Ca. IX

brilliant pictures can be produced by using also a parallelizing lens
as indicated in § 402 A.

If one has an optic bench apparatus (fig. 121, 158, 159) one can
get good results with the condensers of all foci by placing the
microscope so that a diverging cone of light enters the substage
condenser (fig. 1546). It will then be necessary to lower the
substage condenser slightly for the higher powers.

§ 403. Kohler method of using the substage condenser. — The

general principle is shown in fig. 170. The microscope is moved
toward the main condenser until the focus is at the iris diaphragm.
One can tell when the main condenser is focused on the iris dia-
phragm in the same way as that in focusing on the black hood of the
objective (§ 375) viz., by noting when the image of the crater and
the tip of the lower carbon appear on the iris. After the image is
focused on the iris diaphragm the iris is opened to admit the cone
of light, and the substage condenser is raised or lowered slightly to
get the most brilliant light. As one can see by the diagrams of
light cones and the plates of the light rays and the light cones, the
light is diverging beyond the focus so that diverging and not
parallel light enters the substage condenser. As the condenser
cannot focus diverging light at the same level that it would focus
parallel light it may be necessary to lower the substage condenser
somewhat to get the most brilliant image with high powers. Fur-
thermore, if a concave lens of 10 to 20 diopters is put in the fork as
described in § 402 A the image will be markedly brighter unless a
very long focus main condenser is used (fig. 171). (See also Ch.
XIV, § 864).

§ 404. Aperture of the substage condenser. — The purpose of
the substage condenser in projection, as in direct observation
with the microscope, is to increase the aperture of the illuminating
cone. And as it is now one of the fundamental doctrines, that the
resolution or making visible of minute details depends directly
upon the aperture of the objective used, naturally as much as
possible of the aperture of the objective is employed. For this,
the substage condenser diaphragm should be wide open, so that the

CH. IX]

HIGH POWER MICRO-PROJECTION

279

FIG. 155.

THE EFFECT OF USING AN IRIS DIAPHRAGM IN THE CONE OF
LIGHT FROM THE MAIN CONDENSER.

The second element of the condenser is shown at the top. The focus of4the
cone of light from the condenser is shown at F, the axis by A .

B At the right are shown in millimeters, three diameters of the cone of light
with three different openings of the iris diaphragm (22, 33, 44 mm.)

C At the left are shown the apertures corresponding with these openings
in the iris diaphragm (23°, 34°, 45°). The aperture of these openings is also
shown above the circles.

One can see by this diagram what an enormous amount of light is lost by
making the illuminating cone smaller.

280 HIGH POWER MICRO-PROJECTION [Cn. IX

entire beam of light from the lamp condenser may enter. Then,
just as in ordinary observation, one can often make the contrast
more striking by cutting down the aperture somewhat by closing
more or less the substage condenser diaphragm. It must not be
cut down too much, for that will render the image dim and defeat
the very purpose of the substage condenser.

As a general statement, much more of the aperture of the
objective can be used in projection than in ordinary direct observa-
tion in the microscope. Naturally, objectives of relatively large
aperture give the more brilliant images (see § 855).

§ 405. Oculars to use in projection. — Generally speaking, only
low powers arc used (x2, X4, x8). The lower the power the more
brilliant the image. Compensation oculars have been found better
than the Huygenian. A compensation ocular as high as xi2 gives
brilliant images for short screen distances.

One should not forget that the ocular, when used in projection,
is really a second projection system, and hence the image will be
erect on the screen (fig. 207).

§ 404a. Centering the substage condenser. — As the substage condenser
becomes one of the optical elements in projection, its' principal optic axis must
be centered on the common axis of the entire apparatus.

It is assumed that the microscope without the substage condenser has been
properly centered as directed in § 374-375.

To center the substage condenser, use the ocular and objective (x4 ocular,
8, 10 or 1 6 mm. objective), remove the bellows if present (fig. 133), place a piece
of white cardboard at about 45 degrees as shown in fig. 1 16, between the large
condenser and the substage condenser, and light the cardboard well with a
mazda lamp. This will give the light for the microscope.

Now put a preparation on the stage and focus the microscope as for ordinary
observation. Remove the specimen and close the substage iris diaphragm
nearly up. With a pocket magnifier examine the eye-point or Ramsden's disc
(fig. 127 E P) beyond the ocular. This disc of light appears as if on the back
lens of the objective. If the iris is properly made and the substage condenser
is centered with the objective and ocular, the center of light will appear to lie
exactly in the middle of the back lens of the objective (fig. 151). If the sub-
stage is not in the optic axis then the disc of light will appear eccentric; and
if the substage condenser is markedly off the center the spot of light will make
a break in the black ring on one side as shown in fig. 30, 1-4. If it is only
slightly off center, the disc of light will seem to be surrounded by a dark ring
of unequal width. If the substage condenser is not found to be correctly
centered, the centering screws (fig. 150) must be used to move it slightly until
the disc of light is central as shown in fig. 151.

The Abbe condenser found on most microscope's has no centering screws.
Tlu' makers center the instrument carefully and fix it in position. If it is
found badlv out of center it is best to return it to the makers for adjustment.

CH. IX]

HIGH POWER MICRO-PROJECTION

281

§ 406. Range of objectives to use with a substage condenser. —

Objectives of 16, 12, 10, 8, 6, 4, 3, and 2 mm. equivalent focus
are used with the substage condenser. For objectives of longer
focus than 16 the substage condenser of the ordinary form is
rarely used. Either a special long focus substage condenser is used
or the ordinary one is turned aside and the cone of light from the
large condenser used as directed above (§ 376).

§ 407. Change in position of the substage condenser for differ-
ent objectives and thickness of slides. — For the highest powers

FIG. 156. PROJECTION MICROSCOPE OF ZEISS.

(From the 4th edition (1899) of Zeiss' catalogue of instruments and appliances
for Photo- Micrography and Projection).

This projection apparatus, which in its main features was described in Zeiss
microscope catalogue No. 28, (1889), and No. 29 (1891), consists of an
optical bench on which all of the parts needed move separately so that any
desired arrangement can be made for projection of large objects with low power
or smaller objects with high powers.

Commencing at the right:

/ Arc lamp with inclined carbons, and with fine adjustments to center the
source of light (crater of the positive carbon).

2 First element of the condenser consisting of a meniscus and a plano-
convex lens, to render the light beam parallel.

3 Water-cell.

4 Second clement of the condenser to converge the light-beam.

5 Iris diaphragm to cut down the light-cone if desirable.

6 Stage and substage condenser.

7 Projection objective and fine focusing device. In the figure no ocular
is used.

This arrangement of the parts enables the user to employ a microscope with
oculars or amplifiers, or the simple apparatus here shown, or photographic
objectives.

282

HIGH POWER MICRO-PROJECTION

[Cn. IX

(2-3 mm. oil or water immersion) and for the 3 and 4 mm. dry
objectives the condenser is usually very close up to the slide, so
that the object is practically in the focus of the beam of light.

For the 8, 10, 12, and 16 mm. objectives the substage condenser
must be separated sufficiently from the specimen to light the whole
field.

It will be found in practice that one must be more precise in
keeping the substage condenser at just the right level for projec-
tion than for ordinary direct microscopic observation. Hence, it
will be found that for a thin slide the condenser, even for high
powers, may need to be separated slightly from the object, while if
the slide on which the specimen is mounted is thick, the condenser
may need to be as close to it as possible.

§ 408. Screen distance for high power projection. — This
should not be excessive, for even in the darkest room the image will

FIG. 157. LEWIS WRIGHT'S PROJECTION MICROSCOPE.

(From Wright's Optical Projection).
C Condenser of three plano-convex lenses.
-•1 Alum eell for absorbing radiant heat.

P Plano-concave lens of highly dispersive glass to aid in correcting the
aberrations of the condenser and to render the light parallel.

S C Substage condenser. For low powers but one lens is used.

.V Stage.

() Object and objective.

A M Amplifier.

F Fine focusing adjustment.

R2 Rack and pinion, coarse focusing adjustment.

.ft, Coarse adjustment for the substage condenser.

CH. IX] HIGH POWER MICRO-PROJECTION 283

be too dim if the screen distance is over two or three meters (6 to 10
feet).

With objectives of 4, 6, 8, 10 mm. and lower powers, one can
use a greater distance with satisfaction, but for the oil and water
immersions, a distance of one to two meters (3 to 7 feet) gives the
best results. This, of course, refers to minute details. If one
simply wants size, the limit is much greater; but that is not
scientific projection.

§ 409. Kind of screen for high power projection. — The prin-
ciple enunciated by Goring and Pritchard must be kept in mind.
The whiter and smoother the screen, the more brilliant the image
and the clearer the details. Nothing has been found better by
the writers than smooth, white bristolboard. This is also very
easily procured, and when it becomes dirty or discolored, it can be
cheaply replaced. We have also found white cardboard in sheets
of 71 x 112 cm. (28 x 44 in.) good.

§ 410. Specimens to project with high powers. — These must
have in a good degree the qualities of specimens giving clear images
to the eye in direct, microscopic observation. That is, they should
have definite outlines and contrasting colors; for example, well
stained preparations of red and white blood corpuscles mounted in
balsam and projected with the oil immersion objective.

Preparations of bacteria, well stained and mounted in balsam,
may be projected with the oil immersion.

Thin histologic and embryologic sections, if well stained and
mounted in balsam, answer well. The nuclei of cells show well,
also the band of cilia in a ciliated epithelium, and the cells in
mitotic division. Naturally, well prepared plant preparations
have the advantage of very sharp outlines.

§ 411. High powers with the vertical microscope. — Any prep-
aration which can be projected well with high powers may be used
on the vertical microscope (§ 397). Of course, there is some loss
of light in the double reflection required (fig. 147, 176), but if the
screen is within two meters (6 ft.) distance and the observers few
and close, results are fairly satisfactory. For example, if one has

284 USE OF ALTERNATING CURRENT [Cn. IX

water in which there are many large bacteria and infusoria, a most
striking picture on the screen is made. For this projection a water
immersion is excellent. An oil immersion may also be used and
also a dry objective of 4 to 6 mm.

For securing a large field, the objective and amplifier are better
than an objective and ocular (§355).

USE OF ALTERNATING ELECTRIC CURRENT WITH THE PROJECTION

MICROSCOPE

§ 412. It is unfortunate that it should ever be necessary to use
alternating current in micro-projection ; but if that is all which can
be obtained, much can be accomplished with it by skillful handling.

(For a discussion of the difference between direct and alternating
current and the relative amount of light yielded by the two, also
for the possibility of getting direct from alternating current by
means of a motor-generator set, or by a "current rectifier," see Ch.

xin, § 681-683, 751-752).

§ 413. Wiring the Arc Lamp. — This is exactly as for the magic
lantern, (fig. 3). And as with all arc lamp \vork there must always
be present some form of regulating device like a rheostat or induc-
tor (fig. 145, 197, § 748).

§ 414. Arrangement of the carbons. — For micro-projection the
carbons should always be at right angles, and the light will then be
almost wholly from the upper or horizontal carbon (fig. 191). As
this is in the optic axis and looks directly toward the condenser it
is the most satisfactory source of light available with this as with
the direct current lam]) for micro-projection. This is because the
image of the crater of one carbon is as large as can be received by
the projection objective.

It is especially necessary for micro-projection that the lamp have
fine adjustments to keep the crater exactly centered (fig. 3, 14-6).

§ 415. Amount of current necessary. — As the alternating
current gives less than one-third as much available light as the
direct current one cannot project with such high powers nor pro-
duce so large screen images as with the direct current (fig. 302).

CH. IX] MICRO-PROJECTION WITH HOUSE CURRENT 285

For example, with direct current of 10 amperes one can accom-
plish a great deal in micro-projection if the manipulation is skillful.
To get equally brilliant results with alternating current would
require 30 to 40 amperes of current. The heating is also excessive
with the high amperages. (See Ch. XIII, § 768).

If alternating current must be used for projection with the micro-
scope, one should not expect too much, but get as good results as
possible by observing carefully the conditions giving good screen
images, viz., apparatus in perfect order and alignment on one axis;
a good screen and a dark room.

It is not wise, according to our experience, to try to use more than
25 amperes alternating current for micro-projection, and it is better
as regards the specimens and apparatus, to be satisfied with the
results which can be obtained with 15 to 20 amperes. An arc
lamp with carbons at right angles is to be preferred.

§ 416. Centering the apparatus on one axis, separating the
elements properly and the conduct of an exhibition are precisely
as for the direct current light. The results, however, cannot be
made as satisfactory, although, as stated above (§ 412), by care and
skill much can be accomplished.

THE PROJECTION MICROSCOPE ON THE HOUSE ELECTRIC LIGHTING

SYSTEM

§ 417. As with the magic lantern (§ 127), the small electric
current (4 to 6 amperes) available from the regular house lighting
system gives very gratifying results.

Small carbons (6-8 mm. diam.) are employed and either one
of the small arc lamps especially designed for the purpose or an
ordinary arc lamp with adapters or bushings can be used.

Of course the direct current is much more effective, but even with
the alternating current, which is now so common in lighting sys-
tems, successful projection with the microscope can be done.

The small carbons form a minute crater, and thus approximate
closely to a point source of light, which is the ideally perfect source
from the optical standpoint. From our experience this is a

286 MICRO-PROJECTION WITH SUNLIGHT [Cn. IX

better source of light for the microscope than the lime light, and
now electric lighting is so common that one can use almost any
room in a house or laboratory at night for a projection room.

Of course one should not expect too much, but for small audiences
— 50 to 100 — and with a moderate sized screen — 2-3 meters —
(6-10 ft.) astonishingly satisfactory micro-projection can be done.

§ 418. Hand-feed and automatic lamps for small currents. —

Most of the small current lamps are of the hand-feed type whatever
the form of the electric current (a. c. or d. c.) but some automatic
ones have been constructed (fig. 44, 205). Large arc lamps may,
by special arrangement, be so adjusted that they give good results
automatically from 5 to 25 amperes (e. g. the automatic lamp of
A. T. Thompson and of the Bausch and Lomb Optical Co., fig.
186, 187).

As for the usual lantern arc lamps, only those for the direct
current have hitherto been constructed of the automatic form.

For a full discussion of the wiring and setting up of the apparatus
see Ch. Ill and XIII, § 128 and fig. 3, 40, 45.

Do not forget that a rheostat or ballast of some kind must be
used on every outfit where an arc lamp is employed (§ 129, 748).

Remember the precautions for turning on and off the current
when using the house circuit (§ 133). For a further use of these
small currents in drawing, see Ch. X, § 486.

MICRO-PROJECTION WITH SUNLIGHT

§ 419. This was the first light used for micro-projection and
remains the best. If it were only available at all times it would
be universally employed.

§ 420. Arrangement of the parts of the apparatus. — For the

heliostat to keep the sunlight in a constant position one should
consult Chapter VI.

After getting parallel light from the sun in a constant position,
then one should use the proper condenser (fig. 74). The remainder
of the apparatus is precisely as for the projection so far discussed
and all the requirements of centering and arranging at the proper

CH. IX] MICRO-PROJECTION WITH LIME LIGHT 287

distance from one another are as for the electric light described
above.

As the spot of light must remain in exactly the same place to be
received by the small lenses of the projection objective, it is neces-
sary to regulate the hand heliostats oftener than for the magic
lantern.

It may also be necessary to make slight corrections in the mirror
of the clock-driven heliostat from time to time. The law is : The
axial ray must correspond with the optic axis of the apparatus.

§ 421. Use of a water-cell. — The radiant energy of the sun is
so great that a water-cell to remove as much of it as possible except
the luminous part (§ 844) is as desirable as with the electric light.
It is also desirable to have a specimen cooler (fig. 121).

PROJECTION MICROSCOPE WITH THE LIME LIGHT

§ 422. The management of the lime light for the projection
microscope is exactly as for the magic lantern (see § 1 63 , 1 64) , only
more attention will be necessary to keep the best possible light all
the time. The image of the luminous spot should be focused on
the hood of the objective as for the electric arc. While there is
not so much danger from overheating as with the electric light or
sunlight, it is desirable to use a large water-cell. The stage cooler is
also an advantage. For the correct form of a condenser see § 363.

As the intrinsic brilliancy of the lime light is less than that of
sunlight or the electric light one must not expect so much of it as
of them.

§ 423. Other sources of light are insufficient to give good
micro-projection except in a very limited degree, and for some
special purposes. See under drawing, Ch. X, § 463.

HOME-MADE PROJECTION APPARATUS

§ 424. Projection table. — For all kinds of projection the table
should be of convenient height, so that the operator can stand dur-
ing the exhibition. A height of 100 centimeters (40 inches) is
suitable for most persons. The size of the top varies greatly with

288

HOME-MADE PROJECTION APPARATUS

[CH. IX

the work to be done. For the work of micro-projection, drawing,
etc., contemplated in this and the following chapter a table of the
following dimensions has served admirably: Height, 100 centi-
meters (40 in.). Size of top 125 cm. (50 in.) long; 50 cm. (20 in.)
wide. The legs are about 5 cm. (2 in.) square, and have large
screw eyes in the lower ends for leveling. The table should be

FIG. 158. HOME-MADE OPTICAL BENCH.

t t t t The track of rods or tubes on the baseboard.

Radiant The block carrying the arc lamp.

as Asbestos paper between the track rods at the arc lamp end of the optical
bench.

Condenser The block carrying the condenser.

Stage The block carrying the stage of the projection apparatus or the lan-
tern-slide holder.

Microscope The block carrying the projection microscope or the lantern
slide or other projection objective.

//// The railing flanges holding the sockets.

base The baseboard.

rigidly made so that there will be a minimum of vibration. If the
table vibrates there is a disagreeable trembling of the screen image.
(For pictures of such a table see fig. 133, 182).

Carrying out the precautions against reflections from light sur-
faces, the table is made dull black or brown. This is easily
accomplished by using some dull black paint like "dead-black
Japalac" or other dull black, or dull brown paint, thinned some-
what with turpentine.

The anilin black stain used for laboratory tables is also most
excellent (§ 424a).

To the projection table should be fastened the rheostat, and the
ammeter, if one is used ; also the lamp switch and the incandescent
lamp (fig. 133). Then the table can be moved from one place to

CH. IX] HOME-MADE PROJECTION APPARATUS 289

another and be ready for projection by connecting the supply wires
for the lamp to the line at any outlet box (fig. 3).

§ 425. Lathe bed or optical bench for projection apparatus. —

For the projection microscope, and for general experimental pur-
poses there is no form of projection outfit so suitable and flexible
as the lathe-bed type. It is easily and cheaply constructed. Any
teacher with a little ingenuity and the aid of a tin-smith, black-
smith, plumber, and carpenter or cabinet-maker, can construct all
except the optical parts. The optical parts can be obtained of
dealers or manufacturers of microscopes and projection apparatus.
There is this further advantage in getting up a projection outfit,
the person who does it will know enough to use it. He will not

§ 424a. Stain for laboratory tables. — During the last few years an excellent
method of dying wood with anilin black has been devised. This black is
lustreless, and it is indestructible. It can be removed only by scraping off the
wood to a point deeper than the stain has penetrated.

It must be applied to unwaxed or unvarnished wood. If wax, paint or var-
nish has been used on the tables, that must be first removed by the use of
caustic potash or soda or by scraping or planing. Two solutions are needed :

SOLUTION A

Copper sulphate 125 grams

Potassium chlorate or permanganate 125 grams

Water 1000 cc.

Boil these ingredients in an iron kettle until they are dissolved. Apply two
coats of the hot solution. Let the first coat dry before applying the second.

SOLUTION B

Ani^in Oil 1 20 cc.

Hydrochloric Acid 180 cc.

Water 1000 cc.

Mix these in a glass vessel putting in the water first. Apply two coats with-
out heating, but allow the first coat to dry before adding the second.

When the second coat is dry, sandpaper the wood and dust off the excess
chemicals. Then wash the wood well with water. When dry sandpaper the
surface and then rub thoroughly with a mixture of equal parts turpentine and
linseed oil. The wood may appear a dirty green at first but it will soon become
ebony black. If the excess chemicals are not removed the table will crock. An
occasional rubbing with linseed oil and turpentine or with turpentine alone will
clean the surface. This is sometimes called the Danish method, Denmark black
or finish. See Jour. Ap. Micr., Vol. I, p. 145; Bot. Zeit., Vol. 54, p. 326, Bot.
Gazette, Vol. 24, p. 66, Dr. P. A. Fish, Jour. Ap. Micr., Vol. VI., pp. 211-212.
The Anatomical Record, Vol. V. 191 1, pp. 145-146. (Quoted from The Micro-
scope, by Gage, nth cd. 1911, pp. 282-283).

2QO HOME-MADE PROJECTION APPARATUS [Cn. IX

expect the apparatus to do the work of a machine, and also to
supply all of the intelligence to enable it to do so.

§ 426. Baseboard and track. — For the lathe-bed carrying all
the apparatus (fig. 121, 159) a flat board about 2 cm. (J/gin.) thick
is used for the base. The width and length can be made to suit
the apparatus designed. The dimensions for that shown in fig.
i58-i59are: Length 125 cm. (4 ft.); width 22.5 cm. (8^ in.).

The track which serves as a guide to the blocks bearing the differ-
ent pieces of apparatus (fig. 121) is best made of two brass tubes or
rods 12 mm. (^ in.) in diameter and the full length of the base-
board (§ 42 6a).

§ 427. Fixing the track to the baseboard. — For this, holes
should be bored through the tubes or rods, being careful to have
the holes parallel so that there will be no torsion or twist when the
tubes are fastened to the board. If rods are used the screw holes
must be countersunk. If tubes are used then the upper wall
should have a larger hole than the lower and a slender screw driver
used, (fig. 159 ts), then the screw head goes through the upper wall
and presses against the lower side only.

One tube or rod is fixed firmly to the base, thus : With a straight
edge like a T-square make a straight line on the baseboard where
the track is to be laid and then fasten the one track accurately along
this line so that it will be perfectly straight.

Now for the other track lay it as follows : Use apparatus blocks
(§ 428) near the ends of the baseboard and put the loose rod in
place. Press the block down firmly so that the loose track will be
forced into the groove. Put screws in the end holes, but do not
screw them down firmly. If there arc intermediate holes as in
fig. 158-159 move a block near the hole, press it down firmly and
then put in a screw, but do not screw it in firmly.

§ 426a. For the rods, one ean procure the thin, polished or nickeled brass
tubing used for railing, or the thick brass tubes used instead of iron tubing. The
measurement given means the total diameter. Of course one can use any
desired diameter by varying the' size of the V-shaped notches in the apparatus
blocks (fig. 158 A) or the position of the cleats (fig. 159). If brass tubing is
employed for the track, the size known to the plumber is that of the bore, not
the outside diameter. Tubing \vith x'4th or 9'ijtli inch bore answers well. The
outside diameters will be 10 and 13.5 mm. (13/32 and 17/32 in.) respectively.

CH. IX] HOME-MADE PROJECTION APPARATUS 291

This will make a track along which the blocks will move freely.
If both tracks were firmly fixed the blocks would have to be con-
structed with extreme precision or the blocks would bind. They
would also bind if the tracks were not perfectly parallel at all
points. The loose track gives slightly and thus compensates for
any little irregularity of the track or apparatus block.

§ 428. Apparatus blocks. — These are shown in figures 158, 159.
They must be sufficiently heavy so that the various pieces of appara-
tus they carry will be steady ; and finally the sockets for receiving
the stems of the apparatus must be on the blocks in a position so
that the parts like the stage and the microscope can be brought
sufficiently close together.

Size and weight of the different blocks for the apparatus figured
(fig. 158, 159):

1. Arc lamp block. 12^ x 12^ cm. (5 x 5 in.) ; weight i kilo.
(albs.), (fig- 158).

2. Condenser block. 12^ x 10 cm. (5 x 4 in.) ; weight 2 kilos.
(4# Ibs.), (fig. 158).

3. Stage block, 12^ x 6 cm. (5 x 2 in.) ; weight, i kilo. (2 Ibs.),

(fig- 158).

4. Microscope block, 12^2x10 cm. (5x4 in.); weight, 2-3
kilos. (4-6 Ibs.), (fig. 158).

5. Block for lantern-slide carrier, 12^2x6 cm. (5 x 2 in.);
weight Y2 kilo, (i lb.), (fig- 158).

6. Block for lantern objective, or a photographic objective,
(fig. 158), i2>^ x 10 cm. (5x4 in.); weight, 2 kilos. (4^ Ibs).

7. Block for horizontal microscope, 17 x 12^ cm. (7x5 in.),
weight 2^/2 kilos. (5^ Ibs.), (fig. 145).

§ 429. Construction of the apparatus blocks. — If one has the

facilities of a machine shop and foundry at his disposal these
apparatus blocks may be made of cast iron, smoothed and grooved
on a planer. In like manner the lathe bed with V's can be made
(fig. 134). Lacking these facilities one can prepare blocks of wood
which will answer almost perfectly as follows: Select some fine
grained board 2 cm. to 2.5 cm. thick (7/$ to i in.), and cut it into

HOME-MADE PROJECTION APPARATUS

[Cn. IX

blocks of the required size for the special purpose. The blocks can
be made as heavy as desired by adding sheets of lead (fig. I58A).

For the guides to follow the track, one can make V-shaped
grooves, or more easily, strips of the proper thickness can be screwed
to the block (fig. 159). One of the strips should be screwed tightly
to the block, and the other should have screw holes through the
strip considerably larger than the screws, then it will be possible to
make slight changes in position to get an exact fit. When in the
exact place desired the screws can be set firmly. As the large holes
in the strips will be larger than the heads of the screws, metal
washers should be employed (fig. 159).

§ 430. Sockets for the stems of the apparatus. — There is first
screwed to the top of the block, a railing flange, and into this is
screwed a short tube of the size to receive the stem or post of the

FIG. I58A. SHIELD FOR A PROJECTION ORJECTIVE.

This shows the method of supporting an objective in a shield. The shield is
supported by a bolt with a fan shaped end (p).

The bolt or stem enters the socket and is held in place by a screw (s).

U Apparatus block composed of U-ad sheets above and a block of wood
below. The block of wood has V-shaped grooves for sliding along the track.

b Baseboard with track, end view.

CH. IX]

COMBINED PROJECTION

293

apparatus. This tube has a set screw in the side to hold the post
at any desired level (fig. is8F). In order to be able to perfect the
centering of the apparatus, the screw holes in the flanges are made
larger than the screws so that by loosening the screws the flange
can be shifted slightly from side to side. If necessary one can use
washers to increase the size of the screw heads, so that the holes
in the flange can be quite large.

§ 431. Wooden shields for holding objectives, etc. — For hold-
ing projection objectives of low power, shields of thin board (i to

FIGURE 158?. SECTIONAL VIEW OF A RAILING FLANGE AXD SOCKET.

/ The flange in section. The screw holes are made large for centering.
5 The set screw to hold the post in place.

p Post extending down into the socket. It is held at any desired height
by the set screw (s).

sc Socket for receiving the post or stem of any piece of apparatus.

\l/2 cm., l/2 in. thick) can be used, and a post or stem of iron made
from a bolt by hammering out the end in the form of a fan (fig.
i58A). To aid in centering, the screw holes in this post should
also be larger than the screws.

MICROSCOPE AND LANTERN-SLIDE PROJECTION COMBINED

§ 432. With an outfit of the lathe-bed type (fig. 138), it is very
simple to change from lantern-slide to micro-projection and the
reverse. All that is necessary is to put the lantern-slide carrier
next the condenser, and the lantern-slide projection objective on its
block in position. The stage and the microscope must be set off
the track on the table. The only difficulty is that the second ele-
ment of the condenser for the micro-projection is of too short a
focus for most lantern-slide projection. This can be overcome in

294

COMBINED PROJECTION

[CH. IX

table

8 b 2

(yip

;< ii

,'9;

c w (t)

Base Board

Base

Board

t t

•

b 3

t t

Base

Board

Fu;. 159. HOME-MADE OPTICAL BENCH KOR ALL PURPOSES.

Base Board This is drawn at the right as if transparent to show the track
and under side of the carrying 1 flocks, and the various ways in which the guid-
ing cleats can be applied.

t t t t The tubular tracks on which the carrying blocks ride.

CH. IX]

COMBINED PROJECTION

295

b i Block with four guiding cleats of wood. The screw holes in the inside
cleat at the left and the outside one at the right are made large, and washers
are used on the screws. This is to make accurate centering possible.

b 2 Block showing two guiding cleats between the tracks. Only one
cleat has large screw holes for centering.

b j The third carrying block with the guide cleats on the outside of the
track rods. Only one has large screw holes for centering.

b 4 The fourth carrying block with guide cleats at only one end, and with
centering holes in one cleat.

At the left Sectional views of the carrying blocks.

In b I is shown how to make a table for carrying apparatus along the optical
bench, and at / s, the method of screwing the track tubes to the baseboard.

In b 4 is shown how to attach a shield with an opening for lantern slides (0).

two ways: (i) The arc lamp can be put closer to the condenser,
thus making the beam between the elements diverging instead of
parallel (fig. i), or (2) a condenser lens of longer focus can be used
for the lantern-slide projection. In much of the modern projection
apparatus the condenser lenses are easily changed (see fig. 166).

FIG. 1 60. UNIVERSAL LEVEL.
(Cut loaned by the L. S. Starred Co.).

A level like this which serves for vertical and horizontal leveling is very
convenient and essential for projection work.

The second method of combined projection is to have two com-
plete lanterns side by side, one for micro-projection and one for
lantern-slide work. In this case there should be a double- pole,
double-throw switch ; then one can turn either lantern off or on at
will (fig. 162, 164).

Finally, in much of the modern apparatus special provision is
made for combined projection (see fig. 164-176).

296 PROJECTION MICROSCOPES ON THE MARKET [Ctf. IX

FIG. 161. THUMB SCREWS AND THUMB NUTS.

(From the Catalogue of the Hartford Screw Company).

Thumb screws and thumb nuts are necessary if one is to construct home-
made apparatus.

PROJECTION MICROSCOPES OBTAINABLE IN THE OPEX MARKET

§ 433. The projection microscope so far considered in this
chapter was designed to give the range needed for modern micro-
projection in a biologic or other laboratory, that is, for use with
specimens slightly smaller than lantern slides (50 to 65 mm. in
diameter) to those of i mm. or less (fig. 121, 147).

CH. IX]

COMBINED PROJECTION

297

Screen Image

Will W2

FIG. 162. COMBINED LANTERN-SLIDE AND MICRO-PROJECTION WITH
Two COMPLETE OUTFITS SIDE BY SIDE.

Wi W2 The supply wires from the outlet box (fig. 3).
D S Double-pole, double-throw knife switch.
/ Binding post of supply wire W2.
2 Binding post for supply wire W i.

j Binding post for the wire (W j) from the switch to the rheostat at the
right.

4 Binding post for the wire (W 5) to the lower carbon of the arc lamp at the
right.

5 Binding post on the switch for the wire (W 6) to the rheostat at the left.

6 Binding post on the switch for the wire (W 5) to the lower carbon of the
arc lamp at the left.

II i, H 2 Hinges for the switch blades.

j i, j 2, j 3, j 4 Jaws for receiving the switch blades when the switch is
closed.

SH Switch handle for opening and closing the switch. The switch is
closed at the right. On the left the handle, bar and switch blades are shown
with dotted lines.

298 PROJECTION MICROSCOPES ON THE MARKET [Cn. IX

W 8 The wire to the left lamp, lower carbon.

W 6, W 7 Wire including the rheostat, passing to the upper carbon of the
left arc lamp.

L Rheostat Rheostat for the left lamp.

r 3, r 4 The binding posts of the left rheostat.

L Lamp The left arc lamp.

F Feeding mechanism for the carbons.

d Clamp for fixing the arc lamp in any vertical position on its standard.

5 s Set screws for the carbons.

// C Horizontal or upper carbon.

V C Vertical or lower carbon.

R Rheostat Rheostat for the right arc lamp.

r i, r 2 Binding posts for the rheostat.

W 3 W 4 Wire from the switch through the right rheostat to the upper
carbon of the right arc lamp.

W 5 Wire to the lower carbon of the right lamp.

R Lamp The right lamp. It is exactly like the left one.

L Lamp, R Lamp The arc lamps for the two projectors.

Condensers The triple-lens condensers with water-ceils for the two pro-
jectors.

Axis, Axis, Axis, Axis Principal optic axis in the two projectors.

P Objective The projection objective at the left.

Microscope The projection microscope at the right.

Screen Image, Screen Image The images formed on the screen by the two>
instruments.

NOTE. — In using these projectors it is only necessary to turn the switch
handle over to the one desired and that lamp can be lighted. One can turn
from one to the other at will.

A more economical arrangement would be to have a single rheostat inserted
along either Wi or W2 before reaching the knife switch, then the single rheostat
would serve for both lanterns.

With the two rheostats, as here shown, both lanterns could be run at the
same time if there were two switch handles and double blades hinged at the
center (II i, H 2).

The projection microscopes in the open market rarely possess
anything like this range. Very few will project an object as great
as 25 mm. in diameter.

It seems to the writers of this book that the makers have unduly
limited the range of their apparatus by a too rigid insistence on the
use of substagc condensers and projection oculars, and also by the
effort to make combined apparatus. Combination always means
compromise and more or less loss of individual efficiency.

It is certain, too, that most of them have not fully appreciated
the necessity for dull black surfaces. The bright finish is probably
to please the eye when the apparatus is not in operation. It
certainly is not good for the eyes when the apparatus is in opera-
tion.

CH. IX] PROJECTION MICROSCOPES ON THE MARKET 299

However, many opticians are coming to finish their apparatus
in black, and all of them are ready to make modifications in their
instruments which they are convinced will make them more effec-
tive and convenient for those who are to use them. But as many
men have many minds it is not possible for the manufacturers to
please every one in all particulars, hence the apparatus in the open
market must represent a kind of average. While the authors
realize the limitations mentioned above, it is a pleasure to be able
to assert without reserve that the quality and design of the appa-
ratus obtainable at the present time are excellent.

§ 434. As the projection microscopes most common in America
are of German, English and home manufacture some examples are
illustrated below.

FIG. 163. LEITZ PROJECTION MICROSCOPE.

(From Leitz Catalogue}.

1 Arc lamp.

2 Condenser next the arc lamp.

3 Water-cell.

4 The lantern-slide holder.

5 Iris diaphragm.

6 Biconcave, illuminating lens to give the light the right angle before it
enters the substage condenser.

7 Stage and substage condensers, on a revolver for use with different powers.

8 Projection objectives on a revolving nose-piece.
Q Projection oculars on a revolver.

The enclosing curtain is turned over the top to uncover the parts. (Sec fig.
96 for the entire apparatus in its latest form, 1914).

300 PROJECTION MICROSCOPES ON THE MARKET fCn. IX

FIG. 164. PROJECTION MICROSCOPE, MAGIC LANTERN AND MEDIOSCOPE

WITH SINGLE RADIANT.
(Cut loaned by Williams, Broivii & Earle).

The arc lamp and condenser move laterally so that each instrument can he
illuminated at will.

A The medioscope is an achromatic combination of large aperture for
objects of large size, but smaller than lantern slides.

B Projection microscope with large projection ocular. It has a water-cell
(D) in the path of the light. The arc lamp and condenser (N O P) are in place
for micro-projection.

C Projection objective for lantern slides.

FIG. 1 6s.

NE\V REFLECTING LANTERN WITH THE PROJECTION
MICROSCOPE.

CH. IX]

TROUBLES WITH MICRO-PROJECTION

301

removed. For projection the mirror must be in place to reflect the light along
the axis as for lantern slides. The change to the projection of opaque objects
is almost instantaneous, but for lantern-slide projection the projection micro-
scope must be removed and the lantern-slide objective put in place, but^the
apparatus is so constructed that this is easily accomplished.

FIG. 166. IMPROVED, COLLEGE, BENCH LANTERN ARRANGED FOR

M ICRO-PROJECTION.
(Cut loaned by the Mclntosh Stereopticon Co.).

The optical bench consists of a long baseboard with the two guide rods
supported by three brackets.

As each part is independent it can be changed in position or entirely removed
and other apparatus put in its place, thus giving great flexibility.

TROUBLES WITH THE PROJECTION MICROSCOPE

§ 435. The source of troubles with the projection microscope
are mainly the same as with the magic lantern. These have been
fully discussed at the end of Chapter I (§ 62-98). See § i28a for
the blowing of fuses with the arc lamp on the house system.

The special troubles with the projection microscope are almost
wholly due to the smallness of the lenses necessary for micro-pro-
jection; and as the foci of these lenses are relatively short, slight
changes in the position of one of the elements of the apparatus, and
slight deviations from the true axis produce correspondingly great
effects. It is necessary to be more exact in micro-projection, but
the great fundamental principles are exactly as for the magic
lantern.

§ 436. Insufficient illumination on the screen. — Besides those
given in Chapter I the following may be causes :
i. Too large a screen image may be attempted.

302

TROUBLES WITH MICRO-PROJECTION

[CH. IX

FIG. 167. UNIVERSAL PROJECTOSCOPE SHOWING THE ARRANGEMENT
FOR HORIZONTAL TRANSPARENCIES AND FOR MICROSCOPIC PROJECTION.

(Cut loaned by the C. H. Stoelting Co.}.
Commencing at the left :

1 Feeding screws for the carbons.

2 Fine adjustment for moving the arc lamp back and forth along the axis.
3-4 Fine adjustment screws for moving the arc vertically and laterally to

keep the crater in the axis.
L H Lamp and lamp-house.
C First element of the two-lens condenser.
F-F vSupports.
T Water-cell.

M Mirror above the objective to reflect the light to the vertical screen.
M, M, Mirror in position to reflect the horizontal beam directly upward.
C, C3 Second element of the two-lens condenser.
R Projection microscope.

B Optical bench on which slide the different pieces of apparatus.
Bl B, Supports of the optical bench. (See also fig. 16, 102).

2. The object may not be in the best position in the light cone

3. The substagc condenser, when that is used, may be a little
too near or too far from the specimen. Slight changes in

CH. IX]

TROUBLES WITH MICRO-PROJECTION

303

FIG. 168. THOMPSON'S PROJECTION MICROSCOPE.
(Cut loaned by the A. T. Thompson Co.).

The projection microscope with the substage condenser system is attached
to the reflectoscope (rig. 97) in the position where vertical opaque objects are
placed ; this allows the direct beam of light to be utilized in micro-projection.

The stage and the objective holder are independent, and no ocular is used.
This permits the projection of large objects with low powers or smaller objects
with high powers. From the short tube employed, the field is not restricted.

its position often work wonders. The substage condenser
may be too near to the large condenser or too far from it
so that the light cone does not reach it in its most favorable
position.

4. The room may not be dark enough or external light may fall

directly on the screen from some window or open door.

5. Never forget the carbons. A slight mal-position or decen-

tering of the crater may cause all the trouble.

304

PROJECTION AIICROSCOPES ON THE MARKET [Cn. IX

(Balance of descriptive matter on next page}

CH. IX] PROJECTION MICROSCOPES ON THE MARKET 305

Commencing at the left :

Large, well ventilated, light-tight lamp-house. As shown in fig. 104, 105,
the lamp-house with the lamp and first element of the condenser can be inclined
to direct the light downward upon an opaque object.

Following the lamp-house is a dark box for opaque projection. The large
projection objective with mirror is above and the table for the opaque objects
below. Within the dark box is a mirror so inclined that it reflects part of the
scattered light back upon the object (see also fig. 105). Opaque objects up to
20 cm. (8 in.) square can be projected.

Following the large objective for opaque projection is an objective for lantern-
slide or other projection with the object in a horizontal position. Following
this is the polarizing apparatus of glass plates (see § 880). The second element
of the condenser serves for lantern-slide and for low power micro-projection,
but for high powers this is turned out as here shown and a small double convex
lens in the dark chamber near the first element of the condenser is swung into
position and serves to project an image of the crater at the plane of the dia-
phragm of the substage condenser (fig. 170).

Just beyond the bellows are shown the projection microscope and the
lantern-slide objective. These are so hinged parallel to the axis that the
microscope can be turned laterally and thus bring the lantern-slide objective in
position. In the picture the lantern-slide objective is turned aside and the
projection microscope is in position.

The substage condensers for different objectives are shown on a revolving
carrier, as are also the micro-projection objectives and the projection ocular
and amplifier.

FIG. 170. DIAGRAM OF THE ILLUMINATING SYSTEM FOR HIGH POWER

PROJECTION.
(Cut loaned by the Bausch & Lomb Optical Co.}.

This is a modification of the Kohler system ( § 401-403), and consists of the
first element of the triple condenser (meniscus and convex lens) to render the
beam parallel. The small, convex lens near the condenser serves to project
an image of the crater upon the plane of the diaphragm of the substage con-
denser. This is designed to fill the aperture of the substage condenser and,
hence, of the high power objectives.

L The radiant.

C The meniscus and convex lens of the condenser and the small special
convex lens for micro-projection.

L' Inverted image of the radiant.

E Substage condenser.

5 The specimen.

C' Image of the small condensing lens in the plane of the specimen (5).

306

PROJECTION MICROSCOPES ON THE MARKET [Cn. IX

CH. IX] PROJECTION MICROSCOPES ON THE MARKET 307

FIG. 171. NEW STYLE CONVERTIBLE BALOPTICON FOR MICROSCOPE, LAX-
TERN-SLIDE AND OPAQUE PROJECTION, AND FOR THE PROJECTION OF
LARGE TRANSPARENCIES IN A HORIZONTAL POSITION.

(Cut loaned by the Bausch & Lomb Optical Co.).

As shown in the picture, this instrument is designed for projecting all kinds
of objects either in a vertical or in a horizontal position. For the large trans-
parencies the object is placed on the broad plate beneath the objective.
Immediately under the object is the condenser lens of 20 cm. (8 in.) diameter,
thus making it possible to project X-Ray plates, brain sections, etc., 20 cm.
(8 in.) in diameter. A mirror in the dark chamber directs the horizontal beam
from the first element of the condenser vertically as in all projection of this
kind.

For the large transparencies the projection objective is in a vertical position
with mirror to reflect the light to a vertical screen and to overcome the left to
right inversion.

FIG. 172. UNIVERSAL BALOPTICON FOR OPAQUE OBJECTS, MICROSCOPIC

OBJECTS AND FOR LANTERN SLIDES OR OTHER TRANSPARENT OBJECTS

IN A VERTICAL OR A HORIZONTAL POSITION.

(Cut loaned by the Bausch & Lomb Optical Co.).

For the opaque projection and lantern slides in a vertical position see fig. 106.

For lantern slides or other transparent objects in a horizontal position the
arrangement for vertical slides is pushed back, and this brings the condenser
lens and plate for supporting horizontal objects over the opening. The same
mirror is used for directing the beam of light upward as for the vertical slides.

308 TROUBLES WITH MICRO-PROJECTION [Cn. IX

For micro-projection the microscope is placed in line with the objective for
opaque objects, the objective serving as a condenser. The light passes directly
from the radiant through the first element of the condenser and the objective
for opaque objects to the microscope. The microscope is so hinged that it can
be turned aside and the other forms of projection quickly brought into use.

6. There may be mist on some of the glass surfaces as the water-
cell, or some glass surface like the objective front may be
dirty.

§ 437. Unequal illumination of the screen. — This is often due
to the lack of centering of some element.

1. It is usually the crater of the upper carbon that gets out of

the axis. It is easily corrected by means of the fine adjust-
ments of the lamp (fig. 3).

2. There may be some less transparent part of the object over

part of the field. One can easily determine this by moving
the specimen slightly.

3. Part of the mask (§ 384, fig. 143, 148) may be in the field.

FIG. 1 73. BAUSCH & LOME'S SIMPLEST FORM OF PROJECTION MICROSCOPE.
(Cut loaned by the Batisch & Lomb Optical Co.).

This is designed for low power projection and consists of an objective holder,
rack and pinion focusing adjustment, stage and substage condenser for low
powers. The whole is put in place of the projection objective for lantern slides.
This simple outfit added to the magic lantern enables one to do very successful
micro-projection.

§ 438. Hazy images may be due to direct light on the screen
from some window, etc. Keep especially in mind also that internal
reflections in the objective, the microscope tube or the amplifier
tube will cause hazy images (§370, 371), also dirt or balsam on the
front lens of the objective.

CH. IX]

TROUBLES WITH'MICRO-PROJECTION

309

§ 439. Dark spots on the screen.—

i. They may be caused by air bubbles in the water-cell or in
the stage cooler.

FIG. 174. SIMPLE ADDITION TO THE MAGIC LANTERN FOR
MICRO-PROJECTION.

(Cut loaned by the Spencer Lens Co.).

This consists of a jointed frame by which the objective holder and focusing
device can be brought down in position when the lantern-slide objective is
turned aside. No microscope tube is used. This makes a very efficient and
convenient addition to a magic lantern at moderate cost, and with it a great
deal of projection can be successfully accomplished.

For lantern-slide projection the microscope is turned to the top of the lamp
enclosure and the lantern-slide objective is turned on its hinge back into posi-
tion in the optic axis.

2 . They may be caused by dark spots or bubbles in the slide or

specimen.

3 . Dark spots on the condenser, amplifier or ocular may cause

them.

§ 440. General conditions for good micro-projection. — With
good specimens, clean glass surfaces, and all the elements on one
axis, there should be no trouble in getting a good screen image on a
suitable screen and in a well darkened room.

It would be of very great advantage for any man who aspires to
use the projection microscope effectively, if he could see the room,
apparatus, and exact method of work of some one who had mas-
tered the art. Good projection will not do itself.

PROJECTION MICROSCOPES ON THE MARKET [Cn. IX

FIG. 175. MODEL 4-5 DELINEASCOPE WITH THE MICROSCOPE IN A VERTICAL
POSITION* FOR HORIZONTAL OBJECTS.
(Cut loaned by the Spencer Lens Co.).

This figure is to show the course of the rays for lantern slides in a vertical
position and for microscopic objects in a horizontal position.

A mirror M reflects the light vertically through the horizontal specimen, and
by means of a prism (PR) in the tube of the microscope the vertical light is
made to extend out horizontally to the screen.

A joint in the microscope frame makes it possible to turn the microscope
down in front of the instrument after turning the lantern-slide objective aside
on its hinge. Then vertical objects can be projected in the usual manner, or
by using the prism (PR) the image can be reflected down upon a horizontal
drawing surface.

CH. IX] PROJECTION MICROSCOPES ON THE MARKET

FIG. 176. MODEL 8 DELINEASCOPE SHOWING THE POSITION OF THE
MICROSCOPIC ATTACHMENT FOR VERTICAL AND FOR HORIZONTAL

OBJECTS.

(Cut loaned by the Spencer Lens Co.).

This projection microscopic attachment is designed to use with or without
oculars or amplifiers, and for microscopic objectives of all foci from 125
mm. to the highest available. The substage condenser consists of several
lenses which are easily turned in place or out of position. By making a suitable
combination any object and any objective can be used. To enable the opera-
tor to get the object in the right position in the cone of light there is a rack and
pinion movement moving microscope and stage toward or from the condenser.
Th;s is done by the lower milled head shown. The upper milled head is for
the usual coarse adjustment and a micrometer screw is present for the fine
adjustment (see also fig. 177).

312

PROJECTION MICROSCOPES ON THE MARKET [Cn. IX

FIG. 177. DIAGRAM SHOWING THE COURSE OF THE RAYS FOR LANTERN
SLIDES AND FOR MICROSCOPIC OBJECTS IN A. VERTICAL AND IN A
HORIZONTAL POSITION WITH MODEL 8 DELINEASCOPE.

(Cut loaned by the Spencer Lens Co.).

T Table for opaque objects.

W Wheel by which the table is raised and lowered.

D Diaphragm which may be used above the table.

B Bulb which always illuminates the interior of the machine.

C Condensing lenses in front of the arc.

0 Large objective for opaque work.

01 Smaller objectives for vertical attachment.

M Mirror for throwing light downward to the lantern slide.

Ml Mirror for throwing a perpendicular beam out through the lantern-
slide compartment.

M., Mirror used in connection with the projection of the vertical side of an
object.

M., Mirror which assumes a position of 45° when the microscope is used
perpendicularly.

P Prism which is used in the prism chamber when the microscope is used
perpendicularly or for drawing on a horizontal surface when the microscope
is horizontal.

S .Shelf upon which the lantern slide is placed previous to throwing it up
into the optical axis by the handle.

II Handle of the lever for raising lantern slides into position.

CH. IX]

DO AND DO NOT IN MICRO-PROJECTION

313

§ 441. Summary of Chapter IX:

1. Use actual objects in lec-
tures and discussions as well as
diagrams (§352).

2. Employ a projection micro-
scope with equipment for speci-
mens ranging from 60 mm. to
less than i mm. in diameter

(§354).

3 . In demonstrating with the
projection microscope use first a
low power and show the rela-
tions of parts, then use higher
powers to show details.

4. Use objectives without
oculars from 125 mm. to 4 mm.
focus (§355)-

5 . Oculars or amplifiers can be
used with all the objectives on
the microscope (fig. 138), but
preferably with those not higher
than 8 mm. focus.

6. Use a screen distance from
5 to 10 meters (16 to 33 feet).

7. It is better to use a micro-
scope in the usual manner if
very high powers, like the oil
immersion, are to be used (§355).

8. If possible use a triple-
lens condenser (§ 363).

Do NOT

1 . Do not stop with diagrams
where actual specimens can be
shown. Diagrams alone are
liable to give false impressions.

2. Do not use projection
apparatus with a narrow range
of field or of powers.

3. Do not show minute de-
tails without first showing the
object as a whole, so that rela-
tions can be clearly recognized.

4. Do not use oculars for
projecting for large, class demon-
strations. Oculars restrict the
field too much.

5. Do not use oculars or
amplifiers unless for special
reasons.

6. Do not have the screen
distance too great.

7 . Do not try to make out the
finest details by projection, but
use a microscope in the ordinary
way.

8. Do not use a poor con-
denser for micro-projection, the
triple form, meniscus next the
radiant, is best.

DO AND DO NOT IN MICRO-PROJECTION [Cn. IX

9. Make the room dark and
use a perfectly white image-
screen for micro-projection
(§ 36o)-

9. Do not try to project with
the microscope in a room that
cannot be properly darkened or
with a dirty screen.

i o . Use only the direct current
arc light for micro-projection,
unless compelled to use alter-
nating current (§ 412).

11. Use an ammeter and a
variable rheostat or other bal-
ancing device and be very
careful about the wiring (fig. 2-3
188).

12. The arc lamp must have
fine adjustment screws to enable
one to keep the arc centered on
the objective front (§ 362).

13. Always use a water-cell
in micro-projection with the arc
lamp radiant (§ 364).

14. Use a mechanical
for serial sections (§ 366).

stage

15. Blacken the objective
mounts, and all metal parts of
the projection apparatus to
avoid glare; make sure there
arc no shiny surfaces within the
projection apparatus (§ 570-

10. Do not use alternating
current if it is possible to obtain
direct current.

n. Do not neglect the am-
meter and the variable rheostat
when installing a projection
microscope.

12. Do not try to use an arc
lamp without fine adjustments,
otherwise the crater cannot be
kept centered.

13. Do not project with the
arc lamp without using a water-
cell to absorb the radiant heat.

14. Do not try to show
selected sections of a series
without the help of a mechani-
cal stage.

15. Do not leave the objec-
tive mounts with brilliant re-
flecting surfaces to dazzle the
eyes of the operator, and do not
leave shiny surfaces within the
apparatus to give cross lights
and make the image dim.

CH. IX] DO AND DO NOT IN MICRO-PROJECTION

315

1 6. Use a hood on the objec-
tive to aid in centering the light
and in placing the objective the
right distance from the conden-
ser (§ 372); a light shield
beyond the objective to stop
stray light is also an advantage
(§ 373)-

1 6. Do not forget the advan-
tages of an objective hood for
centering the light and prevent-
ing glare; and do not omit the
light shield to cut off stray light.

17. It is of the utmost im-
portance that every part be
accurately centered for micro-
projection (§ 375), and that the
parts should be separated from
one another the right distance
(§ 376, 382).

17. Do not fail to have all
parts accurately centered, and
the correct distance apart.

1 8. Remember that it is a
pure waste to use too great an
amperage (§ 378).

1 8. Do not use a greater cur-
rent than necessary.

19. As the same object is to
be shown entire and with magni-
fied details and different objects
require different magnifications,
it is convenient to have two,
three or four objectives of
different powers in a revolving
nose-piece (§ 379).

20. For exhibition purposes
it is a great advantage to use
carbons whose ends have been
shaped by previous burning in
the lamp (§ 380).

19. Do not show all objects
with the same objective, but
have two or three on a revolving
nose-piece so that different
powers can be used with the
minimum of trouble.

20. Do not forget to shape
the ends of the carbons by burn-
ing them awhile in the arc lamp
before anv formal exhibition.

DO AND DO NOT IN MICRO-PROJECTION

[CH. IX

21. Be sure that the carbons
are in the correct mutual posi-
tion to give a good light. A
screen image of the burning
carbons often is of real help
(§381).

21. Do not omit the correct
setting of the carbons. A good
light cannot be produced with
the carbons in the wrong mutual
relation.

2 2 . Mask the preparations for
exhibition (§ 384).

23. Remember the advan-
tages of a large field for seeing
the relation of parts (§387).

2 2 . Do not exhibit specimens
which are not properly masked.
It is necessary to be able to
work with certainty and rapid-
ity in an exhibition.

23. Do not forget the impor-
tance of a large field so that the
relations of parts can be seen.

24. Remember that one can
do good projection work with
an ordinary microscope (§ 393).

25. For objects which must
remain in a horizontal position,
a vertical microscope must be
used; this involves the use of
two mirrors or of a mirror and a
prism to reflect the light upward
and then horizontally to the
screen (§397)-

24. Do not forget that one
can do very good work by using
an ordinary microscope in pro-
jection.

25. Do not try to use a hori-
zontal microscope when one in a
vertical position is called for.

26. Have everything in per-
fect order and adjustment when-
ever an exhibition of micro-
scopic objects is to be made.
Haphazard work will give only
haphazard results (§ 400).

26. Do not do haphazard
projection.

CH. IX]

DO AND DO NOT IN MICRO-PROJECTION

317

27. For high powers like oil
immersions, the screen distance
must be short, the screen and
light perfect, the room very
dark and the spectators close
to the screen (§ 401-410).

28. Remember the advan-
tages of the small-carbon arc
lamp for use on the house light-
ing system for drawing and for
demonstrating to a few (§ 417).

29. Use sunlight when it is
available (§419).

30. One can do excellent
micro-projection by home-
assembled apparatus (§ 424).

31. For passing from micro-
projection to lantern-slide pro-
jection it must be remembered
that the lantern-slide picture is
much brighter with the same arc
light. To avoid the great con-
trast, one would do well to use
a tinted glass in the magic
lantern to soften the light as
for opaque and lantern-slide
projection (§ 282).

32. Study faithfully the
"troubles" with the magic lan-
tern in Ch. I, and in this chap-
ter (§ 435-439)-

27. Do not try high power
projection for a long screen dis-
tance, a light room or a poor
screen, or anything else not in
accordance with the most exact-
ing work.

28. Do not forget the advan-
tages of the small-carbon arc
lamp on the house lighting
system for drawing and demon-
strations for a few persons.

29. Do not neglect the most
brilliant light, i. e., sunlight,
when it is available.

30. Do not refrain from micro-
projection because you do not
have an expensive special out-
fit. Home-made apparatus is
often more effective and can be
assembled by any one.

31. Do not forget the phy-
siology of vision in passing from
a dim to a brilliant light or the
reverse.

32. Do not expect the appra-
atus to supply the brains.

THE METRIC SYSTEM

[Cn. IX

THE METER FOR
LENGTH

10 CENTIMETER RULE

THE UPPER EDGE IN MILLIMETERS, THE LOWER IN CENTIMETERS, AND HALF

CENTIMETERS

THE METRIC SYSTEM
UNITS THE MOST COMMONLY USED DIVISIONS AND MULTIPLES

Centimeter (cm.), o.oi Meter; Millimeter (mm.), o.ooi
Meter; Micron (/*), o.ooi Millimeter; the Micron is
the unit in Micrometry.

Kilometer, 1000 Meters; used in measuring roads and
other long distances.

THE GR\M FOR f Alilligram (mg-), °-°O1 Gram.

W_T •; Kilogram, 1000 Grams, used for ordinary masses, like

\\ I- 1 GUT

I groceries, etc.
THE LITER FOR f Cubic Centimeter (cc.), o.ooi Liter. This is more com-

CAPACITY j mon than the correct form, Milliliter.

Divisions of the Units are indicated by the Latin prefixes: deci, o.i ; centi,
o.oi; milli, o.ooi; micro, one millionth (o.oooooi) of any unit.

Multiples are designated by the Greek prefixes; deka, 10 times; hecto, 100
times; kilo, 1000 times; myria, 10,000 times; mega, one million (1,000,000)
times any unit.

TABLE OF METRIC AND ENGLISH MEASURES

Meter (M.) (unit of length) = 100 centimeters;

1,000 millimeters, 1,000,000 microns (/*) 39-3& inches; 3.28 feet;

1.094 yard.

Centimeter (cm.)=.oi meter; 10 millimeters,

10,000 microns (/*)• -3937 (I) inches

Millimeter (mm.)=.ooi meter, .1 centimeter,

i, ooo microns (jj.). -03937 (1^ inches

Micron (M) = .OOI millimeter (unit of measure in

micrometry) 000,039,37 inch

1/25000 inch

Liter (L.) (unit of capacity) = I, ooo cubic centi-
meters (i quart approx.)

Cubic Centimeter (cc.) =.oor liter (,',., cubic inch approx.)

Gram (g.) (unit of weight) 15-43 grains.

Kilogram (Kg.) = 1,000 grams 2.2046 (2*) pounds.

Yard =3 feet, 36 inches 91 .44 centimeters.

Foot = one-third yard, 12 inches 30.47 centimeters.

Inch = 3',-, yard, ,'_>- foot 2.54 centimeters.

.001 (, g^jj) inch 0254 millimeters =25.4 M-

Fluid ounce = 8 fluidrachms 29.57 (3°) cubic centimeters.

Pound (LI).) (avoirdupois) = 16 ounces 453-6 grams.

Ounce (oz.) (avoirdupois) = 437^2 grains 28.35 (30) grains.

Ounce (oz.) (Troy or apothecaries) =480 grains . .31.10 (30) grams.

CHAPTER X

DRAWING AND PHOTOGRAPHY BY THE AID OF
PROJECTION APPARATUS.

§ 450. Apparatus and Material for Chapter X :

Room with electric current supply (§ 453); Arc lamp and
rheostat or other regulating device (§ 462, 493) ; Water-cell (§ 504) ;
Carbons of various sizes for small and large currents (§ 486, 488-9) ;
Condenser suitable for the objects to be projected (§467, 533) ; Mi-
croscope with objectives and oculars and with a 45 degree mirror
or a prism (§ 458-459, 493) ; Photographic objectives for projecting
the images of large objects for use with negatives or lantern slides
(§ 534) ; Movable drawing surface (§ 459-460) ; An opaque lantern
(§ 469) ; A photographic camera with ground-glass focusing screen
(§ 471); Metric measures; Transparent micrometer (§ 508 + );
Letters on tissue paper for drawings (§ 528 + ); Photographic
paper, negatives and chemicals (§ 532, 547); See also the needs
in Ch. I, (§ i).

§ 451. For the history of drawing with projection apparatus
see the Appendix, with its references to literature.

It will also be advantageous to consult the works given in Ch. I,
§ 2 and the catalogues of the manufacturers of projection and
photographic apparatus. The Eastman Kodak Co. has published
a very useful booklet on Enlarging. This deals, not with micro-
scopic, but with the moderate enlargements up to 20 diameters with
photographic objectives.

DRAWING WITH PROJECTION APPARATUS

§ 452. The aid which projection apparatus could give for
getting accurate drawings was recognized from the beginning; and,
indeed, this was considered one of its most important uses.

By the aid of projection apparatus accurate drawings can be
made by any careful worker, although artistic perfection can be
added only by those gifted of nature. Even for born artists it is
helpful in getting the details of complex objects in due position and
in correct proportion.

319

320 ROOM FOR DRAWING [Cn. X

The range of possibility is great, for, by the aid of projection
apparatus, one can draw the images produced by the objectives
used with the magic lantern, photographic objectives, and micro-
scopic objectives of all powers. The microscopic objectives may
also be combined with amplifiers or with oculars for projecting the
images to be drawn.

Drawing with projection apparatus has the advantage over
drawing with the camera lucida that one can see the entire specimen
in one field. More important still, the artist can use both eyes.
There is entire freedom of head and eyes, the image remaining
constantly in one place, regardless of the position of the draughts-
man.

ROOM FOR DRAWING WITH PROJECTION APPARATUS

§ 453. Any room suitable for projection is also suitable for
drawing with projection apparatus.

Any laboratory which can be made moderately dark in the
day time is suitable for day work; and, of course, any room is
suitable in the evening.

§ 454. Special photographic and drawing room. — Many labora-
tories have one or more photographic rooms which are also used for
drawing. These are mostly separate rooms. Sometimes they are
adjoining a laboratory, and sometimes they are like a ticket booth
in a large railroad station, i. e., a room within a larger room (fig.
179). This is the plan adopted in the Wistar Institute (Anat.
Record, 1907) and in the author's laboratory (Proc. Amer. Micr.
Soc., 1906, p. 44-45). If these rooms are painted dull black
within, stray light is absorbed, and it is much easier to get sharp
pictures. In a black room the door can be left partly open and
thus secure better ventilation.

As the radiant gives off much heat it is an advantage to have an
electric fan in the room if one works several hours at a time. It
is espccialh' necessary in hot weather, — summer vacations when
teachers have time for research.

§ 455. A drawing room made with screens or curtains. — If one

has not a permanent room or booth in the laboratory, a fairly good

CH. X]

PROJECTION APPARATUS FOR DRAWING

321

substitute can be made by means of opaque curtains enclosing one
corner of a room. This would be something like the early drawing
rooms or tents used by Kepler and others for sketching landscapes
(fig. 88, 89). It is advantageous to have the cloth curtains
rendered fire -proof by saturating them with a solution of sodium
tungstate, or some other fire-proofing solution (see Popular Science
Monthly, Vol. LXXXI, 1912, p. 397).

(Proceedings of the Amer. Assoc. Adv. Science, Vol. XLIII,
1894, p. 119.)

FIG. 179. PHOTOGRAPHIC AND DRAWING BOOTH (P D) IN A LARGE

LABORATORY.

This booth contains water and electric supply for photography and for
projection work including drawing, printing and photo-micrography.

PROJECTION APPARATUS FOR DRAWING

§ 456. The apparatus used for drawing may be the ordinary
magic lantern (fig. 1-2), the projection microscope (fig. 121), the
opaque lantern (fig. 92-111), or a photographic camera (fig. 117,
217).

§ 457. Drawing on a vertical surface. — For this, the only addi-
tion to any of the forms of projection apparatus is a vertical draw-
ing-board, mounted so that it may be moved to a greater or less
distance from the apparatus to get the desired size of image. Or
one may use a fixed wall for the drawing surface and move the

322 PROJECTION APPARATUS FOR DRAWING [Cn. X

apparatus back and forth to get the different sizes required. (For
getting the picture like the object see § 512).

§ 458. Drawing on a horizontal surface. — From the earliest use
of projection apparatus for drawing, it was the custom to draw the
image on a vertical surface, or by means of a plane mirror to change
the direction of the rays of light so that the image would fall on a
horizontal surface. It was found also that when a plane mirror

FIG. 1 80. PROJECTION MICROSCOPE FROM CHEVALIER (Planche 2).

M Mirror reflecting the sun's rays (RR1, rr1) to the condenser (C); from
the condenser they pass to the substage condenser (c) and are condensed upon
the object (o).

L Achromatic objective.

A Amplifier composed of a plano-convex and a double concave lens; this
amplifier makes the rays much more divergent, i. e., BB' instead of bb1.

P Right-angled prism acting as a 45 degree mirror to project the image
down upon a horizontal surface for drawing.

was used, the image on the horizontal surface appeared erect.
Sometimes the mirror was placed before the objective and changed
the direction of the rays 90 degrees (fig. 89), and sometimes it was
used to bend the rays downward after passing through the objec-
tive. With the microscope and magic lantern the mirror is usually
beyond the objective (fig. 182, 193).

Reflecting prisms have been much employed with the microscope
instead of mirrors (fig. 180, 192). The}- have the advantage of
giving more perfect reflection and of avoiding doubling of the
image, as occurs with a plane mirror silvered on the back.

CH. X]

PROJECTION APPARATUS FOR DRAWING

323

§ 459. Drawing table with attached 45 degree mirror. — One of

the simplest and most convenient arrangements for the magic
lantern and the microscope is to have a large mirror attached to
the drawing table. The table and mirror can then be moved
toward or from the projection apparatus to aid in getting the
desired magnification (fig. 182).

FIG. 181. KORISTKA'S SIMPLE DRAWING OUTFIT.
(From Koristka's Microscope Catalogue).

This drawing outfit can be connected with the house lighting system.
/ Nernst lamp for illumination.

2 Condenser connected with the lamp-house.

3 Stage of the microscope.

4 Projection objective.

5 45° mirror for reflecting the rays down upon the horizontal drawing
surface.

6 Horizontal drawing surface. The drawing-board slides along the axis,
thus making it possible to vary the distance and hence to increase or diminish
the size of the drawing at pleasure.

When sitting down to draw, a convenient height for the table is
76 cm. (2^ ft.). The one shown in fig. 182 has a top 100 cm. long
and 75 cm. wide (39 x 30 inches).

The plate glass mirror is 75 cm. long and 60 cm. wide (2^ x 2 ft.) .
It is permanently fixed at 45 degrees inclination; and to avoid the
sharp angle at the base of the mirror it is raised from the table 10
to 15 cm. (4 to 6 in.).

The mirror itself is in a strong wooden frame, and it is supported
by vertical and horizontal pieces, as shown in figure 182.

324 PROJECTION APPARATUS FOR DRAWING [Cn. X

FIG. 182. DRAWING WITH PROJECTION APPARATUS AND A MOVABLE
TABLE WITH 45° MIRROR.

Commencing at the left :

Supply wires to the table switch.

From one pole of the table switch a wire extends to the binding post of the
upper carbon of the arc lamp.

From the other pole of the switch a wire extends to the rheostat (R) and from
the rheostat to the binding post of the lower carbon.

Arc lamp within the lamp-house.

The metal lamp-house is shown as if transparent, as it was left in position
during only a part of the time while the photograph was exposed.

Condenser and water-cell (fig. 121).

Stage of the microscope with stage water-cell.

Projection microscope with objectives in the revolving nose-piece, a shield
to stop stray light and an amplifier in the end of the large tube.

The lamp, condenser, stage and microscope are on independent blocks and
can be moved freely on the optical bench. The picture of the 10 centimeter
rule under the door of the lamp-house gives the scale of the picture.

R Adjustable rheostat.

20-10 These numerals show the range of current which the rheostat per-
mits. The arrow indicates the way to turn the knob to increase the current
(see fig. 281, Ch. XIII).

On the legs at the left is a shelf for the rheostat.

The adjustable drawing shelf has an arrangement for moving up and down
on metal wavs which can be attached to any table, whatever the form of the

CH. X] PROJECTION APPARATUS FOR DRAWING 325

legs. The supporting brackets are so jointed that the shelf can be let down
when the large drawing table needs to be brought up close to the projection
table. This method of moving the drawing shelf and lowering it is due to
Dr. B. F. Kingsbury.

As one must sit close to the table, there should be no vertical rail
under the front edge to interfere with the knees of the artist. At
this edge there is a strengthening piece flat against the top. On
the other edge and at the ends are the usual vertical rails. To
ensure the rigidity of the table, there are pieces passing across the
ends between the legs and near the bottom, and a middle piece
extending lengthwise between these end pieces, thus holding the
table legs at the two ends, so that they cannot spread either side-
wise or endwise (fig. 182).

The legs are 6 cm. (2^ in.) square, and smooth on the lower end
so that the table can be moved easily, or casters may be used. The
entire table is finished in dull black and all the corners rounded.

§ 460. Projection table with drawing shelf. — The simplest of
all arrangements for drawing with the projection microscope and
the magic lantern is a projection table with an adjustable shelf
attached to the end (fig. 183, 187). For this arrangement the
mirror or prism for reflecting the light downward must be close to
the objective or to the end of the microscope.

As the shelf can be raised to the level of the table top or depressed
about 50 cm. (20 in.), it is possible to get quite a range of magnifica-
tion from the different image distances alone, using the same objec-
tive; but, of course, the upper range is not so great as with a
separate drawing table. With the drawing shelf, however, one
can get lower powers, as the image can be closer to the end of the
objective. By using different objectives one can get all the range
desired with either arrangement. The single table and adjustable
shelf is, of course, much the cheaper.

If one uses the table and drawing shelf it is necessary that the
apparatus be movable on the optical bench, so that the objective
ma}* be beyond the end of the table over the drawing shelf. This
is easily accomplished with an optical bench like that shown in fig.
158-159. In case one desires a larger drawing surface than the

326

PROJECTION APPARATUS FOR DRAWING [Cn. X

FIG. 183.

ARRANGEMENT FOR DRAWING OBJECTS THE SIZE OF LANTERN-
SLIDES.

The illumination can l:c by the ordinary heavy lantern-slide current, or by
the small current of the house lighting supply. The 5 ampere current is
sufficient for drawing. If one wishes to draw on a horizontal surface, then a
mirror is put beyond the objective. If the drawing is on a vertical surface, as
for wall diagrams, then the mirror is removed.

w Supply wire cable from the outlet box (fig. 3).

/ u.1 Wires to the arc lamp.

s Table switch.

r Rheostat of the theater-dimmer type with a range of 5 to 35 amperes.

/ Arc lamp.

a a a Axis.

c Condenser and water cell.

Is Lantern-slide support.

o Projection objective for large objects.

m 45° mirror to reflect the light down upon the horizontal drawing shelf.

as Adjustable drawing shelf.

b Baseboard with track along which the, carrying blocks can be moved
independently.

attached shelf, a small drawing-board may be clamped to the shelf
as shown in fig. 183.

CH. X]

PROJECTION APPARATUS FOR DRAWING

327

The size of the projection table is the same as given above (§ 424) .
A convenient size for the drawing shelf is 50 cm. (20 in.) long, and
25 cm. (10 in.) wide.

In fig. 183 the legs of the table are square and straight and the
shelf slides up and down on the legs, being clamped in any desired
position by the thumb nuts.

FIG. 184. DR. RILEY'S ATTACHMENT TO AN ORDINARY MAGIC LANTERN

FOR DRAWING.

(Science, Vol. XXIX, IQOQ, p. 37-38).

AB Mirror support.
CD Mirror and mirror frame.

F Clamp for fastening the mirror support in position in front of the magic
lantern objective.

E Drawing paper under the mirror.

In figure 182 is shown a neat and efficient arrangement designed
by Dr. B. F. Kingsbury, in which the shelf is hinged so that it can
be lowered out of the way when using the drawing table with
attached 45 degree mirror. The guides for sliding the shelf up or
down and clamping it in any desired position, arc of metal and can
be attached to any table whether the legs are square, tapering or
of any other form.

328 RADIANTS FOR DRAWING [Cn. X

RADIANTS FOR DRAWING APPARATUS

§ 461. General. — The best light for projection is naturally the
best light for drawing with projection apparatus. One must
always keep in mind that a rather dim light in a perfectly dark
room, after one has been long enough in it to acquire twilight
vision, may seem quite brilliant. The old observers with their
very dim artificial lights understood this well, and did much with
projection apparatus which at first sight would seem impossible to
us.

The electric arc and other brilliant artificial lights are so common
at the present that many have come to feel that they cannot see at
all unless the object is flooded with light. But, excepting those
who are night-blind, that is, have poor twilight vision, much can be
done with the Welsbach mantle light, the alco-radiant mantle
light, etc. Even a kerosene lamp of good quality is very service-
able, but one must always keep in mind that the dimmer the light-
source, the darker must be the work-room, and the more care must
be taken to avoid stray light. Too high powers should not be used
with weak lights. For high power drawing very brilliant light is
necessary.

§ 462. Arc lamp with direct current. — This is, of all the

artificial sources, the most satisfactory for drawing, as for projec-
tion (fig. 3). With it the drawing room need not be very dark, and
one can obtain sufficient light for the highest powers with which it
is desirable to draw. Ordinarily a 5-10 ampere current is sufficient
(sec also § 485). If low amperages arc used the apparatus is not so
greatly heated as with higher amperages, and furthermore the
specimens are less liable to injury from overheating.

The same lamp that is used for projection is suitable for drawing.
There is some advantage in having an automatic arc lamp, then the
artist will not have to bother about the lamp except to supply it
with proper carbons, and to see that they arc in proper position.
With the hand-feed arc lamps the carbons must be brought closer
together about every 3-5 minutes. It is a convenience if the artist

CH. X] DRAWING WITH THE MAGIC LANTERN 329

has some sort of device, like a Hooke's jointed rod, so that the lamp
may be adjusted without getting up (see fig. 43).

For the arc lamp on the house circuit see Ch. Ill and § 486 below.

§ 463. Other radiants for drawing. — Any of the sources of light
discussed in the first six chapters can be used for drawing. One
must use the precautions given in those chapters for getting a good
screen image by a proper alignment and separation of the elements
of the apparatus, and by suiting the darkness of the room to the
light.

DRAWING WITH THE MAGIC LANTERN

§ 464. Drawing wall diagrams. — The simplest form of projec-
tion for drawing is with the magic lantern. With it the preparation
of wall diagrams is very easy (fig. 185).

If one has a lantern slide of the picture or object to be drawn it is
put into the lantern as for ordinary projection. The drawing-
board is then arranged at a distance to give the desired size, and
then all the lines traced with a crayon, a brush or a coarse pen. One
can use water colors or paints. For the black nothing is better
than India ink.

If one has a smooth wall to which the drawing paper or cloth can
be fastened, then the lantern can be moved closer or farther away
to get the desired size.

If one has no lantern slide, then a negative may be made of the
subject to be drawn, and the negative used in the lantern instead of
the lantern slide. The negative should not be too dense or the
lines will not come out clearly.

For making negatives to draw from, it is advantageous to use
lantern-slide dry plates. These will be of the right size for the
lantern and are more transparent than ordinary negatives.

For lettering diagrams nothing is more convenient than the large
rubber type found in sets used in advertising and sign making.

§ 465. Getting the desired size. — Any desired size may be
obtained by varying the distance between the drawing surface and
the projection objective. Either the lantern or the drawing surface
or both must be movable.

330

DRAWING WITH THE MAGIC LANTERN

[CH. X

The size of the drawing can be varied without moving the lantern
or the drawing surface by using an objective of longer focus for a
smaller diagram, or of a shorter focus for a larger diagram (see also
§ 5°7)-

§ 466. Use of the magic lantern for small drawings. — It fre-
quently happens that a small drawing of some large object is
needed for publication. This may be some natural object or a
piece of apparatus. The object or piece of apparatus is placed in a
good light and a small negative made on a lantern-slide plate, being
careful not to make the negative too dense. After this is dry, it
can be put into the lantern-slide carrier and projected upon the
drawing paper, and the outlines accurately traced. Then with a
pen and India ink one can ink in the lines and add any necessary
shading free-hand, having the object or piece of apparatus in view
so that it can be accuratclv done. The exact magnification or

Condenser

Arc Lamp

FIG. 185. SIMPLEST FORM OF MAGIC LANTERN WITH ARC LIGHT FOR USE

IN DRAWING.

SW Supply wires.

So K Socket with its key switch.

S P Separable attachment plug.

LW Wires extending from the cap of the plug to the knife switch.

KS Knife switch for turning the current on and off.

Rheostat The balancing device for regulating the current.

Arc Lamp The arc lamp with right-angled carbons.

Condenser The two-lens condenser with the first (i) and the second (2)
elements.

LS Position of the lantern slide or other large object.

Objective with r, its center.

Axis Axis The principal optic axis of the condenser and the objective.
The radiant must be centered on this axis.

Image Screen The drawing surface on which the image is projected.

CH. X] DRAWING WITH THE MAGIC LANTERN 331

reduction of the picture can be determined by photographing a
metric or other measure (fig. 178) on the same plate with the
object or piece of apparatus.

§ 467. Size of condenser necessary for making drawings.—

When lantern slides, or negatives made on lantern-slide plates or
other plates of that size are used, the condenser of any magic lan-
tern will answer. Sometimes, however, it is desired to make dia-
grams or drawings from negatives of larger size. There are two
ways of accomplishing this:

(1) A lantern slide can be made from the large negative by the
aid of a photographic objective as described in Ch. VIII, § 329.
This can then be used in the ordirary lantern.

(2) If the large negative is to be used direct, then the condenser
of the magic lantern must be of sufficient size to illuminate the
negative. That is, the condenser must have a diameter a little
greater than the diagonal of the negative to be illuminated and
drawn (see fig. 114).

§ 468. Drawing on a horizontal surface by the aid of the magic
lantern. — This is easily accomplished by using a 45 degree mirror
or a prism beyond the objective (fig. 192).

One must be careful to put the negative or lantern slide in the
carrier in such a way as to give an erect image (§ 512).

If the negative or lantern slide or other object is too dense, so
that the light is relatively dim, the image will be duplicated when a
mirror silvered on the back is used, therefore, one must use a prism
or a mirror silvered on the face for these dark objects. For very
transparent objects the image appears single even with a mirror
silvered on the back, the silver image being so much brighter than
the glass image that the latter does not show.

One can use the magic lantern and separate table with a 45 degree
mirror (fig. 182) or the mirror can be fastened to the projection
table as in Dr. Riley's device (fig. 184) or the mirror may be close
to the objective, and the adjustable drawing shelf used (fig.
183).

332 DRAWING WITH THE EPISCOPE [Cn. X

DRAWING WITH THE EPISCOPE OR REFLECTING LANTERN

§ 469. If one has access to a lantern for opaque objects (Chap.
VII), diagrams may be made from pictures in books and from suit-
able objects without the trouble of making a negative or a lantern
slide. The object is put in position in the reflecting lantern and its
image thrown upon the drawing surface. It can then be traced
as for a lantern-slide image, and the details, shading and lettering
added as described for diagrams made from lantern slides or from
negatives (§464).

§ 470. Drawing on a horizontal surface by the aid of the opaque
lantern. — If the apparatus is suitably arranged, the mirror will
throw the image downward upon a horizontal surface instead of out
horizontally. Then the tracing can be made as for a lantern slide
(§ 468). There is one difficulty with the reflecting lantern in mak-
ing drawings. If the object to be drawn is of some thickness, only
a part of it will be in focus at any one time, hence it is not easy to
get the parts in true perspective. (For erect images see § 514).

If one makes a small negative with a good objective, the perspec-
tive will be good and all the parts will be in focus.

When this negative is projected upon the drawing surface with an
ordinary lantern, all the parts of the image will be in focus.

If one wishes drawings of flat objects, pictures in books, etc., the
opaque lantern answers admirably, but heavy currents are re-
quired, and it is not so safe for inexperienced persons as the magic
lantern with a small current and a negative or a lantern slide (see
further in Ch. VII, § 290).

DRAWING WITH A PHOTOGRAPHIC CAMERA

§ 471. The drawing of enlargements or reductions of opaque
objects with the photographic camera has been much practised.
The object is put in a good light and arranged to show the desired
aspect, then a photographic camera is directed toward it, and the
bellows lengthened or shortened until the picture on the ground-
glass focusing screen is of the desired size. Then the plate holder
with a clear glass or a focusing screen of clear glass is used and over

CH. X] DRAWING WITH CAMERA AND MICROSCOPE 333

it some tracing paper. By covering the head with a focusing cloth
to shut out the surrounding light, one can trace the outlines of the
object on the tracing paper, and transfer these to ordinary drawing
paper, and proceed to ink them in and give the shading necessary
free-hand.

With the magic lantern or with the opaque lantern the image is
projected upon the drawing surface and regular drawing paper can
be used to make the original pencil tracing upon, but with the
camera one must use translucent paper for the tracing and then
transfer it to the drawing paper. (To get an erect image with
translucent paper see § 519).

DRAWING WITH THE PROJECTION MICROSCOPE

§ 472. Range of objects. — For drawing as for projection it is
exceedingly desirable that the projection microscope should enable
the investigator to commence where the magic lantern leaves off,
and to carry the \vork to its utmost possibilities; that is, begin-
ning with large specimens of 50 to 60 mm. (2 in.) in diameter re-
quiring low objectives, and going on from this to the smallest
objects visible and using the oil immersion objective at the other
extreme.

To realize this ideal possibility one must have available for
drawing some such outfit as that described in Ch. IX for projec-
tion ; and in addition suitable arrangements for reflecting the image
down upon a horizontal drawing surface. Fortunately, the addi-
tions are relatively simple and inexpensive.

Finally, for the widest usefulness in drawing there must be the
possibility of using the ordinary house electric lighting system for
an electric lamp with small carbons (see § 486).

§ 473. Drawing large objects with low powers. — For this it is
necessary to have a stage with a large opening (fig. 134), and the
objective must be mounted in a shield with no tube at all (fig. 138),
or the tube must be short and of large diameter, so that the field is
not restricted (fig. 137). Finally, there must be some means of
increasing or diminishing the distance between the objective and
the drawing surface to get the desired magnification.

334 DRAWING WITH PROJECTION MICROSCOPE [Cn. X

• h

FIG. 1 86.

APPARATUS FOR DRAWING WITH THE MICROSCOPE WITHOUT
AN OCULAR OR SUBSTAGE CONDENSER.

The arc'lamp is Mr. Albert T. Thompson's automatic lamp for direct current,
5-25 amperes. This is the first automatic arc lamp for right-angled carbons.

By means of the optical bench carrying all the apparatus, the different
parts are pulled forward so that the microscope tube and mirror project over
the drawing shelf. This is adjustable up and down for varying the magnifi-
cation.

The stage of the microscope (st) is independent and contains a large glass
water-cell against which the specimen rests. It conducts away the heat from
the specimen.

a a a Optic axis.

b Optical bench with track.

c Triple condenser with water-cell.

/ Thompson's automatic arc lamp for 5-25 amperes direct current.

m Microscope without ocular. The 45° mirror reflects the light down upon
the drawing surface.

r Adjustable rheostat.

s Double-pole knife switch (table switch).

st Stage with the stage water-cell for cooling the specimen. The stage is
entirely separate from the microscope.

sh Shield 25 cm. in diameter to stop any stray light from the stage of the
microscope.

•wi Double cable supplying the electric current to the apparatus.

•W2 Flexible cables from the switch to the lamp.

CH. X] DRAWING WITH PROJECTION MICROSCOPE 335

§ 474. Varying the drawing distance. — The drawing distance
is easily .varied by means of a movable table like that figured (fig.
182), or by an adjustable shelf attached to the projection table (fig.

183)-

Another way of varying the size of the drawing is to use higher or
lower objectives, the drawing distance remaining the same (see
§5°7)-

§ 475. Lighting the object. — For large objects and low powers
the best way to illuminate the object is to use the main condenser
only and to put the object in the cone of light where it is fully
illuminated (fig. 132). If the drawing shelf is used this will involve
moving the lamp and condenser toward the drawing-board ; for the
microscope must be beyond the end of the table, so that the image
can be thrown down on the shelf, (fig. 186). The change in posi-
tion of any part or parts is, of course, very easy with an optical
bench (fig. 158-159).

§ 476. Drawings with objectives of 16, 12, 10, and 8 mm. —

With objectives of this range without an ocular, one can draw
objects varying from 5 to 2 mm. in diameter. For lighting, use the
large condenser and focus the image of the crater on the hood of the
objective (fig. 140), and then push the stage up toward the objec-
tive until the object is in focus, finishing the fine focusing with the
micrometer screw of the microscope.

DRAWING WITH THE PROJECTION MICROSCOPE, INCLUDING AN
OCULAR AND A SUBSTAGE CONDENSER.

§ 477. Drawing fine details with high powers (8 to 2 mm. focus) .

— As pointed out for the projection of images showing fine details
(§ 401), it is necessary to use a substage condenser to get the neces-
sary aperture of the lighting beam, and to use an ocular to com-
pensate for objective defects. If one uses a water or an oil immer-
sion objective the proper immersion fluid must be used between the
cover-glass and the objective, as in ordinary microscopic work.

§ 478. Parallelizing the converging beam of light. — The sub-
stage condenser used for ordinary observation is designed for ap-

336

[Cn. X

FIG. 187. AN ORDINARY MICROSCOPE USED WITH THE LAMP AND
CONDENSER OF A MAGIC LANTERN FOR DRAWING OR PROJECTION.

W The supply cable from the outlet box (fig. 3).

s The table, knife switch.

r Rheostat of the theater-dimmer type.

/ The automatic arc lamp. This is the three-wire automatic arc lamp of
the Bausch & Lomb Optical Company for 5-25 amperes. (For the method
of connecting the wires see § 704).

c Triple condenser with water-cell.

a a Principal optic axis.

p The concave parallelizing lens to render the converging cone from the
condenser parallel or nearly so before entering the sub stage condenser of the
microscope.

m The microscope in a horizontal position. If it is to be used for drawing
there must be a prism or mirror beyond the ocular to reflect the light down on
the drawing shelf.

b Baseboard with track serving as an optical bench.

a s Adjustable drawing shelf on the front of the projection table.

proximately parallel light (fig. 150 A. B.), hence it is necessary to
render the converging cone of light from the main condenser
approximately parallel. This is most easily accomplished by using

CH. X] DRAWING WITH HIGH POWERS 337

a plano-concave or double-concave lens. This is mounted in a
fork-like holder and is set in the socket for the mirror stem of the
microscope (fig. 152, 187). Then the microscope is pushed up
toward the condenser until the parallel beam is of sufficient diam-
eter to fill the substage condenser. The substage condenser
diaphragm is opened to its full extent.

§ 479. Concave lens to be employed. — This depends upon the
focus of the main condenser. If the focus is about 15 cm. (6 in.), use
a concave lens of -16 to -20 diopters (§ 356). If the main condenser
has a focus of 20 to 40 cm. (8 to 16 in.), use a concave lens of -8 to
-12 diopters. The longer the focus of the main condenser the.
shallower can be the concavity of the parallelizing lens. Indeed,
for objectives of 16, 12, 10, and 8 mm. focus a condenser lens of 25
to 38 cm. (10 to 15 inches) focus gives very good results, when
the substage condenser is used without any parallelizing lens
(fig. 154).

§ 480. Position of the substage condenser; opening of the
condenser diaphragm. — As pointed out in Ch. IX (§ 407), the posi-
tion of the substage condenser must be very precisely determined
for different objectives and for different thickness of slides.

To begin with, the substage condenser diaphragm is opened to
its full extent. Then in each case one must get the sharpest possi-
ble image by getting the best position of the substage condenser,
and closing the diaphragm more or less. As a general statement,
the diaphragm should be considerably wider open for drawing
than for ordinary observation.

§ 481. Oculars to employ for drawing. — Those of X2, X3, X4, x6,

x8, and xi2 may be used. Naturally, the lower and medium
powers give the more brilliant images as for direct observation,
One will rarely need to use an ocular higher than x8.

§ 482. Mirror or prism for reflecting the image-forming rays
down upon the drawing surface. — For high power drawing it is
better to have the reflecting mirror or prism close to the ocular
(fig. 192) rather than to have it distant, as with the drawing table
in figure 182.

338 DRAWING WITH HIGH POWERS [Cn. X

If a mirror is used it must be a perfect one and preferably slivered
on the face to avoid duplicating the images. If it is silvered on the
back the glass must be thin. A totally reflecting prism is best,
but it is somewhat expensive, costing about twice as much as the
mirror.

§ 483. Avoidance of distortion. — Whichever is used for reflect-
ing, it should be fitted with a stop so that it will be at 45 degrees
with the main axis, then the image-forming rays will be reflected
directly downward and the image will not be distorted, provided
of course, that the mirror or prism is directly above the drawing
surface. If it were turned over to one side more or less, the image
would be correspondingly distorted.

It is a good plan for one to become familiar with the distortions
possible in drawing. For example, if the mirror or prism is not at
45 degrees with a horizontal microscope (fig. 182, 193), the spot of
light on the drawing surface will not be circular but elliptic, the axis
of the ellipse being parallel with the optic axis of the microscope.
If the prism or mirror is not directly above, but turned to one side,
then the spot of light will be elliptic and projected to one side of the
axis of the microscope. If one is familiar with the possible dis-
tortions it will be easy to detect them ; then they can be corrected.
Naturally, a drawing should be accurate when finished.

§ 484. Specimens suitable for drawing with high powers. — Any

object suitable for projection can have its image projected upon a
drawing surface (see also § 410).

§ 485. Amount of electric current required for drawing. — If one
has a direct current, 5 to 10 amperes will be sufficient for all draw-
ing purposes. The specimens must usually be left for a consider-
able time in the focus or near the focus of the light beam, and hence
are liable to overheating. The lower the amperage the less the
danger from the overheating. Then it is not good for the eyes
of the artist to have the light on the drawing surface too dazzling.

With alternating current, 6 to 15 amperes usually suffice.

Here, as in all other projection, skill is of more account than
overwhelming electric currents.

CH. X]

DRAWING WITH HOUSE CURRENT

339

PROJECTION DRAWING APPARATUS WITH THE RADIANT
CONNECTED WITH THE HOUSE LIGHTING SYSTEM.

§ 486. General Statement. — As shown in Chapter III (fig.
41-43), the arc lamps using small, cored carbons (6 to 8 mm. in
diameter) and drawing from three to six amperes may be connected
with any socket for an incandescent bulb of the house lighting
system. The light so obtained is more powerful than the usual
lime light. The carbons being small, the light approaches closely
to the ideal point source. Consequently for all projection pur-
poses, including drawing, this form of arc light is of the greatest
importance and utility. Of course, for projection in a large hall
it is insufficient, but for the relatively small screen pictures needed
in drawing and for small classes, the results are very satisfactory.

§ 487. Wiring, rheostat and connections for the arc lamp
attached to the house lighting system. — This is shown in fig. 188-
189 and described in § 128-135. Remember and practice the
advice given about turning the current on and off (§ 133), and the
possibility of short circuiting and burning out the incandescent
bulb socket. Never use an arc lamp without a suitable rheostat
or inductor. (See § 129, also § i28a for fuses on the house system).

FIG. 1 88. WIRING AND CONNECTIONS OF THE ARC LAMP USED ON THE
HOUSE LIGHTING SYSTEM (See fig. 45).

340

DRAWING WITH HOUSE CURRENT

[Cn. X

FIG. 189. SMALL ARC LAMP FOR DRAWING.

Commencing at the left :

Wi Supply wires

So Lamp socket.

K Key switch in the socket.

Sp Separable attachment plug.

W2 Wires to the arc lamp.

W3 Wire to the binding post of the upper carbon.

W4 Wire to the rheostat (R) and from the rheostat to the binding post
of the lower carbon.

A Support of the arc lamp; the lamp can be raised or lowered on this
support.

F Feeding screws for the carbons.

H C, V C The horizontal and the vertical carbons.

In In Insulation between the carbon holders and the remainder of the
lamp. This prevents the current from taking any path away from the carbons.

Ch Chimney over the arc.

T C The tube and the condenser in the movable inner tube. The con-
denser is at its focal distance from the crater, and therefore the rays are made
parallel.

CH. X] DRAWING WITH HOUSE CURRENT 341

Sh Shield to stop stray light and to aid in centering.

C Carbons with alternating current. They are of the same size.

D Carbons with direct current. The upper one is 8 mm. and the lower one
6 mm. in diameter.

E Shield or disc at the end of the condenser tube showing the opening of
the condenser (C) and the spot of light at the right.

§ 488. Arc lamp and small carbons. — The form of arc lamp to
use on the house circuit is not of particular importance. It may
be very conveniently one of the small lamps shown in fig. 41-44,
201, 205, or it can be an ordinary arc lamp for greater currents,
but supplied with long clamping screws, bushings or adapters for
the small carbons (§ 127). The small lamps are generally of the
hand-feed type and move the upper and the lower carbons equally.

§ 489. Size of carbons for direct current. — A. — The carbons
found useful for direct current are as follows, all being of the soft-
cored variety:

(1) Upper or positive carbon 7 mm. in diameter, lower or nega-
tive carbon 5 mm.

(2) Upper carbon 8 mm., lower 6 mm.

(3) Upper carbon n mm., lower 8 mm.

B. — The carbons for alternating current with an equal feed for
the upper and the lower carbon, should be of the same size, and this
size should not exceed 8 mm. in diameter for 5 to 6 amperes. If
only three or four amperes are used, then it is better to have carbons
not greater than 6 mm. in diameter.

§ 490. Reason for using small carbons. — In order to have the
light steady and thus have the field continuously bright, the entire
end of the upper carbon should be white hot.

If the carbon is so large that the crater covers only a part of the
tip, the crater will wander about on the end of the carbon. Every
change in the position of the crater changes the direction of the
light beam. While the crater is in one position the entire field of a
high power objective may be brilliantly illuminated; if the crater
wanders to a new position, the field will be only partly or not at all
illuminated. In such a case, one must constantly change the posi-
tion of the mirror of the microscope to keep the field bright. If,
however, the crater is nearly as large as the end of the carbon, it

342

DRAWING WITH HOUSE CURRENT

[Cn. X

will wander but little, if at all, and the light will be more con-
stant.

§ 491. Feeding the carbons together. — If one has an alternat-
ing current to work with, the small arc lamp will burn about 10
minutes with 8 mm. carbons before going out. With the right-
angle position the carbon giving the light remains constantly in
the axis. With inclined carbons, it rises constantly above the axis.
The carbons with the right-angle arc should be fed about every
five to seven minutes to insure the best light.

FIG. 190. SIDE AND FRONT VIEW OF SMALL CARBONS WITH FIVE
AMPERES OF DIRECT CURRENT (Natural Size). Compare fig. 191.

FIG. 191. SIDE AND FRONT VIEW OF SMALL CARBONS WITH FIVE AMPERES
OF ALTERNATING CURRENT (Natural Size).

The crater is much smaller than with direct current (fig. 190).

CH. X] DRAWING WITH HOUSE CURRENT 343

If direct current is used, the lamp will burn for about six
minutes and the carbons should be fed together every three to five
minutes. (See fig. 205).

CONDENSER, STAGE AND MICROSCOPE FOR DRAWING WITH THE
HOUSE LIGHTING SYSTEM

§ 492. Drawing outfit. — If one has a drawing outfit consisting
of the projection apparatus shown in figure 182, all that is necessary
to do is to place the arc lamp with its small carbons in the lamp-
house and arrange it exactly as for projection.

The procedure is precisely as described above for the ordinary
arc lamp on the usual special lantern lighting system (Ch. IX).

§ 493. Small Current Outfit. — This consists of an arc lamp
using small carbons (6 to 8 mm. in diameter) and a rheostat or an
inductor (fig. 197) not allowing over 5 to 6 amperes of current to
flow. Instead of the usual large condenser (fig. 1 2 1) , a small, single,
convex lens is used. This is of 70 to 100 mm. (3 to 4 in.) focus,
and 37 to 50 mm. (i^ to 2 in.) in diameter, and is placed in a tube
extending straight out from the upper carbon. Usually, also, the
lens is in a sliding tube, so that it may be varied in distance from
the source of light. If it is at its focal distance from the light, the
beam will be approximately parallel (fig. 189); if farther from the
light, the beam will be converging.

§ 494. Method of using the lamp with a special condenser.—

There are three methods of using this arc lamp and special con-
denser :

(1) The lamp can be put in line with the drawing microscope
and a converging beam thrown directly on the specimen as for the
large apparatus (fig. 132), the mirror and sometimes the substage
condenser having been removed or turned aside.

(2) The mirror is removed from the microscope, but the sub-
stage condenser is left in position, and a parallel beam of light
thrown directly into the substage condenser along the optic axis

(fig. 20lA).

344

Substage
Condenser

DRAWING WITH HOUSE CURRENT

[Cn. X

Microscope _

Objective '' ^ Ocular

FIG. 192.

DIAGRAM OF THE MICROSCOPE ARRANGED FOR DRAWING ON A
HORIZONTAL SURFACE.

The light is from an arc lamp supplied by the house current (fig. 188, 189).

A right-angled prism is used to reflect the rays down upon the drawing sur-
face.

The designations are self explanatory except in the ocular r i means the real
image formed by the objective and field lens (see fig. 207).

The adjustment screw heads at the side of the microscope are:

/ a Fine adjustment.

c a Coarse adjustment.

H The handle in the pillar for carrying the microscope.

(3) From the difficulty of getting the small lamp and condenser
in the optic axis without the use of an optical bench it has been
found much easier to get the light upon the specimen and through
the microscope by placing the arc lamp and its condenser at right
angles to the microscope, and to use the regular microscope mirror
for reflecting the beam through the substage condenser (fig. 193).
If the substage condenser is not used, the mirror reflects the beam
directly on the specimen, as for low power projection.

CH. X] DRAWING WITH HOUSE CURRENT 345

§ 495. Microscope. — Any modern microscope with a good sub-
stage condenser can be used, provided it is supplied with a flexible
pillar, so that the tube can be made horizontal; and provided also,
that the fine adjusting mechanism will work when the tube is
horizontal.

There must be a prism or mirror beyond the ocular to reflect the
image-forming rays downward upon the drawing surface (fig. 192).

The discussion of avoidance of distortion, the proper objectives,
oculars, etc., to use, which was given in the earlier part of this
chapter apply here (§ 452, 483).

§ 496. Position of the microscope for drawing. — In the drawing
outfits thus far devised, the microscope is placed in one of the
following positions :

(1) In an inverted position with the objective pointing directly
upward (as in the large Edinger apparatus, fig. 202).

(2) Inclined at 45 degrees (as in the small Edinger apparatus,
fig. 204).

(3) In a horizontal position (fig. 192).

With the microscope in an inverted, vertical position, there
should be no distortion of the image if the drawing surface is
horizontal.

With the inclined microscope, the mirror used must be so inclined
that it throws the image directly down upon the horizontal drawing
surface, or the image will be distorted. It is not easy to tell just
the inclination of the microscope, and therefore, the exact inclina-
tion to give the mirror, to make the axial ray perpendicular to the
drawing surface. In the small Edinger apparatus (fig. 204), the
directions are to make the inclination of the microscope 45 degrees
and the inclination of the mirror 22^ degrees. This arrangement
will give a correct image. One may need to use a protractor to
make sure that the inclination of the microscope and mirror are
exactly correct.

With the horizontal microscope, the mirror or prism is so
arranged that it is fixed at 45 degrees and therefore if put directly
over the ocular of the horizontal microscope, will reflect the light
perpendicularly upon the drawing surface, thus avoiding distortion
(see § 483).

346

[On. X

With the horizontal microscope, unless one uses a table with a
drawing shelf (fig. 187), the microscope must be raised on a block
or support of some kind and clamped to the block so that it will be
rigid (fig. 193-194). A convenient height is 250 mm. (10 inches).

To vary the magnification slightly, the distance can be made
greater by using an additional block, or it may be made less by
raising the drawing surface. For a very convenient arrangement
for changing the elevation of the microscope see fig. 198, 2oiC.

For obtaining the scale or magnification of the drawing see
§ 508-510.

§ 497. Getting the light through the horizontal microscope
with the plane mirror. — The simplest method is to place the lamp

FIG. 193. DRAWING WITH THE MICROSCOPE IN A DARK ROOM.

In the arrangement here shown the light is from a small arc lamp drawing
current from the house lighting system.

The supply cable and the lamp socket are shown, then the separable attach-
ment plug and the supply wires with the rheostat inserted along one wire (fig.
1 88).

The arc lamp is at the level of the microscope mirror and at right angles with
the microscope axis. The light from the arc lamp is reflected up to the sub-
stage condenser by the mirror and passes on through the specimen and micro-
scope as shown in fig. 192.

The shield between the lamp and drawing surface is to keep stray light from
reaching the drawing surface. The shield is represented as transparent. It
was left in place during only a part of the time of the exposure in making the
photographic negative.

CH. X] DRAWING WITH HOUSE CURRENT 347

and the microscope at right angles. Use a level (fig. 160) and make
sure that the condenser tube is horizontal, and the axis of the con-
denser at the same level as the center of the mirror. Place a disc
of blackened asbestos or tin of about 12.5 cm. (5 in.) in diameter
just behind the condenser as shown in fig. 193-198. This is easily
done by making a hole of the proper size in the disc to go over the
condenser tube (fig. 195). If now the current is turned on and the
arc established the light will extend from the condenser to the plane
mirror and be reflected by it, if it is set at 45 degrees, up into the
substage condenser. From the lower face of the substage con-
denser a part of the light is reflected back to the mirror and from
the mirror back toward the lamp, and is received by the black disc
over the condenser tube. The mirror should then be turned until
the spot of light enters the lamp condenser. The mirror will then
be in position to reflect the light along the optic axis of the micro-
scope.

If the microscope is in focus on the object, the light will traverse
the objective and ocular and be reflected down upon the drawing
surface by the mirror or prism beyond the ocular.

By changing the mirror slightly while watching the circle of light
on the drawing surface, the best illumination can be obtained.

§ 498. Getting the light through the microscope with the con-
cave mirror. — One proceeds exactly as described above, only the
light reflected back to the black disc on the lamp condenser tube
will be a crescent instead of a circle. The middle part of the cres-
cent can be reflected into the lamp condenser and then the light will
pass through the microscope and be reflected down upon the draw-
ing paper, provided the microscope and the arc lamp are at right
angles and at the proper level.

§ 499. Substage condenser. — Use the substage condenser with
objectives of 16, 12, 10, 8, 6, 4, and 2 mm. focus. For objectives
lower than the 16 mm. the substage condenser is turned aside.

With different objectives and slides of different thickness the
substage condenser is changed somewhat in position to get the best
light on the object and to light the entire field.

348 DRAWING WITH HOUSE CURRENT [Cn. X

To start with, the substage condenser diaphragm should be
opened widely. In some cases the picture can be made sharper by
afterward closing the diaphragm somewhat.

For drawing, a skillful use of the substage condenser is very
important. One must be more precise in its use than in ordinary
microscopic observation.

FIG. 194. DRAWING WITH A MICROSCOPE WITH THE ARC LAMP AT RIGHT

ANGLES.

In this picture a prism is placed beyond the ocular to reflect the light down-
ward (fig. 192). The arc lamp is on the back side of the microscope with the
condenser facing the mirror. The spot of light on the shield or disc above the
lamp shows that the light is not centered along the axis of the microscope.
The mirror must be turned slightly until the light reflected back from the
substage condenser and microscope mirror enters the condenser tube of the
arc lamp (see fig. 195).

§ 500. Plane mirror and substage condenser. — Use the plane
mirror and substage condenser for all objectives of 12, 10, 8, 6, 4,
and 2 mm. equivalent focus.

§ 501. Concave mirror and substage condenser. — For the 16 to

1 8 mm. focus objectives use the substage condenser with the con-
cave mirror. It may also be necessary to separate the condenser
somewhat from the preparation to light the entire field.

CH. X] DRAWING WITH HOUSE CURRENT 349

§ 502. Concave mirror without a substage condenser. — For

objectives of 20, 25, 30, 35, 40 and 50 mm. focus use the concave
mirror without a substage condenser.

§ 503. Immersion objective. — -For immersion objectives used
in drawing do not forget to use the proper immersion liquid
between the cover-glass and the objective ; cedar oil for the oil
immersions, and distilled water for the water immersions.

FIG. 195. SHIELD AT THE END OF THE ARC LAMP CONDENSER TUBE TO
AID IN CENTERING THE LIGHT.

This disc is of blackened sheet iron, asbestos or cardboard and is 125 to 150
mm. in diameter. It is placed at the end of the lamp condenser tube. If the
light is centered, then that reflected back from the substage condenser and
microscope mirror will enter the lamp condenser (C). If the light is not cen-
tered there will be a round spot of light somewhere outside the lamp condenser.
In that case the mirror must be turned slightly until the reflected light enters
the lamp condenser.

If the plane mirror is used the spot of light will be nearly circular; with the
concave mirror it will be crescentic.

C The lamp condenser.

I Spot of light outside the condenser showing that the light is off the center.

350

DRAWING WITH HOUSE CURRENT
AVOIDANCE OF HEAT

[On. X

§ 504. When the small currents from the house circuit are used
the heat is not great enough to injure most specimens mounted in
balsam. For live objects and objects mounted in glycerin or
glycerin jelly, etc., it would be wise to place a water-cell in the beam
before it reaches the microscope (see § 364, 3Q4a, fig. 206).

FIG. 196.

DRAWING OUTFIT FOR THE HOUSE LIGHTING SYSTEM WITH A
BLACK CLOTH TENT OVER THE MICROSCOPE.

This arrangement answers well for a moderately lighted room. Naturally
the opening for drawing should face toward some dark furniture or the dark
side of the room, not toward a window.

5 Separable cap to attach to the separable plug in a socket of the house
lighting system.

w r One of the supply wires cut and inserted into the binding posts of the
rheostat (see fig. 188).

/ Small arc lamp for supplying the illumination. It is at the level of the
mirror and at right angles to the microscope.

m Mirror of the microscope.

t Cloth tent over the microscope. It appears semi-transparent as it was
left in position during but part of the time when the photograph was
taken.

CH. X] LIGHT SHIELD FOR DRAWING 351

SHIELDING THE DRAWING SURFACE FROM STRAY LIGHT

§ 505. Shield for working in a dark room. — If one works in a
dark room all that is necessary to screen the drawing from stray
light from the arc lamp, when the lamp is at right angles to the
microscope, is a blackened cardboard shield (fig. 193).

If the lamp is in line with the microscope, it will be necessary to
put a shield with a perforation for the light beam either before the
beam reaches the microscope, or it may be put over the tube of the
microscope so that it will shield the drawing surface.

§ 506. Drawing in a light room. — If this is necessary one should
get in as shaded a part of the room as possible. To screen the
drawing surface there are two ways :

(1) There may be a cloth for enclosing the drawing surface and
the head of the artist. This is like the plan used in focusing a
photographic camera (fig. 204).

(2) By means of cardboard, or of a wire frame and cloth cur-
tains, a box or tent is built around the drawing surface enclosing
also the microscope tube. The end of the box next the draughts-
man is open sufficiently for him to see the image (fig. 196, 198).
The drawing surface should look toward some dark furniture
or a dark or shaded part of the room, and except for the most
exacting work the surface will be sufficiently shaded. For the most
exacting work, and for the greatest freedom from accessories, the
evening or a dark room in the daytime offers the best facilities
(§ 453)-

How TO GET ANY DESIRED MAGNIFICATION IN A DRAWING

§ 507. The magnification can be varied by any of the following
ways, or two or more of the ways may be combined.

(a) By using a higher or lower objective.

(b) By using an amplifier, of greater or less power, with the
objective.

(d) By changing the distance of the drawing surface; the
farther it is away in any given case the larger will be the image, and
the nearer it is the smaller the image (§ 5ioa).

352

MAGNIFICATION OF DRAWINGS

[Cn. X

FIG. 197. DRAWING OUTFIT FOR THE HOUSE LIGHTING SYSTEM, USING
AN INDUCTOR INSTEAD OF A RHEOSTAT (fig. 193).

Commencing at the left :

The supply wires to the lamp socket, and the supply wires from the separable
attachment plug to the arc lamp.

One of the supply wires is connected directly with the arc lamp and one is
cut and the two cut ends connected with the two poles of the inductor exactly
as with a rheostat (fig. 188), and from the inductor the wire is continued to one
of the binding posts of the arc lamp.

The inductor is only for alternating current (§ 736). The amperage can be
varied by sliding the soft iron core in and out of the coil. The more the core
is inserted the greater the inductance and hence the less the amount of current
that is allowed to flow.

As shown in the picture, the core is only partly inserted and a medium
current is allowed to flow.

If one uses alternating current this is a much more economical method of
controlling the current than a rheostat (see § 736 + ) and a steadier light is
produced.

The two most common changes are: (i) Using a higher or
lower objective; and (2) Changing the distance of the drawing
surface.

CH. X] MAGNIFICATION OF DRAWINGS 353

For slight variations in size the change in distance is by far the
best and easiest change to make.

If one has a drawing table (fig. 182) it is very simple to push it
farther from or closer to the projection apparatus.

If the drawing shelf is used it can be raised or lowered (fig. 183).

If the simple apparatus is used on an ordinary table the entire
microscope can be raised for higher magnifications or for lower
magnifications the drawing surface can be raised to bring it closer
to the microscope tube (fig. 193-204), or the microscope can be
lowered on its adjustable support (fig. 198).

DETERMINATION OF THE MAGNIFICATION OF A DRAWING

§ 508. For getting the magnification it is necessary to use for
an object a transparent micrometer with known divisions upon it.
For most of the work done a micrometer with heavy lines every
half millimeter is satisfactory. These lines may be ruled on glass
and filled with graphite, or they may be made by photography
(see § 5o8a).

For example, if the micrometer used has half millimeter spaces,
the image projected on the screen will be magnified more or less
according to the distance of the screen and the objective used.
The exact size of the image is easily measured on the drawing sur-
face with a millimeter scale. Suppose that two of the half milli-
meter spaces were used as object, the object would be one milli-
meter in actual size. If the image of two spaces projected on the
screen or drawing surface measured 2 5 millimeters then the magni-
fication would be 25.

§ 508a. One can make a satisfactory micrometer for determining the
magnification of drawings as follows : Make a negative of the millimeter scale
(fig. 178, 21 1) making the picture exactly half the size of the original scale, then
the spaces will be half millimeters. As the scale is black with light lines the
negative will show dark lines with intervening clear spaces exactly as is a glass
micrometer.

If so desired the micrometer lines may be covered with Canada balsam and
a cover-glass applied as for microscopic specimens. (See The Microscope,
§ 354~5> P- 257)- This would protect the lines and make the specimen more
transparent. A lantern-slide plate is the best for making the negative, as it
gives transparent lines.

354

MAGNIFICATION OF DRAWINGS

[Cn. X

Briefly stated, if one has an object of known size, then the size of
the image divided by the size of the object will give the magnifica-
tion in every case. In the example given above the object is i mm.
and the image is 25 mm. : 25 -=- i = 25. Or, if one took a single
space as object, that is, half a millimeter, then the image would
measure 12.5 mm., and 12.5 -=- .5 = 25 as before (see § sioa).

FIG. 198. SPENCER LENS COMPANY'S APPARATUS FOR DRAWIM; WITH

THE MICROSCOPE.
(Cut loaned by the Spencer Lens Co.).

This consists of a small arc lamp with the proper wiring, rheostat and con-
nections for the house electric supply. The lamp has all the adjustments, and
the condenser tube is telescoping so that the beam of light may be parallel or
converging.

The microscope is supported on an adjustable shelf which can be raised or
lowered on the vertical rods, thus enabling one to get any desired magnification.

The vertical supports for the microscope shelf serve to carry a curved metal
band to support the cloth curtains to shade the drawing surface. There are
two curtains and they hang freely, thus avoiding all interference with the
hands in drawing. If one desires, the arc lamp can be put in line with the
microscope and the mirror turned aside.

For a reflector beyond the ocular a prism is used, thus avoiding any of the
defects of a mirror.

CH. X]

MAGNIFICATION OF DRAWINGS

355

MAGNIFICATION OF WALL DIAGRAMS, AND DRAWINGS MADE WITH
THE OPAQUE LANTERN

§ 509. Scale of wall diagram. — Make the diagram any desired
size as directed in § 457. Then remove the negative or lantern
slide from the holder and insert a lantern slide or negative of the
metric rule (fig. 178, 211). The image of the metric rule on the
drawing surface will, of course, be magnified exactly as much as the
diagram. By using a metric measure, one can find the magnifica-
tion of the screen image exactly as described in § 508.

For wall diagrams, it is not usually very important to know the
magnification. All that is necessary is to get it large enough to be
seen well. But if one wishes to show the relative size of objects
such as blood corpuscles of different animals, then the magnifica-
tion must be known.

\_j

FIG. 199. MODEL 4-5 DELINEASCOPE WITH THE PROJECTION MICRO-
SCOPE IN A HORIZONTAL POSITION.

(Cut loaned by the Spencer Lens Co.).

With the microscope in this position the image may be thrown down on a
horizontal surface for drawing by pushing the totally reflecting prism up in
the microscope tube to intercept the light (see fig. 109, 175).

356

MAGNIFICATION OF DRAWINGS

[Cn. X

FIG. 200. COMBINED DRAWING AND PHOTO-MICROGRAPHIC APPARATUS

OF THE BAUSCH & LOME OPTICAL COMPANY FOR USE ON THE

HOUSE LIGHTING SYSTEM.

(Cut loaned by the Bausch & Lomb Optical Co.).

This is a kind of universal apparatus serving for drawing with the microscope,
projection with a microscope and with a magic lantern; opaque projection,
and finally for photographing with all objectives and with the microscope.
It can be used in a horizontal, an inclined or a vertical position. For
drawing with the microscope in a horizontal position there is an adjustable
drawing shelf with a cloth tent for shutting out daylight in a light room.

The large condenser enables one to use the apparatus on specimens of all
sizes up to lantern slides.

§ 510. Scale of diagrams or drawings made with the opaque
lantern. — If one uses the episcope or opaque lantern, or a photo-
graphic camera for drawing, it is very easy to get the exact magnifi-
cation of the drawing by putting a metric rule upon some part of

CH. X] DRAWINGS FOR MODELS 357

the object, or beside it. It will be at the same scale of magnifica-
tion or reduction as the drawing.

In practice some lines of the image of the scale are made beside
the drawing. For example, suppose the image of one centimeter
measured on the drawing was 10 centimeters long, one would know
that the drawing is 10 times larger than the object. If the length
of the centimeter on the drawing was only one-half centimeter long,
then one would know that the drawing is only half as large as the
object and so on (§ 5o8a, 5ioa).

DRAWINGS FOR MODELS

§ 511. Drawings for models. — These are made much more
easily with projection apparatus than with the camera lucida or in
any other way. The simple drawing outfit for use on the house
circuit described above makes it possible for every laboratory and
indeed every private worker to use this effective method, even if
complete projection apparatus and heavy lantern currents are not
available.

In making drawings for models several steps must be taken in
order that the resulting model shall be anything like a true repre-
sentation of the actual object.

(1) The object (embryo, etc.) should be photographed at a
known magnification before it is sectioned.

(2) The sections should be made of a known thickness
1 5 [A, etc.).

§ 510a. The general law for magnification and reduction. — With a given
object the size of the image depends directly upon the relative distance of the
object and of the image from the center of the lens (fig. 185, 209, 210). If the
image is farther from the center of the lens than the object then the image will
be larger than the object; conversely if the image is nearer the center of the
lens than the object then it will be smaller than the object.

For example, if the image is to be ten times as large as the object the image
must be ten times as far from the center of the lens as the object.

Conversely, if the image is to be one-tenth as large as the object it must be
formed only one-tenth as far from the lens as the object.

In lantern-slide and micro-projection, and in photo-micrography the image
is much larger than the object and correspondingly more distant from the cen-
ter of the lens. In ordinary portrait photography and in landscape photo-
graphy the image is much smaller than the object, and consequently the image
is much nearer the lens than the object (see also § 392a).

DRAWING WITH THE MICROSCOPE

ICH. X

FIG. 201, A, B, C. SIMPLE DRAWING OUTFIT FOR THE MICROSCOPE.

(Cuts loaned by the Bausch & Lomb Optical Co.).

CH. XJ DRAWING FOR MODELS 359

There is a hand-feed, right-angled arc lamp for small carbons, wiring and
connections for the house circuit and a rheostat which will not permit over 6
amperes of current to flow. The lamp condenser is in a telescoping tube so
that either a parallel or a converging beam of light can be obtained. To avoid
stray light the drawing surface is enclosed by a metal box with one side removed.

A Drawing outfit with the lamp and microscope in line.

The microscope is supported on a block to give a drawing distance of 254 mm.
(10 inches).

B Drawing outfit with the arc lamp at right angles to the microscope.

C Drawing outfit with the microscope on an adjustable platform and the
arc lamp at right angles with the microscope.

(3) It must be decided in the beginning how much larger the
model is to be than the original object.

(4) The objective and the drawing surface must be chosen and
mutually arranged so that the desired magnification is attained

(§ 509)-

(5) The object must be placed on the stage of the microscope so
that the image reflected down upon the drawing surface will be
erect, that is, exactly like the object and not inverted in any way
(see below § 512).

(6) Each drawing as it is made must be numbered to correspond
with the number of the section : This is very important.

(7) It is desirable to make a duplicate set of drawings, for one
set is used up in making the model, and one needs a set for reference.

The duplicate drawings are easily made by using thin carbon
paper as in duplicating writing, or in typewriting.

(8) Marking the position of the apparatus. If all the draw-
ings cannot be made at one time, then the objective, the ocular,
if one is used, and the distance of the drawing surface from the
tube of the microscope should be carefully measured or indicated
by chalk marks, so that when working again exactly the same
magnification can be used. It is well also to check up by using the
stage micrometer again (§ 508). Pictures for models may also be
made by photography, (see § 542).

ERECT IMAGES

§ 512. It has been known from the first use of projection appar-
atus that the projected image was inverted, and that this is true
whether a simple aperture, a simple lens, or an objective of several

3 6o

ERECT IMAGES IN DRAWINGS

[Cn. X

The arc lamp is of the Liliput
form with small right-angled car-
bons.

The lamp condenser is large,
such as is used for lantern-slide
projection, hence large as well
as small objects can be illumi-
nated by it.

For convenience in feeding the
carbons there is a rod extending
down within reach of the artist.

The microscope and stage are
separate and independently
movable on the vertical optical
bench. In addition to the lamp
condenser there are two or more
substage condensers of different
foci.

The object is put on the upper
side of the stage.

The microscope can be used
with an ocular, or the draw tube
and ocular can be removed from
the large microscope tube, and
then objectives alone'used, thus
givingf very large fields.

GER'S VERTICAL DRAWING AND PHOTOGRAPHIC
APPARATUS FOR USE ON THE
HOUSE CIRCUIT.

(Cut loaned by Ernst Leitz).

CH. X] ERECT IMAGES IN DRAWINGS 361

lenses is used. The earliest workers also saw that an easy way to
correct for this was to invert the object, then its image would
appear in the natural position. But some objects do not admit of
inversion, hence the effort to obtain erect images by some optical
means.

The first and still the simplest method is by the use of a plane
mirror with a horizontal screen (fig. 88, 89, 181, 204). The mirror
might be put in the course of the beam before or after it has passed
the objective. Figure 89 shows it before and figure 182 after
traversing the objective.

It was demonstrated by Kepler (1611) and practically worked
out by Scheiner (1619) that erect images could be produced by the
use of two objectives in line. The first objective gives a real
inverted image of the object, and the second gives a real, erect
image of the inverted image (fig. 208). This is what occurs when-
ever an ocular is used with an objective in projecting with the
microscope (fig. 207).

The principles for getting erect images with projection apparatus
are very simple, but in practice it is a little puzzling to decide off-
hand just how to arrange the object so that the screen image shall
be erect and not show any of the inversions (fig. 212-214). This
difficulty arises from the fact that in the different kinds of projec-
tion sometimes an opaque object is used, and sometimes a trans-
parent object ; sometimes an opaque and sometimes a translucent
screen is employed; sometimes an objective only, and sometimes
both an objective and an ocular are used for projecting the image;
and finally, sometimes it is necessary to use a mirror or prism as well
as an objective to get the image on the vertical or horizontal surface
where it is to be seen or drawn.

The simplest and surest way to get the microscopic specimen on
the stage of the projection microscope in a position which will give
a correct image for drawing is the following:

i. The prepared microscopic specimen is placed on a piece of
white paper so that it appears exactly as it should in the drawing,
and the letters a and k are written on the cover-glass between
the specimens (fig. 220).

362

ERECT IMAGES IN DRAWINGS

[Cn. X

2 . The slide is then placed on the stage of the projection appara-
tus and its image thrown on the drawing surface. In case the
specimen is wrongly placed to give an erect image the letters will
show it, and the specimen can be rearranged until the images of the
letters are correct in every way, then of course the images of the
microscopic specimens will be erect in every way (see also § 517).

§ 513. Erect images with opaque objects in a photographic
camera with translucent screen. — Place the object upside down in
the holder. On the translucent screen it will be erect (fig. 211).
If the object cannot be put upside down, the image will appear
wrong side up on the translucent screen (fig. 212). It can be drawn

FIG. 203. LARGE EDINGER APPARATUS IN A HORIZONTAL POSITION FOR

PROJECTION ON A VERTICAL SCREEN.

(Cut loaned by Ernst Leitz).

CH. X]

ERECT IMAGES IN DRAWINGS

FIG. 204. EDINGER'S OUTFIT FOR DRAWING WITH AN ORDINARY MICRO-
SCOPE AND SMALL ARC LAMP ON THE HOUSE LIGHTING SYSTEM.

(Cut loaned by Ernst Leitz).

This is the first form of the drawing outfits using the ordinary microscope
and the small arc lamp on the house lighting circuit. It was demonstrated at
the meeting of the Anatomische Gesellschaft at its Leipzig meeting, April, 191 1.

The microscope is inclined to 45° and the mirror at an angle of 22.5°, thus
directing the light vertically down upon the horizontal drawing surface.

For drawing in a light room a cloth tent is provided and is supported above
and on the sides by metal arches. If it is very light one can pull the cloth fever
the head as in focusing a camera. In the evening or in a dark room the cloth
can be opened widely to expose the drawing surface.

or traced in this position and the drawing turned right side up,
when it will appear like fig. 211, that is, correct in every way.

§ 514. Erect images with the opaque lantern or episcope.—

(A) The objective horizontal, the object and the drawing surface

364

ERECT IMAGES IN DRAWINGS

[Cn. X

vertical. The object is placed upside down in its vertical holder.
The mirror reflecting the image upon the vertical drawing surface
will give an erect image (fig. 211).

(B) The objective and the drawing surface horizontal, the
object vertical. The artist with his back toward the apparatus :
Place the object right side up in the vertical holder.

(C) Same as above, but with the artist facing the apparatus as
with the drawing shelf in fig. 183. Place the object wrong side up
in the vertical holder.

(D) Same, except that a vertical translucent screen is used.
Place the object wrong side up in the vertical holder ; do not use a

FIG. 205. SMALL ARC LAMP WITH CLOCK-WORK FOR FEEDING THE CARBONS.
(Cut loaned by Ernst Leitz).

This arc lamp for the house circuit has a clock-work which moves the carbons
continuously. The arc must be started by hand as for a hand-feed lamp, but
when once started the lamp will burn continuously provided the carbons burn
off as fast as they are fed. If the carbons are too large the clock-work will feed
them together faster than they burn away, and if too small the clock-work feeds
the carbons too slowly and the lamp will go out.

The clock-work has a regulating device for speed and the lamp has the usual
feed wheel for hand regulation.

This form of feeding mechanism is equally good for direct and for alternating
current as the movement is entirely controlled by the clock-work. Such a
lamp is especially useful for drawing and for photography.

CH. X]

ERECT IMAGES IN DRAWINGS

365

mirror or prism with the objective, but point the objective directly
toward the screen.

§ 515. Erect images of horizontal objects with the episcope. —

Vertical drawing surface and vertical objective, horizontal object.
The object is placed with its upper edge away from the drawing
surface and the mirror reflecting the image to the vertical screen
will make it erect (fig. 211).

§ 516. Erect images on the drawing surface with the magic
lantern. — (A) With an opaque, vertical drawing surface. Place
the transparency in the slide-carrier as described "for lantern slides
(fig. 7-8), i. e., with the object facing the light and wrong side up.

(B) For a translucent, vertical drawing surface. Place the
object facing the objective and wrong side up.

(C) For an opaque horizontal screen. Place the object so that
it faces the objective and the mirror or prism reflecting the rays
downward will give an erect image (see B and C in § 514).

ERECT IMAGES WITH THE PROJECTION MICROSCOPE

§ 517. Demonstration of the position of objects for erect
images.— The simplest way to determine how a specimen should be
placed on the stage of the microscope to give an erect image on any

FIG. 206. MAGIC LANTERN ARC LAMP AND TWO-LENS CONDENSER USED
IN MICRO-PROJECTION AND FOR DRAWING.

(See fig. 146 for full explanation).

366

ERECT IMAGES IN DRAWINGS

[Cn. X

kind or position of a screen is to use a specimen prepared as follows :
An ordinary microscopic slide is varnished as directed for lantern-
slide glasses (Ch. VIII, §317) and then the small letters a and k are
written in the middle with a fine pen. These letters are selected
because both in script and in printing they indicate clearly which
side up they are and which way they face. With some letters it is
not so easy to determine whether they have suffered an inversion
or not.

A low power, 50 to 100 mm. focus objective, is good for projecting
the image.

One could use a lantern slide with print upon it, or even a nega-
tive. For our experiments we used a lantern slide or negative of
the metric measure (fig. 178, 211) so that cuts could be made for
this book which were exactly like the images obtained on the screen
with the transparency in different positions.

Ocular

Objective

FIG. 207. DIAGRAM OF THE COURSE OF THE RAYS AND THE POSITION OF
THE IMAGES WHEN AN OCULAR is USED.

Object The object whose image is to be projected.

Objective The projection objective.

// Field lens of the ocular. It acts with the objective to give a real,
inverted image r i.

r i The real, inverted image of the object formed by the objective and the
field lens of the ocular.

r1 *'* The inverted image of the object which would be formed by the objec-
tive if the ocular were absent.

e I Eye lens of the ocular. It acts like a second projection objective and
gives a screen image of the real image (r i).

Axis The optic axis of all the lenses.

Screen Image The image projected by the eye lens. This image will be
right side up, but the rights and lefts will be reversed on the ordinary opaque
screen. If a translucent screen is used and the observer is behind it, the image
will appear erect, and the rights and lefts will not be reversed.

CH. X] ERECT IMAGES IN DRAWINGS 367

It is a good plan to have a specially prepared microscopic slide or
a lantern slide with print at hand whenever one is going to draw,
then one can determine quickly and exactly how the specimen
should be placed to give an erect image. A simpler method still
is to write the letters, a, k, on the cover of the specimen to be
drawn (§512, fig. 220).

POSITION OF THE OBJECT FOR ERECT IMAGES WITH THE PROJECTION

MICROSCOPE AND AN OBJECTIVE ONLY, OR WITH AN OBJECTIVE

AND AN AMPLIFIER

§ 518. For an opaque vertical screen. — Place the object on the
stage as a lantern slide is placed in its carrier (§35), that is, with the
specimen facing the light and the lower edge up. With a micro-
scopic specimen this would bring the cover-glass next the stage and
facing away from the objective instead of toward it, as in ordinary
microscopic observation. In this case one must focus through the
slide instead of through the cover-glass. This can, of course, be
done with low, but not with high powers. (See drawing on a hori-
zontal surface § 524).

With the specimen placed as directed, the image on the vertical
opaque screen will appear erect in every way (fig. 211).

If one faces the light and looks at the specimen on the stage it will
look like fig. 214 that is, like print seen through a sheet of paper
wrong side up.

§ 519. For a translucent vertical screen. — If the screen is of
ground-glass like that of a photographic camera, or if it is of tracing
paper or other translucent substance supported by clear glass, the
object should be placed on the stage so that it faces the objective,
and is lower edge up.

When the observer looks at the image on the translucent screen,
i. e., facing the light, the image will be erect like fig. 211.

When he faces the light and looks at the object on the stage it will
appear like fig. 212, i. e., it is simply upside down.

368

ERECT IMAGES IN DRAWINGS

[CH. X

POSITION OF THE OBJECT FOR ERECT IMAGES ON A HORIZONTAL

SURFACE WITH AN OBJECTIVE OR WITH AN OBJECTIVE AND AN

AMPLIFIER AND A 45 DEGREE MIRROR OR PRISM

§ 520. For an opaque horizontal screen. — (A) If for a drawing
table and mirror (fig. 182), place the object on the stage so that
it faces the objective and is right edge up. The image on the
horizontal surface will appear erect when the observer looks at it
facing away from the light.

The object on the stage will appear erect when the observer looks
at it facing toward the light.

Fip 208. KEPLER'S METHOD OF PRODUCING ERECT IMAGES BY MEANS
OF Two PROJECTION LENSES.

(From Scheiner's "Oculus" , 1619).

CH. X] ERECT IMAGES IN DRAWINGS 369

(B) If the mirror is very close to the objective (fig. 183) the
natural position for drawing is to sit facing the light. The object
then is put in position facing the objective as before, but upside
down. The image will appear erect on the drawing surface when
the observer faces the light.

§ 521. For a translucent, horizontal screen. — In some of the
old forms of sketching apparatus the image was reflected upward
by a mirror or prism, and the artist drew on the upper surface.

FIG. 209. DIAGRAM TO SHOW THAT THE SIZE OF THE IMAGE OF AN OBJECT

DEPENDS UPON THE RELATIVE DISTANCE OF THE OBJECT AND IMAGE

FROM THE CENTER OF THE PROJECTION LENS.

(From. The Microscope}.

In this figure the image is four times as far from the center of the lens (cl)
as the object, hence, from the law of similar triangles, the image must be four
times as long as the object.

For such an arrangement, the object is put on the stage facing the
light, but right edge up. The image will appear erect on the
translucent screen when the observer faces the light and looks down
upon the screen. For this experiment the mirror or prism must be
on the lower side of the ocular (fig. 215).

POSITION OF THE OBJECT FOR AN ERECT IMAGE WITH AN
OBJECTIVE AND AN OCULAR

§ 522. For an opaque vertical screen. — The object should face
the light as with a lantern slide, but it must be right edge up.
With a microscopic specimen the cover-glass will be next the stage
as in § 518. On the screen the image will appear erect (fig. 211).
The object on the stage will appear reversed like print seen in a
mirror (fig. 213).

37°

ERECT IMAGES IN DRAWINGS

[CH. X

Objegtive

Objecl-h
Object-a

FIG. 210. DIAGRAM TO SHOW THAT THE SIZE OF THE. IMAGE DEPENDS

UPON THE DISTANCE OF THE OBJECT FROM THE CENTER OF THE LENS.

(From The Microscope).

The object at Object-a necessitates an image at Image-a; while if the same
object is moved closer to the lens, as at Object-b, the image will move farther
from the lens (Image-b) and be correspondingly larger.

// The principal foci of the lens (objective).

axis The principal axis of the lens.

Secondary axis a, Secondary axis b Represent the secondary axes which
mark the limit of the object and the two images.

With the object farther from the lens the secondary axes are in full lines,
while for the object nearer the lens the secondary axes and the image are shown
by broken lines.

§ 523. For a translucent vertical screen. — The object is put on
the stage facing the objective and right edge up. The image will

CH X] ERECT IMAGES IN DRAWINGS

1O CENTIMETER RULE

The upper edge is in millimeters, the lower in centimeters

FIG. 211. CORRECT IMAGE.
\

ni J3/v\o[ sqi 'saapminim ui si sSpa jaddn

H3X3JMIXN3O Ot

FIG. 212. INVERTED IMAGE.

3JEUH

ni iswof sd) .aiaJamillim ni zi 9-gbs isqqu 9iiT

FIG. 213. MIRROR IMAGE.
nbbei. cqSc K in njijjiroG^Gi.3' (p& JOMGX in CGnfiniGfGi.31

TO CEMJJIALEXEB

FIG. 214. INVERTED MIRROR IMAGE.

FIG. 211-214. FIGURES OF A METRIC RULE, FULL SIZE, TO SHOW CORRECT,
INVERTED, MIRROR AND INVERTED MIRROR IMAGES.

These representations of screen images show the result of placing the object
in different positions or of using different means in projection. The determin-
ing factors for the position of the object for a correct screen image are:

(1) Projection by an objective or by an objective and an amplifier (fig. 121,
126).

(2) Projection by means of two lenses or of an objective and an ocular
(fig. 207, 208).

372

ERECT IMAGES IN DRAWINGS

ICn. X

(3) The use of a prism or of a mirror to change the direction of the rays on
their way to the screen (fig. 192).

(4) The use of an opaque screen.

(5) The use of a translucent screen.

appear erect like fig. 211 when seen through the translucent screen
and facing the light.

Facing the light, the object on the stage will also appear erect.

POSITION OF THE OBJECT FOR AN ERECT IMAGE WITH AN OBJECTIVE

AND OCULAR, AND A 45 DEGREE MIRROR OR A TOTALLY

REFLECTING PRISM

§ 524. For an opaque horizontal screen. — (A) For the draw-
ing table and mirror (fig. 182), place the object on the stage so that
it faces the objective and is with the lower edge up. The image will
appear erect on the drawing surface when the observer faces away
from the light.

FIG. 215. EARLY METHODS OF DRAWING WITH PROJECTION APPARATUS.

In the picture at the left (Fig. 6) is shown a drawing tent or box with a 45°
mirror and vertical objective by which an erect image is projected upon the
drawing table as in figures 88-89. The artist sits outside, but has his head
and bust within and the light is excluded by a cloth curtain over the back.

In Fig. 5 is shown a drawing box composed of an objective at the right (CD),
a 45° mirror (E F), and a drawing surface (C) covered by a sloping roof of
opaque material to keep out the light. With this instrument the artist simply
introduces the hand and pencil. The picture will have the rights and lefts
reversed as the drawing is made on the back of the drawing paper, not on the
front as with Fig. 6.

Fig. 4 is to show the course of the rays from an object (A B), and its inverted
image (G H). When the mirror (E F) is introduced the image (/ K) is rendered
horizontal.

CH. X] DRAWING FOR PUBLICATION 373

If the observer faces the light the object on the stage will appear
like a printed page upside down (fig. 212).

(B) For a drawing shelf, the mirror or prism being close to the
ocular and the draughtsman sitting with his face toward the light
(fig. 183, 187), the object is placed on the stage facing the objective
and right edge up.

The image will be erect on the drawing surface (fig. 211).

The object on the stage will also appear erect (fig. 211).

§ 525. For a translucent screen. — For this the object is simply
turned around so that it faces in the opposite direction in each case
but remains the same edge up.

§ 526. For erect images on a horizontal drawing surface with
apparatus like Edinger's (fig. 202). — In this case no mirror or
prism is necessary. The position of the object on the stage for
erect images is precisely the same as for a horizontal microscope
and a vertical screen (§ 518).

This has the disadvantage of requiring one to turn the cover-
glass away from the objective, which prohibits the use of high
powers. If the cover-glass is turned toward the objective the
drawing will be like a mirror image (fig. 213).

DRAWINGS FOR PUBLICATION BY THE AID OF PROJECTION
APPARATUS

§ 527. Projection apparatus can give much assistance in pro-
ducing the outlines and main details of drawings for publication.
The outline drawings should be made on good drawing paper with a
medium lead pencil. When the ink, air-brush, or crayon work is
added for the finished drawing, the pencil lines will be covered or
they may be erased. The finishing must be done free-hand and
constant reference made to the actual specimen, to the image on
the screen, or as looked at through a microscope. The finishing
cannot be done successfully with the image of the specimen pro-
jected on the drawing paper as one cannot tell how the drawing
looks with the image projected upon it. By means of a suitable
screen the image may be cut off of part of the drawing surface while
doing the finishing. By removing the screen the image can be

374 DRAWING FOR PUBLICATION [Cn. X

projected again upon the surface to make sure that all the details
have been correctly drawn.

It is always desirable that drawings accompanying a scientific
article should be at a definite enlargement or reduction, and that
the scale of the drawing be definitely stated (See Style Brief, of the
Wistar Institute, pp. 16-17).

If the drawings have been made without first doing this, then
the magnification can be found by arranging the apparatus exactly
as when the drawings were made and using a micrometer as directed
in § 508.

A plan frequently followed is to have a few lines of the microme-
ter image drawn in one corner near the picture. Then any one
can determine the scale of magnification or reduction (§ 510, sioa).

§ 528, Lettering the drawings. — After the drawings are finished
the various parts can be lettered, or words can be written in where
needed. Most workers, however, cannot letter neatly enough for
publication. For such it is better to use printed words, letters or
numerals.

It is assumed here that the drawings will be reproduced by some
photo-engraving process ; and for this the letters or words pasted
on the drawing would best be printed on tissue paper, (§ 528a);
Gothic type is best. By consulting fig. 216, one can select the
proper size for the reduction to be made (§ 531).

§ 527a. Tracing pictures natural size on drawing paper. — It frequently
happens in making the drawings for a book or for a scientific paper that pic-
tures from other books or scientific papers are desired. Of course, if there are
to be no modifications, the simplest method is to borrow an electrotype or to
have the photo-engraver make a new cut; but sometimes only an outline is
needed or modifications are desired.

If the picture is to be the same size as the one in the book or periodical it
can be easily traced upon the drawing paper as follows: In place of a wooden
shelf on the table (fig. 183, § 460) a piece of thick glass is placed on the brackets
and an incandescent lamp of 40 or 60 watts, surrounded by a lamp shade of
some kind, is turned so that it shines directly upward. It is then placed up
close to the glass and the picture to be traced is placed on the glass, and over
it the drawing paper. The light is so strong that it traverses the picture and
the drawing paper and the picture is clearly seen on the upper side of the
drawing paper. It can be traced almost as easily as if an image were projected
upon the upper face. In tracing drawings for this book, Wattman's hot pressed
paper and Reynold's bristolboard were used in making tracings in the way just
described. Even if there is print on the opposite side of the page the tracing
of the picture can be made successfully.

CH. X] DRAWING FOR PUBLICATION 375

§ 529. Fastening the letters to the drawing. — The letters,
numerals, or words are cut from the printed sheet, with pains to
make straight edges and square corners. Then they are turned
face downward and with a camel's hair brush of small size such
as is used by artists, some freshly made starch paste is put on the
back. As each word or letter is pasted, one uses fine forceps to
pick it up and place it in the desired position, being sure that the
letter or word is arranged properly. In the beginning it is well to
use a try-square or some other instrument to make sure that the
letter or word is arranged correctly. Then it is pressed down, using
some tissue paper over the finger or some fine blotting paper, and
pressing directly downward so as not to disarrange the letter or
word by a lateral thrust.

§ 530. White letters on a black back-ground. — Sometimes it is
necessary to use white letters or numerals on a black ground (e. g.,
see fig. 211-214). In the largest printing houses one might be
able to get these, but they are easily made as follows :

The desired letters, numerals, abbreviations or words are printed
on the white tissue paper as indicated above. A sheet of this
printed tissue paper is used as a negative by putting a clean glass
in the printing frame, placing the printed tissue paper face down
on the glass, and then putting some Velox, Cyco, or other photo-
graphic paper in place and printing exactly as for any negative.
The opaque letters will be in white, and the practically transparent
tissue paper between the letters will give the black back-ground in
the print.

§ 528a. (i) The authors are indebted to Mr. George C. Stanley, Ithaca's
photo-engraver, for the suggestion to use tissue paper for the printed letter s
and words to be pasted on drawings for photo-engraving. The advantage o f
tissue paper is that there is no shadow around the edge of the letter or word .
Where thick, ordinary white paper is used there is frequently left a black line
due to the shadow, and this line must be cut out by the engraver or it will give
a black line in the printed book.

(2) Starch paste for use in sticking on the letters and words should be
freshly made. A good paste is made of dry laundry starch 5 grams, cold water
50 cc. These are put in a small vessel and gradually heated with constant
stirring until the paste is formed. Mucilage and other adhesives make the
tissue paper yellowish ; and paste which has been made some time is liable to
have fine lumps in it so that the letters are torn or distorted in pressing them
down on the drawing.

376 DRAWING FOR PUBLICATION [Cn. X

Paper and developer should be of the contrast variety to give
pure blacks and whites.

These letters, etc., are cut out and pasted on the drawing just as
described above. The photographic paper being rather thick,
there will be a white streak around the letter, etc., where cut out.
This can easily be blackened after being stuck in place by the use
of a pen or a fine brush, using India ink.

SIZE OF DRAWINGS AND THEIR LETTERING

§ 531. It is wise to make the drawings considerably larger than
the desired picture. In reducing, the coarseness and some other
infelicities of the drawing become less noticeable.

Of course if the drawing is made exactly the size of the desired
cut, then it must look exactly as one desires it in the printed book ;
it is not liable to be improved by the process of photo-engraving.
But if the drawing is to be reduced, then the lettering, etc., must
be coarse enough in the drawing to give the proper appearance in
the finished cut.

There is some confusion in the minds of the inexperienced as to
the appearance of a picture half the size of the original. To the
engraver half-size always means that any given line or part is just
half the length of the original. That is, if any line of the original
were 10 centimeters long, the finished cut would show the same
line 5 centimeters long if it were reduced to half the original size.
The appearance is well shown in the accompanying figure (fig. 216).
In the upper half the letters and numerals are of full size ; in the
middle they are of half the original size; and below they are of
one-fourth the original size. This picture will show one also about
the size of type to use for the different reductions. The numerals
on the left indicate the size of the type, as 24 point, 18 point, 12,
10, 8, and 6 point, respectively.

The lettering of pictures in books and periodicals should be
proportioned to the size of the details of the cuts. It is distressing
to have the letters and numerals on figures the most prominent
feature. On the other hand, it is exasperating to have letters
so minute that one needs a microscope to make them out. As

24 Point Type A a
123456789 10

18 Point Type ARS 2 34

12 Point Type ABCabc 1234
10 Point Type ABC abc 12345

8 Point Type ABCD abed 12345
6 Point Type ABCDabcd12345 I II III IV
ABCD abod 123456789 10 I II til IV V VI

24 Point Type A a
123456789 10

18 Point Type A R S234

12 Point Type ABCabc 1234
10 Point Type ABC abc 12345

8 Point Type ABCO aocd 12345

24 Point Type A a
123456789 10

18 Point Type A R S 2 3 4

FIG. 216

378 PHOTOGRAPHIC ENLARGEMENTS [CH. X

FIG. 216. GOTHIC TYPE TO USE ON DRAWINGS AND THE APPEARANCE
WHEN REDUCED.

In the upper half are shown letters and figures of full size with their designa-
tions by the printer, i. e., 24, 18, 12, 10, 8 and 6 point type.

In the lower half are shown the same reduced to one-half the length, and
reduced to one-fourth the length.

shown by the numerals and letters in fig. 216, if the drawing is
not to be reduced at all one can use 6, 8, or possibly 10 point type.
For one-half reduction (one-half off), the lettering should be
with 10 or 12 point type. For one-fourth size (^ off), then the
lettering should be with 12 or preferably with 18 point type.

PROJECTION APPARATUS FOR PHOTOGRAPHIC ENLARGEMENTS

§ 532. Enlarged prints of negatives. — There is great advantage
in making pictures of large objects at a considerable distance so that
the perspective will be correct and all levels in focus. It is also
advantageous to make pictures of microscopic objects without
undue enlargement, then there is greater sharpness of the object
as a whole.

If now one wishes a large picture, any good negative can be
printed by the aid of a photographic objective at almost any
desired enlargement. This can be done with projection apparatus
in a dark room by the following method : The management of the
projection apparatus is as for drawing. The negative is placed in
some kind of a holder and put in the cone of light from the main
condenser where the part to be enlarged will be fully illuminated
(fig. 132, 185). Care must be taken to so place the negative that
an erect image will appear on the printing paper (§ 512).

§ 533. Condenser required with negatives of different sizes. —

Remember that the diameter of the condenser must be somewhat
greater than the diagonal of the part of the negative to be enlarged
(§314 and fig. 114). For example, to use the whole of a lantern-
slide negative (85 x 100 mm., 3^ x 4 in.) the condenser should
have a diameter of 14 cm. (5^ in.).

For a negative 100 x 125 mm. (4x5 in.), the condenser should
be 1 8 cm. (7 in.) in diameter; fora negative 125 x 175 mm. (5x7 in.),
the condenser should be 23 cm. (9 in.) in diameter and for a nega-
tive 200 x 250 mm. (8 x 10 in.), the condenser should be 35 centi-

CH. X] PHOTOGRAPHIC ENLARGEMENTS 379

meters (14 in.) in diameter. Of course, if only a part of the nega-
tive plate contains the picture to be enlarged then a smaller con-
denser in the given case will answer. The above figures are for
the diagonal of the respective sizes. These condensers are usually
of relatively long focus, especially those of the larger diameters,
e. g., the 35 cm. lens ordinarily has a focus of 50 centimeters. The
condensers most used for enlarging are usually of the double form,
the convexities facing each other as for the magic lantern condenser
(%. 185).

§ 534. Objectives to use for printing. — It is necessary to use
an objective which has been corrected for photography. The
ordinary projection objective gives a good visual image, but not a
good photographic image, hence it is better to use a photographic
objective.

§ 535. In focusing, some white paper should be put into the
printing frame or pinned in place and the image focused with care.
The photographic paper when put in the same place will then give
a sharp picture.

§ 536. Photographic paper for printing with projection appara-
tus.— If one has sunlight or the arc light the developing papers like
Velox, Cyco, etc., are plenty rapid enough. If a weak light is all
that is available, then Haloid or one of the more rapid bromide
papers will be called for.

§ 537. Holding the paper while printing. — (A) If the pictures
are of microscopic objects or other pictures of relatively small size
(i. e., up to 30 x 35 cm.; 12 x 14 in.), a good method is to put a
clear glass in a printing frame and press the printing paper down
upon it just as one does for printing from a glass negative by con-
tact. This holds the paper perfectly flat and ensures uniform
sharpness. With the printing frame one can lay it flat if a mirror
or prism is used, or it can stand on edge facing the objective if no
mirror is used.

(B) If the printing paper is large the usual method is to have a
board screen on a track. The picture is then got of the desired
size by varying the distance between the board and the objective,

3 So PHOTOGRAPHIC ENLARGEMENTS [Cn. X

then the image is carefully focused by putting some white paper
on the screen or by having a ground-glass in the middle of the
screen. Then the objective is covered with a dark cap or with a
cap containing ruby glass, and the photographic paper is fastened
in place by thumb tacks or in some other way, care being taken to
stretch it smooth.

§ 538. Exposure. — When the paper is in place the cap is
removed from the objective and the projected image will print on
the paper. The time necessary depends upon the magnification,
the density of the negative, the intensity of the light and the sensi-
tiveness of the paper used . It usually takes about one-fourth the time
one would print by contact using a 1 6 candle-power frosted incan-
descent lamp. A good plan is to try a small piece of the paper and
determine the correct exposure before printing on the large sheet.
After the exposure the objective is covered with the cap and the
paper is developed exactly as for contact printing.

§ 539. Diaphragm of the objective. — In printing, the diaphragm
of the objective is wide open if the unmodified cone of light is used
for illumination. This has one defect with the arc lamp. If there
are any irregularities in the negative, such as minute scratches, etc.,
they would show in the print, whereas if the illumination were from
an extended instead of a very small source like the crater of the arc
lamp, the slight defects would show very much less.

To obviate this defect with the arc lamp one or more plates of
ground-glass or of milk white glass are placed in the path of the
beam before the negative. It must be put far enough from the
negative so that the grain of the ground-glass will not show.

With the ground-glass or the milky glass in the beam the dia-
phragm of the objective can be closed as much as desired. The
use of the ground-glass and the closure of the diaphragm will, of
course, necessitate a longer exposure.

§ 540. Avoidance of stray light. — If one is to do considerable
printing with the projection apparatus a light-tight lamp-house
must be used and light-tight bellows between the condenser and
the negative and objective. A special camera is most satisfactory.

CH. X] PHOTOGRAPHY AND PROJECTION 381

For the occasional use of a laboratory the stray light can be ex-
cluded by means of asbestos paper. Sometimes the arc lamp is
put on the outside of a partition, but that necessitates going out of
the printing room to adjust the lamp. If direct current is available
an automatic lamp is a great convenience.

PHOTOGRAPHING WITH PROJECTION APPARATUS

§ 541. Apparatus which will give good projection of micro-
scopic specimens can, with slight modifications be used for photo-
micrography.

There are three possibilities:

(1) Printing the image directly on a developing paper.

(2) Exposing a dry plate directly to the image as for the paper.

(3) Using a camera and plate holder.

§ 542. Printing the projected image directly on a developing
paper. — With the apparatus set up exactly as for drawing one can
expose a sheet of developing paper to the sharply focused image of
the specimen as described for the enlargement of negatives (§ 532).
The lights and shades will be reversed, but all the outlines and
details will be present. This is a convenient method of getting an
enlarged record of the specimen.

It is also a good method for making pictures for models (§ 511)
especially when there are many details. With the cheap develop-
ing papers in rolls now obtainable the expense is not greater than
for making drawings, and there is liable to be a gain in accuracy.
The main draw-back is that but a single picture is made of each
specimen for a single exposure, while in drawing it is as easy to
make two or three as one, by using carbon paper (§ 511).

§ 543. Exposing a dry plate directly to the image. — A dry plate
may be exposed as just described for the developing paper. The
object must be so placed on the stage of the microscope that the
image on the screen will be a mirror image of the specimen, that is,
the rights and lefts will be reversed as they should be in a negative.
The image is sharply focused, and the light cut off with a deep red
glass so that the plate will not be affected.

382

PHOTOGRAPHY AND PROJECTION

[CM. X

A Set screw holding the rod (5) in any de-
sired position.

P Q Set screws by which the bellows are
held in place.

B Stand with tripod base in which the sup-
porting rod (S) is held. This rod is now grad-
uated in centimeters and is a ready means of de-
termining the length of the camera.

M Mirror of the microscope.

L The sleeve serving to make a light-tight
connection between the camera and microscope.

O The lower end of the camera.

R The upper end of the camera where the
focusing screen and plate holder are situated.

The plate holder is then put in po-
sition, and the dark slide removed.
The red glass is then removed for the
short time necessary for the exposure
O/i oth sec., more or less) and then re-
placed. The dark slide is put back in
the holder. The plate is developed and
printed as usual.

When working with dry plates in this
way great care is required to avoid stray
light. Stray light which would not in-
jure printing papers will fog a dry plate.

§ 544. Using a camera and plate
holder. — When exact results are required
or much photo-micrography is to be undertaken, it is better to use
a camera in connection with the projection apparatus (fig. 219).

The camera and projection apparatus are put on a long labora-
tory table, or the camera may be put on a second table and adjusted
to the height of the projection microscope. The camera is con-
nected with the projection microscope by means of a light-excluding
sleeve such as that used by Zeiss with his photo-micrographic
outfit (fig. 217-218).

The camera serves to exclude all stray light and to hold the
plate holder in the correct position. The camera is supplied with a
focusing screen which occupies exactly the same position as doe&
the plate during exposure.

FIG. 217. VERTICAL PHOTO-
MICROGRAPHIC CAMERA.
(From Zeiss' Photo-micro-
graphic Catalogue).

CH. Xj

PHOTOGRAPHY AND PROJECTION

383

FIG. 2I7A. VERTICAL CAMERA WITH METAL SHIELD.
(From the Transactions of the Amer. Micr., Soc., Vol. XXIII, 1901).

The camera is on a low table and a shield of sheet zinc or roofing tin is on a
stand between the source of light and the camera to protect the camera and
the eyes of the operator. Opposite the light source is an opening with a shutter
The source of light in this case is a kerosene lamp.

384 PHOTOGRAPHY AND PROJECTION [Cn. X

§ 545. Objectives to use. — The microplanars (fig. 123) or
other short focus objectives of the photographic type are used
without an ocular. They can be screwed into the nose-piece of the
microscope or the microscope can be dispensed with and the
objectives put into the end of the camera as with photographic
objectives.

If one wishes to use the ordinary microscope objectives then an
ocular of the projection type is of great advantage although
Huygenian oculars will give good results. The apochromatic
objectives, and the projection or compensation oculars to go with
them, give the best results.

§ 546. Making the negative. — The image is first focused very
sharply on the focusing screen. For lights of high intensity it will
be necessary to soften the light in focusing so as not to injure the
eyes. This can be done by putting a neutral tint glass plate or
several thicknesses of ground -glass or one or more plates of milky
glass in the path of the beam before the object.

The exposure and subsequent development and printing with the
negative are as usual.

§ 547. Photo-micrography with a vertical camera. — If a ver-
tical microscope is to be employed for photography, then it is best
to use a vertical camera (fig. 217). A parallel beam of light is
caused to fall upon the plane surface of the microscope mirror, and
the mirror is turned to throw it directly up through the substage
condenser upon the object. To get the parallel or approximately
parallel beam one uses a condenser lens of very long focus (§ 479,
fig. 154) or a parallelizing lens is used (fig. 153).

TROUBLES MET IN CHAPTER X

§ 548. The troubles liable to arise in the work of this chapter
are those common to the preceding chapters. Those discussed in
Chapter I and III are to be especially reviewed, as the source of
light is most likely to be the electric arc. (See § i28a for the
blowing of fuses).

(i) In drawing with the microscope with the small carbon arc
lamp on the house lighting system, probably the trouble most

CH. X]

TROUBLES IN DRAWING

385

FIG. 218. THE ZEISS PHOTO-MICROGRAPHIC MICROSCOPE.

(From Zeiss' Catalogue).

This is the parent form of photo-micrographic stands with large tube (J1),
handle in the pillar and a special fine adjustment at the side (W). At the top
is half of the light-excluding sleeve.

TROUBLES IN DRAWING [Cn. X

likely to arise is the lack of a brilliant picture on the drawing paper
owing to the light in the room. Remember that to get a brilliant
image the light must come to the eyes from the drawing surface
only, and the drawing surface must receive no light except that
from the specimen. The weaker the light and the greater the
magnification the darker must the room be.

(2) In drawing from negatives or lantern slides remember that
it is necessary to have a condenser somewhat larger than the
diagonal of the object to be drawn (§ 314, 533).

(3) In drawing with the microscope where the substage con-
denser is used the condenser must be in the exact position to give
the best results. If the slide is thick the condenser is a little higher

FIG. 219. MICRO-PROJECTION OUTFIT AND VERTICAL CAMERA ARRANGED

FOR PHOTO-MICROGRAPHY.

(From The Microscope).

The apparatus is set up on a long table or on two tables placed end to end.
The vertical camera (fig. 217) is placed horizontally and the bellows reversed.
For illumination a petroleum lamp with large flat wick (38 mm., \y2 in.)
answers well.

Objects 50 to 60 mm. in diameter may be fully illuminated with the face of
the flame, the lamp being i to 2 centimeters from the condenser. For powers
of 100 to 150 diameters the flame is turned obliquely or edgewise, and placed
5 to 6 centimeters from the condenser. The position shown in the picture
above is for high power work. No water-cell or specimen cooler is needed.

A light-tight connection is made with the large tube of the microscope by a
double sleeve like that employed by Zeiss for the microscope. With low
magnifications no ocular is used, and the objective is placed in the end of the
camera. If one desires to make pictures of a size above the capacity of the
photo-micrographic camera it is possible to use an ordinary camera, (fig. 117-
119), then even quite large objects 50 to 60 mm. long, can be magnified con-
siderably. The petroleum lamp has some advantages over daylight as the
lamp gives an illumination of constant intensity. It is available during the
entire 24 hours of the day, and in all seasons.

CH. X]

TROUBLES IN DRAWING

387

than for a slide which is thin. Attention to the substage con-
denser will make a great difference with one's success.

(4) The right-angled arc lamp should be used in drawing
because if the microscope and lamp are properly arranged the source
of light will remain in the axis no matter how long the lamp burns.
If an inclined carbon lamp or one with both carbons in the vertical
or horizontal position is used the source of light is constantly
getting out of the axis from the burning away of the carbons,
consequently they must be fed up more frequently to keep the
source of light in the field.

(5) The picture will be distorted unless the axial ray strikes the
drawing surface at right angles. Therefore, in using a prism or
mirror for a horizontal surface the microscope must be horizontal
and the mirror or prism at 45 degrees to reflect the axial ray ver-
tically downward. If the mirror or prism is twisted over to one
side the axial ray will not strike the surface at right angles and there
will be distortion. If one has a micrometer in squares it is easy to
determine whether the image is distorted or not.

(6) The image will be erect only when the object is properly
placed on the stage.

(7) If a glass mirror silvered on the back is used, and the object
is quite opaque the secondary image reflected from the face is

FIG. 220. SLIDE OF SERIAL SECTIONS WITH -a, k- ON THE COVER-GLASS

TO ENABLE ONE TO DETERMINE WHEN THE IMAGE ON THE DRAWING

SURFACE is ERECT (See fig. 143, and § 512, 517).

388

DO AND DO NOT IN DRAWING

[CH. X

liable to cause confusion. If the mirror is silvered on the face or
if a prism is used there will be no doubling of the reflected image.

(8) Inverted images. One must follow carefully the directions
or there is liable to be an inversion of the projected image (§ 512-
526).

(9) In printing and photographing with projection apparatus
the difficulties likely to be met with in photography are sure to come
in. Knowledge of the principles underlying photographic pro-
cesses will help one to overcome the troubles.

549. Summary of Chapter X :
Do

i . Have a suitable room or a
suitable shield around the draw-
ing to keep out stray light
(§ 453-455)-

Do NOT

i. Do not try to draw with
the drawing .surface flooded with
stray light. Only the light
from the specimen should reach
the drawing surface.

2. Draw in the evening if a
proper room is not available in
the day time (§453)-

3. Use an arc lamp for light
if possible (§ 461-462, 486-487).

4. Always use a rheostat with
an arc lamp, large or small
(§487,%. 182,185).

5. One can draw images pro-
jected by all forms of projection
apparatus (§452).

2. Do not forget that it is
dark in all rooms in the evening
and, therefore, that is a good
time to draw.

3. Do not use a weak light
for drawing if you can have an
arc light.

4. Never try to use an arc
lamp, large or small, without a
rheostat in series with it.

5. Do not forget that it is
possible to draw the images
projected by any form of appar-
atus.

CH. X]

DO AND DO NOT IN DRAWING

389

6. In drawing with any form
of projection apparatus the
axial ray must strike the draw-
ing surface at right angles or
the image will be distorted
(§483)-

7. Make sure that the mirror
or prism reflects the rays upon
the drawing surface so that the
axial ray is at right angles to
the surface (§482-483).

8. Use a condenser of suffi-
cient diameter to fully light the
object (§ 467, 533).

9. Get the desired size for the
drawing by making the distance
of the drawing surface greater
or less, or by using a different
optical system for the projec-
tion (§465, 507-508).

10. Take great pains to light
as brilliantly as possible (§ 497,
and Chapters I, II, and IX).

11. Take care to have the
images on the drawing surface
erect (§ 512-526).

12. In using projection ap-
paratus for photography, re-
member the principles of good
projection, and the require-
ments for good photography.

6. Do not draw distorted
pictures; therefore do not have
the axial ray strike the drawing
surface obliquely.

7. Do not forget to incline
the mirror used in drawing so
that the axial ray will strike the
drawing surface at right angles.

8. Do not try to project an
object larger than the diameter
of the condenser lenses used.

9. Do not neglect to give the
scale at which every drawing is
made.

10. Do not expect good light
unless the conditions for it are
met.

11. Do not draw inverted
images.

12. Do not expect projection
apparatus to give good photo-
graphs unless sharp, brilliant
images are projected, and
the photographic part is done
correctly.

CHAPTER XI
MOVING PICTURES

§ 550. Apparatus and Material for Chapter XI :

A competent operator (§ S5oa); Moving picture head, or mech-
anism; Rheostat for direct current, or rheostat, inductor or
choke-coil, transformer, rectifier, motor-generator set for alter-
nating current ; Arc lamp and lamp-house ; Condenser, assortment
of different sized plano-convex lenses 14, 15, 16, 17, 19, 21, 23 cm.
focus ($yf, 6, 6^2, 7, 7>£, 8, 9 in. focus); meniscus lens, 23 cm.
focus (9 in. focus); Projection objective, equivalent focus n to
15 cm. (4^2 to 6 in.), preferably of 6.3 cm. (2^2 in.) diameter,
although 4.5 cm. (i^ in.) will answer; Moving picture films ;
Tools, asbestos gloves, pliers, screw driver, copper wire, pins,
film cement; Supply of carbons.

For continuous use a special operating room separated from
the auditorium by fireproof walls, all openings into the auditorium
to have automatic shutters closing in case of fire, the room to be
provided with a large flue connecting to the outside of the
building.

§ SSOa. Competent operator. — As no one can learn a difficult art from book
directions alone without spending an undue amount of time, we strongly advise
every one who wishes to be a moving picture operator or photographer to get
the help of an expert. Every university and technical school worthy of the
name now has laboratories in which the actual operations are learned by the
students in repeated efforts under the direction of expert teachers. Books are
helps, and often give an expert all that he needs to enable him to perform
successfully some difficult or unfamiliar operation. But the living teacher and
the actual experiment serve the beginner most effectively.

We strongly recommend the operator to possess the best works on Moving
Pictures and projection in general, and to subscribe for one or more periodicals.
By studying these he can keep himself informed of all the advances in his pro-
fession. In the long run, the "man who knows" is appreciated.

It was inevitable that with the exceedingly rapid development of the moving
picture business many difficult operations, and the special form of acting
requisite to the production and exhibition of a photo-play were undertaken by
persons without adequate training and experience. It seems to the authors
that it is highly creditable to human intelligence that the work has been so well
done and that the improvement has been so constant and rapid. It seems to
us, furthermore, that an important factor in the present creditable attainments
which have already been reached, has been due to the high standards advocated
by the Moving Picture World in all branches of the art. In particular the
authors wish to commend the work of Mr. F. H. Richardson in his Motion
Picture Handbook and in his weekly discussions and answers to questions in
the projection department of the Moving Picture World.

390

CH. XI] MOVING PICTURES 391

§ 551. For the historical development of moving pictures see
under History in the Appendix.

For works on moving pictures see: Cyclopedia of Motion
Picture Work, 2 vols.; Hepworth, C. M., Animated Photography;
Hop wood, Living Pictures ; Jenkins, C. F., Handbook for Motion
Pictures and Stereopticon Opera; Jenkins, C. F., Picture Ribbons;
Richardson, F. H., Motion Picture Handbook, 2d ed.; Talbot,
F. A., Moving Pictures; Hints to Operators by the Nicholas
Power Company; Periodicals on Moving Pictures, e. g., the Mov-
ing Picture World and catalogues of manufacturers and dealers
in moving picture outfits.

INTRODUCTION

§ 552. The steps that had to be taken in human experience and
knowledge before it was possible to have moving pictures at all,
were many ; and the time between some of the steps was very long.

The first step was a knowledge of the physiology of vision, and
especially a knowledge of the persistence of visual impressions.
Primitive man knew that a glowing torch would make a circle of
fire if it were whirled around rapidly enough. He knew also that he
could see objects illuminated by an instantaneous flash of lightning.

From this power of seeing by an instantaneous illumination, and
the persistence of the impression for a limited time after the light
has gone, arise the possibility of having moving pictures. In a
word, moving pictures are possible because we can see instantly,
but we cannot stop seeing instantly.

To give views rapidly with proper illumination, involved the
discovery of means for artificial light of great brilliancy, and of a
machine by which the views could be lighted and moved along;
and finally the long series of discoveries and inventions in optics
and chemistry before photography was invented to make the pro-
duction of the views cheap and accurate. It was another long step
taken by Newton when he showed that white light in nature is
composed of the rainbow colors. Furthermore, it was shown by
him and contemporary and later physicists and physiologists that a
mixture of less than the seven colors of the rainbow gave to the eye
the appearance of white light. Even two complementary colors

392 MOVING PICTURES [Cn. XI

as red and greenish blue, yellow and indigo blue, etc., give the
appearance of white. With this information it became possible
to add to the photographic black and white moving pictures, the
element of color. This was accomplished by using isochromatic or
panchromatic film, and taking the pictures through colored screens,
the first picture through a red, the second through a green, the
third through a violet screen and this constantly repeated through-
out the whole scene. In exhibiting the picture there is a three-
color screen used so that the picture exposed through the red screen
is projected through a red screen, giving a red image, and the other
colors in like manner. If the film is run through the machine three
times as fast as the black and white film, then the brain mixes the
colors of the successive pictures giving fairly true color values and
black and white. Where only two screens are used — red and green
— the process is the same, but the film has to be run through the
machine only twice as fast as the black and white film as there are
but two colors for the brain to combine. Naturally the combina-
tion of two colors gives a lower range of possibilities than the mix-
ture of three colors, but even this is wonderful, as all will agree who
have seen the colored moving pictures reproducing the gorgeous
scenes of nature or the pageants of human splendor in all their
form and movement and also with a fair approximation to the color
effects.

So perfect have become the materials and processes used in
photography, and the accessory mechanical appliances, and the
artificial lights available, that now the scientist can register
accurately the almost instantaneous movements of an insect's wing,
the flight of a cannon ball, and the numberless actions everywhere
in nature which are so rapid that the unaided eye cannot analyze
them. On the other hand, the movements in the processes of
nature which are so slow that one can only see what has been
accomplished in an hour, a day or a year, can be hastened by the
moving picture machine so that the actual changes can be made to
appear as if they occurred in a brief time, and the actual move-
ments which were too slow for the eye to recognize, are made to
appear rapidly enough for the eye to follow them. In this way the

CH. XI] MOVING PICTURES '393

actual movements in a growing plant or an opening flower are
revealed to the eye ; and the great steps in the evolution of an egg
to a complete animal, swimming, walking or flying, stand out with
startling reality.

The last triumph is the combination of the phonograph and the
moving picture machine so that both the eye and the ear are
appealed to as in real life or in the theater with living actors.
This combination was suggested by Muybridge, the first to analyze
and then combine the movements of animals by photography and
projection. That suggestion was made in 1888, but it is only now
after 25 years that a fair degree of success has been obtained. It
requires two operators, one for the moving picture machine and
one for the phonograph. The phonograph is just behind the
screen, while the moving picture apparatus is in the usual place at
the back of the theater.

The screen is sufficiently transparent so that the phonograph
operator can see the moving pictures, and the moving picture
operator has telephonic connections with the phonograph so that
he can hear accurately the sounds. He can, of course, see the
moving pictures on the screen. The phonograph is made the
master machine and the pictures must be made to follow the sounds.
This is partly accomplished by a direct connection between the two
machines, and partly by the intelligent cooperation of the two
operators.

The first successful efforts in moving pictures were made by
physicists and physiologists who desired to analyze the complex
and rapid movements of men, animals, and machines. The pur-
pose was wholly scientific, but it was early seen that herein lay the
possibility of entertainment and general instruction.

The entertainment or amusement feature is, perhaps, now the
predominant one; but the religious, educational, economic and
scientific use of this powerful means for portraying action has never
been lost sight of, and to-day is more prominent than ever.

Much has been said and written on the moral or social effect of
the moving picture. The writers and their friends have visited
moving picture theaters in many cities and in many lands to see

394 MOVING PICTURES [Cn. XI

the kinds of scenes that were portrayed, and the kinds of people
who crowded the theaters to see them. At the same time they
have also visited the regular theaters to see actual human beings in
the plays, and the kind of plays and the kind of people who were
there to see them.

To some of us, at least, the actual stage and the screen-stage
seem equally real. The screen-stage has the advantage of a
boundless, and untrammeled outlook of land and water, earth and
sky in calm and sunshine and in the resistless action of storm or
volcanic eruption.

In human life it can show actual scenes, commonplace or heroic ;
scenes like a royal coronation, or the barbarisms of war and riot,
and on a scale impossible for a regular theater, and at an expense
which makes them available for all mankind to see and enjoy, each
according to his own knowledge, experience and capabilities.

That some of the scenes in moving picture theaters are neither
inspiring nor uplifting, and that the order in which the scenes
appear is sometimes unfortunate, must be admitted. But these
and all other defects which have been pointed out are not inherent
in the moving picture. They simply indicate human failings.
They can be corrected and are being corrected all the time.

It is perfectly natural to think of the advantages to be gained by
impressing moving pictures into the service of education. The
striking scenes depicted by the moving picture are well adapted for
arousing interest and giving the inspiration which lead to the care-
ful and painstaking effort necessary for a true education. For
example, in the development of a frog or a fish from the egg the
moving picture shows the major changes but not the minor ones
which are the really essential changes. No one would ever become
an embryologist by looking at moving pictures of a developing
animal or plant, and so with all the other subjects the study of
which enters into an education.

There are a good many helps in education, but there is no way
to become really educated in any subject without the continuous
and concentrated study of details as well as of the subject as a
whole, any more than a man can become a skilled mechanic by

CH. XII MOVING PICTURES 395

simply visiting the best conducted machine shop in the world.
Education is personal ; everything gained has to be paid for to the
last farthing in mental effort.

Moving pictures are the offspring of science through some of the
finest minds that the world has known. It is simply for the finest
art, the best science and the highest aspirations of mankind to take
this powerful agent — their offspring — and put it to the real service
of humanity. Let it do what it is so capable of doing in the church,
in general and technical schools of all grades; in scientific, educa-
tional and philanthropic societies; in the theater, in the club, and
finally in the home.

AUDITORIUM, SCREEN AND OPERATING ROOM

First, it is necessary to consider the room for projection, its
arrangement for seats, lighting during and between exhibitions, the
screen and the position of the machine.

§ 553. Auditorium and screen. — The auditorium should be
arranged so that everyone in the room can get a good view of the
screen, there should be a sufficient number of aisles and exits in
order that the room can be filled or emptied quickly and without
disturbance; and provision should be made for giving a sufficient
illumination during the performance so that people can find seats
or leave the room without difficulty.

The screen should be dead white and free from wrinkles. If
simultaneous sound effects are to be produced it is an advantage to
have the screen slightly translucent so that the pictures can be seen
from behind. In a long narrow room one of the metallic screens is
an advantage. These screens are very poor for those on the side
when used in a wide room, as the picture appears very dim when
seen from the side. When the hall is provided with a stage it is well
to hang the screen quite a distance from the front of the stage so
that it will be easier to avoid stray light and in order that the people
in the front seats will not be too close to the picture. A dark
border or frame to the screen is also an advantage. (For the size
of screen and of the screen images see Ch. XII, § 633, 638-639).

396 OPERATING ROOM [Cn. XI

§ 554. Position of the machine. — The machine should be
located so that its optic axis is perpendicular to the screen or the
pictures will be distorted. If the machine cannot face the screen
directly it is better to have it in the middle of the room and pointing
upward or downward, or to have it at the same height as the screen
and pointing slightly to one side. The worst possible distortion
occurs when the machine is pointed obliquely downward as it must
be when placed in one side of a gallery.

§ 555. Tent or booth for temporary operation. — For a single
performance the machine may be laid on a table in the middle of
the auditorium just as with a magic lantern or it may be enclosed
with a temporary booth or tent to enclose any stray light and to
overcome the distracting effect of the machinery.

§ 556. Permanent operating room. — Permanent installation
should include an operating room large enough so that the machine
or machines can be operated without hinderance or loss of time
from lack of sufficient space. This is very essential in any place
where even a short delay is so disagreeable to the audience. The
operating room should be easy to get to and it should be well
ventilated. It should have a large flue at least 50 cm. (20 in.) in
diameter, connecting with the outside of the building. All open-
ings in the operating room should be provided with shutters which
will close automatically in case of fire. The room should be pro-
vided with incandescent lamps and extension cords to use while
working around the machine and finally there should be an electric
fan and a chair for the operator. Every machine should be
accessible from all sides. Film boxes should be placed where they
can be easily reached. Sufficient tools for ordinary operation, a
supply of carbons, pins, film cement, and extra condenser lenses
should also be handy. A shop-room equipped for making repairs
to the machines and for doing jobs of wiring should be near the
operating room. It is not advisable to try to do such work in the
operating room itself.

The operating room is to be at all times kept like a battle-ship in
time of war, with the decks cleared for action, nothing there which
is not actually required.

CH. XI] OPERATING ROOM 397

§ 557. Construction of a modern operating room. — For the

construction of the operating room itself a good description is given
byF. H. Richardson in the Moving Picture World of August 12,
1911, p. 372. See also Richardson's Handbook, pp. 65-93.

(1) "No operating room may have less than 50 square feet of
floor surface, or be less than seven feet, in the clear, from floor to
ceiling at any point.

(2) All operating rooms shall have a vent flue of not less than
1 3/2 square inches area to each square foot of floor area, same to
extend from the ceiling, or a point near the ceiling, to the open air,
above the roof if possible ; provided, however, that no vent exceed
360 square inches in area.

(3) All operating rooms shall be of such fireproof construction
as is approved by the National Board of Fire Underwriters or the
City Fire Marshal.

(4) Every operating room shall have a door, opening outward,
not less than 2x6 feet in size, provided with an appropriate spring
to hold same shut.

(5) Every opening from operating room into auditorium,
except door, shall be equipped with a metal shutter, sliding in
grooves and semi-automatic in action. Same shall be so arranged
that all shutters are held open by a single cotton master cord
passing directly over front edge of upper magazine of each machine,
just high enough to clear operator's head when standing. Shutters
may close by their own weight or by force of a spring. If vent
flue is provided with damper it shall be so weighted that it will
normally stand open and shall only be held shut by cord attached
by master shutter cord before mentioned.

(6) Front, sides, and top of every lamp-house shall be tightly
enclosed, except for vent-holes, protected by wire gauze screen, but
back of lamp-house may be open.

(7) All moving picture projection machines shall be equipped
with approved upper and lower magazines, doors of which shall be
closed when machine is running.

(8) All rheostats shall be located outside the operating room,
but low voltage transformers (inductors, economizers, etc.), used
to control the current may be located inside the room.

3Q8 MOVING PICTURE APPARATUS [Cn. XI

(9) No wire of less size than No. 6 B & S gauge shall be used in
any projection arc circuit.

(10) Only link fuses, enclosed in suitable metal cabinet with
spring door, shall be allowed in any operating room.

(n) All wires, except asbestos covered from outlet to lamps,
shall be in conduits.

(12) All switches shall be enclosed (fig. 278).

(13) All carbon butts shall be deposited, immediately on
removal from lamp, in metal can containing water.

(14) All films shall be kept in solderless metal case with ap-
proved spring-closing cover, or door.

(15) Smoking shall be absolutely prohibited inside the operat-
ing room.

(16) There shall be no reading matter inside any operating
room. Reading matter to be construed to mean newspapers,
novels, etc., but not including catalogues, or books of instruction, or
magazines helpful to the operator in his work.

(17) Not to exceed one ounce of alcohol or one pint of lubricat-
ing oil shall be allowed in the operating room. Benzine, kerosene
and like substances shall not be kept in any quantity in any theater.

(18) Machines may be motor driven.

(19) All machines shall be firmly and effectively anchored to
the floor."

§ 558. Source of electric current. — Next to be considered is the
source of current supply. If one is in a place where there is a good
electric system in operation it is usually much better to buy the
power than to try to run a special power plant. This is because of
the greater certainty of the city power and the absence of responsi-
bility. It is, however, perfectly feasible to generate power with a
gasoline, oil, alcohol or steam engine. When this is done the power
is somewhat cheaper but rather more trouble and without careful
attention it is less certain than a regular supply. Independent
generation of power in small units makes possible the direct con-
nection of the arc to the generator without the use of a rheostat as
will be explained later. (Ch. XIII, § 680, see also § 562).

CH. XI] MOVING PICTURE APPARATUS 399

§ 559. Wiring. — When the supply is decided upon, the wiring
is next installed. This must be heavy enough to carry the greatest
current which is to be used continuously in the lamp. It does not
need to be designed for the rather high current which flows when
the carbons are brought into contact, as any wiring can withstand
a heavy overload for a few seconds without injury.

The wire which enters the lamp-house should be flexible cable,
asbestos covered, and of a carrying capacity at least double the
amount required for use at the arc (§ 694-695) . This is on account
of the high temperature within the lamp-house and consequent
rapid deterioration of a small wire.

§ 560. Fuses. — Fuses should be used in every case and not
circuit breakers. This is because a fuse will not "blow" instantly
when current is drawn greater than its normal capacity (as when
the arc is started) but if this overload is continued, it will melt and
open the circuit. The circuit breaker, on the other hand, will open
the circuit instantly at the same amperage whether the current is
momentary or long continued.

§ 561. Fire underwriters and special regulations. — The wiring
and installation must conform to the fire underwriters regulations
and any special requirements of the city in which the theater is
located. The wiring for moving picture machines is neither
heavier nor more difficult to install than that required for other
forms of projection, notably opaque projection, provision for 25
amperes direct current or 50 amperes alternating current usually
being sufficient for small theaters.

For currents required in different cases; for the size of wire
required for these currents and for fire underwriters regulations see
Chapter XIII, § 691.

§ 562. Rheostat or other ballast. — As with all forms of arc
lamp, the moving picture lamp requires some form of ballast or
regulating device to control the current.

The simplest and cheapest device is of course the resistor or
rheostat. When the electric supply is no volt direct current, a
rheostat is generally used. A rotary motor-generator set or

400

MOVING PICTURE APPARATUS

[On. XI

"current saver" is sometimes used as a ballast with direct current
and effects a considerable saving of power especially when the
supply is 220 or 500 volts. (See Ch. XIII, § 744).

When the supply is alternating current the ballast may be in the
form of a rheostat but reasons of economy exclude this form of
ballast when the machine is used continuously. For continuous
performance an inductor (choke-coil), a special transformer, a
mercury arc rectifier or a motor-generator is used. (See Ch. XIII,
§682-683, 72

FIG. 221. EDISON KINETOSCOPE, PROVIDED

WITH TWO-WING OUTSIDE SHUTTER.
(Cut loaned by the Edison Manufacturing Company).

When power is independently generated, a special dynamo can
be connected directly to the arc lamp without ballast, the dynamo
will be its own regulating device. (See Ch. XIII, § 680).

Whatever form of ballast is used, the quality and workmanship
should be of the best or an endless amount of trouble may be
expected. The rheostat or other ballast must conform to the
underwriters regulations and must be satisfactory to the company

CH. XI] MOVING PICTURE APPARATUS 401

supplying the power. Some power companies object to the use of
an inductor (choke-coil). In such cases a transformer can be used
instead.

§ 563. Stand or table. — A stand or table is provided by the
makers of the machine. The method used to set up the stand will
be fairly obvious from the illustrations furnished by the makers of
the particular machine used. Generally this stand is made of
brass tubes. One maker provides a heavy iron pillar. With this
make provision must be made to anchor this pillar firmly to the
floor.

If the machine is to be installed permanently, it is often better to
use a stand constructed of concrete or a very heavy wooden table
instead of the light stand regularly supplied. A very slight motion
of a rickety stand will cause an enormous movement of the picture
on a screen 15 to 30 meters (50 to 100 feet) away.

§ 564. Unpacking. — The moving picture machines coming from
the factory are very carefully packed. When removed from the
box, it is advisable to take careful notes of just how the different
parts are packed and to number the wooden cleats used to hold
things in place, especially if the machine will need to be shipped
away again.

Be careful in unpacking all parts, especially the lenses. Do not
throw away any wrapping material until sure that no parts are
missing.

§ 565. The moving picture machine. — When unpacked the
moving picture machine will be found to consist of a stand and base-
board, arc lamp, lamp-house, condenser, aperture plate, objective,
shutter, film magazines, and mechanism for moving the film.
There will also be an extra film reel and a rewinder (fig. 221-224).

§ 566. The arc lamp. — The arc lamp usually supplied with
moving picture outfits is of the hand-feed type with inclined car-
bons. The handles for feeding the carbons and for slight up and
down adjustments project backwards so they may be manipulated
without opening the lamp-house. The good makes of arc lamp are
adjustable so that the carbons can be held in the vertical or the

402 MOVING PICTURE APPARATUS [CH. XI

inclined position as desired and each carbon holder can be turned so
that the upper carbon is inclined and the lower one is vertical
The right-angle arc can be used with the moving picture outfit if
desired, but it should not be used with currents much above 25
amperes Twenty-five amperes direct current will be found
sufficient for all but the largest rooms.

Fic^222.. POWER'S CAMERAGRAPH No. 6, SHOWING THE LAMP-HOUSE

IN POSITION
(Cut loaned by the Nicholas Power Co.).

§ 567. Lamp-house. — The arc lamp is enclosed by a metal
house to protect the operator from being blinded by stray light and
to protect the arc from air currents which might blow it out or
otherwise interfere with its performance. The adjusting handles
of the lamp project so that the lamp can be adjusted from time .to
time without opening the doors of the lamp-house.

The house should be provided with doors to enable the operator
to change the carbons and should have a window of dark glass so

CH. XI] MOVING PICTURE APPARATUS 403

that the arc can be watched. This window should be of fairly
large size and directly opposite the crater of the arc. The glass
should be dark enough so that the eyes will not be tired by the too
great brightness and yet light enough so that the whole of the hot
carbon ends can be seen.

Another convenient way to observe the arc is to bore a fine hole
in the side of the lamp-house away from the operator. This acts
like apinhole camera and an image of the arc is seen on the opposite
wall. A sharper image of the arc can be formed by using a long
focus lens over an opening in the wall of the lamp-house to focus an
image of the arc upon the wall. A spectacle lens of about 25 cm.
(10 in.) focus (4 diopters) will answer. The lens may be held by
any convenient clamp but must be adjusted for distance to get the
sharpest image, otherwise it is no improvement over the simple
pinhole.

The lamp-house should be well ventilated as from ^ to 2 kilo-
watts of power, .7 to 3 horsepower, is converted into heat. While
the arc is going there must be some way for this heat to escape,
otherwise everything inside would melt. One of the principal
causes of condenser breakage is poor ventilation of the lamp-house.
The best ventilation is secured by having holes permitting air
circulation but no escape of light, at the top and near the bottom
of the lamp-house. The back of the lamp-house is sometimes
removed.'

In many places the fire underwriters or the city, require that
these ventilating holes be covered with fine wire gauze, to prevent
sparks flying out. This requirement was invented by someone
who had the mistaken idea that an arc lamp was a fiery volcano,
vomiting out sparks and lava in all directions instead of a quiet,
well behaved sort of thing. It is true that a minute spark some-
times does fly up, but it is so light that it cannot do any damage.
Any small piece of the carbon tip which breaks off will fall to the
bottom of the lamp-house where a suitable tray should be pro-
vided to catch it. This tray is also useful to hold the short pieces of
hot carbon just taken out of the lamp when new carbons are put in.

§ 568. Condenser. — The condenser is usually in a box which is
fastened to the lamp-house and moves with it. In front of the con-

404 MOVING PICTURE APPARATUS [Cn. XI

denser is the lantern-slide carrier for use with the magic lantern,
which is usually found in connection with moving pictures.

FIG. 223. NEW STYLE CONVERTIBLE BALOPTICON WITH POWER'S

MOVING PICTURE ATTACHMENT.
(Cut loaned by the Bausch & Lomb Optical Co.}.

The condenser is usually provided with two plano-convexlenses,
each of 18 or 19 cm. focus (7 to ;>£ in. focus). JPP^

The slide-carrier for the magic lantern usually found connected
to the moving picture outfit is generally fastened to the lamp-house

CH. XI]

MOVING PICTURE APPARATUS

405

directly in front of the condenser. This is not a good plan as it
cuts down the light from the condenser, and as the opening is not
round but a quadrangle it often leads to queer shadows on the
screen. Some makers provide a stationary slide-carrier opposite
the magic lantern objective so that the whole face of the condenser
is free when it is opposite the moving picture objective ; this is a
better method than the above.

FIG. 224. DOUBLE DISSOLVING MODEL C BALOPTICON WITH EDISON
MOVING PICTURE ATTACHMENT.

(Cut loaned by the Bausch & Lomb Optical Co.).

§ 569. The moving picture head. — This contains all of the ele-
ments of the moving picture machine except the arrangement for
lighting. The moving picture head holds the objective and con-
tains the film-moving mechanism and the aperture plate.

§ 570. Aperture plate. — Considered optically the aperture plate
which serves as a frame for the picture on the film is the most
important part of the moving picture head.

406 MOVING PICTURE APPARATUS [Cn. XI

The standard aperture plate has an opening 23.08 mm. wide x
17.31 mm. high (29/32 in. x 87/128 in.) with rounded corners.
When the picture is in focus on the screen the edges of the aperture
plate are also in focus at the same time (§ S7oa).

§ 571. The objective. — The objective forms the image of the
film picture upon the screen. It is in design exactly like an objec-
tive for the magic lantern but is of shorter focus.

It is better to have the lenses of large diameter (see § 830).

Moving picture objectives with lenses 45 mm. (iK in.) and
65 mm. (2^2 in.) in diameter are on the market. The objectives
45 mm. (1^4 in.) in diameter will answer but those of 65 mm. (2^2
in.) are to be preferred. The larger objectives will give with less
trouble a screen image without shadows. (See § 829, 830).

One must select an objective of suitable focal length to give a
proper sized screen image for the auditorium to be used. This is
dealt with more fully in § 635. In most rooms a screen image of
suitable size will be obtained with an objective of between 12.5 to
13.5 cm. focus (5 to 5 y£ in.) when the moving picture machine is at
the back of the room.

§ 572. The film mechanism. — This consists in the proper gears
and sprocket wheels for moving the film, and for turning the shut-
ter. The mechanism is complex; differs in different makes of
machines, and no attempt will be made here to describe it in detail.

§ 573. The shutter which cuts off the light during the time
when the film is in motion is located either just beyond the aperture
plate and hence before the objective (fig. 225), or just beyond the
objective (fig. 226, 227). When located between the aperture
plate and the objective, it is called an inside shutter and when
located beyond the objective it is called an outside shutter.

§ 570a. Standard aperture. — As there was some lack of uniformity in the
size of the opening of the aperture plate, the Gundlach-Manhattan Optical
Co. has selected a size for a standard as follows : The aperture has an opening
of 23.08 mm. long and 17.31 mm. high (29/32 x 87/128 inch). This standard
has been adopted by the Nicholas Power Co., the Edison Co., and the Precision
Machine Co. No doubt the other makers of machines will adopt the standard
in due time. Moving Picture World, Vol. 20, April 1 1, p. 210, April 25, p. 512.

CH. XI]

MOVING PICTURE APPARATUS

407

§ 574. The film magazines are large sheet iron.: boxes which
hold the film reels. They are big enough to hold the standard
25 cm. (10 in.) reel and it is a convenience if they are large enough
to hold the larger reels of 30 cm. (12 in.) diameter. The film
magazines are fitted with fire traps to prevent any fire getting into
the magazine if the film should start to burn.

FIG. 225. MOVING PICTURE MECHANISM WITH INSIDE SHUTTER, I S.
For full explanation see Fig. 231.

INSTALLATION OF A MOVING PICTURE OUTFIT

§ 575. After the wiring to the operating room has been installed
in accordance with the fire underwriters regulations and any special
regulations of the city in which the work is done, all is ready to
connect in the rheostat, transformer, or other regulating device
(§ 728, 736) and to attach the wires to the arc lamp.

These connections are exactly like those for the magic lantern
(fig. 3) when a rheostat or inductor (choke-coil) is used. When a
transformer or mercury arc rectifier is used, the primary side is

408

MOVING PICTURE APPARATUS

[CH. XI

connected to the line, and the secondary side is connected to the
arc lamp. (See Ch. XIII, § 683, 739).

The switches should be in a convenient location, so that the
current can be turned on or off without moving from the operating
position.

As soon as the connections are made it is well to use an ammeter
and to find what current the arc will draw with the different set-
tings of the controlling lever of the rheostat or transformer. It is

FIG. 226. MOVING PICTURE MECHANISM WITH OUTSIDE SHUTTER, O S.
For full explanation see Fig. 23 1 .

a good thing also to use a voltmeter to determine the line voltage
on open circuit, also the voltage across the line, between the arc
terminals, across the rheostat or choke-coil, or if a transformer is
used, the voltage given by the secondary both on open circuit and
when the arc is running. The voltmeter or ammeter must be
designed for the kind and amount of current for which it is to be
used, that is, alternating or direct current. When a rectifier or a
motor-generator is used it will be necessary to have both direct

CH. XI] OPTICS OF MOVING PICTURES 409

current and alternating current instruments. (See Chap. XIII
for using these instruments § 662-674).

OPTICS OF MOVING PICTURE PROJECTION

§ 576. For purposes of description the projection of the
individual pictures of a film can be considered apart from the
mechanism which moves the film.

The projection of the film picture has much in common with that
of the ordinary lantern slide but it is somewhat more difficult.

A theoretical treatment of the proper method of lighting the film
is found in § 825. Briefly stated it is this: Light from the arc is
collected by the condenser so as to illuminate the film. This
illumination must be very intense and at the same time must be
evenly distributed over the entire area of the film. To secure this
result with the ordinary large condensers (4^2 in. in diameter)
requires the condenser to be quite a distance away from the film,
the crater of the arc to be of considerable size, and the projection
objective to be of fairly large diameter.

Fig. 228 shows the optical arrangement most commonly used.
Light from the arc is collected by the condenser upon the
film at s, passing through the transparent parts of the film, it is
bent by the objective in such a way as to form a sharp image of
the film s, upon the screen.

Only one picture of the film is seen at a time, the rest being
carried in the magazines or covered with shields. The picture
to be shown is just in front of the opening of the aperture plate.
Optically we are concerned only with the aperture plate and the
short section of film behind it. It is this short section of film which
must be evenly illuminated and projected upon the screen.

Beyond the film is the objective (fig. 229). The objective should
be of good quality as it is the objective which determines the
sharpness of the screen picture. Moreover, the objective must
not be of too small diameter, for if it is too small there is danger
that the screen image will not be evenly lighted although the
illumination of the film may be perfectly even. The focal length

4io

OPTICS OF MOVING PICTURES

[Cn. XI

FIG. 227. MECHANISM OF POWER'S No. 6 CAMERAGRAPH, SHOWING THE

THREE-WING, OUTSIDE SHUTTER.
(Cut loaned by the Nicholas Power Company).

of the objective determines the size of the screen picture for a given
screen distance.

§ 577. Lining up the moving picture machine ; Adjustment of
the light. — The machine being assembled on the board, the parts
lined up mechanically as well as possible (§ 51 +), the final steps to

CH. XI]

OPTICS OF MOVING PICTURES

411

get a good light on the
screen must now be taken.
The moving picture head
as it comes from the fac-
tory should have the
aperture plate and the
center of the objective
mount at the same
height. If they are not,
the aperture plate must
be moved up or down
until its center is on the
same axial line as the ob-
jective. The adjustment can prob-
ably be done with sufficient accuracy
with the eye, when looking through
the lens opening, the lens being in
place. This is a matter which al-
ways should be looked after by the
manufacturer.

Another method would be to re-
move the condensers and adjust the
arc lamp to exactly the same height
as the aperture plate. A piece of
paper put in place of the lens
should, when the arc is lighted,
show the shadow of the center of
the aperture plate in the exact cen-
ter of the circular piece of paper.

FIG. 228. OPTICAL SYSTEM AND ILLUMIN-
ATION OF MOVING PICTURES.

Lamp.

Condenser.

S Lantern-slide holder.

Fire Shutter This is open only when
the machine is running.

s Aperture plate.

Objective.

See fig. 231 for full explanation of the
mechanism.

412 OPTICS OF MOVING PICTURES [Cn. XI

The center of the condenser and the center of the aperture plate
are adjusted to the same height above the baseboard. This is
attended to by the manufacturer, but if a head of one machine is
used with the arc and condenser of another make, adjustment
might have to be made. Make a spot with a pen or a wax pencil
exactly in the center of the front lens of the condenser, measure the

height of this above the baseboard.
Make a similar mark on the aperture
plate at the height of the middle of
the opening and measure its distance
above the baseboard. If the aperture
plate is too low the head should not
be screwed directly to the baseboard
but should be lifted UP sufficiently with
(Cut loaned by the Gundlach- a thin piece of board. If the aperture
Manhattan Optical Co.). plate is too high( the front of the base.

board can be cut down or the lamp-house and condenser can be
raised by using a piece of wood or asbestos board between the base-
board and the lamp-house fastenings.

After getting the objective, aperture plate, and condenser at the
correct height, it only remains to get the arc at the right height.
This is done from time to time by raising or lowering the arc lamp
until the light spot falls exactly over the aperture plate.

The sidewise adjustment of the lamp-house is now made in the
same way by measuring the distance from the edge of the base-
board to the center of the condenser and then to the center of the
aperture plate. This measurement can be made by using a vertical
board (§52).

When the same arc and condenser are used for both moving pic-
tures and lantern slides, the lamp-house should be in the correct
position when pulled on its lateral rods as near as possible to the
operator. If it is not, stops can be fastened on the side rods to hold
the lamp-house in the correct position.

§ 578. Back and forth adjustment of the arc lamp and con-
denser.— This is one of the most important and troublesome adjust-
ments to make. There would be but little difficulty in getting an

CH. XI] OPTICS OF MOVING PICTURES 413

even illumination of the film picture and the screen image if con-
densers were obtainable entirely free from spherical aberration,
but this is not practical. No rule can be given as to the best
position of the arc and condenser but the best position must be
determined for each particular case. Some general hints can,
however, be given.

First — The objective should be of large diameter. This will
allow of a greater range of adjustment through which good illumin-
ation can be obtained (§ 829-830).

The lenses of the double-lens condenser (fig. i) should be as near
together as possible without actually touching. The convex sides
of the condenser lenses should face each other, the plane sides
should face the lamp and the objective.

The condenser lenses should be of fairly long focus, 18 to 19
cm. (7 to y>2 in.).

If the condenser is as far away from the aperture plate as
possible the illumination is usually more even, though less intense
than when the condenser is close to the aperture plate.

When first setting up the machine, it is a great help to have a
series of condenser lenses to try, say such a series as two lenses of
14 cm. focus, one each of 15, 16, 17 cm., two of 19 cm. focus, (two
of 5^" in., one each of 6, 6>^, 7 in., two of 7^ in.). The two con-
denser lenses should be of the same focus, then only one kind of
condenser lens will need to be kept in stock to supply breakage.

When the adjustment for distance is to be made, move the lamp-
house with its condenser close to the aperture plate, fasten it
in position, move the arc in the lamp-house nearer to and farther
from the condenser until the best light is obtained on the screen.
Note how this light appears and whether there are any ghosts or
shadows. Then fasten the lamp-house and condenser slightly
farther from the aperture plate and move the arc until the best
light is again obtained. After repeating this, for every position
of the condenser, the condenser is set at the distance which was
found to be best. It may be necessary to try a different set
of condenser lenses before the best possible result is obtained.
This is a rather tedious process but is well worth while doing.

414 MAGIC LANTERN AND MOVING PICTURES [Cn. XI

ADJUSTMENT OF THE MAGIC LANTERN ATTACHMENT FOR USE IN
CONNECTION WITH THE MOVING PICTURE MACHINE

§ 579. The adjustment of the arc lamp and condenser for the
moving picture part is of much greater importance and is more
difficult than that for the magic lantern attachment, hence, no
attention should be paid to the projection of lantern slides until the
projection of moving pictures is perfect.

In most outfits the lamp-house moves sidewise on some lateral
rods. When pulled towards the operator the lamp is in line with
the moving picture objective, and when pushed away from the
operator until it hits a stop, it is in line with the lantern objective.

Push the lamp-house on these lateral rods until it is held by the
stops. A lantern slide is put in the holder and the lantern objec-
tive support is loosened and the lantern objective moved sidewise
until it is over the spot of light from the arc and moved back and
forth until the image of the slide is in focus on the screen. If there
are shadows on the screen not due to malposition of the carbons,
use an objective with larger lenses.

If the lantern picture does not occupy the same place on the
screen as the moving picture it may be the fault of the side adjust-
ment of the slide-holder, or it may be due to faulty alignment of the
arc lamp and moving picture head. If this should be the case
move the lamp-house sidewise until the lantern-slide picture
occupies the proper position on the screen. Then move the arc
sidewise until the screen is well lighted and clamp it in position.
When, now, the lamp-house is pulled into position in front of the
moving picture objective the spot of light may not fall upon the
aperture plate but to one side. If it is not in the right position do
not alter the adjustment of the arc lamp but move the lamp-house
as a whole to one side until the spot exactly covers the aperture
plate. Then fasten the stop, so that the lamp-house will always
occupy the same position when pulled toward the operator.

§ 580. Management of the arc lamp. — During an exhibition it
is necessary to watch the arc lamp to see that it is burning prop-
erly. There are several ways of burning the arc which will give a
good light :

CH. XI] MAGIC LANTERN AND MOVING PICTURES

415

The carbons may be at right angles (fig. 23 C).

The carbons may be inclined backwards about 30° (fig. 230 a).

The upper carbon may be inclined backward 45°, the lower
carbon being vertical (fig. 230 c).

The carbons may come together in the form of a horizontal V
with the point towards the condenser (fig. 23 D).

Both carbons may be vertical (fig. 230 b).

Whatever carbon setting is used, the arc must be held, so that the
crater or craters face the condenser.

The form of the arc can be watched by observing it through the
smoky glass window or by the pinhole or lens image on the wall
(§ 567). When using alternating current the sound will give an
indication as to whether the arc is of the right length.

Constant vigilance in watching the arc is one of the requirements
for success is showing moving pictures. During an exhibition,
never let the arc go out.

§ 581. Supply of carbons for the arc lamp. — A good supply of
carbons should be provided and placed where they may easily be
reached. The carbons are soft-cored and their size should be
suited to the current used (see § 753a). Generally 16 mm. carbons
(^ in.) are used, both being of the same size.

§ 582. Position of the film in the machine. — When a film is
passing through the machine the rule for its position is the same
as with the lantern slides, that is, the picture should appear correct
when one looks through it toward the
screen but it must be upside down.
To accomplish this one should bear in
mind that as the films are printed they
will appear correct when one looks at
the emulsion side just as with a lan-
tern slide or an ordinary paper print.
Therefore, the light is made to strike
the emulsion side of the film.

§ 583. Mechanism. — Without go-
ing into the details of the special

FIG. 230. POSITION OF CAR-
BONS FOR MOVING PIC-
TURE PROJECTION.

a Inclined.
b Vertical.

c Upper carbon inclined,
lower carbon vertical.

MOVING PICTURE FILM AND MECHANISM [Cn. XI

arrangements employed in the different makes of machine, the
principle is simple, although the mechanical problems in work-
ing out these principles require much care.

Objective

1 1

iin

111 1

FIG. 231. FIGURE TO REPRESENT THE PRiNciPLE7oF THE MOVING

PICTURE MACHINE MECHANISM. "
a b Sprocket wheels moving with uniform velocity.

c Intermittent sprocket wheel which jerks down the^short section^of film
between L and M.

i Idlers to hold the film on the sprocket wheels.

D Gate which holds the film in place in front of the aperture plate.

F Upper film reel, unwinding.

G Lower film reel, winding up.

S Aperture plate.

Objective.

CH. XI]

MOVING PICTURE FILM AND MECHANISM

417

The essential part of the mechanism consists in three sprocket
wheels, a, b, and c, (fig. 231), the two wheels a and b move con-
tinuously at the average rate at which the film is passing (30 cm.,
i foot, per second), and serve to unwind the film from the upper
reel F and feed the film to the take-up reel G at a uniform rate.
The sprocket wheel c, located between the other two, is inter-
mittent in its movements, being stationary for about % of the time
and being in rapid motion for about J"6 of the time. The effect
is, that after the film has been in position for exposure on the screen
this sprocket wheel jerks the small section of film between L and M
forward to the next picture. In fig. 232 is shown one form of
mechanism for causing the intermittent movement of the sprocket
wheel.

When the film is stationary it is projected on the screen by the
objective, but during the short time when the film is in motion a
shutter either before or behind the objective cuts off the light and
prevents any blurring due to the movement of the picture.

The films are made in such a way that if the pictures are right
side up, the later picture will be below the earlier ones, but as in
passing through the machine the pictures are upside down, the
later pictures are above and it is necessary to move the film down-
ward to bring the pictures on
.fiMMtfAtefc*, the screen in due order.

§ 584. Threading the film in
the machine. — The film as
wound on the reel usually is
wound in the correct direction,
so that the first pictures are on
the outside. If this is not the
case, the film must be rewound
on another reel to reverse its
direction. If the direction is
correct the pictures will be up-
side down when the film is in

FIG. 232. INTERMITTENT MOVEMENT the machine, that is, when the

OF POWER'S No. 6 CAMERAGRAPH. ,-•, • -, -, c

,,, , , , , ., ... , , „ film is passing downward from
(L ut loaned bv the Nicholas Power

Company). F (fig. 231).

4i8

MOVING PICTURE FILM AND MECHANISM [Cn. XI

Next, it is necessary to get the film right side out, otherwise,
everything will be reversed and appear as if seen in a mirror, an
especially troublesome state of affairs when titles or letters are
shown. The side of the film which has the emulsion appears
rough, the other side is smooth and shiny. The film often has a
tendency to curl, the emulsion being on the concave side. The
film is turned so that the rough, emulsion side bearing the picture
is toward the light. When it is wound correctly on the reel, and
the emulsion side is turned so it will face the light as the film
unwinds, the reel of film is p'ut in the upper magazine. The end
of the film is pushed through the opening in the magazine between
the rollers of the fire-trap. This can best be done by using the
index and middle fingers to hold the film.

FIG. 233. EDISON KINETOSCOPE MECHANISM.
(Cut loaned by the Edison Manufacturing Company)

The magazine doors are open showing the film reels.
The film is in»place ready to project.

CH. XI] MOVING PICTURE FILM AND MECHANISM 419

The gate D is then opened and the idlers, iii are pushed away
from the sprocket wheels a, b and c. A sufficient length of film
is unrolled from F to reach to the take-up reel G and the film
is put under the sprocket wheel a, so that the teeth fit into the holes
at the edges of the film. Care must be taken that the film goes
over or under the sprocket wheels in such a way that as the crank
is turned forward all of the sprocket wheels tend to move the film
in the same direction, otherwise they will tear it apart. The
arrangement may differ in different machines.

After putting the film on the sprocket wheel a, so that the teeth
pass through the holes of the film, the idler i, is pushed over to hold
the film in place. This can be done with one of the fingers while
holding the film in place with the thumb and forefinger. The film
is then engaged with the lower sprocket wheel b, leaving an extra
length of film to form the two loops L and M. This can best be
determined by experience, it must be enough so that the inter-
mittent sprocket will not jerk the film in two and not long enough
so that the loops will strike any shields there may be to cover them.

The film is held against the intermittent sprocket c, so the loops
L and M, are about equal in size and held straight on the tracks of
the aperture plate when the gate D, is closed.

The end of the film is now pushed through the fire-trap opening
in the lower magazine and fastened to the take-up reel G. This is
accomplished by slipping the end under the spring on the spindle
of the reel, in such a direction that the film will not be folded as the
reel is turned. The reel is turned to insure the end of the film
being well fastened. Fig. 233 shows a mechanism with the film in
position and ready to operate as soon as the magazine doors are
closed.

If the picture is not directly in front of the aperture plate but is
above or below (misframed) , it can be put in its proper position by
a lever which lowers the mechanism and film without disturbing
the position of the aperture plate and objective.

§ 585. Direction of motion. — The normal direction of motion
to secure the proper sequence of events in the order in which they
occurred is secured by moving the film downward, and results

420 MOVING PICTURE FILM AND MECHANISM [Cn. XI

from a right-hand rotation of the crank. If the crank is turned to
the left the film will be pushed upward by the intermittent sprocket
instead of being pulled downwards as it should be. This would
most likely result in crumpling and breaking the film.

§ 586. Operation and speed. — After the machine is threaded
the lamp is pulled toward the operator so that the light shines upon
the aperture plate.

In starting the machine do not start with a jerk but start grad-
ually (1-2 seconds), otherwise an unnecessary strain is put upon the
gears. The crank is turned in a right-hand (clockwise) direction
at such a speed that the film passes at the rate of 16 pictures per
second. If the gearing is arranged so that the intermittent
sprocket would move 16 times for each revolution of the crank, this
would require i revolution per second or 10 revolutions of the
crank every ten seconds. One should practice the speed for a
while with no film in the machine, looking at the second hand of a
watch and turning with a uniform speed until there are just 10
revolutions every time the second hand passes a ten second division.
This should be practiced for some time until the proper speed can
be maintained with certainty. After the film is in, the action in
the scene will serve as a guide for the proper speed, as some films
are improved by being shown at a slower or faster rate than they
were taken, i. e., the standard given above.

See Richardson's Handbook, p. 219.

§ 587. Automatic fire shutter. — As the machine starts, the
automatic fire shutter (fig. 228) opens and allows the light to fall
upon the film. If the picture is not at the right height on the
screen it can be "framed up" by moving a lever which raises or
lowers the mechanism and film.

If an old machine is used that does not have an automatic fire
shutter, one must be extremely careful never to allow the light to
fall upon the film except when it is in motion, otherwise one or two
seconds will suffice either to ruin the film if non-inflammable film
is used or to start a conflagration if celluloid film is used. The
danger from this source is so great that we strongly recommend

CH. XI] MOVING PICTURE SHUTTER 421

that a water-cell be used (§ 848) in cases where an automatic fire
shutter is not provided; where a motor is used to drive the
machine; for all experimental work and for every person running
a moving picture machine who has not had abundant experience in
operating. It is so easy to let the film stop for a second, or to have
the film break leaving a tag end of film in the aperture plate, and
wonder afterward what started the fire.

§ 588. Setting or "timing" the shutter.— The shutter should
be mounted on the spindle used to turn it in such a way that it will
cut off the light from the screen during the time when the film is in
motion. If the shutter is not set exactly right in the beginning it
is often a rather tedious job to correct its position, but by going at
the matter systematically the difficulty is greatly lessened.

Shutters of the one-wing type can, of course, be set in only one
way but shutters of the two- or three-wing types may have wings of
different widths. In this case the widest wing is the one which
should intercept the light while the film moves.

The easiest way to set the shutter would, of course, be to run
the machine very slowly and watch the picture on the screen. If
no shutter were used the picture would seem to jump up, and be
replaced by a picture which comes up from below. When the
shutter is in place, if the picture seems to jump up just before the
light is out, the shutter is said to be too "late" and it must be
loosened on its shaft and turned slightly forwards, that is, in the
direction in which it is turning. The shutter is then fastened
securely in position. If, the picture jumps into place from below
just after the light comes on, the shutter is said to be too "early"
and it must be turned slightly backwards. That the shutter may
be correctly set when it is turning rapidly as well as when it is
moving slowly, it is well to hold the outside of the shutter or the
shaft on which it turns with the finger so as to take up lost motion.
When in rapid rotation all the lost motion is taken up on account
of air friction.

§ 587a. With a two-lens condenser the water-cell can be put between the
condenser and the aperture plate (fig. 206).

422 MOVING PICTURE SHUTTER [Cn. XI

Running the machine slowly with a film in the machine is entirely
practical provided the arc current is not extremely heavy, and
provided a water-cell is used (See § 596, 779-782).

When no water-cell is at hand the machine must be run rapidly.
In this case the rule for changing the position of the shutter is
exactly the same but the motion of each individual picture cannot
be seen. If one has a film which is nearly opaque, but has a few
spots in it, as a period on a title for example, there is an effect
known as "travel ghost" which is seen if no shutter is used or if
the shutter is incorrectly timed. This is caused by the persistence
of vision. As the white spot moves upward, it appears to be a
streak instead of a spot. If, now, the shutter is too late, the light
is not cut off until the spot starts to move upwards and a streak is
seen above the spot. If the shutter is too early, the light is turned
on while the spot is still moving upward and before it comes to
rest; the streak is then seen below the spot.

If the shutter is too narrow the motion of the spot, both before
and after the light is cut off and the streak will be seen both above
and below the spot of light.

§ 589. Rule for setting or timing the shutter.— If the streak or
travel ghost appears above the letters of the title, the shutter is too
late, turn it slightly forward on the shaft.

If the streak or travel ghost appears below the letters of the title
the shutter is too early, turn it slightly backwards on the shaft.

If the streak or travel ghost appears both- above and below the
letters of the title, the shutter blade is too narrow.. Use a shutter
with a wider blade.

§ 590. The best position of the shutter and the speed to prevent
flicker. — The shutter may be placed in either of two positions ; it
may be just beyond the film and between it and the objective
(inside shutter) or it may be placed beyond the objective (outside
shutter). There is a difference in the effect produced depending
on which of these positions is chosen (fig. 225-226).

With the inside shutter, when the machine is turned slowly the
image of the shutter can be seen somewhat out of focus traveling
from one side of the picture to the other.

CH. XI] FLICKER WITH MOVING PICTURES 423

With the outside shutter, beyond the objective, the wing of the
shutter as it advances removes light from the whole of the picture, a
phenomenon which tends to reduce flicker.

The diameter of the inside shutter is limited by the size of the
mechanism, while the outside may be made as large as is desired.

As will be seen below, the diameter of the shutter has an effect
on the light.

The picture should be entirely covered by the shutter before
it commences to move, and it should not be uncovered until it has
ceased to move. This requires that the wings of the shutter need
to be about 3 cm. (i^ in.) wider than the theoretical J^6th of the
circumference of the circle.

The larger the circle the nearer to Yd of the circle is the width
of the shutter wing.

With a shutter of large diameter, the actual velocity is greater
and the interruption of the light is more sudden, therefore a
shutter of large diameter is to be preferred.

§ 591. Flicker. — The standard speed of the film is given as
18 meters (60 ft.) per minute, 30 cm. (i ft.) per second. There
being 16 films per 30 cm. (foot), this gives 16 pictures per second.

It is the general intention to run films at this speed though they
are often run either faster or slower to get the best effects. The
time during which one picture is shown (Vie second) can be divided
into 6 equal periods, during five of these periods the picture is
stationary and during the 6th the film is moved and the next
picture substituted.

One complete change will be called a cycle.

The films could be run through the machine with no shutter at
all, the film being in place an instant and then moved and the next
picture substituted by a quick movement. This will cause a
spreading out of white patches into a vertical streak called travel
ghost, and will also give a general gray appearance and lack of
contrast to the screen image.

To avoid this appearance some kind of a shutter is used to
obliterate the pictures while the film is in motion. The shutter
can be either translucent or opaque.

424

FLICKER WITH MOVING PICTURES

[CH. XI

00 I 234

Logarithm of ILLumination

FIG. 234. THE RELATION BETWEEN THE ILLUMINATION OF THE SCREEN

AND THE NUMBER OF FLASHES PER SECOND AT WHICH FLICKER JUST

DISAPPEARS.

If the flashes are more frequent than indicated by the curve for a given
illumination there will be no flicker, but if less frequent, flicker will be seen.

The solid line represents the observation of T. C. Porter and the dotted line
represents some rough observations made by the authors.

CH. XI] FLICKER WITH MOVING PICTURES 425

If the shutter is translucent the appearance during the change
of pictures is that of a screen lighted to a uniform gray. This kind
of shutter is not much used in practice as it has the disadvantage
of slightly illuminating the parts of the screen which should be
absolutely black.

The opaque shutters were originally made to cover the picture
during the time the picture was in motion, i. e., from % to Y> of the
cycle, the rest of the cycle the screen was lighted, but this was
found to give a very bad flicker.

Recently to get rid of the flicker the shutters have been made
with 2 or 3 opaque wings.

With the one-wing shutter a cycle is made up with

1 . Picture on the screen — screen light — Y* to % cycle.

2. Picture changed — screen dark — Y* to Yt> cycle.

There are 16 cycles per second. The average transmission is
YI to % of the incident light.

It has been found that with a one-wing shutter the flicker is
nearly as troublesome when the opaque part is Yf> as when it is
Yt of the shutter. To avoid this, extra dark wings are added to
the shutter, the form with 3 wings being the best With a three-
wing shutter a cycle is made up of :

1. Picture on the screen — screen light — % cycle.

2. Same picture on the screen but — screen dark — Yt> cycle.

3. Same picture on the screen but — screen light — Yt> cycle.

4. Same picture on the screen but — screen dark — Yb cycle.

5. Same picture on the screen but — screen light — Yb cycle.

6. Picture changed — screen dark — Y> cycle.

The screen is dark Yz and light Y* °f the time : Transmission of
incident light, 50%.

Each picture is thrown on the screen three times before it is
changed for the next. Thus, while there are 16 cycles per second;
there will be 48 flashes per second.

At this speed, 48 flashes, flicker will altogether disappear (See
§ 592).

426 FLICKER WITH MOVING PICTURES [Cn. XI

THEORY AND EXPERIMENTS ON FLICKER

§ 592. Experiments have been made to determine the speed at
which flicker disappears, that is, the speed at which the eye is un-
able to distinguish between a continuous and an intermittent
light.

These experiments show that at a certain speed the appearance
of flicker disappears. This speed is practically the same for
different people.

As the speed is increased the flicker disappears for the center of
the field of vision before it does for the edge. Thus, the light on a
screen may not appear to flicker when looked at directly but it may
appear to flicker when looked at out of the "tail of the eye."

As the brightness of illumination is increased the appearance of
flicker is increased and a higher speed is required for flicker to
disappear. Thus, when showing a very dark film, the light may
not appear to flicker at all, while with a very transparent film or
no film at all the light may appear to flicker violently although the
speed is the same.

If, instead of having the dark period and the light period equal,
either the dark period or the light period is made less in proportion
the flicker appears less violent, and it disappears entirely at a lower
speed. This effect is, however, not very great.

Thus, the flicker with a shutter in which l/6 is light and %
is dark, is the same as one in which % is light and Yt> is dark
(§ SQ2a).

§ 592a. A formula to express these factors numerically was worked out by
T. C. Porter of Eton College and published in the Proceedings of the Royal
Society,, Vol. 63, p. 347; Vol. 70, p. 313-329 (1902).

The constants have been recalculated.

Let f = number of light flashes per second at which flicker disappears when
light and dark flashes are equal.

Let n = number of flashes per second; light and dark flashes are unequal.

w = angle of white space in disc.

(360° — w) = angle of dark space in disc.

I = intensity of illumination in meter candles.

b = constant depending on illumination.

From experimental data the formula comes out

f = 26 -f- 12.2 log I

b = 12.04 + 2-3?8 log I

n = f + b [logw — log (360° — w) — 4.5106].

CH. XI] PRECAUTIONS FOR MOVING PICTURES 427

§ 592b. Table showing Speed at which flicker just disappears. —
FROM T. C. PORTER

Flashes per second

Illumination Logarithm of at which flicker

meter candles illumination just disappears

.0625 8.796-10 17.75

.in 9.046-10 18.08

.25 9.398-10 18.50

i.oo o.ooo 25.08

4.00 0.602 33.50

1.56 0.193 28.00

2.70 0.431 32.00

6.30 0.799 35-50

25.00 1-398 42.66

IOO.OO 2.OOO 5O.I6

IOO.OO 2.OOO 5O.83

178.00 2.250 55.08

400.00 2.602 56.42

1600.00 3.204 65.00

6400.00 3.806 71.00

RESULTS FOUND BY THE AUTHORS WITH A MOVING" PICTURE OUTFIT
32.00 1.500 36

IOO.OO 2.OOO 41

1000.00 3.000 50

3200.00 3.500 54

The curves (fig. 234) are drawn to show the speed at which
flicker disappears for equal light and dark flashes. There is not a
great advantage as far as the speed at which flicker disappears in
having the duration of the dark flash very short. The actual
appearance of flicker is much less violent, however, when the dark
section is narrow.

GENERAL PRECAUTIONS

§ 593. Inspection of films. — Before attempting to show films
to an audience, it is well to inspect them carefully to see that they
are in good condition and wound on the reel correctly.

Use the rewinder to roll the film from the new reel upon an empty
reel. Turn the handle slowly with one hand while holding the
edge of the film between the fingers of the other hand; do not touch
the face of the film. When a patch is met in the film inspect it
carefully to see that: (i) The same side of the film is on top. (2)
The patch is made at the right place so there will not be a misframe,

428 PRECAUTIONS FOR MOVING PICTURES [Cn. XI

i. e., see that the pictures are evenly spaced. (3) The sprocket
holes match evenly. (4) That the patch is strong and no loose
corners stick up.

If the patch is not good in all these particulars, it must be
remedied.

There should be no torn sprocket holes or torn places in the film
or bad scratches in the emulsion. If any such defects are found,
they should be cut out and the film patched together again. Places
may be found where the film broke and was pinned together.
Remove the pin and cement the film.

When the whole film has been inspected in this way, rewind it,
so that it will go through the machine correctly.

See that there is a "leader" or strip of blank film i to 2 meters
(4 to 5 ft.) long to thread through the machine, so the entire title
of the film may be shown. The part of the film used to thread the
machine often becomes broken and a good "leader" saves the film
itself from damage.

If there is time, it is well to run the film through the machine and
watch the screen picture before showing it to an audience.

§ 594. Splicing the film. — When moving pictures are to be
shown the operator will need to patch the film occasionally. Often
a film breaks or an old splice comes in two. A splice is made
by cementing the two ends of the film with "Film Cement."

Cut one end of the film, b, (fig. 235), exactly on the line between
two pictures and scrape the back (shiny side) of the film with a
sharp knife. There may be oil on the film. It must be removed;
cement will not hold otherwise. Cut the other end of the film a,
about 4 mm. (}/% in.) longer than a dividing line between two pic-
tures. Then scrape off the emulsion between the picture division
and the ends of the film. This emulsion can be scraped off
accurately to the line by holding a straight edge over the picture
on a, and letting the end of the film project. Scrape the emulsion
off and right down into the film stock. Scrape the corners as well
as the middle, as the corners usually are the first to work loose.
Film cement is then spread on the back of b, and the front of a,
with a brush or stick, never use the fingers. Be sure to get plenty

CH. XI]

PRECAUTIONS FOR MOVING PICTURES

429

of cement on the corners of the
film. Then immediately press
the two ends of the film together
firmly for a few seconds, being
careful not to push the ends of
the film sidewise in doing so.

Several points must be care-
fully observed in order to get a
splice which is satisfactory and
durable.

i . Cut the film so that the di-
viding lines between two pictures
come exactly together or there
will be a "misframe" when the
film is running through the ma-
chine.

FIG. 235A. EDISON FILM MENDER.

(Cut loaned by the Edison Manu-
facturing Company).

It has three gates or hinges — those
on the sides clamp down and hold
the film while the ends are cut and
prepared and the cement is applied.
The narrow middle clamp is then
closed holding the ends of the film
firmly in contact while the cement
dries. The gauge shown at the left
enables the operator to cut true edges
on the film and scrape the proper
width for the cementing.

FIG. 235. METHOD OF PATCHING A
MOVING PICTURE FILM.

One end of the film B, b is cut on
the line between two pictures and
the other end A , a, is cut a short dis-
tance beyond the line between two
pictures. The film side of one and
the shiny side of the other are
scraped, cement is applied and the
two ends are placed together so
that the sprocket holes will match.

2. Scrape the film well,
both the back side of b, and
the emulsion side of a.

3. Apply the cement and
work rapidly.

4. Be sure to hold the
emulsion side of both films
either up or down.

430 PRECAUTIONS FOR MOVING PICTURES [Cn. XI

5. Get the film together so that the two parts of the film are
in a straight line and not at an angle.

6. Get the sprocket holes together, so that they will match
accurately.

7. Press the film firmly together without any sidewise motion.
It is well to practice on short pieces of scrap film until strong

splices fitting together accurately can be made quickly.

There are two kinds of film cement, one which is good for cellu-
loid films only, the other (NI cement) will work equally well on
non-inflammable film and celluloid film.

For making permanent patches in a routine way there is a film
mender (fig. 23 5 A), consisting of a guide and a pressure clamp, so
that the film maybe accurately held while being cemented together.

All splices should be as far as possible made before beginning a
performance. Any old splices which appear weak and likely to pull
apart should be pulled apart and cemented together again.

With the greatest precaution a film will sometimes come apart
during an exhibition. When this occurs the film is pinned together
to be spliced permanently later. Be sure to remove pins and make
permanent splices before attempting to run the film through the
machine again.

WINDING AND REWINDING

§ 595. A device to wind the film from one reel to another is a
part of any moving picture outfit.

While passing through the machine the film is always wound on
the lower reel in the wrong direction for use, and it is necessary to
rewind it, so that it will be right side out again.

While rewinding is the time to remove pins and splice per-
manently any breaks in the film which occurred during an exhibi-
tion.

In most moving picture theaters one film is rewound while the
next film is being shown, the operator turning the moving picture
crank with one hand and the rewinder with the other hand. When
the rewinding is done this way very rapidly and the rewinder is
fastened to the walls of a sheet iron booth a decidedly terrifying
sound may be produced.

CH. XII DANGER OF FIRE 431

DANGER OF FIRE

§ 596. Before the introduction of non-inflammable films, all
films were made by coating the emulsion upon celluloid. This is a
nitrate (the trinitrate) of cellulose to which is added a certain
amount of camphor. A more highly nitrated cellulose is called gun
cotton.

There is sufficient oxygen in the nitrated cellulose to partially
support combustion and it is the cause of the highly inflammable
nature of celluloid. This was strikingly shown in some experi-
ments made to ascertain the possible danger from an ignited film.
A small reel of film was lighted and put under a tin box so that no
air could get at it. A fire in ordinary combustibles, such as paper
or wood, would soon be smothered, but the roll of film continued
to decompose in the closed box. This shows that if a roll of film,
even in a closed fire proof magazine, once catches fire it will con-
tinue to burn as long as there is anything left of it.

The gases given off from the film decomposing in a closed box are
very disagreeable and will burn in contact with air if they are once
lighted. If celluloid will burn so vigorously in a closed box, what
would be the effect of a large reel of film lying uncoiled in a waste
basket or on the floor should it once catch fire ? This was the prac-
tise in the early days of the art of projecting moving pictures.
Seven to ten meters (twenty or thirty feet) of film piled loosely, will
be completely consumed in a few seconds, burning with a fierce
flair e while it lasts.

In view of this very evident danger, modern, apparatus is
designed to make it as safe as possible. To the good design of the
machine must be added the cooperation of the operator to prevent
a fire.

The fire shutter (fig. 228), automatically closes whenever the
machine is not running. This shutter is placed in front of the film
and prevents the light of the arc from striking it except when it
is in motion. If the film should break, however, a tag end might
remain in the aperture plate and be ignited, the fire shutter remain-
ing open while the crank was being turned. To prevent this
trouble the light should be instantly shut off whenever a film
breaks.

432

DANGER OF FIRE

[CH. XI

The time required for igniting a film was examined. It was
found that an ordinary film, partly black and partly transparent
when held in the condenser focus would first curl and later burst
into flame. The time required for each was noted, first with, then
without a water-cell.

Image of arc

No water-cell
Curl Burn

With water-cell
Curl Burn

20 Ampere D. C. Arc

Concentrated spot 1.3 sec.

vSmall spot 2 sec.

24 Ampere A. C.

Concentrated spot 6 sec.

35 Ampere A. C.

Spot large enough to project

picture, film dead black ... 3 sec.

2.6 sec.
3. 5 sec.

5 sec.
7 sec.

losec.
12 sec.

losec. over 30 sec.

1 2 sec. over 60 sec.

With 3 5 amperes alternating current and the crater image large
enough to project the full size of picture, the film curled in 3 seconds
and burst into flame in 12 seconds. When a water-cell was used
the film was merely slightly warped and not in the least injured
after an indefinite exposure. With larger installations the water-
cell could not be relied on to protect the film indefinitely, though it
would much reduce the risk.

The data given in § 848 (fig. 342), shows the effects of the
water-cell in reducing the radiant energy.

Examination was made of the probable security afforded by the
fire-trap of a fire-proof film magazine. A short piece of film was put
through the fire-trap of a film magazine. This fire-trap consists in
a flat tube, the lower end of which is nearly closed by a pair of
metal rollers. The flame would not follow the film through the
metal tube. When, however, the film was pulled rapidly through
the fire-trap it might or might not be extinguished by the rollers.

With the upper magazine, where the film hangs down, the rising
flames heated the film to such an extent that when pulled upward
through the fire-trap it continued to burn on the other side. When
the film projecting from the lower magazine was ignited and pulled
down through the fire-trap, it was extinguished just as a strip of

CH. XI] MOVING PICTURE EXHIBITION 433

paper would be. The end of the film did not get as hot as that
projecting from the upper magazine because the rising flames did
not tend to play around the unburned part. It would seem, there-
fore, that the fire would probably not be carried into the lower
magazine along with the film. Of course, with the upper magazine
the film is going out of the opening in normal operation. What
would be the effect of the sharp blaze from a meter or more (three
feet) of loose film which would quickly unwind if the film broke can
only be conjectured. It would be likely to get the magazine red
hot and set the film inside on fire.

With these possibilities of risk in mind, one will naturally be very
careful in handling the apparatus, so that nothing shall start to
burn and to follow the precautions of keeping all of the films not
in use inside of fire-proof boxes. The two films in use are : the film
in the machine, and the film which has just been run through and is
being rewound.

When non-inflammable film is used the above precautions are not
necessary from the standpoint of fire risk, but the films might be
spoiled. It is, however, a good plan to be careful even if non-
inflammable films are used, so that habits of carelessness will not
lead to accident should one of the celluloid films be included with-
out the knowledge of its nature.

THE CONDUCT OF AN EXHIBITION

§ 597. Inspection of the plant. — Is the exhibition going to go
smoothly, without hitches, or will the light be poor and go out, the
film be out of focus, and break and everything go wrong? This
depends largely upon the operator and a very careful inspection of
all the apparatus before the exhibition begins.

The principle things to look out for are:

(1) See that all wiring is in good shape, no binding posts loose,
no wires almost burned out in the lamp.

(2) See that the carbons in the lamp are long enough, that extra
carbons are ready, that tools to change carbons are handy.

(3) Burn the arc a little while till the carbon ends are properly
shaped.

434 MOVING PICTURE EXHIBITION [Cn. XI

(4) See that the optical parts are clean, and free from dust.
See that everything is in line and the light is even on the screen.

(5) See that the objective is in focus.

(6) See that the mechanism is oiled and in good order, no screws
loose.

(7) See that the films are properly mended and that there are
no misframes.

(8) See that the rolls of film are in the proper order.

The first reel of film is put in the magazine and the machine is
threaded.

The arc is either pushed away from the operator so it will not
shine on the moving picture head or else the dowser in front of the
condenser is let down.

The arc lamp is lighted. When all is ready the crank of the
machine is started, the arc lamp pulled toward the operator into
position, the dowser is raised, and the house lights turned off
all at the same time.

During the exhibition there should be but two things to watch.

1. The adjustment of the carbons. The carbons need occa-
sional attention to keep a good light.

2. The action on the screen. The action on the screen should
be very carefully followed. It will serve as a guide to the proper
speed to turn the crank of the machine. The lighting of the picture
and the focus of the objective may need attention occasionally as
can be seen by watching the screen.

If the machine or the film is poor various mishaps may occur and
require a short stop.

The most frequent is a misframe. This occurs when a .patch
has not been properly made and the pictures not properly matched.
The difficulty is remedied by raising or lowering the framing lever.
Note the place where the misframe occurs and remove it before the
film is shown again. ,

The film may, break. Turn off the light instantly, or push the
lamp over to the lantern-slide side or lower the dowser. If a tag
end of film is left in the aperture plate, it may catch fire if the light
is not turned off. The film is now threaded through the machine

CH. XI] MOVING PICTURE EXHIBITION 435

again and the ends pinned together in the lower film magazine.
Splice permanently later.

When the end of the film is reached, turn up the house lights and
put out the arc light, or push the lamp over to the lantern-slide side
as the case may require. Turn the crank a few times until the film
has all rolled into the lower film magazine.

The lower reel is taken out and put on the rewinder, the empty
reel from the upper magazine put in its place and a new roll of film
is put in the upper magazine.

FIG. 236. THE EDISON HOME KINETOSCOPE.
(Cut loaned by Thomas A. Edison, Inc.).

At the end of the exhibition all of the films are rewound and put
in the box to be kept until the next day or to be sent away.

§ 598. Home projectors and advertising magic lanterns. — In

addition to the regular moving picture machines there have been
two side-line developments. One of these is a relatively cheap
moving picture machine with a small arc lamp for the house light-
ing system (§ 127) or some other form of radiant (Ch. IV, V).
Some of these small instruments like the "Phantoscope" of Jenkins,
take the standard size of motion picture film. Edison has put out
another form, the "Home Kinetoscope," (fig. 236). This does not
project the ordinary size of moving picture, but very small pic-
tures. Instead of one row of pictures on the film there are three
rows. With the small pictures in three rows, a film 80 feet (24.38
meters) long contains as many pictures as 1000 feet (304.8 meters)
of the ordinary moving picture film, and the mechanism is so.
arranged that the three rows are shown without a break.

436 TROUBLES WITH MOVING PICTURES [Cn. XI

The automatic magic lanterns are devised to show automatically
a series of ordinary lantern slides. One of these instruments is
called the "Advertigraph" by Williams, Brown & Earle and has a
capacity of 24 lantern slides. Another form, designated a
"Stereomotorgraph" by the Charles Besler Co., has a capacity
of 52 lantern slides. These instruments are very effective for
advertising and for exhibitions in museums.

TROUBLES

§ 599. There are two main troubles confronting the moving
picture operator: A poor screen image, and fire in the operating
room.

A poor screen image. This may be due to any one or a combina-
tion of the following:

(1) An operator with insufficient knowledge and experience.
This is probably the most common cause.

( 2 ) A poor pro j ection apparatus .

(3) A bad light due to insufficient current or to a wrong relative
position of the carbons.

(4) The parts of the projection apparatus not on one axis.

(5) The film may be poor; too dark or not sharp, or worn out,
or badly perforated, or scratched, giving rainstorm appearances.

(6) The film may be wrong side up or wrong side out in the
machine.

(7) There may be a "misframe" (§ 584, 597).

(8) The apparatus or the floor may vibrate, giving a jerky
appearance on the screen.

(9) The shutter may not be in the right position or of the right
design, hence flicker, travel ghost, etc.

(10) The general light in the room may be too great, hence, a
gray picture without sufficient contrast. The same effect is pro-
duced by a single room light or the light from a door or window
shining directly on the screen.

Fire in the operating room. This seems inexcusable, but may
occur. To avoid loss of life and of property the operating room
must be (i) truly fire-proof; (2) it must have a large flue leading

CH. XI] TROUBLES WITH MOVING PICTURES 437

to the open air outside the building; (3) all the openings in the
operating room must be closed by fire-proof shutters the instant a
fire starts. In this way the smoke and gases will escape through
the flue, and no one in the audience will know that anything is
wrong.

From the standpoint of the operator, if a fire should start he
should turn off the arc light and turn on the room lights as soon as
possible. If there is a pail of water or a small fire extinguisher
of the wet form in the room the water or the fire extinguisher can
be used to good advantage to prevent the fire from spreading.
The cooling effect will sometimes put out the film, although, as
stated above exclusion of oxygen does no good for the celluloid
contains enough oxygen to support combustion. The real way
after all is to be so careful that a fire never starts. (See Richard-
son's Handbook, zd edition, pp. 65-93).

438

DO AND DO NOT WITH MOVING PICTURES [Cn. XI

§ 5991. Summary of Chapter XI:

1. Learn the principles, and
perfect yourself in the practice
under expert guidance, before
you assume the responsibility of
an independent operator.

2 . Keep your operating room
in perfect order.

3. Light the theater so that
the lights cannot shine directly
in the eyes of the spectators or
upon the screen.

4. Have a perfect screen. If
it is a painted screen, add a fresh
coat occasionally.

5. Use direct current for the
arc lamp if possible (Ch. XIII).

6. Inspect wiring and appara-
tus daily.

7. Keep the lenses of the con-
denser and of the objective
clean, and in the right relative
position.

8. Keep in mind the precau-
tions (§ 593-594).

9. Leairn to conduct the ex-
hibition in the best possible
manner.

10. Remember that it is far
easier to avoid a fire than to put
it out.

Do NOT

1. Do not pretend to be a
competent operator until you
have the requisite knowledge
and experience, and then never
stop learning.

2. Do not have your operat-
ing room in disorder.

3. Do not install room lights
so that they can glare in the
eyes of the spectators or shine
on the screen.

4. Do not project on a dirty
screen.

5. Do not use alternating
current for projection if you
can use direct.

6. Do not neglect a careful
daily inspection of wiring and
apparatus.

7. Do not use dirty lenses or
objectives.

8. Do not fail to study care-
fully the precautions (§ 593).

9. Do not neglect the direc-
tions for the conduct of an
exhibition.

10. Never forget the danger
from fire.

CHAPTER XII
PROJECTION ROOMS AND SCREENS

§ 600. Apparatus and Materials for Chapter XII:

1. Room which can be made entirely dark, or which can be
partly lighted, depending on the kind of projection and the
radiant.

2 . If for exhibitions, the room should have plenty of aisles and
exits, and there should always be lights (red lights) near the exits,
and these lights should be independent of the projection circuit.
The room should be well ventilated, and of a form found suitable
for audiences, e. g., like a church, theater or university lecture
room. The room should be tinted and decorated with light-absorb-
ing colors (§ 604).

3. The lantern or other projection apparatus should be so
placed that it does not interfere with the audience (§ 612-620).

4. Special room for the projection apparatus. If in a moving
picture theater, there should be a fire-proof room for the apparatus.
This should have a large ventilator extending through the roof or
side of the building (§ 556-5 57).

5. Screen upon which the images are projected. This should
receive the image at right angles to avoid distortion (fig. 241), and
be of sufficient size for the room (§ 633).

§ 601. For the historical consideration of rooms and screens
see under history in the Appendix. See also the works referred to
in Chapter I, § 2, and the catalogues of manufacturers of projection
apparatus and materials. Periodicals on moving pictures like the
Moving Picture World; F. H. Richardson's Motion Picture Hand-
book; and F. A. Talbot's Moving Pictures.

§ 602. Suitable room for projection. — Any room which can be
darkened may be used for projection, but to be satisfactory it
should have the qualities of a good auditorium.

(1) There should be plenty of aisles and passages, so that the
auditors can easily reach their seats.

(2) There should be plenty of exits, so that the room can be
quickly and safely emptied.

(3) There should be plenty of fresh air.

439

440 PROJECTION ROOM [Cn. XII

(4) Each seat should have a good view of the stage and the
screen.

(5) There should be enough diffuse light in the room so that
people can find their way around easily and after gaining twilight
vision, be able to take notes.

§ 603. Form of the room. — In general that shape of room which
has been found most satisfactory for churches and theaters and for
science lecture rooms in colleges and universities is well adapted for
projection. As, however, the entire attention must be given to
the images on the screen in the middle of the stage there is a ten-
dency to make the rooms used especially for projection longer than
they are wide. In a room which is approximately square, the
spectators who sit at the sides of the room near the front do not
have so good a view of the screen as those in the middle of the room
and farther back.

With a long narrow room either the picture must be magnified
excessively to enable those on the back seats to see the details,
while for those on the front seats the pictures seem very coarse, or
there must be a compromise so that only for those in the middle of
the hall are the screen pictures of the most favorable size.

We strongly advise any person having the responsibility of
planning a lecture hall for educational purposes or for exhibitions,
to take advantage of human experience and see a considerable
number of halls in various places, and get hints of what not to do
as well as of what to do from those who have had experience.
Then he can combine excellencies and avoid mistakes in planning
his own building or room.

§ 604. Tint and decoration of the room. — In order to get the
best possible results in projection, no light whatsoever should reach
the eyes of the spectators except that reflected from the screen.
With the moderate light available for the earliest users of the magic
lantern it was advised that the walls and ceiling be made black so
that, as they put it, "the room would be as sombre as possible."
For some experiments in projection with polarized light, the
spectroscope, and the highest power micro-projection such a room

CH. XII] LIGHTING THE PROJECTION ROOM 441

would still be an advantage; but for ordinary magic lantern and
moving picture exhibitions total darkening of the room is unneces-
sary and undesirable. But for all projection it is a great advan-
tage to prevent any light from falling upon the screen except that
from the projection apparatus. The room should therefore be
tinted with some light-absorbing color. Nothing is better than the
brownish color of natural wood, such as oak or pine. If natural
wood is not used, the walls and ceilings can be tinted brownish
or olive. For decorations, rich, dark red, orange, green, and blue
may be used. Light orange, green, and blue reflect too much light
but the dark, rich colors give the pleasing effect without making
the room too light.

For mixing these tints, if oil colors are used, much turpentine
should be employed to give a flat or dull finish, not a shiny or glossy
one. If the finish is shiny it will act like a mirror and give an
undesirable glare, and shine in the face of some of the auditors.

§ 605. Light in the exhibition room. — For magic lantern and
moving picture exhibitions, the room should be light enough so
that the spectators can easily find their way about ; and after the
twilight vision is established, the spectators should be able to take
notes easily.

If the room is finished and decorated with light-absorbing colors
and tints as indicated above, there is no danger of making the
screen images gray and dull from reflections from the walls and
ceiling. One has simply to guard against direct light shining on the
screen from a window or from a lamp. (For lighting a black-board
in a lecture room see fig. 240).

§ 606. Lamps for general lighting. — The lamps to give the
needed light should be so arranged, and with such shades that:

(1) They cannot shine directly in the eyes of the spectators; and

(2) That they cannot send any of their rays directly upon the
screen. This is best accomplished by placing the lights along the
sides of the room or on the ceiling or both, and shading them so that
none of their light can extend directly to the screen.

The arrangement sometimes used of a row of lights around the
screen is bad; for, while no light can reach the screen from them,

442

LIGHTING THE PROJECTION ROOM

[On.

the glare in the eyes of the spectators will detract from the effect.

If ceiling lights are used they should be placed close to the ceiling
and on the side of the construction work (stringers, etc.) away from
the screen. Then the light will extend obliquely downward and
backward, but none of it will fall directly upon the screen.

Lights along the sides of the room can be placed behind the
projecting construction work, or shaded so that the light cannot
extend toward the screen.

FIG. 237. METHODS OF INDIRECT LIGHTING.

(Cuts loaned, joy the National X-Ray Reflector Co.).

A Shows an opaque bowl containing the electric light. The light is
reflected upward and is diffused throughout the room.

B and C Illustrate the indirect lighting where the bowl containing the
electric light allows a certain amount of the light to extend downward. The
light is also reflected upward as in A.

In C the bowl is cut away to show the electric bulb, the reflector for throwing
the light upward, and the opal glass diffuser below to give the soft luminous
effect of a very large source in the "luminous bowl."

CH. XII] DARKENING THE PROJECTION ROOM 443

The indirect or concealed light sources which have been recently
developed answer all the requirements for suitably lighting a mov-
ing picture theater or, indeed, any other place where a soft light is
required and the light should not shine directly in the eyes of the
spectators (fig. 237 A, B, C).

It is also an advantage to have the screen in a kind of alcove i to
2 meters (3-6 ft.) deep and the walls on the sides, the floor and the
ceiling dark brown or dark red or olive to absorb any light reflected
upon 'them (6o6a).

For exhibitions, it also adds brilliancy to the picture to have a
black border around the screen. It gives also the effect of a framed
picture.

With such an arrangement of the lights in a suitably tinted room,
no light will reach the screen directly to destroy the contrast and
render the image vague. There can be sufficient diffused light in
the room to enable one on entering to see the aisles and seats, and
go about without stumbling. In a short time twilight vision will
be established and it will then be possible to read or to take notes.

§ 607. Red lights near all exits. Fire escapes. — In public
halls, and especially in moving picture theaters, it is an advantage,
and often a requirement in city regulations, to have red lights near
every exit so that the audience can see exactly where it is possible
to get out of the hall.

The manager of every public hall should look to it every day that
the fire escapes are in working order and before every exhibition
that the doors or gates to the fire escapes are unlocked and easily
opened.

§ 608. Relative darkness of the room for different kinds of
projection. — The amount of diffuse light permissible in the pro-

§ 606a. While it is a great help to have a screen in a dark alcove, still the
general light of the room, although none extends directly upon the screen,
tends, if too great, to make the image less brilliant and definite. Every one
who has studied astronomy at all with a telescope knows full well how the
defmiteness of the image of a nebula or dim star cluster diminishes when the
moon rises and floods the heavens with its diffuse light. One can also see the
effect of too much diffused light by observing a lighted clock face on a dark
night, and the same face with the same light shining from it on a moonlight
night or early in the evening twilight before complete darkness.

444 DARKENING THE PROJECTION ROOM [Cn. XII

jection room depends entirely upon the brilliance of the screen
image. In order to see the screen image clearly there must be
strong contrast between it and surrounding objects. With trans-
parent lantern slides and sunlight or the electric light to illuminate
them one can see the screen images well in a room so light that
everything in the room is visible provided no direct light reaches
the screen except 'that from the projection apparatus. If the
lantern slides are less transparent or the light used for projection
less brilliant, then the room must be relatively darkened to give
the needed contrast. Keeping the principle of contrast in mind,
one readily understands that for some of the experiments in physics
where the light on the screen is very dim, with kinemacolor moving
pictures and with Lumiere colored lantern slides, and with high
power micro-projection, the room must be very dark in order to get
the screen image clearly visible. In like manner if the source of
light for projection is relatively weak, like the acetylene flame or
some other less brilliant light than the electric arc, the room must
be darker than with a more brilliant radiant.

§ 609. Daylight and twilight vision. — It has been known for
time out of mind that with most people the eyes can adapt them-
selves to a dim light or to a bright light. If one goes into a dimly
lighted room from full daylight the room will at first appear per-
fectly black, but in a few minutes objects can be seen fairly well,
and within half an hour the room will appear comparatively light.
On the other hand, in passing from a comparatively dark room to
full sunlight the eyes are so dazzled at first that hardly anything
can be seen, but soon the eyes become adapted to the bright light.
It has been found by careful experiments on large numbers of
people that the main adaptation of the eyes for bright light after
being in a dark room requires only about 6 minutes, while the
adaptation for a dim light after being in full daylight requires
about 30 minutes, although after 10 minutes the eye is about 100
times as sensitive in a dark room as it is in full daylight. While
the pupil expends normally in dim light, thus increasing the aper-
ture of the eye, this is not the fundamental thing in adaptation, but
there is some change in the retina which gives it greater sensitive-
ness.

CH. XII] DARKENING THE PROJECTION R

B C

i — — - — — — — 1 i 1

()(

DM 44

'i A w

'\ / 5:

n sh r1

«$;;; SH ;>.-.

'— ) (~

':Sh

' 1

•<! P

CrG

]

FIG. 238. FACE AND SECTIONAL VIEW OF WINDOW SHADES PLANNED FOR
IN THE CONSTRUCTION OF THE BUILDING.

A Cross section showing the window shade (Sh) in the grooves (W W) at
the sides.

B Face view of the window with the shade (Sh) shown by dotted lines.

C Sectional view of the window showing the window sashes (W S), the
ordinary window curtains (Cr) close to the sash, and the window shade (Sh)
considerably in front of the curtain, i. e., near the front of the window frame.

The coping over the window is shown by dotted lines as turned down.
This exposes the shade roller so that it can be adjusted if it gets out of order.

The window shade is shown as drawn down, and the shade string goes over a
pulley (P) and is caught in a fork-like holder (C) in front of the window frame.

446 DARKENING THE PROJECTION ROOM [Cn. XII

Much of the projection at the present time requires daylight
rather than twilight vision from the brilliancy of the screen images,
but one should keep in mind that good screen images may be
obtained by two methods (i) brilliant illumination and daylight
vision; or (2) moderate illumination and twilight vision.

§ 610. Method of darkening a room. — As many rooms used for
projection are well supplied with 'windows there must be some
method of excluding daylight or other outside light. The two
means usually employed are wood or metal shutters and opaque
cloth curtains.

Shutters may be on hinges and swing sidewise, or they may be
hung, and by means of pulleys raised and lowered. In many
laboratories where the shutters are opened and closed several times
during a lecture, there is a water or electric motor to move the
shutters.

If curtains are used they should be of dark colored opaque cloth
on a spring roller, so that they can be opened or closed as much or as
little as desired. These are usually opened and closed by hand
(fig. 238).

§ 611. Excluding light at the window margins. — As curtains
are usually hung, there is a space admitting light at the top, bot-
tom, and sides of the window. This can be avoided by having the
edges of the curtain in a groove at the sides and bottom of the
window frame, and having the curtain roller above the opening of
the window frame (fig. 238). If one has the designing of the
building, proper grooves can be planned for and put in when the
window frames are made. If this has not been planned for in
designing the building, then the light-excluding devices can be
added afterwards. That is, a light-excluding shield can be put all
around the window frame (fig. 239). This will, of course, cut down
somewhat the opening of the window frame.

POSITION OF THE PROJECTION APPARATUS IN THE ROOM
§ 612. The best position for the projection apparatus in a lec-
ture room or exhibition room is at the back of the room, where it is
entirely free from the audience. This also gives the operator
greater freedom (fig. 240).

CH. XII]

DARKENING THE PROJECTION ROOM

447

FIG. 239. FACE AND SECTIONAL VIEW OF A WINDOW SHOWING HOW THE

LIGHT-EXCLUDING SHADE CAN BE INSTALLED AFTER THE BUILDING is

CONSTRUCTED.

448

POSITION OF PROJECTION APPARATUS [Cn. XII

A Cross section showing the window shade (Sh) behind the thin boards
(W W) which serve to exclude the light at the top, sides and bottom of the
shade.

B Face view of the window with the" light-excluding shade (Sh) shown in
dotted lines, (L) indicates the size of the window frame. The sash cuts this
down somewhat and the thin board frame to cut out the light around the edge
of the curtain cuts it down considerably more.

C Lateral view of the window with the shade in dotted lines. The light-
excluding frame around the edge is in full lines in B and C.

§ 613. Position of the projection apparatus with a level room.—

In a level room, the projection apparatus at the back of the room
must be at such a level that the projection beam goes over the
heads of the spectators. This can be accomplished by building a
platform, or by using a high table. In case the image is still not
high enough on the screen, the lantern can be tilted slightly upward
by putting a wedge under the end of the baseboard supporting it
(fig. 240)..

FIG. 240. SECTIONAL VIEW OF A LECTURE ROOM HAVING A GALLERY.

B Black-board. This is lighted by incandescent lamps behind a curved,
metal shield (H L). This gives plenty of light for the black-board without in
any way injuring the brilliancy of the screen image.

L T Lecturer's table on the platform (P).

Ml The magic lantern in the gallery on its table and special support ( T).

Sc Screenjfor the image above the black-board.

CH. XII]

POSITION OF PROJECTION APPARATUS

449

§ 614. Level room with the apparatus near the screen. — It is

sometimes desirable to put the apparatus near the screen. Then
provision must be made by removing some of the seats if the center
aisle is not wide enough.

The apparatus must usually be raised somewhat also, and some-
times the objective inclined more or less upward. In case it is
desired to have the apparatus very near the screen it must be
pointed upward considerably and then the screen should be hinged
at the bottom so that it can be inclined toward the lantern till it is
perpendicular to the optic axis. The simplest way to fix the
screen in any position, and to change the position is by means of
ropes and pulleys at the top.

FIG. 241.

LECTURE ROOM WITH RISING SEATS, AND THE LANTERN IN THE
MIDDLE OF THE ROOM, NOT AT THE BACK.

B Black-board lighted by the hidden lights (H L) behind a curved metal
shield.

L T Lecturer's table in front of the audience.

Ml E The magic lantern (Ml) ; its rays shown in full lines, and the episcope
or opaque lantern (E) with its rays shown extending from the mirror (M) in
dotted lines.

Sc The screen for receiving the image. As the magic lantern must be
elevated the screen is tipped toward it to meet the axial ray at right angles.

For such a position of the magic lantern the projection objective must be of
shorter focus to give the desired size of image than when the lantern is at the
back of the room (§ 636).

450 POSITION OF PROJECTION APPARATUS [Cn. XII

The lantern should be fastened to a hinged board when it is
elevated considerably (fig. 118, 242).

§ 615. Magic lantern on the lecture table. — Occasionally it is
an advantage to have the magic lantern on the lecture table ; then
the lecturer can manipulate it himself.

There are three arrangements possible: (i) The lantern is
pointed toward a screen at the side of the room (fig. 243). (2) It
is pointed obliquely upward toward the screen in front of the
audience. In this case the screen must be inclined toward the
lantern as indicated above (§614). (3) Occasionally, for ease of
manipulation, the lantern is pointed obliquely upward toward the
audience and a plane mirror reflects the image-forming rays back-
ward to the screen (fig. 244). If a mirror is used, the lantern slides
must be inserted with their faces toward the objective.

FIG. 242. MAGIC LANTERN TABLE WITH HINGED BASEBOARD.
Hinges connect the baseboard to the table at the left. By putting a block
under the board at the right, it can be elevated to bring the screen picture
higher up (fig. 118).

CH. XII] POSITION OF PROJECTION APPARATUS

451

§ 616. Projection with inclined seats or gallery. — If the seats
in the auditorium are raised after the manner of an amphitheater or
if a gallery is present, in many cases the apparatus can go to the
back of the room or in the gallery. This may make it necessary to
point the projection apparatus somewhat downward towards the

FIG. 243. GROUND PLAN OF A LECTURE ROOM WITH THE MAGIC LANTERN

ON THE LECTURER'S TABLE AND THE SCREEN AT THE SIDE OF THE ROOM.

B Black-board.

Ml Magic lantern on the lecture table (L T) and pointing up to the screen
(Sc) on the side of the room.

Sc The screen is shown tipped forward to avoid distortion.

Such a position of the lantern enables the lecturer to perform experiments or
show lantern slides conveniently.

452

POSITION OF PROJECTION APPARATUS [Cn. XII

screen, but as the distance is usually considerable, the screen image
will be good on a vertical screen. The position of the lantern
should never be so high that the screen image will be distorted.

§ 617. Apparatus in the middle of the auditorium with raised
seats. — If the apparatus cannot be at the back of the room in an
amphitheater then a space or alcove must be made somewhere in
the middle by omitting a certain number of seats. The machine
is liable to be more or less distracting if in the middle of the room,
but sometimes this cannot be avoided on account of distance or the
form of the amphitheater (fig. 241).

FIG. 244. PART OF A LECTURE ROOM WITH THE MAGIC LANTERN ON THE

LECTURE TABLE DIRECTED TOWARD THE AUDIENCE AND A MIRROR TO

REFLECT THE IMAGE ON THE SCREEN IN FRONT OF THE AUDIENCE.

B Black-board with hidden light behind the curved metal shield (H L).

Ml M Magic lantern pointing toward the audience. The mirror reflects
the image back to the screen in front of the audience. The mirror also serves
as a shield.

Sc The image screen. By means of the pulley and cord it is inclined on its
hinges at the lower edge toward the mirror of the magic lantern. In this case
it is not inclined sufficiently to meet the axial ray at right angles, hence there
will be some distortion of the image and the upper edge will not be in sharp
focus when the lower edge is.

T Lecturer's table. With such an arrangement the lecturer can demon-
strate with the lantern conveniently, and still have the screen in front of the
audience. If he uses lantern slides they must be put in the holder facing the
objective, not the light or there would be a mirror image on the screen (fig. 213).

CH. XII]

POSITION OF PROJECTION APPARATUS

453

Occasionally when the seats are on a steep incline there is left a
space through which the projection objective can send its beam to
the curtain, the apparatus and operator being under the seats of
the amphitheater.

§ 618. Apparatus on one side of the room. — Occasionally the
apparatus is put on one side of the room and instead of projecting
directly in front of the audience the projection is on one side of the
room. The auditors simply turn in their seats to face the screen.

L T

FIG. 245. SECTIONAL VIEW OF A LECTURE ROOM SHOWING THE POSITION OF
THE PROJECTION APPARATUS WITH A TRANSLUCENT SCREEN.

Ml Magic lantern or other projection apparatus on its table (T) and
raised platform (PI) in a room outside the lecture room.

L T Lecturer's table.

Tr Sc Translucent screen. The audience does not see the apparatus;
only the screen image is visible.

Lantern slides must be inserted in the holder facing the objective, not the
light, or the image will have the rights and lefts changed like fig. 213.

This is not so satisfactory as when the screen is directly in front
(fig. 243).

§ 619. Apparatus wholly without the room. — Regardless of the
form of the room, the apparatus may be placed in a room just back
of the lecture table in front of the audience and a translucent screen
employed. This arrangement has decided advantages, but a
translucent screen is not so satisfactory as a white opaque screen
(see fig. 245).

§ 620. Special operating room. — With the ordinary magic
lantern and projection microscope the apparatus and operator are

454 WHITE IMAGE SCREENS [Cn. XII

usually in the general exhibition room, and there is no special
boxing or enclosure of the apparatus. But in moving picture
theaters, where there is some danger from the inflammability of the
picture films, both the fire underwriters and the municipal regula-
tions usually require some form of fire-proof operating room.

IMAGE SCREEN

§ 621. Next in importance to a suitable room for exhibitions
with projection apparatus is a good screen upon which to project
the image.

No one has ever more briefly and clearly stated the qualities of a
good image screen than Goring & Pritchard: "It should reflect
the greatest -possible quantity of light and absorb the least."
"Every care should be taken to render the surface as smooth, white
and opaque as it can be made" . . . "inasmuch as the bril-
liancy and perfectness of the picture will greatly depend on the
whiteness, and the sharpness of its outline upon the smoothness of
the screen." The screen should be dull white, never shiny.

§ 622. Screens of plaster paris upon the wall. — A screen ful-
filling all the requirements just given is a wall coated with a smooth
finish of pure, fine plaster of Paris.

§ 623. Painted wall screen. — While a plaster of Paris wall
screen is perhaps the best, a smoothly plastered wall, if properly
painted, gives almost as good results and is much cheaper. The
wall, as stated, should be finished as smoothly as possible by the
plasterers, then it is coated with pure linseed oil if porous, or with
a mixture of equal parts of linseed oil and turpentine if the wall is
hard and non-porous. When this is dry, the wall is painted with
either white lead ground in oil and thinned with turpentine, or
with "sanitary paint" thinned with turpentine. The sanitary
paint has the advantage that it does not turn yellow with age, and
that it is more easily cleaned with soap and water.

When the paint is properly thinned it should be strained through
one or two layers of gauze (cheese cloth) to get out any lumps or
coarse particles.

CH. XII] WHITE IMAGE SCREENS 455

In spreading the paint on the wall one should use a soft brush
and apply only the tip of the brush. This will give a smooth finish
and if one uses plenty of paint there will be no joints, but the
whole will appear like one uniform coat. Practical painters call
this "flowing on the paint."

After one coat is well dried another can be put on until the wall is
perfectly white. If plenty of turpentine is used the surface will be
dull. It should not be glossy or shiny.

Whenever the surface becomes dirty it can be washed off with
soap and water. If it is not up to standard whiteness after the
washing and drying, put on another coat of the paint.

Sometimes hot glue, 15% to 20% in water, is used for sizing the
wall. This answers well if the wall is perfectly dry and not subject
to moisture. In general it is safer to use the linseed oil sizing.

In our experiments several white paints were used, but the pure
white lead (sometimes called "flake white") and the non-lead con-
taining paint called "sanitary paint" were found most satisfactory.
The latter has the advantage over white lead that it does not yellow
with age, and gives a very opaque and white surface which stands
washing with soap and water very well.

§ 624. Whitewashed wall screens. — A smoothly plastered wall
that has been carefully whitewashed with milk of lime gives a good,
dull white surface for a projection screen. It rubs off rather easily
and cannot be cleaned. Of course a fresh coat of whitewash will
renew the screen. It. is cheap as well as good. One should take
pains to strain the whitewash, and to apply it smoothly so that a
uniform surface will be produced.

We did not find a kalsomined wall satisfactory for projection.
It is, or soon becomes, too yellow.

§ 625. Painted cloth screens. — A good screen can be made by
stretching some smoothly woven, strong cotton cloth (strong
muslin) upon a frame and painting it as for the wall (§ 623). The
frame must be strong and the cloth stretched tight so that there
will be no wrinkles, and it must not rest against anything.

One could paint directly on the cloth, but it is more satisfactory
to size the cloth in some way first. One of the best methods is

456 WHITE IMAGE SCREENS [CH. XII

to use white linseed oil, raw or boiled. The oil is put on with a
soft brush like paint. It is well to make all the brush strokes in
one direction, so that the lint or nap on the surface of the cloth will
be smoothed down in one direction. After the linseed oil is dry the
cloth is painted, preferably with sanitary paint and turpentine,
although white lead thinned with turpentine answers well. One
coat should be allowed to dry before adding another. It takes
from one to two days for each coat to dry. The screen will be
white and opaque with three to five coats. Care should be taken
to strain the paint as for the walls (§ 623), then there will be no
rough spots (§ 62 $a, 62 sb).

If the curtain gets grimy it can be wiped off with soap and
water, and if necessary after it is dry, a fresh coat of the paint can
be put on.

§ 626. Roller screens. — Cloth screens which have been painted
as just described make excellent roller curtains, for the sizing and

§ 62 Sa. Amounts of sizing oil and paint for a cloth screen. — For oil-sizing
and painting a muslin screen the following times for drying in the summer, and
the following amounts of oil and paint were used to make a perfect screen.
For sizing, white raw linseed oil was used, and only one coat was applied.

For this it required 220 cubic centimeters of the linseed oil per square meter
of cloth, or about one-tenth of this amount per square foot.

For painting, a preparation of sanitary paint known as "Artists' Scenic
White," ready for use on screens was used, two coats were applied. It
required no cc. of the paint for each square meter of surface.

It required about 36 hours for the raw oil sizing to dry; 24 hours was
sufficient time for a coat of the white paint to dry. The finished screen was
flexible and easily rolled.

For a screen 3 meters or 10 feet square it would require for sizing and paint-
ing about two quarts of linseed oil and about the same amount of the "Artists'
Scenic White" or any other white paint for two coats of the paint.

§ 625b. The cloth may be sized by the use of white shellac. This is thinned
about half with denatured alcohol and painted on the surface just as described
for the oil size. It gives a good surface to paint on, but does not leave the
curtain so flexible.

A hot 15% to 20% solution of white glue in water may also be used as
described for the oil or shellac size. This has the advantage of pasting down
the nap of the cloth and of giving a very good surface to paint on. It has the
disadvantage of expanding and contracting greatly with different conditions of
moisture. If the glue size is used the curtain should have at least one coat of
paint on the back, so that the glue size cannot be so easily affected by moisture.

§ 625c. The authors wish to express their appreciation for information on
paints and the painting of wall and cloth screens for projection, to Mr. A. E.
Nash, Superintendent of the Cornell University paint shop.

CH. XII] WHITE IMAGE SCREENS 457

painting leave the cloth flexible, and without liability of cracking
and peeling. They are mounted on heavy spring rollers as
ordinary window curtains are so commonly mounted, and can be
rolled up when not in use.

§ 627. White cloth screens without paint or other facing. —

White cloth such as a bed sheet has always been and still is used.
The cloth should be as white as possible, and of good thickness.
It is also advantageous to have the screen of one piece without
seams. Bed sheets may be obtained in large dry goods houses
about 3 meters square (10 ft. sq.) without seams. These make
very good curtains when the folds are ironed out, and the sheet
stretched to hold it flat. It is not easy to stretch a sheet so evenly
that there will be no folds or wrinkles. Fortunately, a slight
unevenness is not noticeable in the screen image. A screen which
appears quite uneven to the naked eye in daylight may give very
good screen images and appear perfectly smooth, when giving an
exhibition.

Cloth screens have the disadvantage that they are not suffi-
ciently opaque. If one goes behind the screen the image is almost
as well seen as in looking at the face of the screen. This means
that almost as much light traverses the screen as is reflected from
the face. Naturally, it takes much more light for a brilliant screen
image than with an opaque screen (§ 632).

For some purposes it is advantageous to be able to see the image
on the back, then assistants behind the screen can make the
appropriate noises to make the scene seem more real. For exam-
ple, in a moving picture scene, sounds can be made to imitate
the breaking of the waves on the shore, the clatter of horses hoofs
on a pavement, etc., etc. Unless the assistant could see the
image it would not be possible to suit the sound so accurately to
the scene.

Sometimes so large a screen is needed that strips of white cloth
are sewed together. If this must be done the seams should be very
smooth. On such screens the seams show like lighter streaks on
the image, as more light is reflected from the double thickness of

458 WHITE IMAGE SCREENS [CH. XII

cloth. Behind the screen the seams show as black or dark streaks,
as less light traverses the screen along the seam (§ 627 a).

§ 628 Paper screens. — The suitability of white paper screens
has been recognized for a long time. One of the best possible
screens is a large sheet of white cardboard. As shown by photo-
metric measurements, the reflections from a white cardboard are
almost as great as from the standard surface of oxide of mag-
nesium (§ 632). The white cardboard is especially suitable for
the images of the high power projection microscope, and if it could
be had in sufficiently large sheets it would make an almost perfect
screen for large rooms. (In large paper stores one can get sheets
71x112 cm. (28x44 in.). The paper used for drawings by
architects and engineers and 69 x 102 cm. (27 x 40 in.) in size is
also excellent for screen purposes. It is not so easy to get a smooth
surface as with the cardboard).

Finally, cloth is sometimes faced with paper to give a more
opaque and perfect screen.

SCREENS WITH METALLIC SURFACES

§ 629 Dull white surfaces reflect almost equally throughout
the whole hemisphere (fig. 248) and therefore the image appears
almost equally brilliant in any position. Those near the axis of
the projection apparatus in the middle of the room do not see the

§ 627a. Screens for traveling exhibitions. — When exhibitions must be
given in school-houses and in halls where there is no lantern and no screen, the
exhibitor must supply both. In traveling it is inconvenient to carry a roller
screen, and usually the screen is folded so that it can be packed in a small
space. This, of course, makes creases in the screen, and besides there is noth-
ing to support it so that it will hang smooth and even.

For a traveling screen a heavy, seamless bed sheet is excellent. Bed sheets
in one piece as large as needed are to be had. To hang these sheets there
should be a strong cord along the upper edge either in a hem or in curtain rings.
From the corners of the sheet should be strong cords by which the sheet can be
stretched out smooth and held in position by passing the cords through screw
eyes or attaching them to other fixed supports.

It is well also to have rings along all the edges to attach strings to, to pull
the edges taut, and to support the curtain at the upper edge.

For temporary use, a sheet may be stretched and held in position by tying
strings to the corners and by fastening the strings along the edges by safety
pins.

CH. XII]

SCREENS WITH METALLIC FACIN G

459

image much more brilliantly illuminated than those at the side.
Sir David Brewster in 1832 advocated and used the bright metallic
surface on the back of looking glasses, which at that time was
composed of mercury and tin. Later, surfaces covered with
silver-leaf, silver particles or particles of aluminum have been tried.
Last of all, plate glass has been ground on one side, and the smooth
side silvered. The ground surface of the glass is turned toward the
projection apparatus and facing the spectators who get the image
reflected from the mat surface of the glass and transmitted from
the mirror through the mat (§ 6 2 pa).

FIG. 246. DISTRIBUTION OF LIGHT
REFLECTED FROM A WHITE

SCREEN.

It is approximately uniform through-
out the entire hemisphere.

FIG. 247. DISTRIBUTION OF LIGHT
FROM A SEMI-DIFFUSELY RE-
FLECTING SCREEN.

The closeness of the arrows indicates

the apparent brightness as seen

from different directions.

§ 630. Suitability of metallic screens. — Metallic screens are
not suitable for micro-projection, or, indeed, for any projection if
fine details are to be studied close to the screen, but details which
can be seen at a distance of 2 to 3 meters are very well brought
out on the mirror screen, and other metallic screens. In com-
paring a mirror screen, an aluminum bronze screen and one of
plaster of Paris or cardboard if the image was observed within
the narrow angle of 15 degrees to the right or left of the axis, 30
in all, the mirror screen was brightest, the aluminum next, and
finally the plaster of Paris or cardboard, the screens being in the
field at the same time so that the comparison was under identical

§ 629a. The authors wish to acknowledge their indebtedness to The Motion
Picture Screen Company of Shelbyville, Indiana, U. S. A., for their courtesy in
sending a sample of their "Mirror Screen" for experiment; to the Bausch &
Lomb Optical Company for the loan of the two metallic screens of Zeiss; to
the J. H. Gentner Company of Newburgh, N. Y., for samples of Mirroroide;
and to other screen manufacturers for courteous answers to inquiries.

460

BRIGHTNESS OF SCREENS

[CH. XII

CH. XII] TRANSLUCENT SCREENS 461

On the curved surface of the diagram are given the degrees of inclination of
the light. On the diameter, and on the radius at right angles to the diameter
are given the percentage of apparent brightness. Magnesium oxide is taken
as the standard and called 100%.

The data shown on the diagram are given in figures in the table, § 632.

Curve i. Screen coated with magnesium oxide. It is to be noted that, it is
only in the central region that the full 100% of reflection occurs.

Points 2222 Plaster of Paris screen.

Curve 3 Cardboard screen.

Points 4 4 Screen painted with white lead.

Points 5 5 Screen painted with Artists' Scenic White.

Points 6 6 Screen painted with zinc white.

Curve 7 Cardboard screen painted with aluminum.

Curve 8 Zeiss metallic screen.

For 9 see the table, § 632.

For the Mirror Screen, see the table, § 632.

Points 10 10 10 Reflection and transmission of a white muslin screen.
Note its uniformity (§ 632).

Points ii ii ii Reflection and transmission of white gauze (Griswoldville
gauze, No. 10). With this screen more light is transmitted than reflected.

Point 12 Transmission of ground-glass.

Point 13 Reflection of bristolboard.

conditions. At an angle of 30 degrees and upward the metallic
screens appeared almost black, and the white screens pure white.

§ 631. Translucent screens. — For the old phantasmagoria and
for many appearances given by shadow pictures it is necessary to
have a translucent screen like ground-glass or translucent cloth
or paper. The paper or cloth is rendered as translucent as desired
by the use of water, water and glycerine, or oil. Tracing cloth
makes good translucent screens of moderate size.

With a translucent screen the apparatus is entirely out of sight
behind the screen and only the picture shining through the screen
is seen by the audience. This is not so good and effective a method
of showing projection images as the opaque white screen or the
metallic screen, for much more light is lost (fig. 248). It is still
used in some institutions, as it entirely eliminates the projection
apparatus and the operator from the auditorium (§ 631 a).

The ground-glass screen is excellent, but this, like a metallic
screen restricts the brilliant image to a rather narrow angle (see
§ 630, 632 and fig. 250). The ground surface should be fine or there
is given the appearance of looking toward a bright light in a snow
storm, this is especially marked if one is near the ground-glass and
looking nearly along the axis.

462 REFLECTION OF DIFFERENT SCREENS [CH. XII

The cloth screens were not so satisfactory as the ground-glass
because the crossing threads make a kind of grating and one sees
diffraction images ; and if one is in direct line with the arc lamp
the cloth acts almost as if it were transparent. The translucent
mercerized paper used in making tracings is practically as good as
ground-glass, but it is difficult to hold it smooth and even. The
tracing cloth used by architects and engineers is good for a trans-
lucent screen.

There is a practical difficulty with all translucent screens. On
account of the poor reflection, the operator cannot tell with the
same certainty when the image is in focus as with a white, opaque
screen.

§ 632. Table of the reflection of different screens compared
with magnesium oxide.

No. At 15° At 45° At 60°

1 Magnesium Oxide 100 83

2 Plaster of Paris 95.6 88.7 78

3 Cardboard 84.5 67

4 White Lead 88.5 79.4

5 Century Company's White 89.4 81

6 Zinc Paint 84.4 76.5

7 Aluminum Paint on Card 210 18

8 Zeiss Metallic Screen, smooth .... 136 14

9 Mirror Screen 200

10 White Muslin, Reflection 73.4 69.1 66.9

Transmission 39 30

11 Gauze, Reflection 33 27

Transmission 35

12 Ground-Glass, Transmission 300 14.2

13 Bristolboard, Reflection 91.5

§ 631a. For example, in the anatomical institute at Munich. Here all the
projection, whether with the magic lantern, the projection microscope or the
opaque lantern, is upon a translucent screen; also in some of the lecture rooms
in Holland.

CH. XII]

BRIGHTNESS OF SCREENS

463

FIG. 249. DISTRIBUTION OF APPARENT BRIGHTNESS WITH DIFFERENT
SCREENS WHEN VIEWED FROM DIFFERENT DIRECTIONS.

On the sides and curved surface are given the degrees of inclination of the
reflected light.

The numbers along the central radius indicate the relative brightness of each
screen, magnesium oxide being used as the standard and called 100%.

A Magnesium oxide screen. It gives the standard brightness of 1 00% and
reflects nearly equally throughout the entire 180 degrees.

B White cardboard.

C Screen with aluminum bronze facing. This gives 3.2 times the bright-
ness of magnesium oxide in the center, but it falls off rapidly at the sides.

E Mirror screen. This gives 7. 1 times the brightness of magnesium oxide
in the center.

It is to be noted in general that the mirror screens, (C. E.) give great inten-
sity when seen near the center, and that this intense light is restricted to an
angle of about 25 degrees. Farther to the side the light falls off rapidly,
being in marked contrast with the white screens (A B).

464 SIZE OF SCREENS CH. XII]

SIZE OF SCREENS AND SCREEN IMAGES

§ 633. The size of screen images which will give the best
results in a given case can only be determined by trial. The size
should be great enough so that the people sitting on the back seats
can see all the details to be shown and still not so large that those
sitting near the front will be repelled by the coarseness of the image.

As a result of experiments to determine the best size of screen
picture for the average seat in a room the following general rules
have been worked out so : —

§ 634. Size of the screen for lantern slides. — The screen image
must be large enough so that details are visible to the most distant
spectator. For example, in teaching work and in demonstrations
at scientific meetings, etc., lantern slides often contain tables of
figures and printed sentences. Naturally, the farthest sitter
should be able to see the figures and to read the words easily.

This could not be done by those on the back seats if the letters
were much smaller than six point. Of course, if the letters on the
slide are as large as eight or ten point type (fig. 216), they can be
read at a glance.

In long, narrow rooms the magnification necessary to enable the
people on the back seats to see the details well will make every-
thing gigantic for those sitting near the screen.

For a well atranged auditorium, if the letters and numerals on the slide are of the size of
6 point type, such as shown in this sentence, and the screen image is from one-fourth to one-,
fifth as wide a1-, the distance from the farthest seat in the room to the screen, all in the
audience should be able to read the print on the lantern slide with ease.

§ 635 Projection objectives necessary to give the proper
screen image with the magic lantern. — If the lantern can be at the
extreme rear of the room, and the image of the slide is to be one-
fourth or one-fifth as wide as the room is long, as stated above
(§ 634), a projection objective of 30 cm. (12 in.) focus will give the
desired screen image for a properly made lantern slide, no matter
what the size of the room. This is because the 30 cm. objective
gives an image on the screen, regardless of its distance, which will
appear to the observer standing by the lantern, like the same lan-
tern slide held 30 cm. (12 in.) in front of the observer's eyes. If
the lantern slide is well made and properly proportioned all the

CH. XII]

BRIGHTNESS OF SCREENS

465

details should be plainly visible when the slide is 30 cm. in front of
the eyes, and therefore are plainly visible in the screen image as
far back as the lantern.

FIG. 250. DISTRIBUTION OF BRIGHTNESS OF TRANSLUCENT AND REFLECT-
ING SCREENS WHEN SEEN FROM DIFFERENT DIRECTIONS

Reflected Light from Magnesium Oxide and Mirror Screen.
Light Transmitted through Ground Glass.

Note that the mirror screen when seen perpendicularly reflects 7 times
as much light as does magnesium oxide, and ground-glass 19 times as much.

But this great brightness of the mirror screen and the ground-glass is
limited to a very narrow angle, while the white magnesium oxide reflects
nearly equally throughout the entire hemisphere.

Brightness of MgO is taken as unity and the figures on the radius indicate
the number of times brighter the screen appears than this.

466

SIZE OF SCREENS

[CH. XII

If the letters and numerals and other details on the slide are too
small to be seen by the normal eye when held 30 cm. (12 in.) away,
then they will not show clearly in the screen image with this objec-
tive at the back of the room, although they may be plainly visible
to those near the screen.

As the lantern is frequently not quite at the extreme back of the
room, an objective of 25 cm. (10 in.) focus is more commonly used
than the one of 30 cm. (12 in.). It makes the image somewhat
larger, and for many people is more satisfactory.

§ 636. Objective to use when the lantern is not at the back of
the room. — Regardless of the position of the lantern a screen image
must be large enough for all in the room to see the details as stated
above (§633-634).

FIG. 251.

SIZES OF SCREENS NECESSARY FOR DIFFERENT
SCREEN DISTANCES.

This shows that the same object and objective will give a screen image of a
size directly proportional to the screen distance.

CH. XII] SIZE OF SCREENS 467

If the lantern cannot be at the back of the room, but must be
closer to the screen, then the projection objective must be of shorter
focus than 25 to 30 cm. (10-12 in.).

To determine the proper objective to use to give the desired size
of image in any case one must proceed as follows :

1 i ) The size of screen image is decided on by remembering that
it should be between one-fourth and one-fifth the distance to the
farthest seat in the room.

(2) The distance from the screen to the lantern must be
measured.

(3) Following the simple optical law founded on the geometry
of similar triangles that: "The size of object and image vary
directly as their distance from the center of the objective," one
can by simple proportion get the focus which the objective should
have for a given screen image.

§ 637. Examples. — For example, suppose the distance from the
screen to the farthest seat is 20 meters (66 ft.), the width of the
screen should be not less than one-fourth this distance, i. e., five
meters (16.5 ft.).

Now suppose that instead of the lantern being 20 meters from
the screen it is only n meters (36 ft.) from it, what should be the
focus of the projection objective to give a screen image 5 meters
(16.5 ft.) wide?

The formula best adapted for this calculation is:

f d

where f is the distance of the object from the center of the objective
(focus of the objective).

0 is the size of the object.

d is the distance from the objective to the screen.

1 is the size of the screen image.

It is assumed in all the calculations for the magic lantern that
the width of the lantern-slide opening or picture is 7.5 cm. or 3
inches.

468 SIZE OF SCREENS [Cn. XII

In the above example
f is unknown.

0 the size of the object is 7.5 cm. 3 in.
d the distance of the screen image is n meters 36ft.

1 the size of the screen image is 5 meters 16.5 ft.
Substituting the values in the formula we have, for metric values,

f ll £ 7-SXI1 r r .LI. t.- .,_•

— = — or f = - - = 16.5 cm., focus or the objective.
7-5 5 5

For English values

- = — or f = - — — = 6.5 in., focus of the objective.
3 16.5 16.5

§ 638. Size of screen image for moving pictures. — As the
scenes depicted by the moving picture are so largely of human
action, and thus resemble a theater play, one would think that the
standard should be the representation of people in their natural
size. The fact is, however, that in most picture theaters the people
represented are of heroic or semi-heroic size, being from 1^2 to
two times the natural size of ordinary people.

The large size of the moving picture on the screen has come about
naturally, as the details of movement and the facial expression of

§ 636a-637a. In the formula here given it is assumed that the objective will
always be at its principal focal distance from the object regardless of the screen
distance. This is not strictly true, but as the screen distance is so great rela-
tively to the distance of the objective from the object, the slight error involved
in the above assumption is negligible. If the screen distance and the principal
focal distance were more nearly the same, the error would be altogether too
great to be neglected (see fig. 210).

It follows, naturally also, from this formula that, if any three of the elements
are given, the fourth can be found. Ordinarily, it is the proper focus of the
objective to use that is unknown, but any one of the elements might be desired,
and it can be found if one knows three of them.

As it is the focal length of the objective that is most often required, the
following may be of assistance; it simply states in words what the formula
shows:

To find the focal length of the objective needed, the screen distance and the
size of the screen image being known: Multiply the screen distance in meters
by 7.5, and divide the product by the size of the screen image in meters and the
result will give the focus of the objective in centimeters. For English measure :
Multiply the screen distance in feet by 3 and divide the product by the size of
the screen image in feet, and the result will give the focus of the objective in
inches.

CH. XII] SIZE OF SCREENS 469

the actors could not be seen if they were only of the size of average
human beings. On the theater stage the action is made more
intelligible by the spoken words; but where there is only pan-
tomine one must see the details of the action and the facial expres-
sion to make the play fully intelligible.

To enable those seated in the extreme rear seats to see the action
on the screen without getting the picture too large for those on the
front seats, the width of the picture should be between /^ to Ya of
the distance of the farthest seat to the screen. The width of
^6 is on the whole the most satisfactory if the end of the room is
large enough to permit a screen of this size.

§ 639. The size of the screen limited by the room. — It some-
times happens that the size of the screen which can be used is
limited by the size of the wall on which it can be placed. The size
of the screen may also be limited by the height of the ceiling above,
and the heads of the spectators below. This is true of some lecture
halls, and of many of the moving picture theaters which are re-
modeled store buildings. If the screen image is limited in size by
any of these factors thus requiring a smaller picture than that hav-
ing a width of ;^th the distance from the screen to the farthest seat
for the magic lantern or >6th the distance for a moving picture,
it is necessary to use an objective of longer focus accordingly.

If the width is limited and one can use any height desired, the
calculation is made exactly as in the previous section.

If the height is limited, then the calculation is made in the same
way except that the height of the object instead of its width is
taken; that is, for lantern slides the extreme opening of the mat is
taken as 7 cm. (2^4 in.), or for moving pictures 23.08 mm. long,
17.3 mm. high, 2%2 in. X 87/i2s in. (see § 57oa).

For example, in a university lecture room the greatest height of
the screen which could be used was 2.9 meters (9.5 ft.), and the
room was 14.3 meters (47 ft.) long. The question was: What
focus of objective would give this size of screen image with the
lantern at the back of the room?

47° SIZE OF SCREENS [Cn. XII

In this example

f is unknown, (i. e., the focus of the objective).

0 the height of the object is 7 cm. (2^4 in.).

d the distance of the screen is 14.3 meters (47 ft.).

1 the size of the image is 2.9 meters (9.5 ft.).
Substituting the values in the formula we have :
For metric values :

— or f = — -^- = 34.5 cm., focus of the objective.
7 2.9 2.9

For English values :

or f = -^ = 1 3. 6 in., focus of the objective.

2-75 9-5 9-5

An objective of 13.6 in. or 34.5 cm. would have to be specially
constructed. Those on the market and easily procurable were of
30 cm. (12 in.) and 38 cm. (15 in.). The shorter focus objective
gave considerably too large a screen image and could not be used,
therefore the one of longer focus was taken, and a correspondingly
long focus condensing lens used.

Second example. In another lecture room the lantern must be
16.75 meters from the screen and the screen could not exceed 3.35
meters in width, what should be the focus of the objective and the
second element of the condenser to meet these conditions?

Applying the formula :

- = — '-^- whence f = 37.5 cm. the focus of the objective needed.
7-5 3-35

In English measure :

- = -— whence f = 1 5 in. That is, a 1 5 inch objective is demanded.
3 ii

§ 640. Size of screen and screen images for micro-projection.

—Here the law holds, that to be satisfactory, the details to be
shown must be large enough so that they can be seen with ease.
The microscopic specimens vary so greatly in character that no

CH. XII] TROUBLES WITH ROOMS AND SCREENS 471

general rule can be given for the size of screen necessary. For
large halls the screen used for the magic lantern usually answers.
In small rooms for special demonstrations it is advantageous to
have a movable screen on a stand that can be varied in distance for
different conditions. The magnification and the objective neces-
sary for the same must be determined in each case by the lecturer
before the lecture or demonstration (see § 400, Ch. IX.)

§ 641 . Troubles with Rooms and Screens :

1. Poor image on the screen. This may be due to

(a) Insufficient light from the radiant;

(b) Too much light in the room;

(d) If an approximately square room is used, then the
mirror and other metallic screens will appear very dark and un-
satisfactory for the spectators outside of an angle of greater than
15 to 20 degrees from the axis, and the farther outside the 15 de-
gree position the darker will appear the screen image (fig. 247).

(e) The objective and second element of the condenser may be
improperly proportioned, i. e., focal lengths too different (§ 89-90).

2. Oppressive in the room. Too little fresh air.

3. Room lights shining in the eyes of the spectators. Not
properly placed or shaded.

4. Distorted image. The screen and the axial ray from the
projection apparatus not at right angles.

5 . The details of the picture not visible for the spectators on the
back seats. The objective is of too long a focus and it does not
magnify enough. Use a shorter focus objective.

6. The screen picture altogether too large. Too short a focus
objective; use one of longer focus, and adapt the condenser to it.

7. There is a glare in the room from the ceiling or walls or both.
The paint used in finishing is shiny, not dull and flat. Use more
turpentine and less oil in the paint.

8. The room too dark. Use more room lights properly placed
and shaded.

472

DO AND DO NOT WITH ROOMS AND SCREENS [Cn. XII

§ 642. Summary of Chapter XII:
Do

Do NOT

1. Use a room properly
equipped for projection if good
results are expected (§ 602).

2. Use light-absorbing tints
for tinting and decorating the
projection room (§ 604) .

3. Make all paints dull or
flat, never shiny, for a projec-
tion room.

4. Light the projection room
sufficiently, so that the specta-
tors can find their seats without
trouble (§ 605).

5. A perfectly darkened room
is only necessary for special
projection (§ 608).

6. Lamps for general lighting
should be shaded or so arranged
that their light cannot shine
directly in the eyes of the spec-
tators or upon the image screen
(§606).

7. Have red lights near all
exits (§ 607).

8. Take the necessary pre-
cautions to prevent light enter-
ing the room at the edges of the
window shades (§ 611).

1. Do not expect good pro-
jection in a room not equipped
for it.

2. Do not use light-reflecting
colors like yellow, white, light
red or green for decorating
the projection room; but the
dark, rich, light-absorbing col-
ors, dark red, brown, etc.

3 . Do not use paint that gives
a shiny or enamel surface, for
this will produce a glare by the
reflections. Use flat paint.

4. Do not have the projection
room darker than necessary.

5. Do not attempt the most
difficult projection unless the
room can be made perfectly
black.

6. Do not use unshaded
lamps for the general lighting.
Light shining directly in the
eyes of the spectators is very
distressing.

7. Do not fail to have red
lights by the exits.

8. Do not leave the window
shades without protection at the
margins.

CH. XII] DO AND DO NOT WITH ROOMS AND SCREENS

473

9. If possible place the pro-
jection apparatus at the back of
the room (§612).

10. The axial ray of the image
beam should strike the screen at
right angles (§614).

1 1 . Incline the screen if neces-
sary (§614).

12. Light the black-board by
lights behind a curved, metal
shield (fig. 240).

13. If a translucent screen is
used the objects must be put
into the apparatus with the pic-
ture facing the objective (§516).

14. Use a good screen (§ 621).

15. If you use a mirror or
metalliic screen remember that
it does not reflect equally
throughout the 180 degrees
(§ 629).

1 6. Make the screen image
large enough so that the most
distant spectator can see the
details (§633, 639).

9. Do not place the projec-
tion apparatus in the middle of
the room if it can be avoided.

10. Do not let the axial ray
strike the screen obliquely.

1 1 . Do not incline the screen
unless necessary.

12. Do not try to use the
blackboard without lighting it
by hidden lights.

13. Do not forget the rules
for erect images when using a
translucent screen.

14. Do not use a dirty screen.
Wash it or give it a fresh coat of
white paint.

15. Do not use a mirror or
metal faced screen in a square
room ; such screens are not good
for the fine details necessary in
micro-projection (§ 360).

1 6. Do not have the screen
image too small nor too large.

CHAPTER XIII

ELECTRIC CURRENTS AND THEIR MEASUREMENT;
ARC LAMPS, THEIR WIRING AND CONTROL;
CANDLE-POWER OF ARC LAMPS FOR
PROJECTION

§ 650. Apparatus and Material for Chapter XIII:
Direct and alternating current ; Voltmeter, ammeter and watt-
meter for direct and alternating currents ; Shunt generator, motor-
generator set, mercury arc rectifier ; Polarity indicators ; Arc lamp ;
Rheostat, inductor, transformer and other ballast; Carbons;
Water-cell; Insulated wires, flexible cables, insulators, switches,
separable plugs, caps, etc. ; Fuses and circuit breakers; Wire, iron
plates, etc., for home-made rheostats.

§ 651. Historical development of electric lighting. — See the
Appendix.

ELECTRIC CURRENTS: KINDS AND COMPARISON

§ 652. Direct current. — The earliest electric currents studied
and experimented with were produced by the voltaic cell. These
currents have a constant polarity, always flowing in the same
direction.

A direct electric current may be produced by a voltaic battery,
or by a dynamo (§ 652a).

The first installations of electric plants were all for direct current,
now they are mostly for alternating current (see below).

The principal use of direct current at present is for trolley cars,
and other apparatus where variable speed motors are necessary;
in electrolysis such as charging storage batteries and the decomposi-
tion of chemical compounds; for electric lighting in some of the
more densely populated cities and for projection purposes.

§ 652a. Generator, Dynamo. — Generator is a comprehensive term in-
cluding all means of producing electric currents whether these means be
chemical or mechanical.

Dynamo, on the other hand, is a special term denoting a generator by which
mechanical is transformed into electrical energy; for example, a steam engine
may give motion to the dynamo and thus produce an electric current. In a
word, a dynamo is a generator of electricity, but all generators of electricity are
not dynamos.

474

CH. XIII] ELECTRIC CURRENTS 475

§ 653. Alternating current. — This is characterized by flowing
first in one direction and then in the opposite direction ; the polarity
is therefore constantly changing. (See § 676).

Alternating current is pro-
duced only by dynamos. It is
used especially for the trans-
mission of power to great dis-
tances, incandescent lighting,
arc lighting, for motors and for

FIG. 252. CONNECTIONS OF A th electric furnace, as in the
VOLTMETER TO MEASURE THE

LINE VOLTAGE. manufacture of carborundum

G Dynamo. and graphite.

A Arc Lamp. Alternating current has the

R Rheostat. advantage of being more easily

Note that the terminals of the volt- produced, as the dynamo is Sim-
meter are connected to the two points * .
between which it is desired to measure pier; but its great superiority
the potential difference. In this case lies in the fact that practically
it is the main supply (across the line).

without loss it can be stepped

up or down in voltage by stationary transformers. This makes it
possible to raise it to a very high voltage (1000 to 100,000 volts)
for transmission to a distance over wires of moderate size. It is
then stepped down in voltage before it is used. In this process of
stepping up or down in voltage the amperage takes the reverse
direction, so that the product of the volts by the amperes is a
constant quantity.

The disadvantages of alternating current for the arc lamp are:

1 . The arc is not as bright as with the same amperage of direct
current.

2. The light is intermittent.

3. The alternating current arc is noisy.

ELECTRIC UNITS AND THEIR MEASUREMENT

§ 654. Electric Units. — For the purposes of this book it is
necessary to refer frequently to electric units, like the volt, the
ampere, the ohm and the watt; it seems proper therefore to give
a brief discussion of these units.

476 DIRECT CURRENT UNITS [Ca. XIII

DIRECT CURRENT UNITS

§ 655. The Volt. — This is the unit of electromotive force, that
is the electric force or pressure necessary to produce one ampere of
current in a circuit with a resistance of one ohm.

The difference of potential between the two poles of a Weston
standard cadmium cell is i.o 19 volts. The ordinary battery used
for ringing door bells has approximately one volt pressure.

Voltage is a general term representing the pressure in volts in an
electric circuit.

If the difference of pressure between the two given points is
great, then the voltage is said to be high; if the difference is slight,
then the voltage is low. For example, in projection one might use
55 volts for the arc lamp, or 220 volts, or 500 volts. Ordinarily
neither the low voltage of 55 nor the high voltage of 500 or 220
is used, but an intermediate voltage of no.

§ 656. The Ampere. — This is the unit of current. It is the
current which will deposit .001118 gram of metallic silver per
second from a 15% solution of silver nitrate in water. It is the
current which one volt will maintain in a circuit with one ohm
resistance (see below).

Amperage is the term by which is designated the amount of

current in amperes flowing at
any given moment. If a large
amount of current is flowing the
amperage is said to be high or
great, if a small amount, then it
is said to be low or small. For

example, in projection, the am-
FIG. 253. CONNECTIONS OF A VOLT- , , .

METER TO MEASURE THE ARC perage needed for drawing with
VOLTAGE. the microscope on the house cir-

V VcTtnSer. cuit (§ 493) is small (3-6 am-

A Arc lamp. peres) , while for opaque pro-

R Rheostat. jection (§ 289), and for moving

Note that the terminals of the volt- J . ' . fe

meter are connected to the two points pictures (§ 693) in large halls

between which it is desired to measure the amount of amperage needed

the potential difference. In this case . . .

it is the two carbons (across the arc), is great (20 to 100 amperes ).

CH. XIII] DIRECT CURRENT UNITS 477

§ 657. The Ohm. — This is the unit of resistance to the flow of
an electric current. It is represented by the resistance, at zero
centigrade, of a column of mercury 106.3 centimeters long, of uni-
form cross sectional area, and weighing 14.4521 grams. Such a
column of mercury will have a cross sectional area of one square
mm.

Ohmage is a term analogous with voltage and amperage. It is
used to designate the amount of resistance in ohms of an electric
circuit.

A conductor may have little resistance, as copper, etc., or it may
have great resistance like German silver. Naturally then copper
wire is used largely for electric circuits, and German silver wire for
making resistors or rheostats (§ 7240.}.

§ 658. The Watt. — This is the unit of activity and is the rate
at which work can be done by a current of one ampere under a
pressure of one volt. One watt means the doing of work at the
rate of io7 ergs per second, or one joule per second. This is approx-
imately equal to the lifting of i kilogram, io centimeters every
second.

§ 659. Kilowatt. — A kilowatt is 1,000 watts. This term is
more common than watt. It is equal to 1.34 horse power.

§ 660. The watts which any direct current represents are

obtained by multiplying the quantity of current flowing by the
pressure — that is, the amperes by the volts. Thus, if there were
an amperage of one and a voltage of one, there would be an
activity of one watt. If the voltage were io and the amperage 100,
or the voltage 100 and the amperage io, there would be an activity
of 1,000 watts, or one kilowatt.

§ 661. Kilowatt-hour. — This is the unit of electrical energy or
work, which is in commercial use and which is used as a basis for
making the charges to consumers. A kilowatt-hour is the work
represented by one kilowatt when acting for one hour.

In order to find the amount of work done by an electric current
it is necessary not only to know the rate at which the work is being
done but also the time during which this rate is continued. Thus,

478 ELECTRIC MEASUREMENTS [Cn. XIII

take the example of an arc lamp which uses 20 amperes direct
current from a no volt line. The line then supplies 20 x no =
2,200 watts or 2.2 kilowatts. If this arc were used for only a few
minutes, the energy supplied would be comparatively small, but if
the arc were used all day, the energy supplied and hence the coal
or other fuel consumed in generating this power would be compara-
tively large. In order to measure this energy, the power measured
in kilowatts is multiplied by the time the power is used. In the
above example, if the arc were run for eight hours the electrical
energy used would be 2. 2x8 = 17.6 kilowatt-hours.

ELECTRIC MEASUREMENTS: VOLTMETERS, AMMETERS, WATT-
METERS FOR DIRECT CURRENT

§ 662. Voltmeter for direct current. — This is an instrument for
measuring in volts the difference of potential between two points
of an electric circuit.

The voltmeter must be adapted to the kind of current — direct
or alternating — and for the pressure, low voltage or high voltage.
It consists of a delicate galvanometer of exactly the same type as
that for an ammeter, but it has a high resistance in series with it.
This high resistance allows but a small current to flow through the
galvanometer; and this small current is proportional to the differ-
ence of pressure or voltage between the binding posts of the volt-
meter, and causes the needle of the voltmeter to be deflected.
Numbers on the dial indicate the voltage for different amounts of
the deflection.

§ 663. Connection of the voltmeter with the circuit to be
measured. — One pole of the voltmeter is positive and one negative.
To connect the instrument with the circuit for determining the
voltage between two points, the positive binding post of the volt-
meter is connected by a wire to the positive point in the circuit, and
the negative binding post with the negative point in the circuit
(fig. 272). This gives the full electric pressure between the two
points connected with the voltmeter, although only a very small
current flows through it on account of its high resistance. The

CH. XIII]

ELECTRIC MEASUREMENTS

479

main current continues to flow in the electric circuit between the
two points exactly as though the voltmeter were not in use.

The voltmeter should not be connected in series with the line as
with the ammeter (§ 665). While no particular harm might be
done, the high resistance of the voltmeter would allow only a small
current to flow in the line and if one were using an arc lamp it
would go out from the insufficient current.

If one does not know the direction of the flow in the circuit to be
tested, the voltmeter can be correctly connected by trial as follows :
Connect the positive binding post of the voltmeter by a wire to one
of the points, and the negative binding post by a wire to the other
point. Turn on the current, and if the connection is right the
needle of the instrument will point to the voltage ; if the connection
is wrong the needle will tend to be deflected off the scale below the
zero point. If it is wrong, turn off the current and reverse the
position of the wires in the binding posts.

§ 664. Ammeter. — This is
an instrument to show the
amount of current flowing
through a given circuit at a
given instant. It consists of a
galvanometer of the particular
type adapted to the current
used, that is, direct or alter-
nating current. It is also
adapted to the amount of cur-
rent to be measured, that is
small currents and large cur-
rents, say from o to 10, o to 25,
o to 50, o to 100, etc.

The galvanometer part of the
ammeter is a delicate instru-
ment so that the whole current
used in projection is not sent
through it, but a definite frac-
tional part goes through it and

FIG. 254. CONNECTING AN AMMETER

IN THE LINE TO MEASURE THE

CURRENT FLOWING.

a Ammeter, A, with external shunt,

b Ammeter with self-contained
shunt. The shunt in this type is in-
side of the instrument case.

Note, the ammeter is connected
along one wire so that the entire cur-
rent flows through the instrument and
its shunt.

480 ELECTRIC MEASUREMENTS [Cn. XIII

the main part of the current goes through a special wire, known as
a shunt (fig. 254).

In some ammeters the galvanometer, and the shunt are in the
same box (self-contained ammeters) , in others the shunt is outside

(fig- 254).

When an electric current flows through the ammeter, the
galvanometer needle is deflected, the amount of the deflection
measuring the amount of the current. With the ammeters used
in projection, the galvanometer has been calibrated so that the
needle points to the number of amperes of current flowing in a given
case (fig. 145).

§ 665. Connection of the ammeter with the projection circuit.—

If one is to use an ammeter in an electric circuit, the instrument is
connected with the line in series, that is along one wire. Further-
more, it is necessary to connect the positive pole of the ammeter
with the positive end of the wire, and the negative end with the
negative pole. In most cases when installing a projection outfit
the direction of the current flow is not known, and the proper
connection of the apparatus is found by trial (see § 702 for the
direct current arc lamp).

To install an ammeter cut one of the wires, and insert one cut end
in the positive, and the other cut end in the negative binding post
of the ammeter. Then the arc lamp and the rheostat are wired as
shown in fig. 270.

Now close the switch and cause the arc lamp to burn. If the
ammeter is correctly connected, the needle will point to the number
of amperes of current flowing. If the connection is wrong, then
the needle will tend to move off the scale below the zero mark. In
case the connection is wrong, open the switch and reverse the posi-
tions of the wires in the binding posts of the ammeter. When the
current is turned on again the needle will be deflected until it
points to the number of amperes.

By looking at fig. 273 it will be readily seen why one of the cut
ends of the same wire will be positive and why one will be negative.
That is, if the whole circuit is considered from the dynamo back to
the dynamo, it will be seen that starting from the positive pole of

CH. XIII] ELECTRIC MEASUREMENTS 481

the dynamo, any point in the circuit toward or nearer this positive
pole will be positive in comparison with any other point nearer the
negative pole. Then if the circuit is cut at any point the end of
the wire next the positive pole of the dynamo will be positive, and
the end nearer the negative pole of the dynamo will be negative.
Now if the cut end of the wire nearer the positive pole of the dynamo
is inserted in the negative binding post of the ammeter, and the
other end in the other binding post, the needle tends to be deflected
in the wrong direction. If the two ends of the wire are correctly
connected with the ammeter, the needle will be deflected in the
right direction, and indicate the amperage.

§ 666. Ammeter for projection. — In projection the ammeter is
usually all that is required, for the voltage on a given line is nearly
constant, and can be found easily by inquiring of the central station.
On the other hand, the required amount of current for different
purposes varies greatly and the factors in the production of a good
image are so many that an ammeter to show at a glance what
amount of current is flowing is of the highest importance, for with
a given amount of current the operator knows at once what kind
of a light can be reasonably expected in the different cases. If the
screen light is not good with the adequate amperage for the purpose
then he can look to the other possible causes of failure (see § 61-96).

If one is to be able to determine for himself all the electric fac-
tors in projection work, then a voltmeter and a wattmeter should be
added to his apparatus.

§ 667. Precautions for the ammeter. — In connecting the am-
meter be sure not to connect the ammeter directly to both line
wires. As the ammeter has very little resistance, putting it across
the line would have practically the same effect as connecting the
two points with a heavy wire, that is a short circuit would be
formed and the fuses would be blown. Besides the very heavy
current which would flow momentarily might be sufficient to
seriously damage the delicate instrument.

§ 668. Safe rules for the beginner to follow when connecting
instruments may be stated as follows:

482 ELECTRIC MEASUREMENTS [Cn. XIII

For the voltmeter. — After all the connections for the circuit are
complete, connect the two terminals of the voltmeter to any two
parts of the line between which it is desired to measure the differ-
ence of potential.

For the ammeter. — After all the connections for the circuit are
complete, and after the arc has been found to work satisfactorily,
cut one of the wires and insert the self-containing ammeter in
between the two cut ends just as if it were a short piece of heavy
wire. If there is an outside shunt connect the ends of the supply
wire to the large binding posts of the shunt and the wires of the
ammeter to the smaller binding posts of the shunt.

§ 669. The Wattmeter. — This is an instrument for measuring
electrical power or activity. There are two types of wattmeter —
the portable or indicating wattmeter and the integrating or supply
wattmeter. Both work on the same principle, but the method of
indication is different.

The wattmeter has two sets of terminals or binding posts. One
set is connected with the line in series — along one wire — like an
ammeter, while the other set is connected in multiple — that is, to
both lead wires — like a voltmeter. In fact this instrument is a

sort of a combination voltmeter
and ammeter, as it measures
the product of the volts times
the amperes.

In connecting the wattmeter
FIG. 255. WATTMETER TO MEASURE p.reat care must be taken to ret
THE POWER CONSUMED AT THE ARC.

G Dynamo. the sets of binding posts correct-

W Wattmeter. ly joined with the line. That is

R Rheostat. the binding posts for the current

The heavy line wire passes to the terminals of the wattmeter must

wattmeter and from it to the upper car- be connected in series or along

bon. From the lower carbon the heavy

wire passes to the rheostat and back one Wire like the ammeter (fig.
to the dynamo. The fine wire passes 2_3) while the voltage binding
from the upper carbon to the watt-
meter, and from the wattmeter to the posts must be connected in para-
lower carbon. With this connection of llel or across the line, like a
the wires the power consumed at the
arc is measured. voltmeter (fig. 272).

CH. XIII] ELECTRIC MEASUREMENTS 4§3

If the wattmeter were wrongly connected, the instrument could
not register the watts on the one hand and on the other it might be
injured.

§ 670. Portable wattmeter. — This has a pointer which shows
directly in watts, the power consumed at a given instant.

§ 671. Stationary or house
wattmeter. — The wattmeter for
the electric supply looks some-
thing like a gas meter for the gas
supply. It is of the integrat-

FIG. 256. WATTMETER TO MEASURE ing ^P6- is permanently con-

THE POWER DELIVERED BY THE nected with the line, and con-

DYNAMO. taing & wheel the ed of whOSe

G Dynamo. . ,

W Wattmeter. rotation is directly proportional

A Arc lamp. to fae power consumed. This

R Rheostat.

The fine wire connects the watt- wheel turns pointers over the
meter with the line where the power is dials on which are indicated

kilowatt-hours. The numbers

toward which the pointers are directed indicate the kilowatt-
hours which have been used, just as the pointers in a gas meter
indicate the number of cubic feet of gas which have been used.
For example, by consulting the wattmeter before and after an
exhibition one can see how much work, measured in kilowatt-hours
has been consumed by the arc light.

Suppose the voltage of the line were no, and the voltage between
the carbons is 55 volts. Suppose the amperage is 20, then the
watts should be (volts times amperes) 55x20 = noo watts at any
instant, and for an hour, for example, it would be 1 100 watt hours,
or i.ioo kilowatt-hours.

§ 67 la. With both direct and alternating current, when a rheostat is in
the circuit, the amperage may be found by the aid of the stationary wattmeter,
this is always present in a house supply of electricity, as is the gas meter for the
gas supply, and one does not always possess an ammeter.

It is necessary to know the voltage of the line. This is usually 1 10 or 220.

One must also know the watts, or kilowatts at any given instant. This can
be found by the wattmeter as follows: Suppose the reading is 1.87 kilowatt-
hours. As this number was obtained by multiplying volts x amperes x time,
and the time is one hour, then the kilowatts of power consumed is 1.87. The

484 ALTERNATING CURRENT UNITS [Cn. XIII

UNITS AND THEIR MEASUREMENT WITH ALTERNATING CURRENT

With alternating current there is, strictly speaking, no such
thing as voltage and amperage as the electric potential is varying
from instant to instant. Consequently a kind of average value of
the electric pressure and amount of current is used instead.

§ 672. Alternating current voltage. — When alternating current
is measured, the voltage indicated on the voltmeter is the mean
effective voltage.

In order that this average effective value for a volt shall corres-
pond as nearly as possible to the analogous value with direct cur-
rent, the value taken is the square root of the average of the squares
of the instantaneous values of the potential difference during an
entire cycle. Or briefly, it is the root mean squares of the instan-
taneous pressure.

§ 673. Alternating current amperage. — The number of amperes
indicated on an ammeter when using alternating current represents
the mean effective amperage. The average effective value of the
ampere is, as with the volt, the square root of the average of the
squares of the instantaneous values of the current during an entire
cycle.

voltage with a rheostat is the line voltage. Now as the kilowatts are the pro-
duct of voltage by amperage divided by i ,000 and both the voltage and the
kilowatts are known the amperage can be found by multiplying the kilowatts
by 1,000 to reduce them to watts, and dividing the watts by the voltage = 1 10.
1870 -H 1 10 = 17 amps. With alternating current, if an inductor (choke-coil)
is used for regulating the current, the wattmeter can also be utilized for deter-
mining the amperage at the arc, for by experiment it is known that no matter
what the line voltage is, the voltage across the arc is usually about 34 volts.
The fall of potential across the inductor does not count. The wattmeter only
records the power consumed by the lamp. The amperage, assuming the same
number of watts as in the above example, would be found this: 1870 -=- 34 =
55 amperes. That is, with an inductor in place of a rheostat one could use
several times the amount of current and use only the same number of kilowatts
of power. As it is the power consumed that must be paid for, one can appre-
ciate the saving by using an inductor or choke-coil rather than a rheostat.

The two cases just given are the only ones in which the wattmeter can be
used to find the amperage. If a current-saver, transformer, rectifier, or other
similar device is used in the circuit the amperage in the arc cannot be deter-
mined by the wattmeter, one must use an ammeter of the proper type for the
current.

CH. XIII] ALTERNATING CURRENT UNITS 485

§ 674. Watts with alternating current.— With alternating as
with direct current, the instantaneous watts are equal to the pro-
duct of the instantaneous volts by the instantaneous amperes.

As the voltage and amperage with alternating current vary
from instant to instant over the entire cycle, it follows that the
instantaneous watts must also vary from instant to instant. To
obtain the average watts over an entire cycle, the arithmetical
mean of the instantaneous watts is taken. This average of the
watts may be anything between zero and the product of the mean
effective volts times the mean effective amperes, depending on the
character of the circuit, i. e., whether the circuit contains resistance
only or whether it contains both resistance and inductance.

§ 675. Power factor.— When alternating current is used with
inductance in the circuit as described in § 736 (where an inductor
or choke-coil is put in series with the arc) the power transformed
into heat or work, and hence which must be supplied to the dynamo
by coal or other fuel is less than the product of the mean effective
volts by the mean effective amperes. This is because most of the
energy required to magnetize the iron core of the inductor when the
current is increasing is returned to the line when the current is
decreasing. In the case mentioned the line voltage was no; at
the arc the voltage was 34, and 55 amperes were drawn. The power
consumption at the arc, which is unable to return any absorbed
energy to the line, is the product of the volts by the amperes, i. e.,
34 x 55 = 1,870 watts. In this case the power factor is unity. In
the case of the entire circuit, however, by multiplying the line
voltage by the amperage, i. e., 1 10 x 55 we get 6050. A wattmeter
would register only the 1870 watts consumed at the arc. The
power factor is the value by which we must multiply the product of
volts x amperes in order to get watts. Thus, if we multiply 6050
by .3 1 we get 1870. The power factor is of course obtained in prac-
tice by dividing the watts by the product of volts by amperes, i. e.,
P. F. = Watts -4- Volts x Amperes; and Watts = Volts x Amperes x
Power factor. Nothing comparable to this effect is possible with
direct current, that is, with direct current the power factor is always
unity.

486 DYNAMO FOR ARC LAMPS [Cn. XIII

§ 676. Cycle. — With alternating current where the current
flows first in one direction and then in another with a change in
polarity for each reversal, a cycle includes a change in polarity to
the opposite, and back to the starting point. That is, a cycle
includes flow in two directions and consequently includes two
polarities ; and this is repeated over and over again.

§ 677. Frequency. — The number of cycles per second with an
alternating current is called its frequency. The frequencies in
most common use are: 25 cycles, 60 cycles and 135 cycles per
second. The 60 cycle frequency is most generally used for lighting
circuits and the 25 cycle frequency is mostly employed for long
distance transmission, and frequently for motors. The 130 or 135
cycle frequency is now uncommon.

SPECIAL DYNAMO FOR ARC LAMPS

§ 678. The characteristics of the arc are that the potential
difference between the electrodes is dependent upon the arc length
but not upon the current (see § 743). It is required to supply this
arc with a constant current regardless of the differences in arc
length. This may be done with a constant potential supply by
using a rheostat in series with the arc, or it may be done by using a
constant current generator. Since the early days of arc lighting,
street arcs have been connected in series and are supplied by a
direct current dynamo of this type, no resistance being used. These
dynamos have an automatic controlling device which increases the
voltage when the current falls slightly below the rated value (6.6
amperes) and which decreases the voltage should the current rise
slightly above this value. For street lighting this regulation must
be very close, but for projection purposes the regulation need be
only approximate. There are some types of dynamos which have
the proper characteristics to be connected directly to an arc lamp
without intervening resistance. The characteristics of such a
dynamo must be that a slight momentary increase in current
caused by a lowering in the potential difference at the arc will be
met by a decrease in the voltage generated, and conversely a

CH. XIII] DYNAMO FOR ARC LAMPS 4§7

decrease in current will be met by an increase in the voltage
generated.

§ 679. Shunt generator. — The connections for a shunt genera-
tor or dynamo are shown diagrammatically in fig. 257. A is the
revolving armature from which the current is drawn. N and S are
the poles of the field magnet and F is the field coil which keeps it
strongly magnetized. The stronger the magnetization of this field
magnet the higher the voltage furnished by the machine. As
usually operated the field rheostat R must be continually adjusted
so that the right current is supplied to the field coil F to keep the
machine at the desired voltage.

§ 680. Adaptability of a shunt generator for direct connection
to an arc lamp. — If instead of continually adjusting the rehostat R
so that the dynamo will supply a constant potential, the machine
is left to itself it will be found that when no current is supplied, i. e.,
the dynamo is running on no load, the potential difference between
the terminals a and b is greatest and consequently the current
flowing in the field coil F is greatest. If now current is drawn from
the dynamo the potential difference between a and b will drop
slightly. This will result in a decrease in the current flowing in the
field coil F, a decrease in the magnetization of the field magnets and
hence a decrease in the voltage generated. The result is in the
direction desired, namely, that an increase in the current will be
met by a decrease in the voltage.

Whether or not a shunt generator connected directly to an arc will
work satisfactorily, or whether the arc will be unstable and want to
either "run away" or "die out" will depend upon the details of the
design of the dynamo ; that is, the voltage at no load, the resistance
of the shunt field coils and the resistance of the armature and also
on the resistance of the wiring to the arc. Some dynamos have
been designed which will work satisfactorily when connected
directly to the arc without any intervening resistance. Such
dynamos may be run directly by some form of engine or they may
be part of a motor-generator set in which high voltage, direct
current or alternating current is used to furnish the power. (See
also § 682, 684).

488

DYNAMO FOR ARC LAMPS

[Cn. XIII

FIG. 257.

SHUNT GENERATOR CONNECTED DIRECTLY TO THE ARC LAMP
WITHOUT INTERVENING RESISTANCE.

N S Poles of field magnet.

A Armature rotating between the poles of the field magnet.

a and b Terminals of the Dynamo.

F Field coil; current through this coil magnetizes the iron of the field
magnets.

R Adjustable field rheostat controlling the current flowing through the
field coil.

L Arc lamp.

+ and — indicating the polarity of the wires connected to the arc.

This will maintain a uniform current in the arc regardless of its length in
case the dynamo is properly designed and proportioned for the purpose.

CH. XIII]

CURRENT RECTIFIERS

489

CURRENT RECTIFIERS

§ 681 . While alternating current is mo/e cheaply generated and
transmitted, especially if the distance is great, the available light
given by the alternating arc is much inferior to that given by a
direct current, as can be seen by consulting the table of available
candle-powers for different amperages (§ 756). On this account
and from the noiseless character of the direct current arc, efforts
have been made to utilize alternating current to get direct current.

Up to the present time two methods of doing this for projection
purposes have proven themselves successful :

FIG. 258. MOTOR-GENERATOR SET.
(Cut loaned by the General Electric Co.).

The alternating current motor is at the left, the direct current generator is
at the right. The two armatures are mounted on the same shaft.

§ 682. Motor-generator sets. — This is an indirect way of
getting direct current from alternating. It consists of an alternat-
ing current motor and a direct current dynamo attached to the
same shaft. The alternating current is not converted into direct
current but is used to furnish mechanical power which drives the
direct current dynamo just as it could be driven by a water-wheel,
a gas or other engine.

The efficiency of a motor-generator is about 60%.

If the dynamo is specially designed for the purpose, the arc lamp
can be connected directly to it without using a rheostat so that
there is no loss from this cause as must be the case when the rheo-
stat is used. (See above § 680).

490

CURRENT RECTIFIERS

[Cn. XIII

§ 683. Mercury arc rectifier. — This is a method of securing
direct current from alternating. It is a utilization of the mercury
arc, and gives an efficiency of about 70%. The current is slightly

FIG. 259 FIG. 260

FIG. 259. MERCURY ARC RECTIFIER, FRONT VIEW.

(Cut loaned by the General Electric Co.).

This size is designed for 30 amperes. It requires 14.5 amperes alternating
current at 220 volts or 29 amperes at 1 10 volts, and delivers 30 amperes direct
current at 62 volts. (See tests § 754). It consumes 2600 watts alternating
current and delivers 1860 watts direct current which gives 8,600 candle-power
with the projection arc.

FIG. 260. MERCURY ARC RECTIFIER, REAR VIEW.

(Cut loaned by the General Electric Co.).

This gives a good view of the rectifier bulb and the inductor directly below
the rectifier bulb which serves to limit the current in the arc by acting upon the
alternating current primary. The iron case on the floor contains a com-
pensating reactance which serves to smooth out the fluctuations on the rectified
current.

CH. XIII]

CURRENT RECTIFIERS

491

FIG. 261. MERCURY ARC RECTIFIER, DIAGRAM OF CONNECTIONS.

(Cut loaned by the General Electric Co.).

The alternating current supply comes in at the upper part of the transformer.
This supplies alternating current at 220 volts (for a no volt arc) between the
points C and H. The arrows indicate the direction of flow of the current dur-
ing one-half of the cycle and the arrows enclosed in circles indicate the flow of
current during the other half of the cycle. Taking the time when H is the
positive pole of the transformer, the current flows down this wire and over to
the point A. Here the current flows through the tube to the cathode B,
through the battery J (or the arc lamp situated at J) to D. It then flows to
the right through E and up to G.

When the current is reversed, current cannot follow this path because
between A and B the rectifier tube acts as a valve, as the mercury arc allows
current to flow towards B but never away from it, hence the current must flow
from G to A l to B through J to D, through the coil F to the left and up to the
point H.

The function of the coils E and Fis to act as an auto-transformer, for without
them current could flow directly from G to H without passing through the
rectifier tube. In actual practice both coils E and F are wound on the same
iron core.

492 CURRENT RECTIFIERS [Cn. XIII

The small electrode in the bottom of the tube, at C is used in starting the
tube. In starting, the tube is first rocked making and breaking a mercury
contact. A small amount of current flows through between C and B and starts
the arc going, after which it will continue to burn as long as B is the cathode,
but if the arc is extinguished even for an instant, it will go out and the tube
must be tilted again before it will work.

pulsating, but the current is always in one direction and the pulsa-
tions are so slight that the crater of the positive carbon remains
almost as constant as with the direct current furnished by a motor-
generator set.

Both the motor-generator set and the mercury arc rectifier are
necessarily expensive. For a small plant to be used much of the
time for the arc lamp, and where power is needed for other pur-
poses, like the lighting of the house, pumping water, running
machinery, etc., etc., it would be cheaper to install one of the
modern forms of engines. The cost of running these is relatively
very little, much less than for the current supplied to the rectifier
or for the motor-generator set. It is also very easy to care for the
modern engine used with the generator.

By adapting the generator set for low voltages (60 volts) it is
possible to connect the arc lamp directly without a rheostat, thus
saving the energy wasted by heating the rheostat. A rheostat
may also be used but if so it is called upon to give very slight reduc-
tion in voltage, and therefore uses up but little energy.

PROJECTION WITH 135 CYCLE AND 25 CYCLE CURRENT

§ 684. In most places where alternating current is used for
lighting, the supply has a frequency of 60 cycles per second, and
in this chapter it has generally been assumed that the alternating
current has this frequency. There are, however, places in which
the supply has a frequency of 135 cycles per second and there are
others, especially small towns in the neighborhood of large hydro-
electric plants, in which the supply has a frequency of 25 cycles.
The authors of this book have had practically no experience with
other frequencies than 60 cycles. We have reason to believe how-
ever, that with 135 cycle current the arc will give as good results as
with 60 cycles and will perhaps have less tendency to show a flicker,

CH. XIII]

CURRENT RECTIFIERS

493

especially when used with moving picture projection. When 25
cycle current is used directly (is used raw) to supply the arc, the
result is very bad. The screen shows a violent flicker. The
general appearance is much the same as when a pan of mercury is
jarred rapidly, the surface appears covered with ripples. This
effect is naturally very trying to the eyes.

FlG. 262. OSCILLOGRAMS OF THE ALTERNATING CURRENT SUPPLY AND THE
DIRECT CURRENT DELIVERED.

(Cut loaned by the General Electric Co.; made from the original photograph').

Curve A The direct current delivered.
B The direct current zero line.
C The alternating current voltage curve and its corresponding zero

line.

The height of the curve A above its zero line B represents the instantaneous
value of the direct current. Note that while there are slight fluctuations in the
current, i. e., it is slightly pulsating, the current is always in the same direction
and that these fluctuations amount to only about 30% of the average value.
Note also that the maximum current value corresponds to a maximum positive
value or to a maximum negative value of the alternating current voltage as
shown in curve C given just below.

494

CURRENT RECTIFIERS

CH. XIII]

In order to get good projection when this current supply only is
available, a motor-generator set can of course be used, that is, the
2 5 cycle current is used as power to drive a direct current dynamo
(§ 682). The 25 cycle current can be changed to direct current by
the use of a rectifier (§ 683). Such current would of course be
pulsating although always in the same direction. As the authors
have never seen an arc supplied from a rectifier on 25 cycle current
we can rrake no recommendation except to examine one of these
machines in actual operation. If the arc should prove sufficiently

FlG. 263. OSCILLOGRAMS OF THE POTENTIAL DIFFERENCE BETWEEN THE

ANODE AND CATHODE. IN RELATION TO THE IMPRESSED ELECTROMOTIVE

FORCE.

(Cut loaned by the General Electric Co.; made from the original photograph).

Curve A Potential difference between anode and cathode.

Note that during half of the wave this difference is equal to the full impressed
(line) voltage while during the other half wave the potential difference increases
until the voltage has reached the constant value of 14 volts. When this occurs
current is caused to flow through the arc and is used on the direct current side
of the rectifier.

Curve B Impressed electromotive force, i. e., instantaneous value of the
line voltage.

CH. XIII]

CURRENT RECTIFIERS

495

free from flicker the rectifier would doubtless answer perfectly in
all other particulars. There is no doubt about the motor-genera-
tor; it will give perfect direct current for projection.

§ 685. Need of apparatus designed for the frequency used. —

All alternating current apparatus is designed to work with one
frequency only, that is a transformer, for example, if designed for
use on 60 cycle current will not work satisfactorily on either 135
or 25 cycles. Hence, in ordering apparatus for alternating current
it is necessary to ascertain and specify the frequency as well as the
voltage and other particulars of the supply. This information
can be furnished by the power company.

FlG. 264. OSCILLOGRAMS OF THE ANODE CURRENTS.

(Cut loaned by the General Electric Co.; made from the original photograph).

Curve A Portion of the current furnished by one anode.

Curve B Portion of the current furnished by the other anode.

Note that from a single anode, current flows in one direction only, the mer-
cury arc acting as a valve which prevents the current from flowing in the
opposite direction. When current ceases in one anode the other anode com-
mences to furnish the current.

4Q6 WIRING FOR AN ELECTRIC CURRENT [Cn. XIII

WIRING FOR AN ELECTRIC CIRCUIT FROM THE DYNAMO BACK TO

THE DYNAMO

§ 686. For the purposes of projection by the aid of an arc lamp,
the electric current required, whether it be direct current or
alternating current, is practically always furnished by a dynamo.
To make the electricity available there is a conductor of some kind,
usually a copper wire extending from one pole of the dynamo to the
arc lamp or lamps, and from them back to the other pole of the
dynamo. Such a loop of wire from pole to pole of the dynamo
forms an electric circuit, regardless of the length of the wire. With
direct current, any part of the wire nearer the positive pole of the
dynamo is positive to any part of the wire nearer the negative pole
of the dynamo, hence the wire extending out from the positive pole
of the dynamo is often designated the positive wire, and the wire
received into the negative pole of the dynamo is called the negative
wire. It will be seen from fig. 275, 280 that the circuit is a loop of
wire with the two ends connected with the two poles of the dynamo.

With alternating current, as stated above, there is no constant
polarity, hence it is not correct to speak of negative and positive
wires or positive and negative poles of the alternating current
dynamo.

§ 687. Amperage for different purposes. — As the quantity of

electricity needed for different
purposes varies, the capacity of
the generator or dynamo must
vary. Also the carrying capac-
R ^/ ity of conducting wires is in genr

eral proportional to their size,
FIG. 265. SHORT CIRCUIT. hen(;e for ,arge mmnts it is

G,c GSndauSo,°exd,Sg across the ^cessary to have larger wires

circuit making the path back to the than for small currents (see the

dynamo (G) shorter than the course , i i -i i e <• \

through the arc lamp (A) and the rheo- table below § 694).

If a wire is put across the points s § 688. Short circuit. — By a

and c the electricity will take that path shOrt circuit is meant the short-

mstead of the longer path through the ,. , . , ,

arc. enmg of the distance which the

CH. XIII] WIRING FOR AN ELECTRIC CURRENT 497

current must travel to get back to the dynamo. In figure 265 if a
wire were put across the circuit at the points s. c. instead of the
current extending entirely around the circuit, it would take the
shorter course. Short circuits are undesirable for two reasons :
(i) the current is not available where wanted; (2) it may be
dangerous.

§ 689. Ground. — With many electric circuits such as with
street railway circuits, one terminal of the dynamo is permanently
connected with the earth. If now the wire connected to the other

FIG. 266. AN ELECTRIC CIRCUIT WITH A SINGLE GROUND.
C D The two poles of the dynamo.
G Generator (dynamo).

BI A conductor extending from the electric circuit to the ground (g1).
If all the rest of the circuit is insulated this will do no harm, but see fig. 267.
g1 The earth into which the conductor, B , extends.
A Arc lamp.
R Rheostat.

terminal of the dynamo should also become connected with the
earth, as through a water or a gas pipe, current would wholly or in
part take that path back to the dynamo.

When any part of the circuit is connected with the earth it is
called a "ground."

In case the dynamo and circuit are entirely insulated from the
earth, a single ground will result in no flow of current outside the
wire. If, however, two points in a circuit are connected to the
earth the effect will be the same as if the two points of the circuit
were connected to each other, by an additional wire (fig. 266, 267).

§ 690. Insulation of wires. — To avoid short circuits and the
consequent danger to men and animals and also the danger from

498

REGULATIONS FOR WIRING

[Cn. XIII

fire by the wires coming in contact with inflammable material, the
wires are carefully insulated so that the current is kept in the circuit
and not allowed to escape by taking short cuts or by going to the
ground. Two things are necessary for this: (i) The naked wires
must in no case touch each other at any point, for that would make
a short circuit. (2) The naked wires must not touch anything
which is a conductor.

The wires are insulated by covering them with a coating of
rubber, asbestos, silk, etc., that is, some substance which will

FIG. 267. AN ELECTRIC CIRCUIT WITH A DOUBLE GROUND.
C D The two poles of the dynamo.
G Generator (dynamo).
B1 A conductor extending from the circuit near the pole C to the ground

<«').

Bs A conductor near the pole D extending to the ground (g2).

In this case the current will short-circuit, passing from the point B1 to gl and
from g1 to g', B2 and back to the dynamo at the pole D instead of passing
through the arc lamp (A ) and the rheostat (R). The single ground is dangerous
only in that there is liable to be formed a second ground from some other part
of the circuit.

g1, g2 The earth into which the conductors, B1, B3 extend.

A Arc lamp.

R Rheostat.

not serve as a conductor. Where the wire must be uncovered, as
at switches, etc., some solid substance like porcelain, slate, hard
rubber, glass or some other non-conducting substance is used, for
the naked wires to rest against.

REGULATIONS FOR WIRING: PRECAUTIONS
§ 691. National Electric Code. — To make the wiring and con-
nections of electric apparatus good and safe in every respect, the
electrical engineers, architects and fire underwriters have formu-

CH. XIII] REGULATIONS FOR WIRING 499

lated definite rules for wiring, insulation and the character and
construction of fittings, the installation of apparatus and of light-
ing plants, etc. This national code of rules, with all authorized
modifications found desirable from time to time, is published in
pamphlet form by the National Board of Fire Underwriters for the
guidance of those having electric wiring to do and apparatus to
install. This board also publishes a list of electric apparatus and
fittings which conform to this code. The two pamphlets can be
secured by any one interested by sending five cents in stamps to
cover postage, to the National Board of Fire Underwriters, 135
William St., New York City, N. Y.

General precautions : In wiring or changing wires and in work-
ing about the arc lamp, rheostat, etc., the current should always be
turned off at a switch which will render all the wires and apparatus
to be changed in any way entirely without voltage ("dead"), so
that no matter what is done there is no danger of receiving a shock
or of short-circuiting.

If "live wires" must be worked with, use the asbestos-patch
gloves, and wrap the naked wires in asbestos paper so that it will
be impossible to bring naked wires in contact. Remember also
that a concrete floor, if at all moist, makes an excellent "ground"
for the wires, and if a person stands on the moist floor with the
wires in his hands the current is liable to pass through his body to
the ground. It is safer to use a dry board or rubber mat on the
concrete floor to stand on, or to wear rubbers.

§ 692. Municipal regulations for wiring, etc. — In addition to
the regulations of the National Board of Fire Underwriters, it
frequently happens that there are special regulations by the
municipality concerning the number and character of the general
lights in a theater, etc., and also the source of the electricity for the
arc lamp and for the general lights. There may also be special
regulations for the number and color of exit lights and the source
of the current for supplying them. It is necessary then to know,
not only the latest regulations of the National Fire Underwriters,
but the regulations of the city or state where the electric plant is
installed.

Soo

INSTALLATION OF ARC LAMPS

[Cn. XIII

INSTALLATION OF AN ARC LAMP FOR PROJECTION

§ 693. In the first place it is necessary to know the maximum
amperage to be used for the projection. The wiring, the fuses and
the ballast (rheostat, inductor, etc.) must be adapted to this
maximum current.

If the installation is adequate for the highest current that may
need to be used, it will of course be adequate for any smaller
current.

For drawing, and much of the ordinary magic lantern work, the
current varies from 5 to 15 amperes, and if the installation were
for such work alone, wiring and accessory apparatus which is safe
for 15 amperes would suffice. If, on the other hand, the line were
to be used for large halls also, and especially for opaque projection
(Ch. VII), then the wiring and accessory apparatus would reed to
have a maximum capacity of 50 to 60 amperes. For moving pic-
tures, the line should safely carry a maximum of 75 amperes, or
finally if kinemacolor moving pictures are to be shown in a large
hall, the wiring and accessory apparatus must be adapted for an
amperage of 100 to 200.

The size of solid wires for different currents is given in the follow-
ing table :

§ 694. Table of allowable carrying capacity of single copper
wires of 98% conductivity.*

AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS

No. Brown and
Sharp Gauge

Diameter in
Millimeters

Diameter in
inches

Circular
Mils

With Rubber
Insulation
Amperes

With other
Insulation
Amperes

No. I

7.248

.289

83,690

107

156

No. 2

6-543

•257

66,370

131

No. 3

5.826

.229

52,630

No. 4

5.189

.204

41,740

No. 5

4.620

.182

33,100

No. 6

4-U5

.162

26,250

No. 8

3.264

.128

16,510

No. 10

2.588

.IO2

10,380

No. 12

2.053

.O8l

6,530

No. 14

1.627

.064

4,107

No. 16

1.291

.051

2,583

No. 18

I.O24

.040

1,624

*From the 1913 National Electrical Code, § 18, pp 32-33.

CH. XIII]

INSTALLATION OF ARC LAMPS

501

§ 694a. The carrying capacity of the different wires in this table is the
amperage which can be safely and continuously carried by the wires without
injury to the insulation or to the wire. The rubber covered wire is capable of
carrying as great an amperage as the wires with more resistant insulation, but
the amperage given, is that which experience has shown can be carried without
undue injury to the rubber insulation, and with entire safety in continuous use.

Furthermore, it should be said that the carrying capacity given in the table
is by no means the maximum capacity which the wire could carry. For exam-
ple, one might send a current of 20 amperes through a No. 18 wire, but this
would soon injure the insulation from the overheating. By following the
Electrical Code, one is on the safe side.

§ 695. Table of allowable carrying capacity of
cables and cords composed of several small wires.

flexible

B & S Gauge

No. of Wire

Number of
wires

Rubber Insulation
Amperes

No. 18
No. 1 8
No. 18

7
19
61

25
50
1 2O

No. 1 6
No. 1 6
No. 16

7
19
61

35
70
170

No. 14
No. 14

61
91

235
320

ESTIMATED CARRYING CAPACITY

No. 32
No. 32

40
80

No. 30

15
30

3
6

1913 National Electrical Code, § 94, p. 186-187.

§ 695a. This estimate is based upon the law that "The conductivity of a
wire is directly proportional to its sectional area." Thus, No. 30 wire has a
diameter of .01003 m- and an area in circular mils of 010.03 x 010.03 = 100.6.
The area in circular mils of No. 1 8 wire is 1624 The allowable carrying capa-
city of No. 1 8 wire is three amperes when there is rubber insulation (see table
above). Assuming the same proportional carrying capacity for the No. 30 wire

then its capacity would be l624 = IO0'6, whence I624X =301.8 andX = .18

3 X

amp. If one small wire can carry .18 ampere, 15 should carry .18 x 15 = 2.7
amperes or in round numbers, 3 amperes. If both cords are united into one
conductor there would be 30 small wires with the capacity of .18 x 30 = 5.4
amps, or 6 amperes in round numbers.

For No. 32 wire in the same way: Thus, No. 32 wire has a diameter of
.00795 in. The circular mils = 7.95 x 7.95 = 63.21 for each wire.

502 INSTALLATION OF ARC LAMPS [Cn. XIII

§ 696. Selection of material for installing the arc lamp. — After
determining the maximum amount of current needed for the arc
lamp, then the wire of proper size and quality and insulation to
conform with the National Electrical Code should be obtained.
The simplest way to do this is to go to some reliable dealer in elec-
trical supplies and get the standard material.'

Standard switches, etc., are all marked plainly so that there is no
difficulty in selecting the correct sizes. In America, wire is more
often designated by some standard wire gauge, e. g., that of Brown
&• Sharp, than by the actual diameter in millimeters or inches. In
the above table the sizes in millimeters and inches corresponding
with the B & S gauge numbers are given, also the area measured in
circular mils.

One must not forget that everything that is used wears out, and
when any piece of apparatus or the wire becomes deteriorated by
use it should be replaced.

WIRING FOR THE ARC LAMP, THE RHEOSTAT OR OTHER BALANCING
DEVICE, AND THE LAMP SWITCH

§ 697. Connection with the electric supply. — It is assumed that
the electric supply has been properly installed by an electric com-
pany, or from a private dynamo, to within a short distance of the
arc lamp. This supply will be in a proper outlet box, with fuses
and switches in accordance with the National Electrical Code. In
case the outlet box is on the wall close to the arc lamp, the simplest
and most convenient connection between the lamp switch and the
supply in the outlet box is by means of a separable attachment of
the proper capacity for the maximum current. (See the table of
flexible cables, § 695.) If the current is direct, then it is a conve-
nience to have this attachment irreversible, or polarized so that

For No. 18 wire, as before, the circular mils are 1624 and the relative carrying

capacity is assumed to be — — = ^'2I, whence X = .116 amperes. If there

3 X

are 40 wires in each cord then each cord should carry .1 16 x 40 = 4.64 amperes,
or in round numbers 5 amperes. If the double cord were used for each conduc-
tor to the lamp, then in like manner twice as much could be carried, as there
are 80 wires: .116 x 80 = 9.28 amperes or 10 amperes in round numbers.

CH. XIII]

INSTALLATION OF ARC LAMPS

503

one cannot make a wrong connection (fig. 268A). Such an attach-
ment would also serve for alternating current, but is unnecessary,
as it makes no difference which way the attachment is connected.

The conductor from the electric supply in the outlet box to the
lamp switch, if the distance is small, not over 2 to 3 meters (6 to 10
ft.), is most conveniently made of flexible cable of the proper
carrying capacity (see the table of carrying capacity of flexible

FIG. 268. SEPARABLE WALL RECEPTACLES, POLARIZED (A) AND

NON-POLARIZED (B).

(Cuts loaned by H. Hubbell, Inc.).

With direct current, a polarized attachment insures the same polarity with-
out attention on the part of the operator; with the non-polarized form there
is liability of reversing the polarity unless the connections are specially marked,
and care is taken in putting the separable cap in position. Either form can be
used with alternating current also.

electric cables). The two wires or cables are often enclosed in a
common sheath.

§ 698. In connecting the two wires to the attachment cap, the
insulation is removed for a short distance (i to 2 cm. y£ in.), the
wires scraped clean, twisted all together, and then turned to a loop
to surround the set screw. Great care must be taken to avoid
leaving any of the strands free; this would lessen the carrying
capacity, but more important still, they might become displaced
and make a short circuit (§688).

INSTALLATION OF ARC LAMPS

[CH. XIII

The wire is fixed firmly under the set screw, and if the current is
to be large, 30 amperes and more, the wire should be soldered to its
connection after the screw is firmly set down.

§ 699. Connecting the conductors to the switch. — This is done
exactly as for the attachment cap.

In case direct current is used it is important to know which is the
positive and which the negative wire. This should be determined
before clamping the wire to the switch. The best method is by the

FIG. 269. SEPARABLE ATTACHMENTS, POLARIZED (A) AND NON-POLARIZED

(BC).

(Cuts loaned by H. Hubbell. Inc.).

The attachments A and B are for the ordinary bulb socket.

A is polarized so that the same polarity of the wires is insured, for the connec-
tion cannot be reversed.

B is non-polarized and the polarity may be reversed every time the connec-
tion is made.

C is for receiving an incandescent lamp ; connection is made with the supply
by inserting the prongs into an attachment plug which has been screwed into a
lamp socket.

use of the arc lamp (§ 702), after the arc lamp and rheostat have
been properly connected.

§ 700. Wiring the arc lamp, including the rheostat or other
balancing device. — From one pole of the switch (fig. 270), a wire
of the proper size and insulation is carried directly to the
negative binding post of the lamp, i. e., to the post for the lower
carbon. From the other pole of the switch a suitable wire is
carried to one binding post of the rheostat. From the other bind-

CH. XIII]

INSTALLATION OF ARC LAMPS

505

FIG. 270. WIRING OF THE ARC LAMP FOR PROJECTION.
For full explanation, see fig. 3 and fig. 40.

ing post of the rheostat a suitable wire is carried to the positive
binding post of the arc lamp, that is to the binding post for the
upper carbon. This puts the rheostat, or other balancing device
in one wire, or in series, not in parallel, or across both the wires of
the circuit.

In securing the ends of the wires to the binding posts, scrape
them, and twist the strands, then make a loop and put under the
binding screw of the switch as described for the attachment cap.
Usually for the rheostat, and the arc lamp, the wires are twisted
and kept straight, then inserted into a hole, and a set screw turned
down upon them.

If flexible cord or cables are used for these connections, the wires
on the end, after being scraped clean should be twisted and
soldered, then none of the strands will escape to lessen the carrying
capacity, or possibly to make a short circuit.

5o6 POLARITY TESTS [Cn. XIII

DETERMINING THE POLARITY WITH DIRECT CURRENT

§ 701. General statement and precautions. — With direct
current it is necessary, in most cases, to install the apparatus, like
the ammeter, the voltmeter, the lamp, etc., in a very definite man-
ner so that the current extends through the instrument in a given
direction. That is, the positive end of the wire must be attached
to the positive binding post. But when ready to install any piece
of apparatus with direct current one rarely knows which is the
positive and which the negative wire. It is necessary to find out
by experiment.

Precautions in making polarity tests. — If possible, have a rheo-
stat in the circuit before making the tests. One of the small
rheostats for use with the small current arc lamp can be very easily
introduced into the circuit (see fig. 188, 270 for wiring). If an
adjustable rheostat is already in the circuit, set it for the least
current.

In making the tests never allow two naked wires to come in
contact for that would complete the circuit and might burn out a
fuse or do something worse.

Never use a piece of metal, or a metal dish for holding the testing
materials. Always use glass, porcelain or wood or some other
non-conducting material. The tests are perfectly definite and safe
if applied with due care.

Remember also that when repair work on the line is done, the
polarity of the supply wires may be changed. This would of course
change the polarity of the arc lamp and a good light could not be
obtained. One must be on the lookout for every possible trouble
and have the knowledge and the resourcefulness to make the neces-
sary modifications.

DETERMINING THE POLARITY WITH AN ARC LAMP, WITH A
VOLTMETER OR AN AMMETER

§ 702. (A) If an arc lamp and rheostat are available the
simplest test is to connect the arc lamp, large or small, and rheostat
as directed above (§ 700). With proper carbons in place turn on

CH. XIII]

POLARITY TESTS

507

the current and strike the arc. After the lamp has burned a
minute or two open the switch or pull the separable plug apart and
watch the ends of the carbons. The one that remains red-hot the
longer is the positive one, and the wire leading to it is the positive

FIG. 271.

SIDE AND FRONT VIEWS OF THE RIGHT-ANGLE CARBON ARC WITH
CORRECT AND INCORRECT POLARITY.

A The upper figures show the correct polarity, that is, with the positive
crater on the upper carbon.

B The lower figures show reversed polarity, that is, with the lower carbon
positive and hence the large crater on it.

The photographs were made with a color screen in order to bring out the posi-
tive and the negative craters with the greatest clearness. The exposure for
the craters was instantaneous, then there was an additional exposure of 90
seconds without a color screen, and with an illumination from a mazda lamp
to bring out the carbons and give the appearance seen by the human eye (see
also fig. 292-293).

So8 POLARITY TESTS [CH. XIII

wire. The method in practice is to watch the burning carbons
through smoked glass or smoky mica. The positive one is markedly
brighter than the negative one (fig. 271).

If the upper carbon is positive the lamp is correctly installed, if
the lower carbon is positive then it is improperly installed for
ordinary projection. If the positive wire goes to the lower carbon,
turn off the light by opening the switch or pulling the separable
plug apart. Now reverse the position of the wires in the binding
posts of the lamp, and this will bring the positive wire in connection
with the upper carbon, and the negative wire in connection with the
lower carbon (fig. 2, 270).

If a non-polarized separable plug is used (fig. 268 B), the simplest
way to reverse the polarity is to pull the cap off, turn it half way
round and insert it again. When found to be in the correct posi-
tion mark the socket, the plug and the cap in some way so that the
connections can be made at some future time with certainty.
There are polarized plugs (fig. 268A) in which the connections are
so arranged that the attachment plug can be inserted only in one
way, thus avoiding the change of polarity when once the wiring is
correctly installed.

When the polarity is found to be correct it is advantageous for
future work to mark the insulation of the positive wire near the
switch with red paint. The positive side of the table switch
should also be marked with a + sign or with P. using black or red
paint. In like manner the insulation of the wire near where it is
connected with the binding post of the arc lamp should be marked
red, and a + or P. should be put alongside the binding post for the
upper carbon unless it is so evident that no mistake is likely to
occur.

(B) Testing the polarity with a direct current voltmeter —
To do this connect the voltmeter with both wires (fig. 272).
Turn on the current by closing the switch and if the positive wire
is connected with the positive binding post the voltmeter will
record the voltage in the line. If the wires are wrongly connected
then the hand will try to move off the dial face below zero. If the
hand does not register the voltage, open the switch, and reverse the

CH. XIII]

POLARITY TESTS

509

position of the wires in the binding posts of the voltmeter. Turn
on the current and the voltmeter will register. It is well to mark
the insulation of the positive wire with red, or in some .other way.

FIG. 272. VOLTMETER FOR TESTING POLARITY.

G Dynamo for direct current. The positive pole is above and the negative
pole is below, as indicated by the arrows.

Vm The terminals of the voltmeter are correctly connected across the line
(in multiple) or to both wires and the hand indicates the voltage on the dial.
If the terminals were wrongly connected the hand would not register.

A Arc lamp.

R Rheostat.

The arrows indicate the direction of the current.

The -}- and — signs indicate that any point in the circuit nearer the positive
pole of the dynamo is positive to any point nearer the negative pole.

FIG. 273. AMMETER FOR TESTING POLARITY.

G Dynamo for direct current. The positive pole is above and the negative
pole below.

Am Direct current ammeter. The terminals a +, — b are connected along
one wire (in series). If the positive pole of the ammeter is connected to the
circuit next the positive pole of the dynamo, and the negative terminal in the
wire toward the negative pole of the dynamo, as here shown, the hand will
register when there is current flowing. If the connections are reversed the
hand will not register when the current is flowing.

a+, — b The positive and the negative terminals of the ammeter.

A Arc lamp.

c+ The positive carbon.

c — The negative carbon (the minus sign is put parallel with the carbon to
show the direction of the current).

R Rheostat.

The + and — signs and the arrows are as with the voltmeter (fig. 272).

5io POLARITY TESTS [Cn. XIII

(C) Testing the polarity with a direct current ammeter —
The circuit should be connected with a rheostat and an arc lamp
or one or more incandescent lamps in series (along one wire) then
the switch is opened and the ammeter is inserted in one wire (in
series), (fig. 273). Now turn on the current and light the lamp
(§ 30). If the wires are correctly connected the ammeter will
indicate the amount of current flowing; if it is wrongly connected
then the hand will try to move off the dial below zero. That is, the
positive wire has been inserted in the negative binding post of the
ammeter, and the negative wire in the positive binding post.
Open the switch, and reverse the position of the wires in the binding
posts ; turn on the current and the hand will register the amperage.
The positive wire can then be marked red or in some other way.

CHEMICAL POLARITY INDICATORS

§ 703. Litmus, iodized starch, salt solution and potato indica-
tors.— (A) Litmus indicator. — Take some blue litmus or other
acid-alkaline testing paper, about 10 cm. (4 in.) long and place it
on a pane of glass or a porcelain plate. Moisten it well. Separate
the ends of the wires as indicated in the testing lamp (fig. 21).
Put the two ends about 10 centimeters (4 in.) apart on the mois-
tened litmus paper. Turn on the current. The positive wire will
turn the blue litmus paper red when the current flows. Turn
off the current and mark the positive conductor red, or white.

(B) Iodized starch polarity indicator. — Make some starch paste
by mixing 15 grams (y£ oz.) of dry starch (corn starch, laundry
starch or wheat flour) with 300 cc. (10 oz.) of cold water. Add y£
gram (7 or 8 grains) of iodide of potassium. Now heat the mixture
with constant stirring until the starch is cooked. Put some of the
iodized paste in a glass or porcelain dish and insert the separated
wires to be tested in the paste. Turn on the current and the starch
at the positive pole will be turned blue. Turn off the current and
mark the positive wire in some way. (The iodized starch test is

§ 702a. If one uses a voltmeter or an ammeter of the new, soft-core type
(Eclipse Volt — and Ammeters) which register both alternating and direct cur-
rent, one cannot determine polarity with them, for they register whichever
way they are connected with the circuit.

CH. XIII]

POLARITY TESTS

the one commonly employed for weak currents for it is very
sensitive; it is, however, equally good for large currents).

(C) Salt and water polarity indicator. — Make a y£% solution
of common salt (NaCl) in water. Place the solution in a glass or
porcelain dish about 10 cm. (4 in.) across. Insert the two separ-
ated wires to be tested in the liquid and turn on the current. When
the current is on, many small bubbles will appear at the negative
pole. In making this test remember the precautions (§ 701).

(D) Raw potato polarity indicator. — Cut an ordinary uncooked
potato in half. Insert the wires into the potato having the wires
as far apart as possible. Turn on the current. The potato
around the positive pole will turn greenish. If the potato is quite

FIG. 274. THREE-WIRE ARC LAMP OF THE BAUSCH & LOME OPTICAL COMPANY
For a full explanation see fig. 14.5 and § 704.

512 WIRING FOR ALTERNATING CURRENT [Cn. XIII

moist, many small bubbles will appear around the negative pole.
But the greenish color given at the positive pole is the most certain.
Turn off the current and mark the positive wire red.

With the other chemical tests (A, B, C) the indications are in no
way dependent on the metal forming the wire, but with the potato
test the poles entering the potato must be copper or contain
copper.

§ 704. Wiring the three-wire automatic lamp of the Bausch &
Lomb Optical Company. — This lamp is regulated wholly by elec-
tricity, there being no clock-work. In wiring the lamp one pro-
ceeds exactly as described above (§ 693-700), except that a wire
is carried from the positive side of the switch to the middle binding
post of the lamp directly. Another wire from the same point is
carried down to the resistor or rheostat, and from the rheostat a
wire to the positive or upper binding post of the lamp. From the
negative pole of the switch a wire is. carried directly to the lower
or negative binding post of the lamp. This wiring gives the full
voltage of the line for the electric mechanism governing the lamp
(see fig. 145).

WIRING FOR ALTERNATING CURRENT

§ 705. This is precisely as for direct current, and one does not
have any trouble about the polarity. It makes no difference
which supply wire is connected with the upper carbon and which
with the lower.

§ 706. Insertion of the rheostat, inductor or other balancing
device. — It makes no difference in which of the lead wires the
rheostat, etc., are inserted. Just as with direct current, however,
the balancing device must be inserted along one wire (fig. 281),
otherwise the current would not traverse the entire circuit.

§ 707. Position of the rheostat, etc. — The balancing effect of
the rheostat is the same no matter where it is installed in the special
circuit for the arc lamp. For convenience it is frequently put on
or near the projection table. This is especially necessary if the
rheostat is adjustable. With a fixed rheostat it is sometimes safer

CH. XIII] WIRING FOR ALTERNATING CURRENT 513

to put it near the supply intake, especially if that is at a consider-
able distance from the lantern or other projection apparatus, then
in case of a short circuit in working about the lamp or table switch,
an excessive current could not flow, and there would be much less
danger from fire or the burning out of fuses. (See also § 708).

§ 708. Wiring when the arc lamp is far from the supply. —

When the supply is at a considerable distance from the arc lamp
the flexible wire connection is sometimes used for temporary work,
but is not suitable for permanent installation.

Instead of a conduit, well insulated wires are sometimes used
from the general supply box to the neighborhood of the arc lamp.
The wires must be secured by porcelain or other non-conducting
supports every meter (3 or 4 feet) which will separate them from
the wall i to 2 cm. ($4 in.) and from each other 5 to 7 cm. (2^/2 in.)
and hold them in place. Where the wires pass through partitions,
each wire should have its own porcelain tube so that is does not
come in contact with the partition. The safe rule in wiring is to
treat the rubber covered wires as if they were naked. At the end
it is desirable to have a metal box for the special fuse block and
switch. An attachment fixture is also very convenient (fig. 270).

For the position of the rheostat, etc"., see § 707.

§ 709. Wiring an arc lamp for large currents. — Arc lamps for
opaque projection (Ch. VII) and for moving pictures (Ch. XI)
require large amperages, and frequently the lamps become very
hot, especially if the lamp-house is not large and well ventilated.
For lamps requiring the large currents it is best to use flexible
cables of higher capacity than is needed outside the lamp-house.
The wire should also be insulated with some fire-proof material like
woven asbestos.

The ordinary, rubber insulation will answer for low amperages
especially when the lamp-house is well ventilated. An excellent
wiring material is the flexible cord used for heating apparatus.
This has rubber insulation, and also woven asbestos, and the
outside is covered with, woven cotton to protect the asbestos. Of
course a flexible cord of the proper carrying capacity should be
selected.

SU SWITCHES, FUSES, CIRCUIT BREAKERS [Cn. XIII

If it is difficult to get double cord of the right size, then each of
the wires to the lamp can be composed of the double cable. This
is easily done by removing the insulation at each end of the double
cord and twisting both the wires together. (See the tables § 694,
695, for the carrying capacity of flexible cord and cables).

§ 710. Wiring the arc lamp with a three-wire supply. — Only
two wires go to the arc lamp, then if one must connect the arc lamp
for projection to a three-wire supply system it is necessary to
remember that the middle (neutral) wire and either outer wire will
give 1 10 volts the same as the two-wire no volt circuit.

If connection is made with the two outer wires then 220 volts
will be used in the arc lamp. In this case a rheostat for a 220 volt
circuit must be employed, or two no volt rheostats in series (fig.
287).

Naturally one would connect with the middle or neutral and an
outside wire and employ the usual no volt rheostat but for the
fact that such an arrangement would badly unbalance the work of
the line, and might cause trouble if the electric circuit was running
nearly on full load. It is therefore safer to connect with the out-
side wires and use the requisite amount of ballast.

SWITCHES, CIRCUIT BREAKERS AND FUSES; THEIR CHARACTER,

INSTALLATION AND USE

§ 711. A switch is a device by means of which a gap (fig. 275
and 276) can be made in an electric circuit thus stopping the flow

of current.

A switch should be so con-
structed that when it is opened
\ it makes a gap in all the wires
B of the circuit. For example, in
J a two-wire circuit, the switch
should make a gap in both wires,

FIG. 275. CIRCUIT WITH A BREAK and in a three-wire circuit, a gap
OR GAP. 11 .-, • rf

.. . it in all three wires. If such a
Unless the metalhe circuit, irom the

dynamo, G, back to the dynamo, is switch IS used the line beyond
complete, no current will flow. A gap the switch is "dead, "and no CUr-
m the circuit (B) prevents the now ot
current. rent can be drawn from it.

CH. XIII] SWITCHES, FUSES, CIRCUIT BREAKERS

SIS

FIG. 276. SNAP AND KNIFE SWITCHES SHOWING OPEN AND CLOSED CIRCUIT.

A Snap switch with circuit closed (current on).

B Knife switch with circuit closed (current on).

C Snap switch with circuit open (current off).

D Knife switch with circuit open (current off).

A W Wires from the switch to the arc lamp.

Base The insulating support of the knife switch.

H Handle of the switch blades.

L W Supply wires for the electric current to the switch.

There are two main forms of switches: The knife switch like
that shown in fig. 276 B, D, and the snap switch, which rotates
(fig. 276 A, C). Any switch to be installed should conform in its
construction with the National Electric Code and be plainly marked
with its capacity — -voltage and amperage — and the maker's name.

§ 712. Installation of a switch. — The non-combustible, non-
conducting base should be fastened to some support, and then the
wires of the line cut and scraped and connected firmly in the bind-
ing posts or under the binding screws. If the current is over 30
amperes the wires should also be soldered to the switch after the
screws are well set down. A switch at the supply for the building

SWITCHES, FUSES, CIRCUIT BREAKERS

[CH. XIII

or special plant should be enclosed in a metal box where it can be
easily got at, but not where the naked metal parts might inad-
vertently become short-circuited.

It is necessary also to put the switch in such a position that
when it is opened it will not close of itself by gravity. If the
switch is in a vertical position it must be placed with the hinge
below, so that gravity will tend to open it, never to close it (fig.
277).

If the switch is horizontal, then the hinge should be tight enough
so that the blades will remain in any position in which they are
placed. For a double-pole, double-throw switch for two lamps see
fig. 162.

A knife switch has an appreciable amount of naked metal
exposed. It therefore makes a short circuit easily possible. For
use with projection apparatus, especially if high amperages are to
be used as with opaque projection and with moving pictures it is

Base

FIG. 277. OPEN KNIFE SWITCH IN A VERTICAL

POSITION, WITH THE HANDLE BELOW so THAT

THERE IS NO DANGER OF THE SWITCH

CLOSING BY GRAVITY.

L W Line wires from the electric supply (fig.
270) to the switch.

A W Arc lamp wires from the switch to
the arc lamp. A rheostat is inserted in one of
them (fig. 270).

5 C Spring clamps pressing against the switch
blades when the switch is closed, thus making
good metallic contact.

Base The insulating base of the switch. It
is held in position by two or more screws.

Hg Hinges of the switch blades.

5 B Switch blades. When the switch is
closed these blades make a continuous circuit,
and when the switch is open the circuit is
broken.

Cb Cross-bar of insulating material to
which the switch blades and the handle are
attached.

H Handle for opening and closing the switch.
It is of insulating material

CH. XIII]

SWITCHES, FUSES, CIRCUIT BREAKERS

517

advantageous to enclose the switch in a metal box with a slit
allowing the handle to project and move so that the switch can be
opened and closed. As only the handle is exposed with this arrang-
ment the operator is safe when manipulating the switch in the dark
(fig. 278). See also § 714.

§ 713. End of the switch to connect with the supply wires. —

Sometimes the supply wires are connected with the hinge end of the
switch as in fig. 2. This has the disadvantage that the switch is
then energized up to the break at the handle, when the main supply
is on. As the switch is liable to get out of order and need screwing
up occasionally it is better to insert the lead wires in the opposite or

FIG. 278. ENCLOSED SWITCH IN A HORIZONTAL POSITION.

Commencing at the right:

L W Supply or line wires from the outlet box (fig. 270) to the table switch.

k Key for locking the metal cover when it is closed.

H Handle of the knife switch. It projects through the slot (5) in the cover.
In the position shown the switch is open.

sb Switch box. This is a sheet iron box enclosing the switch so that noth-
ing can come in contact with the naked metal of the switch. Only the switch
handle projects beyond the box. The enclosing box is represented as trans-
parent in order to show the switch and its connecting wires within. The bot-
tom of the enclosing box is covered with asbestos board and the switch base
rests on the asbestos, not on the metal of the box.

hg Hinge of the metal cover. By turning the cover over to the left the
entire switch is exposed.

A W Wires from the switch to the arc lamp.

5i8 SWITCHES, FUSES, CIRCUIT BREAKERS [Cn. XIII

jaw end of the switch as in fig. 277, then when the switch is open
the hinges and blades are "dead" and can be put in order with
safety.

§ 714. Snap Switches. — These are sometimes used for turning
on and off the current at the operating table. They are mounted
on insulating material like porcelain, and are enclosed by a metal
covering which is lined with insulating material. The key or
button for turning on and off the current is also of insulating
material. This form of a switch around the work table is con-
venient, and avoids any danger of accidentally short-circuiting the
line. It should turn on the current and turn it off with a snap. It
is also desirable that there should be a sign indicating when the
current is on or off, as one cannot see directly as with the knife
switch. If such a switch is used, make sure that it is of the right
capacity for the maximum current and that it conforms in every
way with the standard requirements. It will be plainly marked so
that after it is installed one can see at any time the current and
voltage for which it is designed. Snap switches are better adapted
for small currents, than for large ones. Knife switches are to be
used on lines with large currents.

FUSES AND CIRCUIT BREAKERS

§ 715. Fuses and circuit breakers are devices for opening or
breaking the circuit whenever the current in any particular situa-
tion becomes too great. For example, if a part of the line should
be short-circuited.

The devices used are of two kinds ; fuses, and magnetic cut-outs
or circuit breakers.

§ 716. Circuit breakers. — The circuit breaker is a device by
which a magnetic trip releases a catch which allows a large switch
to open, thus breaking the circuit.

The great advantage of a circuit breaker is that nothing is
burned out or melted. It is only necessary to close the switch
again and the current will be on. It acts instantly whenever the
current rises above the amperage for which it is adjusted.

CH. XIII] SWITCHES, FUSES, CIRCUIT BREAKERS 519

§ 717. Fuses.— A fuse is a wire of low melting point forming
part of the circuit. If the current becomes too great this fuse is
melted, thus making a gap in the line. The fuse is then said to
burn out or to "blow." If the current becomes much too great as
in a short circuit the fuse will "blow" instantly, if however, the
current is only slightly larger than the fuse is designed for— as for
example, when striking the arc in an arc lamp — the fuse will not
"blow" instantly, and if the overload is only for a short time it will
not melt at all. If the overload continues for some time, however,
the fuse will get hotter and hotter until its melting point is reached,
when it will melt and open the circuit. This property of the fuse is
of great advantage when using arc lamps, for the temporary over-
load in lighting the arc lamp is unavoidable.

§ 718. Location and installation of fuses. — Like the switch, the
fuses should be placed in the path of all the wires of a circuit — i. e.,
with a two-wire system two fuses, and with a three-wire system,
three fuses, etc. The wiring of a fuse block is the same as for a
switch (§ 712).

There is always a switch in the supply box from the electric
lighting system or from the private dynamo. In this box are also
fuses to open the circuit in case of accidental short-circuiting. The
fuse block, whether for cartridge fuses or for plug fuses should be
selected with care to make sure that it is of the right capacity
for the maximum current and conforms to the standard code. The
fuses are plainly marked, so there need be no mistake.

One should not use fuses of higher capacity than the line was
designed for, for fear of fire or other accident.

If the supply box is some distance from the arc lamp, many
careful operators have fuses as well as a switch at the supplemen-
tary supply box in the operating room, when a conduit or fixed
wires are carried from the main supply to the operating room. The
fuse nearest the arc lamp is preferably of somewhat less capacity
than the ones farther away, then if a fuse is blown it will be the
handiest one to renew.

§ 719. Fuses and the wattmeter. — If but a single meter is used
to measure the current for arc lights, house lights, heating appara-

520 SWITCHES, FUSES, CIRCUIT BREAKERS [Cn. XIII

tus, etc., then each group should be separately fused after the
wattmeter, for then if one part of the line is cut out the rest can go
on drawing current. For example, if the arc lamp were misman-
aged it ought not to be possible to blow out the fuse for the house
lights, and the reverse.

§ 720. Location of fuse blocks. — The general rule is that there
must be a fuse block wherever there is a change in the size of the
wire used. These fuse blocks must be in cabinets in plain sight and
readily accessible. Usually also, with every fuse block there is a
knife switch.

§ 721. Capacity of fuses. — The rated capacity of fuses should
not exceed the allowable carrying capacity of the conducting wire
(see tables § 694, 695), and circuit breakers should not be set more
than 30% above that allowable capacity.

The allowable capacities for incandescent lamp lines are as
follows :

55 volts or less 12 amperes

55-125 volts 6 amperes

1 25-250 volts 3 amperes

For electric lighting each special circuit or line should not be used
for a current greater than will give a power of 660 watts. This
would mean for example, that if one wished to use 60 watt lamps
there could be only 1 1 of the lamps on a single line. If 40 watt
lamps were used then there might be as many as 16 lamps on a line,
etc.

In using flat-irons and other heating devices on an electric lamp
circuit, care must be exercised not to turn on any lights on that
branch of the circuit.

Likewise in using the small arc lamp for drawing with the micro-
scope, ultra-microscopy, etc., where from four to six amperes of
current are needed, one should not use incandescent lights on
that line at the same time, for the current would exceed the allow-
able amount and probably blow a fuse.

§ 722. Replacement of fuses. — As fuses are liable to blow out
it is well to have a supply on hand, then the burnt out ones can be

CH. XIII] RHEOSTATS AND OTHER BALLAST 521

quickly replaced. To replace a fuse, open the nearest switch which
will turn off the current from the line. Take out both fuses, and
examine them; only one is likely to have melted. It is usually easy
to tell which. Discard that one, then insert two good fuses of the
proper capacity, close the switch, and the current will be available
again.

If the lights on a particular line go out from the blowing of a fuse,
and one is not sure which branch it is in the fuse box, the one is
easily found by using the testing lamp (fig. 21) beyond the fuses.
The lamp will light on all the lines with perfect fuses when put
across the blades of the special line switch, or when put in contact
with any naked metal part across the line. The line with a burned
out fuse will not light the testing lamp, when it is applied beyond
the fuse.

RESISTORS OR RHEOSTATS: INSTALLATION AND USE

§ 723. Resistor or rheostat. — A rheostat is a conductor having
considerable resistance ; it is placed in an electric circuit to regulate
the amount of current. In passing through the rheostat much heat
is developed by the energy consumed in overcoming the resistance.
This energy consumption is a dead loss.

The conductor used is ordinarily in the form of wire or strips of
metal such as German silver, iron or nickel.

§ 724. Amount of resistance needed. — Electricians have
worked out with much accuracy the resistance of different metals
and by consulting the tables furnished in books on electrical
engineering one can find how great a length of a given size iron
or German silver wire is necessary to afford the proper resistance
for any given constant voltage, as no or 220. See §

§ 724a. Ohm's Law and its application to projection apparatus. — While the
units, volt, ampere and ohm (§ 654-657) might be worth defining, still it would
lead to no very practical results unless there was a definite relation between the
electric quantities for which these units stand.

It has been found by experiment that there is a very definite relationship,
known as Ohm's Law. (For a history of the discovery of this law by Ohm, see
Dr. Shedd in the Popular Science Monthly for Dec., 1913).

Briefly stated Ohm's law is: "The current in a given circuit is directly pro-
portional to the electromotive force, and inversely as the resistance:"
Nichols, p. 294.

522 RHEOSTATS AND OTHER BALLAST [Cn. XIII

As stated by Norris it is: "The electromotive force consumed in the
resistance of a conductor, is proportional to the current." P. 8.

Using the terms now employed in place of electromotive force (voltage),
resistance (ohmage) , and current (amperage) , the law can be stated thus :

(1) The voltage in a conductor is equal to the amperage multiplied by the
ohmage: V = A O.

(2) The amperage is equal to the voltage divided by the ohmage: A = —

(3) The ohmage is equal to the voltage divided by the amperage: O = —

As V = A O = i . From this form is derived the very simple dia-

A O

gram used practically in getting the formula for the value of any single quantity
if two are known. The formula for the unknown quantity is found thus :

Cover the letter representing the unknown
V quantity, and the remaining letters will indicate

the value of the unknown quantity.

" Examples:

FIG. 279. DIAGRAM OF i. If the voltage and amperage are known,

OHM'S LAW FOR SOLV- what is the ohmage?

ING PROBLEMS (§ 7243). Cover the O and there remain V/A and this is

V = Voltage equal to O, i. e., O = V/A. Suppose the volt-

A _ . age is 1 10 and the amperage is 20, what is the

O = Ohma e ohmage? Applying the formula, O = 1 10/20,

2. If the voltage and the ohmage are known what is the amperage? Here
if A is covered there is left V/O, whence the amperage equals the voltage divided
by the ohmage. If the voltage is 220 and the ohmage is 5.5 as before, what is
the amperage? A = 220/5.5 =40 amperes. This example also illustrates the
fact that if the ohmage remains constant the amperage will increase in direct
proportion to the voltage. (See Dr. Nichols' definition above).

3. If the amperage and ohmage are known what is the voltage? Here the
unknown quantity is represented by V. If this is covered there will be left
A O, whence V = A O. If the amperage is 15, and the ohmage 8 then the
voltage must be 15 x 8 = 120, i. e., V = 120 volts.

As a further example suppose one wished to make a water-cooled rheostat
(fig. 283) and he had some wire which had an ohmage or resistance of 0.25 ohm
per meter, how much wire would be needed with a voltage of no and an
amperage of 15? Here voltage and amperage are known. From the formula
it is seen that ohmage equals voltage divided by amperage: whence 1 10/15 =
7.33 ohms, the total resistance required.

Now as 55 is the voltage required by the arc with the direct current arc

lamp, the lamp itself must offer a resistance of . ~ f°r 3-66 ohms.

A = 15

As the total ohmage needed is 7.33, the rheostat must possess the difference
between 7.33 and 3.66 or 3.67 ohms.

If each meter of the wire to be used offers a resistance of 0.25 ohm, it will

require for 3.67 ohms, ^' ? = 14.68 meters of the wire for the rheostat. (For

0.25
the wattage of the current see § 660).

CH. XIII] RHEOSTATS AND OTHER BALLAST 523

§ 725. Getting rid of the heat developed. — As much heat is
developed in the rheostat, it is necessary to so arrange the coils of
wire, etc., forming it, that the heat can easily escape, otherwise the
wire might get so hot that it would melt. Provision for carrying
away the heat then is of prime importance. For example, a large
iron telegraph wire would get red hot in the air if it were used for
100 amperes, while a much smaller wire if immersed in water would
carry the current easily on account of the rapid dissipation of the
heat in the water.

Ordinarily the resistance wire is in coils, and these are hung on
non-conductors in such a way that there is free circulation of air
around and through the coils to carry off the heat.

Sometimes the wire or strips of metal serving for the resistance
are imbedded in porcelain, and a considerable surface of the porce-
lain being exposed to the air, the heat readily escapes. This is
often the method with the rheostats used for dimming the lights in
theaters (theater dimmers). (See fig. 183, 186 for a theater
dimmer used as a rheostat).

In fig. 198 is a small rheostat with the metal in a helical coil and
wound around a porcelain core. This rheostat is for the small arc
lamp to be used on the house lighting system, and restricts the
current to 4-6 amperes.

§ 726. Fixed rheostat. — This is a rheostat in which the entire
amount of resistance wire must be traversed whenever the current
is on, the amperage of the current is then practically constant.
For example in using the arc lamp if the rheostat is designed for 1 5
amperes, that current must always be used. The fixed rheostat is

best adapted for any place where
many use the same apparatus
(fig. 280).

§ 727. Adjustable rheostat.—
The adjustment consists of an
arrangement by which a greater

or less length of the resistance
FIG. 280. CIRCUIT WITH DYNAMO (G) • , . , j , • ,,

ARC LAMP (A), AND FIXED RHEO- Wlre can be included in the cir-
STAT (R). cuit at will. The more resis-

524 RHEOSTATS AND OTHER BALLAST [Cn. XIII

tance in the circuit the less will be the amperage, and the less resis-
tance the higher the amperage.

In some forms it is possible to have a great range of current, say
from 5 to 45 amperes (fig. 281); in other forms the range may be
limited, say from 1 5-2 5 amperes.

For the projection microscope and the magic lantern it is desir-
able to have a rheostat giving a range of amperage from 5 to 25

FIG. 281. THE USE OF AN ADJUSTABLE RHEOSTAT AS BALLAST FOR AN ARC

LAMP

G Generator (dynamo).

A Arc lamp with right-angle carbons.

AR Adjustable rheostat.

5 If the movable contact-arm is at 5, the resistance allows but 5 amperes to
flow.

25 If the contact-arm is at 25 then only half of the resistance is in the cir-
cuit and 25 amperes of current can flow.

45 If the contact-arm is opposite 45, only a small amount of resistance is in
the circuit and forty-five amperes of current is allowed to flow.

The arrow indicates the direction to turn the contact-arm to increase the
current.

amperes. Such a rheostat is not difficult to construct, nor is it
expensive. The theater dimmer shown in fig. 183 is of this range.
But an adjustable rheostat requires judgment for its proper use;
for apparatus to be used by everybody it is better to have a fixed
rheostat (§ 726).

§ 728. Installing the rheostat. — It is usually placed close to the
arc lamp, i. e., inside the lamp switch, so that when the lamp switch
is open the current is entirely off the arc lamp and its rheostat.

CH. XIII] RHEOSTATS AND OTHER BALLAST 525

In wiring the rheostat, it is to be placed in one wire, (in series) as
all the current must pass through it (fig. 188, 281). It makes no
difference whether it is placed in the wire going to the upper carbon
or coming from the lower carbon.

§ 729. Calibration of a rheostat. — The maker of a rheostat
should mark plainly upon it its capacity if it is of the fixed form.
If it is adjustable, then the range of the rheostat should be given.

Furthermore, the lower range should be plainly marked at the
lowest step and the highest range at the highest step. The user of
a rheostat like that in fig. 145 could not tell easily which way to
turn the knob to increase or diminish the current unless the maker
indicates the amperage at the two ends of the steps. In case there
is no indication a person can determine it for himself if he has an
ammeter.

Insert the ammeter in one wire of the line (fig. 273). Turn the
knob of the rheostat to the middle step, insert proper carbons in the
arc lamp, and turn on the current. When the lamp is burning
properly note the reading on the ammeter. Turn the knob toward
one side and the ammeter will indicate whether there is more or less
current. One can in this way find the amount of current delivered
for the different positions. It is well to mark on the rheostat face
with white paint the amperages corresponding to these positions.
It is also a help to have an arrow pointing from the lowest to the
highest amperage (fig. 182, 281).

§ 730. Home-made rheostats. — While it is altogether false
economy to use anything but the best in the form of a rheostat it
is worth while knowing how one could be made in case of urgent
need.

§ 731. Barrel or bucket type of salt water rheostat. — A wooden
bucket or barrel is used. In the bottom is placed a large plate of
iron, and one end of the supply wire is firmly fixed to this. The
other end of the wire is fixed to another mass of iron. The barrel
or bucket is then filled nearly full of water, and enough common
salt added to make about a ^4% solution. The water should be
well stirred to evenly distribute the salt. The upper iron and

526

RHEOSTATS AND OTHER BALLAST

[CH. XIII

wire are then covered by a burlaps sac so that there can not be a
metallic contact between the masses of metal. This upper wire
and its iron are then immersed in the barrel. If now the arc lamp
is fitted with carbons, and the switch closed the arc will form as
usual, the salt water and the iron plates serving as a rheostat.

wt ._ wi

FIG. 282. SALT WATER RHEOSTAT.

Wlt Wa Conductors. One end of conductor W, is connected to an iron
plate P3 in the bottom of the dish. The other end is connected to the plate P,
which is suspended by a string wound around the clamp. The burlaps sack S,
prevents contact of Pf and P, with resulting short circuit should the upper
plate be let down too far. It is safer still to have both plates covered, and
the container must be of wood, glass or stoneware, i.e. some non-conductor.

The jar contains a %% solution of salt. The resistance is regulated by
raising or lowering the plate P,. If more current is required, lower the upper
plate, if less current, raise P, so that the two plates will be farther apart.

CH. XIII]

RHEOSTATS AND OTHER BALLAST

527

If one wishes a greater amperage the upper wire is lowered in the
barrel and if less current is desired the upper iron is lifted higher
in the barrel (fig. 282). Of course there must be some means of
holding the upper wire in position when it is at the right height
in the barrel.

FIG. 283. WATER COOLED RHEOSTAT.

Wt, W2 Conductors.

R Rheostat composed of the proper length of small naked wire wound
around a frame of wood. The two ends of this resistance wire are soldered to
the cut ends of the supply wire W3 Wa. The rheostat is then immersed in
running water and the containing vessel of wood, glass or stoneware is placed in
a sink.

528 RHEOSTATS AND OTHER BALLAST [CH. XIII

In no case should one use naked wires for this rheostat, but the
rubber, water-proof insulated copper wires required by the National
Electric Code. The ends of the wires must be scraped and fastened
to the plates of iron. This is rather a poor make-shift for a rheo-
stat. The water soon heats up, and as it heats the resistance
becomes less so that more current flows. Then to counterbalance
this, fresh cold water can be added or the upper plate lifted to make
the distance between the iron plates greater. Furthermore, increas-
ing the amount of salt lessens the resistance. If there is too much
salt there will be too much current, if too little one cannot get
enough current without bringing the iron plates very close together,
and this is not safe.

§ 732. Home-made water cooled rheostat. — A home-made
rheostat can be constructed of small, naked wire of the proper
length as shown by calculation or by the electrical tables. The
wire is wound around a wooden frame in a single layer, care being
taken that the different turns do not touch one another. The cut
ends of one of the heavy insulated supply wires are then soldered
to the two ends of the coil. The coil with the soldered junctions is
then immersed in a glass or porcelain dish containing pure water,
no salt being used (fig. 283). If the current is to be on for some
time it is a great advantage to have the vessel containing the rheo-
stat stand in a sink or in some place where water can drain away,
and then to keep a stream of cold water flowing into the vessel to
keep the wire cool.

This general scheme is used in making tests of the gigantic
generators used in large power plants. For such tests the wire used
is naked telegraph wire of the right resistance and length laid out

§ 371a. With such a bucket rheostat, 12 liters (12 quarts) of l/2% salt
solution were used, and the distance between the iron discs could be as great
as 15 cm. (6 in.). With the discs 15 cm. apart and the solution at 23° centi-
grade a current of 10 amperes flowed. After an hour, when the temperature
had risen to 43° C., 12 amperes of current flowed. With the discs nearly in
contact 20 amperes were given.

In this experiment the iron discs were 18 cm. (7 in.) in diameter. By in-
creasing the size of the iron discs the current could be increased, and by
diminishing it the current could be diminished. Iron (tin) funnels are some-
times used instead of discs. It is safer to have both discs covered with the
burlaps, and the conducting wires soldered to the discs or funnels.

CH. XIII]
W,

RHEOSTATS AND OTHER BALLAST

529

FIG. 284. ADJ USTABLE RHEOSTAT
MADE OF SHEETS OF TIN.

A, B, C, D Clip-connectors to hold
the ends of the wires.

Permanent connectors c n 1-2 are
used to join the further ends of the tin
strips 1-2 and 3-4 and a connector (c n)
is used between B and C.

J, J' Movable adjusters to include more or less of the resistance in the
circuit and thus increase or diminish the amperage.

This rheostat is composed of four sheets of tin cut as shown in fig. 285. It is,
therefore, four rheostats in series (see fig. 287). As here connected all four
sheets are used. By putting supply wire W2 from A to C or from D to B only
two of the sheets would be used. Then by means of the adjusters / and /' the
amount of resistance can be increased or diminished at will.

The small diagram at the left shows how the pairs of strips of each side are
connected with each other at the far end.

At the near end of the frame the arched wire connects the two pairs of plates
of both sides at B and C.

530

RHEOSTATS AND OTHER BALLAST

[Cn. XIII

FIG. 285. To SHOW THE TIN PLATE CUT
IN INCOMPLETE STRIPS FOR THE
RHEOSTAT.

Cut in this way the tin plate is like a
continuous flat wire.

straight in the bottom of a
river or creek. The flowing
water keeps the resistance
wire cool.

§ 733. Home-made rheo-
stat of tin strips. — A good
adjustable rheostat for experi-
mental purposes can be cheap-
ly made by cutting tinned
sheet iron into strips as shown
in figure 284, 285, and nail-
ing these strips to a wooden
frame. One end of the con-
ductor is fastened to one end
of the sheet, and the other to
the other end of the sheet.

To make this an adjustable
of heavy copper wire or of sheet copper
to the other as shown. By this

rheostat, a "juniper
is put across from one sheet
means the current can be sent through as much or as little of
the resistance as desired, thus giving a great range in the
amperage. As the surface is very great in the thin sheet iron, the
air currents carry off the heat developed so that this rheostat does
not become unduly heated. It is a very common form around
physical laboratories, but is bulky and not very well adapted to a
magic lantern or a moving picture installation. Furthermore, such
a rheostat does not fulfill the requirements of the National Elec-
trical Code, as there is too much
combustible material in connec-
tion with it, and the resistance
is not boxed in.

§734. Rheostats in series.—
If one has two rheostats, less
current will be allowed to flow FIG. 286.
if they are connected to the line
in series, that is, so that all the

AN ELECTRIC CIRCUIT AND
GENERATOR.

C Generator.
A Arc Lamp,
current must flow through both R Rheostat.

CH. XIII]

RHEOSTATS AND OTHER BALLAST

531

rheostats. According to Ohm's
law (§ 724a), the amount of cur-
rent varies inversely as the resis-
tance, then if two equal rheo-
stats were used only half as
much current would flow as FlG 28? RHEOSTATS IN SERIES.
when one rheostat is used. Also c Dynamo,
if the voltage is increased the A Arc lamp.

•i-, • • ,-, R,, R, Rheostats in series, all the

amperage will increase in the cun4nt must pass through both of them
same ratio provided the resis- (compare fig. 288).

mi The two rheostats R, and R, are con-

tance remains constant. Then nected in series to get a smaller current
if one has two rheostats, each than can be obtained by the use of one
of the right capacity for an arc

lamp with a 1 10 volt circuit, the two in series would give approxi-
mately the correct number of amperes on a 220 volt circuit. The
amperage would be somewhat higher on the 220 volt circuit because
when used singly on a no volt circuit each is somewhat reinforced
by the resistance of the arc lamp. When both are used for one
lamp on a 220 volt circuit there is not twice the resistance, hence
the amperage will be somewhat greater than with one rhostat on
a no volt circuit.

§ 735. Rheostats in parallel.

parallel as shown in fig. 288, two

FIG. 288. Two RHEOSTATS IN PAR-
ALLEL, GIVING Two PATHS
FOR THE CURRENT.

G Dynamo.

A Arc lamp.

RIt R2 Rheostats in parallel.

With two or more paths for the cur-
rent, the total amperage will be the
sum of the amperages going over each
path (§ 735).

—If two rheostats are inserted in
paths are furnished for the cur-
rent. The amperage given by
both will be the sum of that given
by each separately, for example,
if one had two fixed rheostats,
each one giving five amperes of
current, if they were connected
with the line in parallel, 10 am-
peres would be allowed to flow.
On the other hand if they were
connected in series (fig. 287) so
that all the current had to flow
through both of them then only
2^2 amperes of current would be
available. (See § 724 a).

532 RHEOSTATS AND OTHER BALLAST [Cn. XIII

§ 736. Reactors, inductors, choke-coils, economy-coils, com-
pensator-coils, etc. — When alternating current is used the wasteful
method of current control by means of a resistor or rheostat where
so much electrical energy is transformed into heat should be
avoided whenever possible.

In place of a rheostat such as is described above (§ 723 + ) an
inductor is used. This consists of a soft -iron core around which is
wound a coil of insulated wire. The alternating current passes
through this coil; this alternately magnetizes and demagnetizes
the soft-iron core and limits the flow of the current. But the
energy is not dissipated, for the energy used in magnetizing the
core is given up again when the core is demagnetized. It is true
that a small amount of the energy is wasted in heating the appar-
atus, but the amount is so small (5% to 8%) as compared with that
lost in a rheostat that it is negligible.

Variable amperage can be obtained with an inductor by having
the soft-iron core movable so that a greater or less amount of it
will be within the coil.

The more of the soft -iron core within the coil the greater will be
the inductance and hence the less the amperage; and conversely,
the less of the soft-iron core within the coil the less will be the
inductance and the greater the amperage. In fig. 197 the core
is only partly inserted in the coil and a medium amount of current
is therefore allowed to flow.

§ 737. Wiring the inductor and transformer. — The inductor is

inserted along one wire (in series)
exactly as the rheostat is inserted
(fig. 289). With a special arc
lamp transformer the line is con-
nected to the primary of the trans-
former and the arc lamp is con-
FIG. 289. INDUCTOR IN SERIES WITH j , ,, , .^,

AN ARC LAMP. nected to the secondary without

G Dynamo. the use of resistance (fig. 290).

=t Alternating current circuit.

A Arc lamp with right-angle car- § 738. Comparison of the

bons- T ... amount of energy used with an

L Inductor to serve as ballast with

alternating current. inductor and with a rheostat. — (A)

CH. XIII] RHEOSTATS AND OTHER BALLAST 533

With an inductor. — Let the line voltage be no and the amper-
age 55 as shown by the ammeter; the voltage across the arc will
be 34 volts. The power consumption will be volts times amperes,
that is, in this case, 34 x 55 = 1870 watts or 1.87 kilowatts. As
the inductor does not absorb an appreciable amount of energy,
the 1.87 kilowatts represents the energy needed to produce the
arc light.

(B) With a rheostat. — If now a rheostat is used, the watt-
meter will record not only the energy required to maintain the arc
light, but also the energy wasted in heating the rheostat.

For example, suppose as above that the line voltage is no, the
amperage 55, and the voltage across the arc is 34. Then as before
the arc light requires 34 x 55 = 1870 watts or 1.87 kilowatts.

But the difference between the 34 volts at the arc and the no
volts in the line (76 volts) is used in heating the rheostat.

The energy used in heating the rheostat is then 76x55 = 4180
watts or 4.18 kilowatts. Both this wasted energy as well as the
actual energy used in the arc will be recorded on the wattmeter
and the user of the arc lamp will have to pay for 1.87 + 4.18 or 6. 05
kilowatts to run his lamp instead of the 1.87 kilowatts when the
inductor is used. That is it will cost more than three times as
much to run the arc lamp with a rheostat as with an inductor or
choke-coil.

STATIONARY TRANSFORMER FOR ALTERNATING CURRENT

§ 739. Transformer. — A transformer is a device for changing
the voltage of an alternating electric current. This change may
be an increase in the voltage — step-up transformer, or a decrease
in the voltage — step-down transformer. The device consists in a
soft -iron ring wound with coils of insulated wire. In the simplest

§ 738a There is no simple method of economizing with direct current
comparable with the use of an inductor with alternating current. Sometimes
when one must draw on a current at 220 volts pressure there is used a motor
generator set. The motor is driven by the 220 volts current and the genera-
tor produces current at 60 to 70 volts pressure. At this voltage only a
limited amount of resistance is necessary (§ 747), and there is some saving,
but not so much as by using an inductor with alternating current.

534 RHEOSTATS AND OTHER BALLAST [€H. XIII

case there are two coils (fig. 291). If an alternating current supply
is connected with the primary coil an alternating current can be
drawn from the secondary coil.

The voltage and amperage
which can be drawn from the
secondary coil will depend upon
the electric supply and upon the
relative number of turns of wire
FIG. 290. USE OF A SPECIAL TRANS- in the primary and in the second-
FORMER WITH AN ARC LAMP. ary coiis. jf the number of turns
G Dynamo. is the same in both, then the

? £anS±e^UrrentCirCUit' voltage and amperage remain
A Arc lamp. practically the same as if the

The primary of the transformer is -i T , ,

connected to the dynamo while the colls were not Present. In other
secondary is connected to the arc words the circuit is in every way

laThe transformer has sufficient "re- almost as if the wire were contin-
actance" to serve as a ballast for the uous. If the transformer were

tO "* " * SteP"°Wn Perfect the voltage and amperage
would be exactly the same as if it

were not present. In practice they are a little less, but a good
transformer gives an efficiency of 95% to 98%.

If the secondary coil has a different number of turns from the
primary coil then the voltage will vary directly as the ratio of the
number of turns in the two coils, and the amperage will vary
inversely as that ratio. That is, assuming that there is no loss in
the transformer, the watts delivered will remain constant as the
product of volts x amperes remains the same.

For example, suppose the secondary coil has %'th as many turns
as the primary coil, then the number of volts across the secondary
will be %ih the number across the primary and the number of
amperes delivered by the secondary will be four times the number
drawn by the primary. If now the primary is connected to a 220
volt line there will be a potential difference of one-fourth that
number or 55 volts across the terminals of the secondary coil.
Suppose the secondary coil supplies 60 amperes, as might be the
case with an arc lamp, then the primary coil would draw one-fourth

CH. XIII] THE ELECTRIC ARC 535

of this number, or 15 amperes from the line. The watts in the two
cases are theoretically exactly the same.

The watts for the primary are 220x15 = 33 °°-

The watts for the secondary are 55 x 60 = 3300.

(1) Volts secondary _ Turns secondary

Volts primary Turns primary

(2) Amperes primary Turns secondary
Amperes secondary Turns primary

FIG. 291. DIAGRAM OF A TRANSFORMER.

Two coils of a wire, Primary and Secondary, are wound on an iron ring. An
alternating current in the primary sets up an alternating magnetic flux in the
iron ring, which in turn sets up an alternating electric potential in the secondary
coil.

THE ELECTRIC ARC

§ 740. The construction of an electric arc is very simple. Two
electrodes are taken which may be made of any conducting material.
One electrode is connected directly to one of the wires of a direct
current supply of over 40 volts, the other electrode is connected
through a rheostat to the other wire (fig. 280). When the two
electrodes are brought in contact an electric current will flow
between them. If now, the electrodes are slightly separated, the
current will not be immediately interrupted, but will flow through
the air gap between the electrodes.

536 THE ELECTRIC ARC [Cn. XIII

The exact nature of the resulting phenomenon will depend upon
the material of which the electrodes are made, upon the voltage of
the current supply and the resistance of the rheostat, and the kind
of gas surrounding the electrodes.

§ 741. Arc lamp. — Any arrangement for holding the electrodes
and feeding them together as they wear away may be called an arc
lamp.

It consists of three essential elements: — (i) A clamp for holding
the positive electrode; (2) A clamp for holding the negative elec-
trode; (3) A mechanism for moving the holders and therefore the
electrodes nearer together or separating them farther apart.

The electrode holders must be insulated so that the current must
flow through the electrodes and not follow any short circuits (fig.
270).

For the hand-feed and the automatic types of arc lamps see
Chapter I, § 9-11.

§ 742. With direct current, the arc is made up of three parts.

1. The arc stream; a highly heated, incandescent gas which
conducts the current between the electrodes.

2. The positive crater; where the current leaves the positive
electrode to enter the arc stream.

3 . The negative crater ; where the current leaves the arc stream
to enter the negative electrode (fig. 292).

§ 743. Electrical behavior of the direct current arc. — Measure-
ment of the voltage drop in various parts of the carbon arc reveals
the fact that the potential difference between the two electrodes
(§ 743a) is made up of three parts. Starting from the positive
side, the potential difference between the positive electrode and the
arc stream is about 32 volts. The potential difference between the
arc stream and the negative electrode is about 9 volts, thus the
potential difference between the electrodes with the shortest possi-
ble arc is about 41 volts (§ 743b).

As the arc is lengthened there is an additional drop in potential
in the arc stream which depends mainly on the length, but partly on
the cross section of the arc stream. As the arc length is changed,

CH. XIII]

THE ELECTRIC ARC

537

FIG. 292.

THE VERTICAL CARBON ARC WITH 20 AMPERES OF DIRECT
CURRENT.

a Vertical carbons with the positive carbon above and the negative carbon
below. This shows that the large crater is on the positive carbon and the small
crater on the negative carbon. Between the two craters extends the arc stream
of hot gases.

This photograph was made with an exposure of i/ioo second, the aperture
being F/22. A color screen was used to cut out most of the violet, so that the
arc stream would not obscure the craters. A subsequent exposure of 90 seconds
was made without a color screen and with an aperture of F/8. The illumina-
tion during this exposure was by means of a 40 watt, mazda lamp.

b Vertical carbons with a 20 ampere direct current. No color screen.
Exposure i/ioo sec.; opening F/22.

This shows the size of the two craters ; it also shows the conical arc stream
almost as light as the craters. This is because the violet light which has
relativelv little effect in illumination has a great effect on the photographic
plate.

This picture shows how the carbons, the craters and the arc stream appear
in an instantaneous view to the photographic plate, while the one at the left
(a) gives much more nearly the appearance to the human eye. with an instan-
taneous view.

538

THE ELECTRIC ARC

[Ca. XIII

the change in voltage is almost entirely due to the change in the
length of the arc stream.

When the arc is of medium length, as for use in projection, the
potential difference between the two carbons averages about 55
volts. This would mean that there is a drop of 32 volts between
the positive carbon and the upper end of the arc stream, a drop of
14 volts between the upper and lower ends of the arc stream, and 9
volts between the lower end of the arc stream and the negative
carbon.

If the electrodes are made of other substances than carbon, the
potential drop is differently distributed. Thus in the "Luminous"

FIG. 293.

SIDE VIEW OF THE RIGHT-ANGLE CARBON ARC WITH 10 and WITH
20 AMPERES OF DIRECT CURRENT

A 10 ampere arc, B 20 ampere arc. The size of the positive crater is
markedly larger with the higher amperage.

The lower pictures were made by an instantaneous exposure.

The upper pictures were made by a double exposure, that is, an instantaneous
exposure with the current on, to show the craters and the arc stream, and then
an additional exposure of 90 seconds with the current off to bring out the car-
bons. For the second exposure a 40 watt, mazda lamp was used for illuminat-
ing the carbons.

CH. XIII] THE ELECTRIC ARC 539

arc which consists of a copper positive electrode and a negative
electrode made of a mixture of iron and titanium oxides, the lowest
arc voltage is about 30 volts. The lowest arc potential between
electrodes of other substances than carbon are, magnetite 30;
platinum 27; iron 26; nickel 26; copper 23; silver 15; zinc 16;
cadmium 16; mercury 13.

The potential differences in the arc lamp are practically constant
no matter what current is flowing, but there is a small change with
change in current. This is generally such that the greater the
current the less the potential difference, and may be explained as
follows :

Suppose a current of 10 amperes to be flowing between the two
electrodes of an arc lamp. This will be carried by a small cone
shaped mass of conducting gas (fig. 293 A). If the current is
increased to 20 amperes the extra heat developed is sufficient to
bring more air to a high enough temperature to conduct current,
and the cone of conducting gas increases in diameter (fig. 293 B).

A large cone of conducting gas will be losing heat at a relatively
less rate than will a small cone, hence its temperature will be higher
and its resistance will be less. As a result of the increased con-
ductivity of the hot gases of the arc stream, the greater the current
the lower will be the potential difference between the electrodes.
There is also a slight lowering of the contact potential difference
between the electrodes and the arc stream as well as a lessening of
the potential drop in the arc stream.

THE USE OF BALLAST

§ 744. The need of a ballast in series with the arc to control the
current. — On account of the peculiar electrical behavior of the arc
lamp it is necessary to use a ballast such as rheostat, or an inductor
in series with the arc, or else to use an especially designed generator.

§ 743a. While the two electrodes of an arc lamp may be of any conducting
material, with projection arc lamps the electrodes are always made of carbon
and are generally referred to simply as carbons.

§ 743b. These figures are approximations and vary slightly with arc
length and current but are general averages for the usual arc lengths employed :
3 to 10 mm.

See Mrs. Ayrton, The Electric Arc.

540 USE OF BALLAST WITH ARC LAMPS [Cn. XIII

With a metallic wire, the resistance is nearly constant, and the
potential difference is greater the greater the current flowing. Any
change in resistance is due to the rise of temperature when a current
is flowing. The higher the temperature, the greater the resistance.
An arc, on the other hand, has no definite resistance, but its resist-
ance varies with the current flowing. This variation is such that

A B

FIG. 294. FACE AND LATERAL VIEWS OF THE RIGHT-ANGLE CARBON ARC
WITH 10 AND WITH 2O AMPERES OF DIRECT CURRENT.

A With 10 amperes, B with 20 amperes of direct current.

The size of the crater in the two cases is very strikingly brought out.

The middle figures had an additional exposure to bring out the carbons (see
fig. 292-293), while the lateral views above and the front views below had only
an instantaneous exposure.

The positive crater above and the negative crater below are clearly brought
out in all the pictures (see fig. 292).

CH. XIII] USE OF BALLAST WITH ARC LAMPS 541

the potential difference across the arc remains nearly the same
regardless of how much current is flowing.

The commercial electric supply is designed to furnish current for
incandescent lamps, and is maintained at a nearly constant voltage
no matter how much current is used. The arc lamp, on the other
hand, is to be supplied by a constant current. If one were to
attempt to connect an arc directly to the terminals of the supply

FIG. 295. LATERAL AND FACE VIEW OF THE RIGHT-ANGLE CARBON ARC WITH
20 AMPERES OF DIRECT CURRENT.

No color screen was used with the lateral view so that the arc stream would
show. In the front view a color screen was used to bring out clearly the large
positive crater above and the small negative crater below.

This figure is for comparison with the alternating current arc in fig. 296.

To bring out the carbons, an additional exposure was made as for fig. 292-
293-

line without an intermediate rheostat, as soon as the two electrodes
were brought in contact an extremely large current would flow.
Theoretically, this current would be infinite, but practically the
flow is limited by the very small resistance of the supply wires and
the capacity of the dynamo. In a modern installation the current
would be immediately interrupted by the circuit breakers and burn-

542

USE OF BALLAST WITH ARC LAMPS

[CH. XIII

ing out of the fuses before any serious damage could result. Even
after the arc is burning, if one were to remove the resistance by
short-circuiting it, the current would increase to an enormous
value.

§ 745. Example with 110 volt supply, using a rheostat. — If we
assume that the arc is of such a length that the potential difference
between the electrodes is 10 volts, and that this potential difference

FIG. 296. LATERAL AND FACE VIEWS OF THE RIGHT-ANGLE CARBON ARC
WITH 25 AMPERES OF ALTERNATING CURRENT.

By comparing this picture with fig. 295 it will be seen that in this both
craters are of the same size; and that, although 25 amperes of current are
flowing, the crater on the upper carbon from which the light is derived is much
smaller than with the direct current. The sizes of the upper crater give a good
idea of the amount of illumination furnished in the two cases.

An additional exposure was made to bring out the carbons as in fig. 292-293.

remains practically the same if the current is diminished or in-
creased, and if the supply is no volts, and that this voltage is
practically independent of the current used, it is evident that
between one of the electrodes and one of the supply wires there must
be a potential drop of 60 volts. By using a rheostat at this point
the current is controlled. Thus suppose that the rheostat has a
resistance of 6 ohms, then according to Ohm's law (§ 724a), as the
potential difference across its terminals is 60 volts, the current will

CH. XIII]

USE OF BALLAST WITH ARC LAMPS

543

be 10 amperes, V/O = A. Now suppose the arc length were
changed say by bringing the electrodes in contact. In this case
there would be the full line voltage, no volts across the rheostat
and the current would be 1 10/6 = 18.3 amperes. Suppose the arc
length were increased until the potential at the arc was 60 volts.
The potential across the rheostat would then be no — 60 = 50
volts. The current would then be 50/6 = 8.2 amperes. In this
example the conditions are what is known as stable, that is, as the
arc length is decreased the current is increased, but does not reach
an infinite value, and as the arc length is increased • the current
decreases but it does not become zero.

FIG. 297.

LATERAL AND FACE VIEWS OF AN INCLINED CARBON ARC WITH 20
AMPERES OF DIRECT CURRENT.

This picture shows that with the inclined carbons in proper position, the
positive crater on the upper carbon faces toward the condenser. It is evident
also that as the carbon burns away the crater will get farther and farther above
the principal axis of the projection apparatus.

An additional exposure was made to bring out the carbons as with fi'g. 292-
293-

544 USE OF BALLAST WITH ARC LAMPS [Cn. XIII

§ 746. Line voltage exactly equal to arc voltage. — It would
appear that it might be desirable to use a line voltage of exactly
what is required by the arc and omit the rheostat. Suppose in the
above example that this were done by using a line voltage of 50
volts. Now as the arc voltage is constantly varying owing to slight
irregularities in the carbons, to the wearing away of the carbons and
to other causes, it is evident that for an instant the arc voltage
might drop below 50 volts or it might rise above 50 volts. If the
arc voltage should rise above 50 volts, the arc would immediately go
out as the supply is but 50 volts, and if the arc voltage should drop
slightly below this value, the current would rapidly increase. The
result would be that the arc would either go out or else would act
like a short circuit. In this example the conditions are unstable;
that is, no definite current can be maintained.

§ 747. Intermediate voltage. — In practice an intermediate
voltage is sometimes used, that is, dynamos to be used for projector
arcs are sometimes designed for about 70 volts. Here the arc is
sufficiently stable for practical purposes but requires more atten-
tion than with the higher supply voltage. Taking the above
example. The arc voltage at 50 volts leaves 20 volts across the
rheostat. To give 10 amperes requires 20/10 = 2 ohms resistance.
If now the electrodes are brought in contact to start the arc the
current will be limited only by the resistance in the rheostat and
the current will be 70/2 =35 amperes. If the arc gets long enough
to take 60 volts, the difference to be taken up in the rheostat is but
10 volts, and the current will drop off to 10/2 = 5 amperes. This,
therefore, means that with the smaller margin between the line
voltage and the arc voltage, the arc becomes less stable.

§ 748. Ballast with alternating current. — With alternating
current, an inductor (choke-coil) is often used instead of a rheostat.
This behaves as a ballast in a somewhat similar way to the rheostat
but to explain the exact process of regulation would require a more
exhaustive discussion of alternating currents than is justified in
this book, but see § 736.

CH. XIII]

USE OF BALLAST WITH ARC LAMPS

545

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546 LIGHT FROM THE ARC [Cn. XIII

THE LIGHT PRODUCTION OF THE ARC

§ 749. Cause of light from the arc. — The light production from
the carbon arc is due entirely to the high temperature to which the
tips of the carbons are raised, i. e., they become white hot. The
practical problem in projection with the arc deals with the best
method of producing this white heat and of utilizing it.

When the electric current passes between the two electrodes the
heating effect in the different parts is proportional to the power
consumed in them.

The current being the same in all parts, the heating effect must
be in proportion to the potential drop (or voltage consumed) in the
different parts.

Counting the total drop 55 volts, it is divided into:

+ crater drop = 32 volts = 58%

— crater drop = 9 volts = 17%

arc stream = 14 volts = 25%

Total, 55 volts 100%

We see from this that the heating effect will occur principally
at the positive carbon.

Carbon being rather a poor conductor of heat, the heat generated
within the small area of the crater must escape mainly by radia-
tion.

At the negative electrode the heat production is less rapid and
not so high a temperature is reached.

Between the electrodes the heat production is fairly rapid, but
the hot gases of the arc stream with the carbon arc are nearly
transparent and radiate energy very slowly.

Furthermore the violet lines of the spectrum in the arc stream
are brighter than from the crater itself (§ 749 a).

§ 750. Temperature of the crater.— The temperature of the
positive crater rises until such a temperature is reached that carbon

§ 749a. The great brilliancy of the violet lines in the arc stream has received
two explanations: (i) That the arc stream is higher in temperature than even
the crater itself; (2) That the electric current passing through the gas causes
the gas to glow irrespective of its temperature. That is, it causes electro-
luminescence as in the vacuum tube or the aurora borealis.

CH. XIII] LIGHT FROM THE ARC 547

is volatilized. This is the highest temperature which it is possible
to obtain artificially. The temperature of the positive crater of the
carbon arc has been estimated at about 3700° absolute, that is,
3427° Centigrade or 6200° Fahrenheit (§ 7Soa). Compare this
with the temperature of the sun, about 6750° absolute, 6477° C;
the acetylene flame, 2330° absolute, 2057° C.; the gas flame, 1830°
absolute, 1557° C. (§ 75ob).

§ 751. Parts of the light source. — Considered as a light source,
the direct current arc may be divided into four parts.

1. The positive crater.

2 . The negative crater.

3. The hot ends of the carbons adjacent to the craters.

4. The arc stream.

The light emitted by the hot electrodes depends upon their vis-
ible radiation being approximately proportional to the 5th power
of their absolute temperature. The positive crater is the hottest
part of the arc and furnishes most of the light. The negative crater
furnishes much less light than the positive crater, being smaller and
not as hot.

The carbons are white hot for some distance away from the
craters and furnish some of the light of the arc. In calculating the
total light from the arc it would be necessary to consider the entire
area included between the line surrounding the positive carbon
which is at red heat and the corresponding line on the negative
carbon.

The arc stream with the carbon arc emits but little useful light.
When flame-arc carbons are used, however, the greater part of the

§ 750a. Bulletin of the Bureau of Standards, Vol. i, p. 909 and reprint 8.

§ 750b. Absolute temperature. — The absolute zero is defined as the tem-
perature at which a perfect gas would exert no pressure. This is about -2 73°
centigrade, i. e., 273° centigrade below the melting point of ice. In calcula-
tions of high temperature and radiation, all formulae are based on absolute
temperature, that is, the temperatures where the zero is the absolute zero and
where the degree is the degree centigrade.

To find the absolute temperature of a body add 273° to its temperature on
the centigrade scale. Thus ice melts at o° centigrade or 273° absolute, and
water boils at 100° centigrade or 373° absolute. The temperature of the
human body, 37.5° C. is 310.5° absolute. If the absolute temperature is given,
subtract 273° from this value to find the centigrade reading.

548

LIGHT FROM THE ARC

[Cn. XIII

FIG. 299

CH. XIII] LIGHT FROM THE ARC 549

FIG. 299. SIDE AND FRONT VIEWS OF THE INCLINED CARBON ARC WITH 15
AMPERES OF DIRECT CURRENT (EWON'S AUTOMATIC LAMP).

The upper carbon (+c) is soft-cored and 18 mm. in diameter; the lower
carbon ( — c) is solid and 12 mm. in diameter.

This is to illustrate an automatic lamp with a magnet (m) to control the
magnetic blow; the use of a large, cored upper carbon (+c) 18 mm. in diame-
ter; and a small solid lower or negative carbon ( — c) 12 mm. in diameter.

Incidentally there is shown the wandering of the crater in the right hand
lower picture. When the crater wanders in this way the source of light is
outside the principal optic axis.

Photographed with an instantaneous exposure for the arcs and with an
additional exposure of 90 seconds for the carbons and the blow magnet (see fig.
292-293).

light is furnished by the incandescent gases of the arc stream.
Flame-arc carbons are not ordinarily used in projection.

For purposes of projection, only the light from the positive crater
of the direct current arc, or usually from only one of the craters of
an alternating current arc need be considered. The large objective
of the rragic lantern utilizes the light from both carbons with
alternating current and this is important.

§ 752. The alternating current arc. — Most conducting materials
when used as the terminals of an arc lamp will not allow a reversal
or even a very short interruption of the current without going out.
This property is used in the mercury arc rectifier.

When carbon electrodes are used, however, the current may be
interrupted for a short interval, or the current may be reversed
without putting out the arc.

When the alternating current is used, first one carbon and then
the other is positive. Craters of equal intensity are formed on both
carbons, but neither is as bright nor as large as is the single positive
crater when direct current of the same amperage is used.

The light from a single crater is not steady but is intermittent.

The process during one cycle can be described as follows :

When the current is reversed so that, say, the upper carbon
becomes positive, the crater is fairly cool. For the short time it is
the positive crater, its temperature rises very rapidly. Whether
or not it momentarily reaches the temperature which it would if
permanently the positive crater is uncertain. The current dies
out and the crater cools rapidly. When the current has reversed

550 LIGHT FROM THE ARC CH. XIII

its direction the crater is negative. The heating effect of the
current is small and the carbon tip continues to cool until the
current has again died out. This cooling still continues until the
current has again reversed its direction, and increased to a con-
siderable positive value.

The temperature of an alternating current arc crater is at no
instant higher than that of a direct current arc crater with the same
amperage, and, as part of the time its temperature is much lower
than this, the average temperature will be lower than with a
direct current crater, hence the light will be less and of a yellower
color.

FIG. 300. SOME POSITIONS OF THE CARBON ELECTRODES USED IN PROJECTION

LAMPS.

A Vertical carbons. This position gives the least light along the principal
optic axis.

B Inclined carbons.

C Horizontal carbons. This arrangement is common for the search light,
and for the reflectors used in projection (see fig. 95).

D The usual arrangement for the carbons when at right angles. The
upper or horizontal carbon is positive with direct current. '1 he crater on it is
in the optic axis and serves as the source of light with both direct and alter-
nating current.

E Right-angle carbons in which the horizontal, positive carbon is below.
This is an unusual arrangement.

V V-arrangement of the carbons for alternating current. With this
arrangement both craters supply light for the projection of lantern slides or
opaque objects.

THE ARC LAMP AS AN ILLUMINANT

§ 753. The arc lamps suitable for projection purposes may have
the carbons in any one of five positions.

1. With inclined carbons (fig. 297).

2. With carbons at right angles (fig. 295).

3. With converging carbons (fig. 300).

4. With vertical carbons (fig. 292).

5. With horizontal carbons along the axis (fig. 300).

CH. XIII]

LIGHT FROM THE ARC

551

Most of the light from the arc, and all of the light which is useful
for projection comes from the craters of the arc ; from the positive
crater, if a direct current arc.

§ 753a. Table showing the proper size of cored-carbons for
different amperages, and the rate of wear in millimeters per
hour; also the relative rate of burning in length and in weight.

(For the small carbons to be used on the house electric light-
ing system see § 123, 131, 417-418.)

Direct Current, Right-Angled Carbons.

AUTOMATIC LAMP

Amperes

Size Carbons

Burns, mm.
per Hour

Relative
Volume

Relative
Burning Weight

II upper

1.8

8 lower

ii "

1.65

1.69

14 "

i-75

1.67

14 "

1.87

1.77

14 "

i-53

1.36

14 "

i-95

1.92

14 "

1.61

I-3I

HAND-FEED LAMP

1.65

I-5I

II
14 "

32
37

I
1.62

H
I4 "

23
48

I
2.00

2.OO

15 "

24
44

I
2.2

I
2.26

552

LIGHT FROM THE ARC
Direct Current, Inclined Carbons.

[Cn. XIII

HAND-FEED LAMP.

Amperes

Size Carbons

Bums mm.
per Hour

Relative
Volume

Relative
Burning Weight

14 upper
ii lower

41
41

1.62
I

_L33
i

14 "

1.84

i-77

14 "

1 .81

i 8 cored
12 solid

26
32

1.8

2.32

14 "

'•4s

1.72

14 "

2-4

2.32

Alternating Current, Hand-Feed Lamp.

RIGHT-ANGLED CARBONS.

Ampeies

Size Carbons

Burns, mm. per Hour

1 1 upper
ii lower

.34

14 "

INCLINED CARBONS

14 upper
14 lower

15 "
15 "

"30

15 "

16 "

~28

CH. XIII] CANDLE-POWER OF ARC LAMPS

CANDLE-POWER OF ARC LAMPS

553

§ 754. A number of measurements of the candle-power of arc
lamps have been made, partly in the Physical Laboratory at
Cornell University, and partly in the Illuminating Engineering
Laboratory of the General Electric Company at Schenectady.
The experiments made at Cornell were for the higher currents and
were made primarily to ascertain the efficiency of the mercury arc
rectifier and the power consumption with different forms of ballast
(§ 754a).

FIG. 301.

CARBONS IN THE CORRECT RELATIVE POSITION FOR BOTH DIRECT
AND ALTERNATING CURRENTS.

A Inclined carbons in the correct position for alternating current.

B Inclined carbons in the correct position for direct current.

C Carbons at right angles in the correct position for either direct or
alternating current. Direct current is indicated.

§ 755. Variation of Candle-Power with current. — Candle-power
measurements were made in the horizontal direction, that is, along
the axis of the lantern, using different currents and with both the
right-angle and the inclined-carbon arrangements. Great care
was taken to hold the position of the electrodes and craters as
shown in fig. 301, as these positions furnish the greatest amount of
light. With direct current especially, it is necessary that the crater

§ 754a. The results of the Schenectady tests were published in the Electrical
World, Oct. 13, 1911.

554

CANDLE-POWER OF ARC LAMPS

[Cn. XIII

/<•

FIG. 302. VARIATION IN INTENSITY OF LIGHT FROM PROJECTION ARC LAMPS
WITH DIRECT AND WITH ALTERNATING CURRENT.

x Right-angle arc.

o Inclined carbon arc.

The small dotted curve is for small currents with the right-angle arc burn-
ing 6mm. carbons.

As shown by these curves, the right-angle arc lamp gives a greater candle-
power for the same current than does the inclined carbon arc lamp, with both
direct and alternating current. Also that direct current gives about four times
the light that the same number of amperes of alternating current gives.

CH. XIII] CANDLE-POWER OF ARC LAMPS 555

face forward as is shown in figure 297 for the inclined electrodes and
in figures 294-296 for the right-angle arc. The results of these
measurements are shown in curve form in fig. 302. These curves
show that the greater the current the greater is the amount of light
given by the arc. The increase in light is, however, more rapid
than the increase in current and no simple mathematical statement
of the relationship is possible. The crosses indicate the individual
measurements with the right-angle arrangement and the circles,
measurements with the inclined carbon arc. The upper curve is for
the right-angle arc with direct current. In this case the highest
candle-power for the same current (amperage) is obtained. The
next curve is for the inclined carbon arc with direct current. The
light is not quite as much with this arrangement as with the right-
angle arc.

The two lower curves are for alternating current. It will be
noticed that there is a greater difference in candle-power depending
on the electrode arrangement with alternating than with direct
current. The short dotted part of the curve for the right-angle
arrangement is for 6 mm. carbons and small currents, while the
main part of the curve is for larger carbons.

A table showing the results of the individual measurements
might be misleading, as large variations in the light of the arc are
continually occurring and a given measurement might be made
when the arc was giving its greatest or its least light. For this
reason the values given in the table (§ 756) for the candle-power of
the arc with different currents were taken from the curve instead of
being from individual observations. These values are good
averages and may be accepted as close enough to the actual candle-
powers for all practical purposes in projection.

556 CANDLE-POWER OF ARC LAMPS [Cn. XIII

§ 756. Table of Candle-Power and Current with Arc Lights.

Size Carbons

Amperes

Direct Current
Carbon?

Alternating Cunent
Carbons

Right-angle

Inclined

Right-angle

Inclined

6 mm.

2OO

3 400

4 650

IOO

5 900

2OO

8 mm.

7-5 1,500

1 ,400 400

300

it mm.

2.2OO I.QOO 500

4OO

12.5

2,9OO 2,5OO

6OO

450

3-700

3.2OO

7OO

500

17-5

4-500

3,800

950

625

13 mm.

5,400

4-550

1,200

750

7-500

6.2OO

1,750

I.IOO

15 mm.

9.500

8,100 2,300

1,400

10,000

3,000

1,900

12,000

2,500

3,200

3.700

4,<Soo

§ 757. Direct current; inclined electrodes.

Amps

Volts

Watts

WATTS
no- Volt Line
With Resistance

Candle-Power

15
20

25
30
40

50
50

51
53
51

750
I ,OOO
1,270
1.590
2,040

1,650
2,2OO
2,750
3,300
4,400

3,490
4,9OO
6,22O
8,750
12,350

Mean

§ 758. Direct current; electrodes at right angles.

Amps

Volts

Watts

WATTS
no-Volt Line
With Resistance

Candle-Power

15
20

25
30

56
50

52
62

560
750
I,O2O
1,550
1,740

I, IOO
1,650
2,200
2,750
3,300

2,300
3,680
6,230
7-500
10,150

Mean

55-6

CH. XIII] CANDLE-POWER OF ARC LAMPS

§ 759. Alternating current; inclined electrodes.

557

LINE WATTS

Amps

Volts

Watts

With Trans-

Candle-
Power

With Resistor

former, 96 per

cent Efficiency

28 560

2.2OO

585

62O

27-5

687 2,750

715

894

26.5

795 3.300

830

IJOO

1 ,080 4,400

1,130

1,830

1,750 5,500

1,830

4-566

1,920 6,600

2,OOO

4,650

Mean

29.2

Power factor (P. F.) at arc nearly i.oo.
§ 760. Alternating current; electrodes at right angles.

430

I,IOO

450

590

600

i ,650

625

763

920

2,200

960

i ,050

i,370

2,750

i,43o

1,690

57 i, 600

3,300

1,670

2,540

Mean

49.6

Power factor (P. F.) at arc 0.964.

§ 761. Rectifier; inclined electrodes.

DIRECT CURRENT
SECONDARY

ALTERNATING CURRENT PRIMARY

Amp?

Volts

Watts

Amps

Volts

Watts

Volt-
Amps

P. F.

Eff.

C. P.

765

175

I,IOO

1,225

.898

•695

3,IOO

54-5

1,090

9-5

188

1,500

1,786

.84

•727

4,720

1,350

194

1,900

2,330

.816

.711

6,470

1, 860

14-5

220

2,600

3,190

.8l6

.7I6

8,600

2,IOO

215

3,120 4,070

.768

.672

12,150

Mean

54-7

.828 .704

558

CANDLE-POWER OF ARC LAMPS

[Cn. XIII

§ 762. Rectifier; Electrodes at right angles.

58o

5-5

195

850

1,070

•794

•683

1,900

675

1 80

1,000

1,260

•793

•675

3,000

1,020

203

1,500

2,030

•739

.680

5,600

1,650

235

2,300

2,820

.816

.718

7,370

1, 860

233

2,600

3,260

.798

.716

9,450

Mean

56.4

.786

.694

§ 763. Power in kilowatts drawn from the line for different
values of light. Inclined electrodes, 110-volt supply, transformer
96 per cent efficiency.

KILOWATTS

Candle-Power

D. C.

at arc

D. C.

Resist.

A. C.

Trans.

A. C.
Resist.

Rectifier

I.OOO

2.7

1,500
2.OOO

•4

I.I

i.i

3-2

3-75

2,500

•55

1-3

1.2

4-3

•9

3,000

1-5

1.4

4-9

i.i

4.OOO

.76

1-9

i-7

5-8 i-3

5,000

i.i

2.25

2.O

6.9 1.5

6,000

1.2

2.6

1.8

7>5°°

•4.i

T 8

•I

i A

2-I5

§ 764. Light given for different values of kilowatt con-
sumption.

Kilowatts

Candle-Power

I.O

i-5

2.0

3-0

4.0
5.0

5,500

7,800
12,000

2,000
3,000
4,3oo
7,300
1 1,000

2,200
3,400
4,800

3-200

4,800
6,900
1 1 ,000

500
1,300
2,200
3,100

§ 765. Candle-power measurements with direct current sup-
plied by a mercury arc rectifier. — By using a mercury arc rectifier
to convert alternating current to direct current, very nearly the
same light intensity is obtained as if the same amperage of direct

CH. XIII]

CANDLE-POWER OF ARC LAMPS

559

current were supplied by a dynamo. This is shown in figures 303-
304 and in the tables which give the results of the Schenectady
tests (§ 757-764).

§ 766. Relation between the power consumption and candle-
power. — Besides the current passing through the arc, it is necessary
to know the power consumption, as it is the power consumption
which determines the cost of maintaining the arc.

With direct current, the right-angle arc, for example, gave 2300
candle-power and required 56 volts potential difference at the arc.
This means a power consumption of 560 watts at the arc with 10
amperes. Under most circumstances, however, the current would
be supplied from a no volt line and this would represent power

FIG. 303. RELATION BETWEEN CURRENT AND CANDLE-POWER.

A The candle-power variation with right-angle carbons, with alternating,
direct and rectified current.

B The candle-power variation with inclined carbons with alternating,
direct and rectified current.

These curves show that rectified and direct current give approximately equal
illumination and that alternating current gives a much lower candle-power with
a given amperage.

560 CANDLE-POWER OF ARC LAMPS [Cn. XIII

drawn from the line to the extent of 15x110 = 1650 watts.
Hence, in fig. 304 there are drawn two curves for direct current, one
for the power consumed at the arc, and the other for the power drawn
from the line with a no volt supply when used with resistance.

With alternating current there are even more possibilities.
There is the power consumed at the arc, the power drawn from the
no volt line with resistance, and the power drawn from the line if a
suitable transformer of high efficiency is used. In calculating the
power consumption when using a transformer the actual power
consumed at the arc was divided by the efficiency of the trans-
former. Thus with 10 amperes alternating current the right-angle
arc consumed 430 watts at the arc. The transformer had 96%
efficiency, hence the power drawn from the line was 430 -=- .96 =
450 watts. In addition, curves were drawn showing the power
consumption when a rectifier was used.

§ 767. Results. — The results as shown in figures 302-304 are,
that with the same amount of power drawn from the line, the least
light is given when alternating current is used with a rheostat and
the most when alternating current is used with a rectifier. With
the right-angle arrangement there is more light for the same power
with direct current and a rheostat, than with alternating current
and a transformer, but with inclined carbons there is but very little
difference in the light given for the same power supplied, whether
alternating current is used with a transformer or direct current is
used with a rheostat. It is to be noticed, however, that by using
sufficient power it is possible to get more light by the use of direct
than with alternating current.

The power drawn from the line depends on the power consumed
at the arc and the efficiency of the ballast or transforming device.

§ 768. Efficiencies with different arrangement of carbons, and
different forms of current. — The efficiencies of these devices are :

With ripht-angle Inclined
ca.bons carbons

Direct Current and rheostat = 50% = 46%

Alternating Current and rheostat = 45% = 27%

Alternating Current and transformer = 96% = 96%

Alternating Current and rectifier = 70% = 70%

CH. XIII]

CANDLE-POWER OF ARC LAMPS

-t-

«./

/•

Q,/

-r- L

,0-

VG"

Power Consumed

—

Irxc! LI

rved Car

bo us

4°

(

_ (

x^'

(.S^

a^cn

O 1 _J— • —

4- — t

_^^

^— -^

/\ c

-J+--

r
<t

1- IT

low/at

(X*

— ^

[— —

FIG. 304. RELATION BETWEEN POWER CONSUMPTION AND CANDLE-POWER.

A Lamp with right-angle carbons.

Alternating current with a rheostat gives the least light.

Alternating current with a transformer gives more light than when a rheostat

is used.
Alternating current with a rectifier gives the greatest amount of light.

562 CANDLE-POWER OF ARC LAMPS [Cn. XIII

Direct current with a rheostat gives less light than alternating current with a

rectifier.

Direct current, if only the power consumed at the arc is counted, gives the
greatest illumination of all for a given power input, (left upper curve),
i.e., 10,000 candle-power for less than two kilowatts of power.
B Lamp with inclined carbons.

Alternating current with a rheostat, the least light.
Direct current with rheostat, next.
Alternating current with a transformer, next.

Alternating current with a rectifier gives the greatest illumination for the
power consumed.

The upper left hand curve shows that direct current gives the greatest
amount of light if only the power consumed by the arc is considered and that
wasted in the rheostat is not counted.

If the sets of curves for the right-angle lamp and those for the
inclined-carbon lamp are compared it will be found that the right-
angle lamp gives the most light for the same current in every case.
The light given for the same power input is the same with rectified
current for both styles of lamp. With either alternating or direct
current and resistance, the right-angle lamp gives the greater light,
but with alternating current and a transformer the right-angle lamp
gives less light. This is due to the higher voltage of the right-angle
arc when used with alternating current, the right-angle arc requir-
ing about 50 volts while the inclined carbon arc requires but 30
volts.

In the table (§ 763) is shown the power in kilowatts drawn from
the line for different intensities of the light. This table was made
from the curves in fig. 3046 and applies to the inclined carbon
lamp, with no volt supply.

In the table (§ 764) is shown the candle-power for different
amounts of power consumption.

§ 769. Distribution of intensity in the different directions with
the different forms of projection arc.— Fig. 305-306 show the dis-
tribution of light around the different forms of arc lamp. The
distance from the center to the curved line gives the candle-power
of the lamp in the given direction. Fig. 305 shows that the right-
angle arc has 3,750 c. p. in a horizontal direction, 4,000 c. p. 15°
below the horizontal, and 2,900 c. p. 15° above the horizontal.
These curves show the results of actual experiments. The light
coming mostly from the crater, a slight change in the position of the

CH. XIII]

CANDLE-POWER OF ARC LAMPS

563

FIG. 305. DISTRIBUTION OF LIGHT INTENSITY ABOUT RIGHT-ANGLE ARCS.

564 CANDLE-POWER OF ARC LAMPS [Cn. XIII

o Direct current (D. C.).

x Alternating Current (A. C.).

The direction of a given point on the curve represents the direction in which
the light intensity was measured. The distance of the point from the center
of the figure represents the intensity in the given direction. For example, 15°
above the horizontal the direct current arc has 2,900 candle-power while the
alternating current arc has 850 candle-power.

The numbers around the outside represent the angle in degrees while those
on the radius represent candle-power.

carbons or the angle of the craters on the carbons causes a great
change in the distribution of light.

§ 770. Right-angle electrodes. — If the right-angle arc is used,
take care to hold the crater in the best position, i. e., facing the
condenser, otherwise a poor light will result. Fig. 294-295 show
about the best position which can be maintained. The distribu-
tion from this arc with direct current is shown in fig. 305. The
distribution of light with an alternating current right-angle arc is
shown in fig. 306.

§ 771. Converging electrodes. — The distribution of light with
converging carbons (55°) with alternating current is shown in fig.
306.

CANDLE-POWER OF ARC LAMPS

§ 773. Intrinsic brilliancy of the crater. — Blondel found that
the intrinsic brilliancy of the positive crater of the carbon arc was
nearly constant, irrespective of the current, at about 158 candle-
power per square millimeter for solid carbons, and 130 candle-power
per square millimeter for cored carbons. This is equivalent to
97,000 candle-power per square inch for solid, and 84,000 candle-
power for cored carbons (§ 7733).

The increase in candle-power of the arc caused by an increase in
current is due, not to an increase in the brightness of the crater,
but to an increase in its area. This is illustrated in fig. 294, which
shows a photograph of a right-angle arc with 10 amperes and with
20 amperes direct current. The increase in the size of the crater
is apparent.

As has been pointed out elsewhere (Ch. IX, XIV), with small
openings such as with microscopic objectives, when the crater

CH. XIII] CANDLE-POWER OF ARC LAMPS

565

FIG. 306. DISTRIBUTION OF LIGHT INTENSITY ABOUT ALTERNATING CURRENT
ARCS WITH CARBONS AT 90 AND AT 55 DEGREES.

566 CANDLE-POWER OF ARC LAMPS [Cn. XIII

90° Right-angle arc (dotted lines).

55° Arc with V-arranged carbons (full lines).

The numerals around the semicircle represent degrees, while those along the
middle radius represent candle-power. It is to be noted that with the V-
arrangement where both craters supply light that there is considerable gain
over the right-angle arrangement.

image becomes too large to enter the opening (objective front),
there is no advantage to be gained by increasing the current, as
this merely increases the size and not the brightness of the crater
and the crater image.

§ 774. Visible and invisible radiation. — It is a well known fact
that, of the total energy supplied to an arc lamp, but a small part

FIG. 307. NORMAL SPECTRUM ILLUSTRATING THE SEGMENT OF RADIATION

WHICH is VISIBLE.

The longest radiation represented in this diagram has a wave-length of 2 f-<
and is at the base of the triangle. The intermediate wave-lengths occur in
regular sequence.

The segment of visible radiation occurs between wave-lengths .68 M and .40 M.
Other waves shorter than .40 n form the ultra-violet, and those longer than
.68 M the infra-red part of the spectrum.

Under some conditions waves longer than .68 M and shorter than .40 M may
be seen, but the radiation for useful vision falls between those wave-lengths.
The height of the lines in this diagram represents the wave-lengths magnified
20,000 times at that particular point in the spectrum.

If the visible radiation is passed through a prism or a diffraction grating, the
wave-lengths are arranged in regular sequence from the longest to the shortest
as shown in the diagram. The longest visible waves appear red to the normal
eye and the shortest violet, with the orange, yellow, green, blue, and indigo in
between.

§ 773a. Blondel, Proceedings of the International Electrical Congress.
Chicago, 1893.

Bulletin of the Bureau of Standards, Vol. r, p. 122 and reprint 8.

CH. XIII] RADIANT EFFICIENCY OF ARC LAMPS

567

appears in the form of radiation visible to the eye as light. A large
amount of energy is radiated in the form of ether waves of such
great length that they do not effect the eye and are called infra-red
radiation. A small amount of energy is radiated in the form of
very short invisible waves capable of exciting fluorescence and
affecting a photographic plate, this is called ultra-violet (fig. 307).

RADIANT EFFICIENCY OF ARC LAMPS

§ 775, 776. In 1911 some experiments were made to determine
the entire energy radiated by the arc, and the relation of this energy
to the visible part of the radiation (§ 776a).

Briefly, the method consisted in getting side by side two patches
of light, which are photometrically equal. One of these patches

FIG. 308. ARRANGEMENT OF APPARATUS TO MEASURE L'/R.
(From the Physical Review).

Energy from the source L can reach the thermo- junction of the radiomi-
crometer Ra by either of two paths, (a) direct, no absorption except by air,
(b) through the prism train P.

Light from the source is focused by the condenser Cx on the adjustable slit
SIt is rendered parallel by the lens C2, dispersed by the prism P and focused as
a spectrum R-Vby the mirror MT. The screen S2 is placed in the red end of the
spectrum so that it cuts off all of the infra-red to .68/n. The mirror M2 reassem-
bles the spectrum to a patch of white light at the radiomicrometer.

The intensity of the patch of direct light is fixed by the brightness and dis-
tance of the source L, but that of the other patch Wean be varied by widening
or narrowing the slit 5, until it is of the same brightness as the direct light.

The prism consists in a 60° hollow prism of carbon bisulfide immersed in a
square glass cell filled with distilled water. It gives a good dispersion with a
deviation of but 20° from a straight line.

The lenses are of glass. The mirrors are plano-concave lenses, silvered on
the concave side. The focal length of JW, is 50 cm. and of Mz is 25.

568 RADIANT EFFICIENCY OF ARC LAMPS [Cn. XIII

falls on the comparison screen either directly, or after passing
through an 8 cm. layer of water, as the case may be. The other
patch of light is robbed of all of its infra-red by the system of
prisms and lenses shown in fig. 308.

The energy in these two light patches was measured by a radio-
micrometer. The screen S2 could be set to remove all of the infra-
red radiation to any desired point in the spectrum. In this work,
after careful experiment, it was decided to adopt the wave-length
.6&(i as best representing the dividing line between the visible part
of the spectrum and the infra-red. The screen was accordingly set
to remove all radiation of greater wave-length than this.

By this method it was possible to measure the energy repre-
sented by the total radiation of the arc, and that of the visible
portions. It was also possible to insert a water-cell between the
source L and the radiomicrometer and compare the light energy,
with that part of the energy passing through an 8 cm. layer of
water. In order to simplify the discussion, the total radiation of
the arc is called R, the portion getting through the 8 cm. water-cell
is called W and the luminous energy is called L. The measure-
ments were made in such a way that the ratio of L/R or radiant
efficiency was determined, or else the ratio of L/W was measured.

In addition to these values, the transmission of layers of water of
different thickness was measured, that is, the ratio of W/R was
determined. This ratio is called "the Water-Cell Efficiency" and
was determined for a layer of water 8 cm. thick. The transmission
of layers of other thicknesses is shown in § 849.

§ 777. The results of these measurements for various sources
are shown in the table (§ 778). The most important values are for
the positive crater of the right-angle carbon arc and for the right-
angle arc with alternating current.

The positive crater shows a radiant efficiency (L/R) of roughly
10%, that is 10% of the energy radiated is visible as light, the other
90% of the energy is mostly in the infra-red. "The water-cell
efficiency" (W/R) varies from 18% to 28%, averaging roughly
25%, that is, one quarter of the energy radiated gets through the

§ 776a. See H. P. Gage, The Radiant Efficiency of Arc Lamps, Physical
Review, Vol. 33, p. in, Aug., 1911.

CH. XIII]

RADIANT EFFICIENCY OF ARC LAMPS

569

water-cell. Of this 25%, 43% is light and the rest is infra-red.
This shows the advantage of using a water-cell as there is only
one quarter the heating effect with the water-cell as without it.

With the alternating current arc, the corresponding figures are
approximately; Radiant efficiency (L/R) 6.4%, Water-cell effi-
ciency (W/R) 15.6%, and of the energy getting through the water-
cell (L/W) 41% is light. In this case it is seen that the water-cell
removes an even greater proportion of energy and hence its bene-
ficial effect is even greater with alternating current than with
direct current. For the practical application of these values, see
§ 850, and fig. 342.

§ 778. Table showing the relation of light energy to the
total radiation of various light sources.

(From the Physical Review, August, 1911)

Source

&
1

C. P.

L/W
Per
Cent.

W/R

Per
Cent.

L/R
Per
Cent.

WATTS

As RADIATED

AT 100% EFF.

W.P.C.

C.P.W.

o
A

C.P.W.

g.S

|&g

Carbon arc

7-5

1550

I8.5

7-9

607

•39

2.6

.031

+ Crater . . .

10
15

2300
3850

42.9

21.7
27.0

9-3
n.6

785
1148

73
133

•34
•30

3-0
3-4

.032
•035

31
29

5600

28.7

12.3

1614

199

.29

3-5

.036

— Crater . . .
A. C.

(est.)

8.25

3-3

377

Shaded . . .

700

40.7

17.8

7-2

457

•65

i-5

.047

Entire ....

20
15

1 200
700

20.9
15-6

8-5
6-3

618
590

53
37

•51

.84

2.O
1.2

.044

•053

23
19

264.

Arc stream ...

6-5

*"T

Flame arcs

Entire arc

Yellow .... 13.5

2580

57-i

26.9

15-4

430

•17

6.0

.026

29O

White ....
Arc stream

13-5

1440

45-7

31-8

14.6

476

•33

3-o

.048

264

Yellow .... 13.5

49-5

White 13.5

54-5

50-5

27-5

Nernst through

copper sulphate

26.4

IOO

-665

.025

39-5

497

Hefner (Angstrom)

•9

•363

H-3

.032

12.3

.08

.047

21.3

268

A. C. Alternating current.
Amps. Amperes.
C. P. Candle-Power.

570 ENERGY FOR MOVING PICTURES [Cn. XIII

L/W The ratio of the luminous energy (L) and the total energy getting
through the water-cell (W) (Water-cell 8 cm. thick).

W/R The ratio of the energy getting through the water-cell (W) and the
total energy (R) radiated by the light source.

L/R Ratio of light energy (L) and the total energy (R) radiated by the
source.

R Total energy radiated by the source.

L The light energy radiated by the source (fig. 307).

W. P. C. Number of watts required for each candle-power with the different
sources.

C. P. W. Number of candle-power given by each watt with the different
sources.

In the right-hand column are given the meter candles or lumens for each watt
of energy in the luminous part of the spectrum with the different sources.

CALCULATION OF THE ENERGY REQUIRED FOR THE PROJECTION OP
MOVING PICTURES

§ 779. It is interesting to calculate, from the data on radiant
efficiency, how much energy is required to project a moving picture.
This has an important bearing on the fire risk with such projection.
Suppose, for example, the picture is to be 3.7 x 5 meters in size
(12 x 16.5 ft.), a suitable size for a 30 meter (90 ft.) hall. Its area
will be 18.5 square meters (298 sq. ft.). A suitable average illumi-
nation of the screen would be 100 meter candles or about 10 foot
candles. As the revolving shutter removes half the light, the
actual momentary illumination of the screen must be 200 meter
candles or 200 lumens per square meter.

Basing the calculations on this, it is seen that 18.5 x 200 or 3700
lumens will be required. When using the right-angle carbon arc
with direct current the light represented by one watt when radiated
in the visible part of the spectrum is 377 lumens (§ 778). In order
to get 3700 lumens it requires 3700/377 =9.8 watts of light energy.
This energy must get through the aperture plate which is 2.5 cm. x
1.75 cm. and which has an area of 4.2 square centimeters (i in. x ^
in., area ^ square inch) hence the light energy per square centi-
meter of film area is 9. 8 74. 2 = 2.34 watts per square centimeter
(§ 779a)- When, however, the entire radiation from the arc is
used, only 10% of which is light, the energy is 10 times as great, and
even when a water-cell is used where 43% of the energy is light, the
energy is 2.3 times as great. These results are shown in tabular

CH. XIII] ENERGY FOR MOVING PICTURES 571

form below, together with the corresponding values when alternat-
ing current is used.

§ 780. Radiant energy passing the aperture plate when using
right-angle lamp with direct current. — Power passing Powei for each

through aperture square centi-
plate meter of fi'.m

Total radiation of arc, of which 10% is light. 98 watts 23 .4 watts
Radiation passing through water-cell, of which

43% is light 22.8 watts 5.44 watts

Visible radiation only, 377 lumens per watt . 9.8 watts 2.34 watts

§ 781. Radiant energy passing aperture plate when using right-
angle lamp with alternating current.— Power passing Power for each

through aperture square centi-
plate meter of film

Total radiation of arc, of which 6.4% is light 222 watts 53.0 watts
Radiation passing through water-cell, of which

41% is light 34. 4 watts 8.2 watts

Visible radiation only, 264 lumens per watt . 1 4 watts 3 . 2 watts

§ 782. Effect of opacity of the film. — When a nearly trans-
parent film is used, a large proportion of this radiation passes
through, but when a nearly opaque film, such as the title is shown,
almost all of this energy is absorbed and converted into heat.
From these tables it is not difficult to understand why, if there is
no water-cell used, the film is likely to spoil or ignite if it is stopped
for a few seconds while the light is falling on it. Take the example
of the light furnished by the alternating current arc such as is used
in a great many places. Here the film is absorbing energy at the
rate of 53 watts per square centimeter, which is faster than the
surrounding air can cool it. If now a water-cell is used, the energy
rate is reduced to 8.2 watts. Experiment has shown that under
these conditions with the water-cell, the heating effect is not great
enough to ignite even a black celluloid film if for any reason it
should stop moving. But it must be remembered that even if a
water-cell is used the film would catch fire if held in an extremely
concentrated beam. (For the time of ignition of film see § 596) .

§ 779a. If the new standard size for the opening in the aperture plate
(§ 5?oa) were used, the figures in the example would be slightly different, but
the principle is shown just as well in the statement here given.

CHAPTER XIV
OPTICS OF PROJECTION
§ 790. Apparatus and Material for Chapter XIV :

See the optical apparatus in Chapters I to XL

§ 791. History of the optics of projection and references to
literature. — See the appendix and the works of reference in Ch. I,
§ 2 ; works on general physics, optics and astronomy.

§ 792. For the most successful use of projection apparatus it
is necessary to understand some of the simplest principles of
optics, and to keep in mind that in the projection of images two of
the fundamental phenomena of optics are constantly present.
These two phenomena are: (i) Reflection and (2) Refraction.

§ 793. Reflection. — By this is meant the change in direction of
rays of light when they meet a surface. The change in direction
of a beam of light striking a surface depends upon the character of
that surface. The principle kinds of reflection are, regular reflec-
tion, irregular reflection, and semi-regular reflection.

§ 794. — Regular reflection. — If the surface is smooth, as in a
mirror, the incident and reflected ray will be in the same plane and
will make equal angles on opposite sides of the normal erected at

FIG. 309. REGULAR REFLECTION AT A POLISHED SURFACE.
The angle of incidence i, is equal to the angle of reflection r; and the incident
and reflected ray are in a plane perpendicular to the reflecting surface.

572

CH. XIV] REFLECTION AND REFRACTION 573

the point of reflection (fig. 309). Most cases of irregular and
semi-regular reflection if considered from the standpoint of a small
enough part of the surface are really cases of regular reflection;
that is, any small particle of which the surface is made reflects the
light striking it regularly, but each particle of the surface reflects
the light in a different direction. Hence, taken as a whole, such a
surface will not reflect the light regularly. (See Mirrors § 800).

§ 795. The use of regular or mirror reflection in projection is
illustrated by the mirrors used in opaque lanterns (fig. 95-1 10) and
by the mirrors and prisms used with drawing apparatus (fig. 180-
204).

With regular or mirror reflection the observer can only see the
light when he is in the path of the rays either before or after the
reflection (§ 796a).

§ 796. Irregular reflection. — If the surface is irregular then
the light striking it is reflected in various directions depending upon
the position of the irregularities on the surface receiving the light.
If these are very small, as in dust or smoke or as on the surface of
snow, white cloth or paper, etc., then the reflected light is scattered
practically equally throughout the entire hemisphere toward
which the surface faces (fig. 310).

§ 797. The use of irregular reflection is illustrated by the
reflected rays from the white screen upon which the image is pro-
jected by the magic lantern, pro-
jection microscope, etc. The im-
age appears almost equally bright
from any point in the room.

§ 796a. If there is dust, smoke or
fog in the path of the beam of light either
before or after reflection, the minute
particles in the smoke or fog irregular-
ly reflect some of the light and one can
see it at any angle (fig. 320-323). Dust
or scratches on the surface of themir- FlG- 3™- IRREGULAR OR DIFFUSE
ror enables one to see where the beam REFLECTION.

of light strikes its surface. If there is A ray of light striking a rough sur-
no irregular reflection then one can only face is scattered equally in all direc-
see a beam of light when in its path. tions.

574

REFLECTION AND REFRACTION

[Cn. XIV

FIG. 311. SEMI-REGULAR REFLECTION.

Light striking some surfaces is scattered
unequally, being reflected to a greater extent
in one direction than in others. This repre-
sents the kind of reflection produced by me-
tallic-faced screens.

§ 798. Semi-regular
reflection. — This occurs
when a surface is imper-
fectly polished, that is, if
the surface is an irregular
one but not sufficiently
irregular to scatter the
light equally in all direc-
tions. The most famil-
iar example is a surface
coated with silver or
aluminum powder. Here
the individual metal par-
ticles reflect the light

regularly, but the different particles lie at different angles and
reflect the light in different directions. The result is that
the light is scattered in all directions, but the greater part of
the light is reflected in the same general direction as it would be if
the surface were perfectly polished. This is shown in fig. 247, 311,
where the length or number of the rays after reflection indicates
the amount of light reflected in that direction.

§ 799. The use of this semi-regular reflection in projection is
in the metallic screens or mirror screens sometimes used in long,
narrow auditoriums. Such screens do not appear equally bright
when seen from all parts of the room but appear brightest when
seen in the direction of the regular reflection from the lantern, that
is, when the observer is nearly in line with the lantern, and they
appear very dim when seen from the sides of the room, 1 5° or more
from the axis of the lantern (§ 630).

§ 800. Mirrors. — It is possible to construct surfaces of metal
or silvered glass which are sufficiently smooth to reflect nearly all
of the light in accordance with the law of regular reflection (§ 794).
Such a surface is called a mirror. It may be plane or curved (con-
cave or convex).

If the mirror surface is plane, it follows from the law that the
rays will have the same angular relation to one another after

CH. XIV]

REFLECTION AND REFRACTION

575

. . _
\ 2 3

FIG. 312. REFRACTION AT PLANE AND AT CURVED SURFACES.
(From The Microscope)

A-C The refracted ray, changing its direction at the point of contact with
the denser medium (line shading).

N-N' Normal to the refracting surface.

B Point of refraction.

A-C1 In (i ) The course the ray would have taken if the medium from A-C1
had been homogeneous.

C-C'1. Course the ray would have taken from the point C if the medium
from C' to C11 had been homogeneous.

(j) Refraction from air to water.

(2-j) Refraction from air to crown glass. As shown, if the incident ray is
at 45°, the refracted ray will be at approximately 28° with the normal.

reflection as before, that is, if the rays were parallel before reflection
they will be afterward; and if they were converging or diverging
before they will converge or diverge after reflection.

With a curved mirror the angular relation after reflection is not
the same as before. For example, with a concave mirror, parallel
rays are bent towards one another and finally meet at what is
called the focal point. If the mirror is convex then the rays are
made to diverge on leaving the mirror.

i Incident ray in the air above the glass.

r Ray of light below the glass, after refraction.

i' Course of the ray of light if the glass were
absent.

r' The refracted beam traced backward above
the glass to show its apparent origin.

n n' Normals where the ray enters and leaves
the glass.

This figure shows the displacement of the light
by refraction through media with plane surfaces,
and that the refracted light is parallel with the
incident light.

Air

Glass

FIG. 313. REFRACTION BY

GLASS WITH PARALLEL

FACES.

576 LENSES AND THEIR ACTION [Ca. XIV

§ 801. Refraction. — By refraction is meant the change in
direction of a ray of light in passing from one transparent medium
into another.

The amount of bending depends upon two conditions :

(1) The greater the angle of incidence of the light, that is, the
farther from the perpendicular or normal that the light strikes the
surface, the greater will be the bending on entering the second
medium. And this increase is not simply with the increase of the
angle of incidence, but proportionally greater, that is, in accordance
with the law of sines (fig. 312).

(2) The bending also depends upon the difference of density
of the two transparent media. If the difference is great, the
refraction will be great, and if the difference of density is small, the
refraction will be proportionally small. See also chromatic aberra-
tion (§8 10, fig. 337).

The phenomena of refraction were worked out with great
accuracy by Ptolemy in the first and beginning of the second cen-
tury A.D. ; but the precise mathematical expression for the law of
refraction was not found until about 1500 years later (Snell's and
Descartes' law of sines). This law of sines includes both elements
mentioned above, and is expressed thus :

Sine of the angle made by the incident ray

-- = index of retraction.

Sine of the angle made by the refracted ray

§ 802. Lens.— By making one or two bounding surfaces of a
transparent body curved, rays of light traversing the body are
made to converge or to diverge. Any transparent body having one
or both of its opposite faces curved is called a lens. The curved
surfaces are usually segments of spheres, as a spherical surface can
be ground and polished more accurately than can any other.

§ 803. Principal axis. — The straight line passing through the
centers of the two spheres of which the surfaces of a lens are seg-
ments is called the principal axis. This axis is perpendicular to
both surfaces of the lens (fig. 314).

§ 804. Optic center. — This is the point in a lens, or near it,
through which light rays pass without angular deviation, that is,

CH. XIV]

577

LENSES AND THEIR ACTION

FIG. 314. THE OPTIC CENTER AND THE PRINCIPAL OPTIC Axis OF VARIOUS

FORMS OF LENSES.

(From The Microscope)

c c' Centers of curvature of the different lenses.

As shown in all the figures each curved face of the lens is a part of a sphere
of greater or less size.

c.l Optic center of the lens.

r Radius of the sphere from which the lens is derived. (Radius of curva-
ture).

1 . Double-convex lens, the two faces having different curvatures.

2. Double- concave lens, the two faces of different curvatures.

3. Plano-convex lens.

4. 5, 6. The same showing the optic center (cl).

7. Plano-concave lens showing optic center (cl).

8. Converging, meniscus lens. The optic center (cl) is outside the lens,
on the convex side.

9. Diverging meniscus lens with the optic center (cl) outside the lens, and
on the concave side.

578 LENSES AND THEIR ACTION [Cn. XIV

the ray before and after it passes the center of the lens extends in
parallel lines. As shown by the following diagrams the optic
center is found by drawing parallel radii from the two curved sur-
faces, or from the curved and plane surface, and joining the ends
of the radii. The center of the lens is the point where the line
joining the outer ends of the parallel radii cross the principal axis
(fig. 314).

The reason why light rays traversing the optic center have no
angular deviation is as follows : The radii are perpendicular to the
surfaces of the lens; and the tangent plane perpendicular to the
radius, is tangent to the sphere at the end of the radius. As the
two tangents to parallel radii must themselves be parallel, it follows
that a ray of light passing from one tangential point to the other is

FIG. 315. CONJUGATE Foci C,C2 ON THE PRINCIPAL Axis.

traversing a body with parallel surfaces at the point of entrance
and departure, and hence it will suffer no angular deviation
although the ray may be displaced, as in traversing any thick
transparent body with plane faces (fig. 313). With meniscus
lenses the crossing point (optic center) is on an extension of the line
joining the centers of curvature (fig. 314).

§ 805. Secondary axis. — Every line traversing the optic center
of a lens, except the principal axis, is a secondary axis. It follows
therefore that every secondary axis must be more or less oblique
to the principal axis (fig. 317).

§ 806. Principal focal point. — The principal focus or focal
point of a lens or of a lens system like a condenser or a projection
objective, is the point on the principal axis where rays of light
parallel with the principal axis before entering the lens or combina-
tion, cross the principal axis after leaving the lens or objective.
It is also sometimes called the burning point (fig. 319).

CH. XIV]

LENSES AND THEIR ACTION

579

With a concave mirror it is likewise the point on the principal
axis where rays parallel with the principal axis before striking the
mirror are made to cross the principal axis after being reflected by
the curved face of the mirror. This point is situated half way
between the mirror face and the center of curvature.

§ 807. Conjugate foci, and the mutual relation of images.—

In figures 3 1 5-318, are shown conjugate foci on a principal and on a
secondary axis. In each case the object and the image might

FIG. 316. METHOD OF IMAGE FORMATION ON THE PRINCIPAL Axis.

In this case the object (CJ and the image (C2) are equally distant from the
center of the lens, hence they are of the same length, and the distance between
them is four times the principal focus of the lens.

FIG. 317. CONJUGATE Foci ON A SECONDARY Axis.

The secondary axis passes through the optic center of the lens, and the con-
jugate focus C.^ is below the principal axis if the point CT is above it.

change places without any change in the mutual relation of the
object and image. For example, if the screen picture with a magic
lantern were an actual scene and the magic lantern pointed toward
it as in projection, a small image exactly like the lantern slide
would be formed at the level of the lantern slide. It is from this
mutual relation of object and image that they are said to be
conjugates.

§ 808. How to obtain the principal focus experimentally.—
This is accomplished by holding the lens or combination, or the
mirror, with the principal axis pointing directly toward the sun.

580 LENSES AND THEIR ACTION [Cn. XIV

The point where the image of the sun appears indicates the prin-
cipal focal point, or the burning point.

Another way to get the equivalent focal length or focus of an
objective is to put it in position on an optical bench like that shown
in fig. 159 and then to use a metric rule (fig. 178), or a lantern slide
of such a rule as object, and a white screen or a ground-glass on the
other side of the objective. The object and the screen are then
moved toward and from the objective until the image is of exactly
the same size as the object. The distance apart of the image and
the object is four times the focal length of the objective (fig. 316).

FIG. 318. IMAGE FORMATION ON A SECONDARY Axis.

With a good lens the arrows C, and C2 are both perpendicular to '

the principal axis.

C, Object.

C2 Image. When object and image are of the same size, as here, the image
is as far below the principal axis as the object is above it.

SPHERICAL AND CHROMATIC ABERRATION AND MEANS OF CORRECT-
ING THESE DEFECTS

§ 809. Spherical aberration. — By this is meant the unequal
bending of the light rays in different zones of a lens. As shown in
fig. 320, the rays passing through the outer zones of a spherical
lens are proportionally more bent than those which pass nearer

§ 808a. Equivalent focus. — The term equivalent focus is often employed
for compound optical systems like objectives. This means simply that the
objective gives the same magnification or reduction in a given case as a simple
lens of that focus would give.

For example, the simple lens in fig. 209, with the object 2 cm. from the
center of the lens, gives an image at 8 cm., four times as large as the object.
Now any compound system of lenses which gives a magnification of four under
similar conditions is said to be equivalent to this simple lens. The expression,
equivalent focus is frequently designated by the initial letters of the words,
e. f.

CH. XIV]

LENSES AND THEIR ACTION

the axis. It results from this that the border rays cross the axis
considerably nearer the lens than the central rays, hence, with
parallel rays, instead of one focus, there are many foci drawn out in
a line. This is shown by the bright core in the photograph of the
cone of rays in fig. 322.

Except with a symmetri-
cal, double convex lens the
amount of spherical aberra-
tion depends upon which face
of the lens receives the inci-
dent light, and whether the
incident light is parallel,
diverging or converging.

With plano-convex lenses,
as shown in fig. 320-323, the
spherical aberration with
parallel incident light is less
when the parallel light is
incident on the convex face
than when the lens is turned

Fic.~3i9. THE PRINCIPAL Focus OF A

CONVEX AND OF A CONCAVE LENS.

(From The Microscope)

Axis, Axis. The principal optic axis of
the lenses.

F The focus. In the convex lens it is
where the light rays actually cross the axis.
In the concave lens it is where they would
cross if produced backward as indicated
SO that the light IS incident by the broken lines.

upon the plane face.

For diverging rays the plane face should receive the incident
light, and for converging rays the convex surface should receive the
light to insure minimum spherical aberration. With all lenses,
the general rule to follow is that for minimum spherical aberration,
the light rays should be equally bent on entering and on leaving the
lens i. e., at both refracting surfaces. Furthermore, with the same
light beam, the aberration is greater for lenses of large curva-
ture than for lenses of small curvature.

To overcome this aberration, a concave lens is combined with a
convex lens, and so proportioned that the too great converging
effect of the outer zone of the convex lens is just counterbalanced
by the diverging effect of the concave lens in its various zones (fig.
324). A perfectly corrected, or aplanatic combination brings all
the parallel rays to one focus.

LENSES AND THEIR ACTION

[CH. XIV

FIGS. 320-321. BEAMS OF PARALLEL LIGHT TRAVERSING PLANO-CONVEX

LENSES TO SHOW MAXIMIUM AND MINIMUM SPHERICAL

ABERRATION.

Figure 320 shows the lens in the position to give the maximum aberration.
That is, the border rays cross the axis much nearer the lens than the inter-
mediate rays.

Figure 321 shows the lens in the position to give the minimum aV erration.
The border and the intermediate rays cross the axis mere nearly in the same
place.

This picture was made in a dark-room (fig. 179). The room was filled with
smoke, and the light was partly scattered by the smoke, thus making the rays
visible from the side (§ 7963). The first element of the triple condenser was
covered in the parallel beam, with a perforated metal disc. This permitted only
minute cylinders of light to escape along the diameter of the condenser.

The lens in fig. 320 appears dark as it was clean . The one in fig. 321 appears
white because there was some very fine talcum dust on the face.

CH. XIV] LENSES AND THEIR ACTION 583

§ 810. Chromatic aberration. — By this is meant the separation
of the images produced by the different wave lengths of which
white light is composed. Newton thought this was a purely
refractive action and therefore could not be corrected without at ]
the same time overcoming all the refraction, hence he thought
there could be no images formed by lenses or combinations of
lenses without the presence of the color defect. But later it was
found that some glass separated the light into colors more markedly
than others of the same refraction. Now by combining two kinds
of glass which act differently in this respect it was found possible

FIG. 324. ACHROMATIC LENSES.
(From Lewis Wright, Optical Projection).

By combining a convergent or convex crown glass lens with a divergent or
concave flint glass lens it is possible to get a combination which is largely free
from chromatic as well as spherical aberration. In all but D and the right-
hand combination, but two lenses are used; in those, one flint and two crown
glass lenses are used.

to bring two or three of the colors to one focus, and thus to produce
practically colorless images by means of lenses (fig. 324).

Usually an objective for forming images — photographic objec-
tive, microscopic objective, projection objective — is corrected both
for spherical and for chromatic aberration, so that the image is
correct in every way. This is accomplished by combining concave
and convex lenses of the right form and composition. Sometimes
also, as with the apochromatic, microscope objectives, a natural
mineral — fluorite — is introduced to make a more perfect correc-
tion than could be accomplished by artificial glass.

584 IMAGE FORMATION, INVERSION OF IMAGES [Cn. XIV

IMAGE FORMATION WITH THE MAGIC LANTERN

§ 811. Ideal case. — When using transparent lantern slides
with a magic lantern and a small source of light the ideal arrange-
ment is that shown in fig. 325.

L, is a point source of light (crater of the arc light). The con-
denser C, focuses this light at the point O,in the optical center of
the objective. The slide-carrier S, is placed just in front of the
condenser. The objective O, is at the proper distance from S, to
formja real image of the slide on the screen. All of the rays of
light from S pass directly through the center of the objective O, and

FIG. 325. LANTERN-SLIDE PROJECTION; No SPHERICAL ABERRATION.

This shows an ideal case where there is a point source of light, and a con-
denser without spherical aberration. The light from the condenser crosses at
the center of the objective (O) and goes on without deviation to the image
screen.

L Light source.

C Condenser.

S Lantern slide.

O Projection objective.

hence undergo practically no deviation. If the source of light L,
were really a point source, and the condenser C, had no spherical
aberration, the shadow of the lantern slide S,in the screen without
an objective would be just like the image which is projected by the
objective.

§ 812. Inversion of the image. — In their passage from the
lantern slide to the screen the rays pass from the top of the slide
to the bottom of the screen, and from the bottom of the slide to the
top of the screen. In like manner the rays from the two sides of
the slide cross before reaching the screen (fig. i.)

This crossing of the rays gives what is known as an inverted
image.

CH. XIV] IMAGE FORMATION, INVERSION OF IMAGES

585

§ 813. Actual case. — The actual case differs from the ideal case
in that the condenser has a considerable amount of spherical aber-
ration and that the source is not a point but is somewhat extended.

§ 814. Spherical aberration of the condenser. — The effect of
the spherical aberration of the condenser has not been sufficiently
studied up to the present, but it exerts a good deal of influence in
projection especially with micro-projection and with moving
pictures.

FIG. 326A. THE PATH OF THE LIGHT RAYS FROM THE DIFFERENT
ZONES OF THE CONDENSER.

FIG. 3266. THE APPEARANCES ON A CARD HELD IN THE DIFFERENT PARTS
OF THE CONE OF LIGHT.

a, b, c, d, e, f Lines showing where the card was held in the light cone to
give the appearances in B.
g Diaphragm.

As was shown in § 809, the light from a point source will not come
to a focus in a point, but the rays passing through the margin of the
lens will be relatively more bent and will cross the axis sooner than
those which pass through the lens near the axis (fig. 320-323, 337).

A curious phenomenon is the effect on the illumination of the
screen when a diaphragm is used to cut off the margin of the cone.

586 IMAGE FORMATION, INVERSION OP IMAGES [Cn. XIV

If placed at b or e (fig. 326), it cuts off the margin of the cone and
lets the center through but if placed at g, light from the center and
the margin of the condenser gets through, but light from a zone part
way out is removed (fig. 327).

The result on the illumination of
an object placed in the converging
cone of light will be as shown in fig.
3266. An object placed near the
condenser will be evenly illumina-
ted. As it is moved away from the
condenser face towards the crossing
of the rays the outer edge first be-
comes more brightly illuminated
than the center and then the spotted
effects shown in the figure will be

FIG. 327. APPEARANCE ON THE seen. At no position will there be
SCREEN WHEN ILLUMINATED BY ... ,. . ,, , .

THE CONDENSER SHOWN IN an even illumination of the object

FIG. 326A IF DIAPHRAGM is when using a point source except

HELD IN THE POSITION g. , , , , . , . , ,

when the object is placed next to the

condenser face, a. If it is necessary to eliminate the spotted effect
due to spherical aberration as when exhibiting moving pictures one
must use an extended source of light, so that the aberration figures
from the different points of the source overlap. The arc lamp with
15 to 20 amperes direct current is sufficiently extended to give an
even illumination provided a short focus condenser is used.

§ 815. Spherical aberration of the condenser with the magic
lantern.— When using the magic lantern the spherical aberration
of the condenser, unless exceedingly great, is of no special disad-
vantage. The rays from the different parts of the slide will not all
cross at the center of the objective but will cross at different points
on the axis (fig. 320, 337). If the objective is of good quality and
of large enough diameter to include all of the beam of light there
will result a good screen picture.

§ 816. Effect of an extended source. — Let a, fig. 330, represent
a point in the slide S. Light which has come from all parts of the

CH. XIV[ IMAGE FORMATION, INVERSION OF IMAGES 587

source L, after passing through (a) will spread out over an angle
and strike the objective lens O. The purpose of the objective lens
O, being to collect all of the light from the point (a) on the slide and
to bring it together at the point (a') on the screen. This is exactly
similar to the case of image formation of a self-luminous or diffusely
reflecting surface described in (Ch. VII, § 273, fig. 90) except that
the light from the point (a) does not spread out in all directions,
but only over the angle x' ay'.
c

FIG. 328. INTERCHANGEABLE MAGIC LANTERN AND MOVING PICTURE

PROJECTION WITH POINT SOURCE AND CONDENSER FREE FROM

SPHERICAL ABERRATION.

a Magic lantern arrangement.

b Moving picture arrangement.

L Crater of the arc lamp as a source of light.

c Condenser.

s,s The lantern slide in (a), and the film in (b).

o, o Projection objectives.

§ 817. Simplicity of the Magic Lantern. — From the above it is
seen that with the arc-light magic lantern the actual case is nearly
the same as the ideal case and the manipulation of the apparatus
is relatively simple.

RELATION OF THE FOCAL LENGTH OF THE CONDENSER TO THE
FOCAL LENGTH OF THE PROJECTION OBJECTIVE

§ 818. Types of condensers. — There are in general use two
main types of condenser; the two-lens type, as shown in fig. 331,
and the three-lens type, as shown in fig. 332. The two-lens type

588

CONDENSERS FOR PROJECTION

[Cn. XIV

has the advantage of simplicity and cheapness while the three-
lens type has the advantage that it has very little spherical aberra-
tion and at the same time it is possible to bring the lamp closer to
the first surface of the condenser, thus utilizing a greater proportion
of the light of the illuminant (§ 8i8a).
c

FIG. 329. ARRANGEMENT FOR INTERCHANGEABLE LANTERN-SLIDE AND
MOVING PICTURE PROJECTION WHEN THE CONDENSER HAS SPHERICAL

ABERRATION.

a Projection of lantern slide.

b Projection of moving picture film.

L Arc-lamp crater, the source of light.

C Condenser of two plano-convex lenses.

S In (a) the lantern slide near the condenser.

S In (b) the moving picture film.

0 Objective for projection.

c' The image of the condenser face.

§ 818a. Types of condensers. — In the development of projection apparatus
almost every form of condenser has been used from a single piano- or double-
convex lens to one composed of three or more lenses. In form, the lenses have
been plano-convex, double-convex, meniscus and parabolic.

For artificial light the condenser is now almost always composed of two or
more lenses.

First element (fig. 332). — The first element may be composed of a single
plano-convex lens or a meniscus, or it may be composed of two lenses. A
meniscus and a plano-convex, or a meniscus and a double convex or finally of
two meniscus lenses. The first element in all cases collects the light from the
source and renders it more or less parallel.

Second element, (fig. 332). — The second element of the condenser may be
a plano-convex or a double convex lens or an achromatic combination.

We have tried the different forms of condensers and have found those com-
posed of two plano-convex lenses, or those with two plano-convex lenses and a
meniscus next the light, most satisfactory (fig. i, 2).

Finally there has been recently produced a parabolic condenser for projec-
tion with the microscope. This form eliminates almost all the spherical
aberration and is promising.

CH. XIV]

CONDENSERS FOR PROJECTION

589

§ 819. The two-lens type of condenser. — In choosing the
objective and other optical parts of the lantern one must first
consider the room in which the projection is to be done and then
choose an objective of such a focal length that the picture will be
of the desired size (Ch. XII, § 635). After the objective is deter-
mined upon, it is necessary to select the condenser lenses of such
focal length that the best results may be obtained. There are two
factors which must be balanced in this choice. First; the closer
the light is to the condenser, that is the shorter is its focus, the

FIG. 330. EFFECT OF AN EXTENDED SOURCE OF LIGHT.

L' (w, x, y, z) Extended light source.

Single Lens Condenser near the source.

5 Lantern slide.

a and b Points on the lantern slide.

O Single lens objective.

y', x' Image of the extended light source on the objective.

/, a Screen image of the lantern slide.

If the light source xy, is not too large, all of the light collected by the con-
denser gets through the objective.

If the light source wz, is too large, the image w' z', will be larger than the
objective and much light will be lost by falling outside the objective.

greater will be the amount of light which it will collect. Second;
the shorter the focus of the condenser, the greater will be its
spherical aberration. In order to get the minimum of spherical
aberration with two plano-convex lenses such as are generally used
for condensers it is necessary to turn them so that parallel or nearly
parallel light strikes the curved surfaces and the diverging light
from the source strikes the plane surface (fig. 321, 323). When the
parallel beam strikes the curved surface of the lens, all of the rays
come to a focus more nearly at the same point than when the
parallel beam strikes the plane surface of the lens (fig. 320, 321).
This requires that the curved surfaces of the lenses shall be turned
towards each other as in fig. 329.

590

CONDENSERS FOR PROJECTION

[Cn. XIV

In order that the combination of the two lenses of the condenser
shall have as little spherical aberration as possible they should be
of about equal focal length. If there is a difference in focal length,
the thicker lens, i. e., the one of shorter focal length, should be
placed next the radiant and the thinner lens, i. e., the one of greater
focal length, should be away from the radiant. The best lenses to
use in a given case can only be determined by experiment but as a
first trial we would suggest the following foci for condensers of 1 1.4
cm. (4^2 inches) diameter for magic lantern work and moving
picture projection.

§ 820. Table of condenser lenses.—

Focus of objective or Distance
of Aperture Plate

FOCUS OF THE LENSES OF THE CONDENSER

Lens next the radiant

Lens away fiom the radiant

15 cm. ( 6 in.)
1 8 cm. ( 7 in.)
20 cm. ( 8 in.)
25 cm. (10 in.)
30 cm. (12 in.)
38 cm. (15 in.)
45.7 cm. (18 in.)

15 cm. ( 6 in.)
15 cm. ( 6 in.)
16.5 cm. ( 6K in.)
16.5 cm. ( 6^2 in.)
1 7. 8 cm. ( 7 in.)
1 7. 8 cm. ( 7 in.)
19 cm. ( 7>i in.)

15 cm. ( 6 in.)
16.5 cm. ( 6^2 in.)
16.5 cm. ( 6}4 in.)
17.8 cm. ( 7 in.)
17. 8 cm. ( 7 in.)
19 cm. ( il/2 in.)
19 cm. ( Tl/£ in.)

With these lenses the light from the first lens will be somewhat
diverging before it strikes the second lens. The best results are
obtained when the two lenses are as close together as they can be
put without touching.

§ 82 1 . The three-lens type of condenser. — The three-lens type
of condenser illustrated in figure 332 is designed on a different
principle. Here the first combination, consisting of a meniscus
lens and a plano-convex or a double-convex lens (fig. ITT), is
designed to render the light from a point source parallel with but
a very small amount of spherical aberration. In order that the
light shall be focused at the center of the objective it is then neces-
sary that the last lens of the condenser shall bring the parallel beam
to a focus where it is wanted, that is, it must have a focus approxi-
mately equal to that of the objective. For long focus objectives
38-46 cm. (15-18 in.) this of course necessitates a rather thin lens
next to the lantern slide (fig. 332).

CH. XIV] IMAGE FORMATION WITH MOVING PICTURES 591

In microscopic projection (Ch. IX) and in drawing with the
microscope (Ch. X) the three-lens condenser with its small spheri-
cal aberration is of great advantage. For micro-projection without
a substage condenser, the final plano-convex lens of the triple con-
denser should have a fociis of about 15-20 cm. (6-8 in.). This will
answer well for objectives as high as 4 mm. equivalent focus (/^
in.). Where a substage condenser is used, the focus of the last

FIG. 331. TWO-LENS CONDENSER FOR PROJECTION.

The condenser (Cond) is shown in connection with the lamp-house and right-
angle arc lamp.

The first lens of the condenser (i) is of shorter focus (i. e., thicker) than the
second lens (2). The condenser is nearer the source of light (L) than the
principal focus of the first condenser lens, hence the light beam between the
condenser lenses is diverging. With this arrangement and the lenses close
together a wider beam of light can be utilized for projection than as if the
condenser were farther from the lamp (see fig. 343). (For a more complete
explanation of this figure see fig. 379.)

plano-convex lens of the large condenser should be longer than
15 cm. One of 25-40 cm. (10-16 in.) is more satisfactory (See
Ch. IX, § 402).

§ 822. Image formation with moving pictures. — When moving
pictures are to be projected, the conditions to be met are not so
simple as with the magic lantern, and one must bear in mind the
actual requirements.

§ 823. Practical requirements. — These requirements are : (The
figures for equivalent focus refer to an actual case). The moving

592 IMAGE FORMATION WITH MOVING PICTURES [Cn. XIV

picture objective was 13.3 cm. e. f. (5^ in.) and the focal length
of the magic lantern objective to go with it is indicated.

1. The dimensions of the aperture plate were 23 mm. x 17.3
mm., diagonal 28.0 mm. (|-| in. x H in., diagonal i^ in.) (For
new standard aperture see § 5;oa).

2. The moving picture is to be thrown on the screen with an
image either as wide or as high as the magic lantern picture or of
the same diagonal.

Lantern slides have a maximum opening of 7.5 cm. wide, 7 cm.
high, diagonal 10.2 cm. (3 in. wide, 2^ in. high, 4 in. diagonal).

FIG. 332. THREE-LENS CONDENSER.

With a three-lens condenser the source of light is placed at the principal
focus of the first element of the condenser, the meniscus and the plano-convex
lens, the position of the lenses being as here shown. This gives a parallel
beam of light. The second element of the condenser should then be of a focal
length to cause the rays to cross at the center of the projection objective (fig. 2).

That one dimension shall be the same with both the moving pic-
ture and the magic lantern, requires that the magic lantern objec-
tive have a focus which is from 3 to 4 times as long as the moving
picture objective, as is shown in the following table :

The two pictures are to be of

Lantern slide objective has an equivalent
focus of

The same width
The same height
The same diagonal

3.17 times that of M. P. objective
4.00 times that of M. P. objective
3.65 times that of M. P. objective

CH. XIV] IMAGE FORMATION WITH MOVING PICTURES 593

In the above case, with a moving picture objective of 13.3 cm.
focus, the focus of the magic lantern objective to use with the
moving picture objective is, for the same:

Width of picture 42.1 cm. 1 6 y2 in .

Height of picture 53.1 cm. 21 in.

Diagonal of picture 48.5 cm. 19 in.

3. The same arc lamp, condenser, etc., are to be used inter-
changeably for either films or slides by simply pushing the appara-
tus sidewise. Usually the slide-carrier is mounted permanently
with the condenser so that the opening is not a circle of the diame-
ter of the condenser but a rectangle 7.5 cm. x 10 cm. (3 in. x 4 in.).

4. Even illumination of the screen. — If the light is not quite
uniform it is better to have the center the brighter rather than the
edge.

§ 824. Ideal case, moving pictures. — The ideal case of projec-
tion (shown in fig. 328 a and b) is where the light is a point source
and the condenser has no spherical aberration. This is the case
which is usually figured, but it is not the best in practice if an
extended source is used.

When changing over to moving picture films the lamp and
condenser are moved to the position b. The objective O, is still
45 cm. (18 in.) from the condenser face where the rays will cross
in the diaphragm plane, and the film is placed 13.3 cm. (5^ in.)
from the objective so that it will be in focus on the screen.

§ 825. Illumination of moving pictures, practical method.—

The method which has been found most successful in lighting
moving pictures is to focus the image of the crater not on the objec-
tive but on the aperture plate. This is because a moving picture
objective usually has a diameter greater than the diagonal of the
film (40 mm. to 65 mm. against 28.5 mm. diagonal; 1^2 in. to 2^in.
against i^s m- diagonal), hence the important point is to get the
light through the film ; the large objective will take in all the light
which can get through the film.

Figures 333-335 show the effects of different methods of lighting.
In practice all three are used together, that is, the film is illum-
inated by the area of the condenser which is not covered by the

594 IMAGE FORMATION WITH MOVING PICTURES [Cn. XIV

slide-carrier and is evenly illuminated by the combined effect of
spherical aberration and an extended image of the crater.

§ 826. Image formation with moving pictures. — Let us trace
the course of the rays from the condenser to the screen assuming
a

°- 333- IMAGE FORMATION OF A MOVING PICTURE FILM WITH AN EXTENDED

SOURCE OF LIGHT.

a,b Second element of the condenser.
L' Image of the source of light.
s, t Film.
Objective.

y, x, z Points on the face of the objective.
b' Image of the condenser.
/' Image of the film on the screen.

FIG. 334. IMAGE FORMATION OF THE MOVING PICTURE FILM WHEN USING
A POINT SOURCE AND A CONDENSER HAVING SPHERICAL ABERRATION.
a, b, c, d, e, f, g Points on the condenser face.
r, s, t Film.
D Diaphragm in front of the objective.

the crater image to cover the entire opening of the aperture plate.
From every point of the condenser as a, fig. 333, light spreads out
over the area s t. Light will reach the point 5, on the film from
every part of the condenser between a and b. From 5, light
spreads out in every direction between the limiting rays b s w, and
a s y, and the objective 0, collects all of this light to the point s' on
the screen. Light from s, reaches 5', between the limiting rays

CH. XIV] IMAGE FORMATION WITH MOVING PICTURES 595

s w sf and s y s.' In the same way light from t, reaches t', between
the limiting rays txt' and tzt'.

The objective 0, will bring an image of the condenser face to a
focus somewhere between it and the screen. In fig. 333 the image
of the condenser face is at the point a' b'. With the magic lantern
the condenser face and the lantern slide being so close together the
image of the condenser face is nearly in focus on the screen.

FIG. 335. THE DOTTED LINES SHOW THE MARGINAL RAYS REMOVED BY THE

SLIDE-CARRIER.
s, t Film.
O Objective.

FIG. 336. SMALL CONDENSER FOR MOVING PICTURES.

This is exactly comparable to lantern projection except that the condenser
and the object are smaller.

§ 827. Image formation when using a point source and a
condenser with no spherical aberration. — The crater image in this
case would be focused at o, and only the rays a s y s', and b t x t',
would be used (fig. 333).

§ 828. Image formation with a point source and a condenser
having spherical aberration. — The condenser must have either no
spherical aberration at all or just the right amount. Fig. 334
represents a condenser having the right amount of spherical aberra-
tion. Consider the effect of each zone of the condenser in illum-
inating the film. The center zone from d to e, lights most of the
center of the film. With this zone onlv, the illumination would

5Q6 IMAGE FORMATION WITH MOVING PICTURES [Cn. XIV

be dim but fairly uniform. The zones from c to d, and e to f, light
a narrow ring of the film near 5 and t, i. e,, a dim center and a bright
outside ring would be produced by the zone from c to/ (fig. 326,
position c). The zone b to c, lights the part of the film between 5
and r, and / to g, lights the part between t and r, the addition of
these zones is to increase the illumination of the center, making the
illumination more uniform. The narrow zones a-b, and g-h, out-
side this, further illuminate the region in the center of the film. It
is necessary to remark that with an actual point source the illum-
ination with this arrangement can never be really uniform but the
"aberration figure" will consist of a bright ring 5 t, a bright point r,

FIG. 337. CONVEX LENS SHOWING CHROMATIC ABERRATION.

(From The Microscope)

The ray of white light (w) is represented as dividing into the short waved,
blue (b) and the long waved, red (r) light. The blue (b) ray comes to a focus
nearer the lens and the red ray (r) farther from the lens than the principal
focus (/). Principal focus (/) for rays very near the axis; /' and/", foci of blue
and red light coming from near the edge of the lens. The intermediate wave
lengths would have foci all the way between /' and /".

at the center (white ghost) and between will be a more or less
evenly lighted disc. When a slightly extended source is used how-
ever, the aberration figures for the different points of the source will
overlap and if the dimensions of the crater image are about one-
third as great as the aberration figure an even illumination may be
secured.

§ 829. Effect of the diameter of the objective. — If for any
reason, as the insufficient diameter of the objective lenses, some
of the light rays are lost after passing the film, the effect on the
screen image is the same as if these rays never reached the film.
Thus, if the objective O, fig. 333, has such a small diameter that it
would not admit the ray b s w, the effect would be the same as if no
light reached s, from the point, b, of the condenser.

CH. XIV] IMAGE FORMATION WITH MOVING PICTURES 597

By reference to fig. 334, it can be seen that if the objective has a
small diameter, or if an iris diaphragm D, with a small opening is
present, only light from the central zones from b to e, is permitted
to pass. Increasing the diameter of the objective or diaphragm
opening has practically the same effect as increasing the diameter
of the condenser. As the diaphragm is opened the effect is striking,
it is as if there were three layers of light upon the screen: First,
the bright spot in the center of the screen increases in size until it
covers the entire opening of the aperture plate, then the light has
the appearance of folding over on itself and the second layer spreads
over the picture starting from the edges. During this stage the
illumination is uneven, there being a dark spot in the center of the
field (dark ghost). The second layer of light reaches the center
and goes beyond so there is a layer of light which starts at the
center and spreads out towards the periphery of the field. In this
stage the illumination is brighter at the center of the field than at
the edges, there being a bright spot in the center (light ghost).
With a larger aperture yet the third layer spreads over the entire
field. For this reason one is more likely to secure an evenly
illuminated field having no shadows in the center if an objective
with large lenses is used than if one with small lenses is used.

§ 830. Advantage in using a large diameter objective. — The

difficulty of lighting a picture evenly when using an objective of
small diameter is often very great and requires a good deal of
rather careful adjusting to eliminate a shadow in the center of the
field and to get rid of the reddish brown corners at the same time.
It is necessary to try various distances from the lamp-house to the
aperture plate, different positions of the arc with respect to the
condensers and it will perhaps be necessary to try condensers of
different focal lengths. It will generally be found more satisfactory
and convenient to have an objective of large diameter which will
allow quite a range of adjustment either side of the very best
without materially damaging the result.

, § 831. Special condenser for moving pictures. — If the con-
denser of a moving picture outfit were designed especially for that

5Q8 IMAGE FORMATION, PROJECTION MICROSCOPE [Cn. XIV

purpose and was not intended to serve for lantern slides also, the
design would be exactly similar to that for lantern-slide projection
except that everything would be on a smaller scale, the condenser
lenses being of smaller diameter and of shorter focal length. This
would, of course, necessitate placing the lamp very close to the
lenses, but they will be small and correspondingly thin and will not
crack as easily as larger ones. Whether or not this would be a good
design for a large size outfit using 35 to 50 amperes is not certain,
but there is no doubt that good results can be obtained for projec-
tion on a small scale using three to four amperes which would not
be possible on account of the difficulty of getting even illumination
if the big standard size condenser were used.

§ 832. Experiment with small size condenser. — The method
of image formation using a small size condenser is shown in fig. 336.
L,is the source, an arc using 5 mm. carbons and 3 amperes of cur-
rent. It is practically a point source. Condenser lenses 58 mm.
(2>£m.) in diameter and 63 mm. a^in.) focal length placed 25 mm.
(i in.) from the source were used.

Even when using this very small source (3 mm. circle) a perfectly
uniformly illuminated field was obtained, a thing which could not
be done when a large condenser having the usual amount of
spherical aberration was tried.

The diameter of the light cone through the objective was 2 cm.
(24 in.). When large carbons were used and the current increased
to 1 2 amperes the effects were to increase the brightness of the pic-
ture and to increase the diameter of the cone of light through the
objective to 3 cm. (i^ in.). It is seen that in either case the lenses
of the objective did not need to be of as large diameter as when
using the ordinary large condenser.

IMAGE FORMATION WITH THE PROJECTION MICROSCOPE

§ 833. Illumination for low powers. — For low powers (20 to 100
mm. objectives) the principle is that the focus of the condenser
should fall at the center of the projection objective and that the
object should be placed in the converging cone of light in the posi-

CH. XIV] IMAGE FORMATION, PROJECTION MICROSCOPE 599

tion to give a sharp image on the screen. To accomplish this best,
the objective is so placed that the focus of the condenser is at the
center of the projection objective, and then the stage of the micro-
scope is moved back and forth until the image is sharp upon the
screen. If a three-lens condenser, without a substage condenser is
used, it will be found best for low powers (20-125 mm. focus) to
have a condensing lens next the objective (2d element of the con-
denser, fig. 332) of 20 to 25 cm. (8 to 10 in.) principal focus. For
the higher powers where greater numerical aperture is needed, a
condenser lens of 15 cm. focus is better.

FIG. 338. IMAGE FORMATION WITH AN AMPLIFIER.

O The back lens of the objective.

A Amplifier (divergent lens).

/' The image which would be projected by the objective if no amplifier were
in place.

/ Image projected with the amplifier in place.

Note that the rays from A diverge more rapidly than from 0 making the
image larger than without the amplifier. (See also fig. 126).

§ 834. Illumination for high powers. — In all high power micro-
scopic projection (2 to 16 mm. objectives) any source of light should
be considered as an extended source whether lime light, arc light,
or the sun is used.

The best method of illuminating microscopic specimens has been
found to be to place the microscope so that the front lens of the
objective is in the image of the crater (or the sun), (fig. 140) and
then the specimen is moved up toward the objective until its image
is in focus on the screen. Light will extend from every point of the
object as shown in fig. 347 and strike the front lens of the objective.
The action of the objective is to bring all of the light leaving a point
of the object to a single point on the screen. The details of image

600 IMAGE FORMATION, PROJECTION MICROSCOPE [Cn. XIV

formation are taken up later in § 858, in connection with
aperture.

§ 835. Amplifiers and oculars. — When using the projection
microscope it is often desirable to magnify the screen image without
changing to another objective. This may be done with an ampli-
fier or an ocular.

§ 836. Image formation with an amplifier. — The amplifier is
a negative lens or combination placed some distance beyond the
objective. Without the amplifier the objective would form an
image at /' (fig. 338). The effect of the amplifier (A) is to cause
rays to cross at / which would otherwise cross at /' and at the same
time the light from the objective O is rendered more divergent and
it covers a larger area on the screen than it would without the
amplifier (Fig. 126).

When using the amplifier one must focus the objective slightly
farther from the specimen.

§ 837. Magnification due to the amplifier. — The magnification
due to the amplifier is greater the shorter its focal length and the
farther it is from the objective. The same principle is employed
as with the telephoto-attachments to photographic objectives. It
has been found that an amplifier of -5 diopters (20 cm. focus) 1 1.3
cm. from the objective will give a magnification of 1.68 times and
an amplifier of -10 diopters (10 cm. focus) at the same distance will
magnify 2.5 times. (See also § 356a for diopter, and for the
amplifier § 3Q2a).

§ 838. Projection ocular.— A projection ocular is required for
certain apochromatic objectives which are designed to be used only
with a compensation ocular, and when the microscope is used with
polarized light, otherwise it is not necessary to use an ocular,
although one may be used with any microscopic objective, see Ch.
IX and Ch. X under demonstrations and drawing with high powers
(§ 401, 405, 477).

The field lens Oi (fig. 339) in connection with the objective
forms an inverted image of the object at D. This image is in turn
projected by the eye lens or combination 0%, to the screen at /.

CH. XIV] LIGHT AND ENERGY LOSSES IN PROJECTION 601

This image is inverted with respect to D, but erect with respect to
the original object (fig. 207). A diaphragm at D, limits the size
of the field and makes its boundaries sharp. Often owing to the
small size of the diaphragm D, the field is not as large as desirable
on the screen.

Besides limiting the size of the field there is a greater loss of light
with the ocular than with the amplifier as the ocular is made of at
least two separated lenses while the amplifier consists of but one
lens or a cemented combination.

FIG- 339- IMAGE FORMATION WITH A PROJECTION OCULAR.

O Objective forming a real inverted image D, with the help of the field lens
of the ocular 0,.

0. Field lens of the ocular.

O2 Eye or projection lens of the ocular. It projects a screen image /, of
the real image D.

The image D, wa? inverted by the objective. 02, also inverts ths image D,in
projecting it, hence the final image / is erect like the object. (See also fig. 207).

APERTURE AND LIGHT LOSSES

§ 839. So far the path of the light from the source to the screen
has been considered mainly from the standpoint of image formation,
no account having been taken of the amount of light needed or of
the losses of light and energy in the apparatus.

Light losses may occur from three causes:

1. Removal of the margin of a beam of light due to lenses of
insufficient diameter.

2 . Reflection of light both regular and diffused, at the surfaces
of the lenses.

3. Absorption of light by the glass of the lenses, by the partial
opacity of the object and by dirt.

4. Special light losses due to the nature of the experiment.

602 LIGHT AND ENERGY LOSSES IN PROJECTION [Cn. XIV

§ 840. Losses by the removal of the margin of the beam.—

From the source, light spreads out in all directions. Only the light
that strikes the front surface of the first lens of the condenser is
available, hence the first lens should be of such a diameter and so
placed that it takes in as large an angle of light from the source as
possible. The use of a meniscus lens next the radiant allows a much
larger angle of light to be used than does a condenser without such
a lens (fig: 332, § 821).

The lenses after the first, should not remove any of the border
rays of the light transmitted by the first lens, or the first lens need
be of only sufficient diameter to furnish a beam of light which will
just fill the opening of the other lenses.

After passing through the condenser the light is available for
illuminating the object. With the magic lantern the entire
diameter of the cone of light passes through the objective and
reaches the screen. With moving picture projection the entire
cone of light may or may not get through the objective (§ 825, 829).
With the microscope, except for the lowest powers, the objective
lenses are smaller than the image of the crater which is thrown on
the front of the objective, and much loss of light occurs from this
cause.

§ 841. Losses by reflection. — The polished surfaces of a lens
reflects some light, about 4 to 5 per cent, at each surface between
glass and air; 8 to 10 per cent, for each lens or plate of glass. If
the surfaces of the glass are not perfectly clean or perfectly polished
the light losses may amount to much more, sometimes 15% at each
surface. All reflected light being lost, this effect is generally much
more important than the slight absorption in the body of the glass
itself. A good illustration of this reflection by glass surfaces is the
brilliant reflection from windows often seen at sunset.

§ 842. Light losses by absorption. — The object (slide, film, or
specimen) absorbs some of the light incident upon it. This is a
necessary accompaniment of showing the object at all, but an
object which does not absorb too much light is to be preferred
whenever obtainable.

CH. XIV] LIGHT AND ENERGY LOSSES IN PROJECTION 603

The glass of which lenses are made is not perfectly transparent
but absorbs some light. This is especially true of the thick con-
denser lenses which usually look green when laid on a piece of white
paper. Such green lenses will be found to absorb an appreciable
amount of light. Some condenser lenses made of cheap glass will
turn purple after being in use for some time.

§ 843. Special light losses. — The use of polarized light neces-
sarily entails the loss of one-half of the light in the polarizing nicol.
The analyzing nicol may transmit most of the remaining light but
generally it is turned to transmit but a small portion of it (§ 884).

In moving picture projection the shutter covering the lens while
the film is in motion removes part of the light. In this case it has
been found by careful experiment that removing all of the light
part of the time has exactly the same effect as removing part of the
light all of the time. Some shutters remove but /^ of the light
while others remove ~%. of the light (§ 591). The latter are, how-
ever, sometimes to be preferred, the avoidance of flicker being of
more importance than the slight dimming of the image.

ENERGY LOSSES

§ 844. Of the energy which is radiated by the source only a
comparatively small part, from 2 to 10 per cent, is of those wave
lengths which affect the eye, the major part of the energy being in
the infra-red part of the spectrum (fig. 307). This infra-red
radiation accompanies the light radiation and is bent by a lens in
very nearly the same manner. It has been found that the differ-
ence in focus between the infra-red and the red, for glass, is no
greater than the difference in focus between the red and the blue.
This is due to the special dispersive qualities of glass.

§ 845. Disadvantage of the infra-red. — As the infra-red
radiation has such great energy and consequently so great a heating
effect wherever it is absorbed (fig. 307), and as at the same time
it has no effect upon the eye, it is advantageous to remove it as far
as possible. Energy losses, in so far as they are not accompanied
by light losses, are of advantage.

604 LIGHT AND ENERGY LOSSES IN PROJECTION [Cn. XIV

§ 846. The energy losses. — The energy losses occur principally
in three places (fig. 342).

1. In the condenser lenses.

2. In the water-cell, if there is one.

3 . In the specimen.

Energy losses beyond the specimen are not considered separate
from light losses.

§ 847. Losses in the condenser. — The glass of which the con-
densers are made, even if perfectly transparent to visible light,
absorbs a large amount of infra-red. A piece of condenser glass
2 cm. thick was found to absorb 41% of the radiant energy from
the positive crater of the right-angle arc incident upon it, while
absorbing but 10% of the incident light. This has two effects.

1 . The absorbed energy heats the condenser very greatly.

2. The light which gets through the condenser has a much less
heating effect on the specimen than it would have otherwise.

The first effect (heating the condenser) is a distinct disadvantage
to the condenser as it is one of the causes of condenser breakage.
Most of the energy absorbed will be by the first lens, and in that
one, more will be absorbed at the surface near the lamp than away
from it ; a circumstance which leads to unequal heating and puts a
strain on the lens.

Different kinds of glass, equally transparent to visible light
absorb different amounts of infra-red. For example, crown glass
will be found to be opaque to some of the longer waves to which
flint glass is perfectly transparent.

The second effect of heating the condenser is an advantage, as
the specimen is relieved of a good deal of the heating effect. Lan-
tern slides are less likely to be cracked, moving picture films are
less likely to curl or catch fire, and microscopic specimens can be
shown for a longer time before they are injured.

§ 848. Energy losses in the water-cell. — Water is very opaque
to radiation of great wave length, even the thinnest films being
absolutely opaque to certain wave-lengths.

The table (§ 849) and the curves (fig. 340-341) show the energy
of the positive crater of the right-angle arc transmitted by layers

CH. XIV. LIGHT AND ENERGY LOSSES IN PROJECTION 605

of water of different thickness. The lower line represents the
energy transmission. About 10% of the light (and energy) is lost
by non-selective reflection. If one wished to know the amount of
energy transmitted to get the same light transmission, it is neces-
sary to add 10% to the above values. The upper line represents
the transmitted energy after the correction for reflection has been
made. These curves show that after the first four or five centi-
meters of water, increased thickness does not reduce the energy
very much. A 6 cm. layer of water transmits 22.5%, a 10 cm.
layer transmits 20%, the difference absorbed in the last 4 cm. being
but 2.5% of the incident energy.

The water-cell, by absorbing a great deal of the energy, reduces
the heating effect of the light on the specimen. This energy

0 246 8 1.0 \i 1.4

Thichness Centimeters

0 Z 4 6 8 1.0 II
Thichness Centimeters

FIG. 340-341. PERCENTAGE OF ENERGY FROMTTHE'CRATER OF THE RIGHT-
ANGLE ARC TRANSMITTED BY LAYERS OF,WATER OF DIFFERENT THICKNESS.

The lower curve shows the actual energy[ transmission in each case, and
the upper curve shows the actual energy transmission corrected for reflection.

606 LIGHT AND ENERGY LOSSES IN PROJECTION [Cn. XIV

absorbed in the water-cell heats the water, but water is peculiarly
adapted for this purpose for it is the best known absorbent of the
infra-red, "heat rays."

Water is easily obtained and put into the cell. It has the highest
specific heat of any known substance; i. e., a given quantity of
water will absorb more energy when being warmed a given amount
than will anything else. If the water in a water-cell becomes so
hot that it gives off bubbles, a cool cell can be substituted for it.
Cooling a cell by the circulation of cold water through it has not
proved successful.

The temperature of the water has no appreciable effect upon the
energy absorption, boiling water serving as well as ice water. The
energy transmission for a water-cell was found when hot (80° C.)
to be 18.4%; when cold (22° C.) 19.2%. The water was slightly
turbid, being more so when hot than cold.

§ 849. The energy transmission of layers of water of different
thickness. — The source of light is the crater of the right-angle
carbon arc.

WITH 12 AMPERES DIRECT CURRENT

Thickness of the layer
of water

ENERGY TRANSMITTED

Observed

Corrected for reflection

.15 cm.

43-5%

484%

•30

38.9%

43-2%

•45

35-8%

39-8%

.60

34-1%

37-9%

•75

32.9%

36.6%

.90

3i-9%

35-4%

I.OO

29-5%

32.8%

1.05

30.8%

34-2%

1.20

27-8%

30.8%

i-35

27-0%

30.0%

6.00

22.5%

25.2%

8.00

22.0%

24-0%

10.00

20.0%

22.5%

WITH 15 AMPERES ALTERNATING CURRENT

8.00 cm.

14-4%

CH. XIV] LIGHT AND ENERGY LOSSES IN PROJECTION 607

§ 850. Table of the energy and light transmission and
absorption.

ABSORBING ELEMENT

PHOTOMETRIC

ENERGY

Absorp.

Trans.

Absorp.

Trans.

Plane Glass . .

10.55%
10.75%
28.8 %

11.3 %

22.6 %

7-9 %
14-7 %
II. I %

15-3 %

1 1.6 %

25-3 %
36.0 %
24.6 %

8945%
89.25%
79-2 %
88.7 %

77-4 %

92.1 %
85.3 %
88.9 %

84.7 %
88.4 %

74-7 %
64.0 %

75-4 %

41-5 %

77-1 %
82.0 %

77-9 %
78.8 %

81.6 %
80.8 %

79-6 %
77-2 %
75-0 %
27-5 %
35-0 %
68.6 %

32-3 %
23-5 %
37-0 %
27-4 %

58.5 %
22.9 %

1 8.0 %

22.1 %
21.2 %

18.4 %
19-2 %

20.4 %
22.8 %
25-0 %
72-5 %
65-0 %
31-4 %
67-7 %
76.5 %
63.0 %
72.6 %

Six Centimeter Cell

PI. glass and 6 cm. Cell . ...

Condenser Cell Clear

Condenser Cell, Muddy

Condenser Water-Cell, Hot.
and turbid

Same, Cold and turbid

Stage Water-cell Clr

Stage Water-cell, Muddy . . .
Ten Centimeter Cell

Alum 6 cm. Cell

Glycerin 6 cm. Cell

Mica, thin

Mica, thick

Balsam, Stage Cell

Glass Slide, green

Glass Slide, white

Green Slide with Balsam
White Slide with Balsam

§ 851 . Other substances dissolved in water. — Other substances
dissolved in water have not been found to improve its energy
absorbing qualities. For a long time it was supposed that a
saturated solution of alum was more effective than pure water but
this is not so; moreover it is very difficult to prepare a saturated
solution of alum which is not turbid. Tests show energy trans-
mission of 22.8% for an alum solution as against 22.9% for clear
water, while alum absorbs 15% of light as against only 10% for
clear water. The alum only serves to dilute the water. Crystals
of alum, K2 SO4 A12 (S04)3 24 HgO, absorb much energy, but it has
been proved that this is entirely due to the water of crystallization.

§ 852. Energy losses in the specimen. — All the light energy
absorbed by the specimen is converted into heat ; hence, an opaque
specimen or one which is black would become heated in the con-
centrated beam of light necessary for the microscope even if only
the radiation in the visible spectrum were used. As the visible

608 LIGHT AND ENERGY LOSSES IN PROJECTION [Cn. XIV

radiation constitutes only about 10% of the energy radiated by the
arc, this effect is insignificant in comparison to the heating due to
the infra-red. Even when the water-cell is used only 43% of the
energy which gets through is visible as light. A greater thickness
of water would reduce this effect but little, hence it is necessary to
carry the heat away from the specimen as rapidly as possible.
This is done by the stage cooling cell which is in contact with the
glass slide. The effect is purely one of conduction, and a thick
piece of any transparent substance would answer. But water has
been chosen because of its great specific heat, and the comparative
cheapness of hollow glass cells.

3,000 CP

FIG. 342. ILLUSTRATION OF THE LIGHT AND ENERGY LOSSES IN THE

PROJECTION MICROSCOPE.

Starting from the arc lamp the light and energy reaching the first face'of
the condenser are each designated by 100%. Opposite each element of the
optical system is given the percentage of light and of energy transmitted by
each. With the 16 mm. objective only about 6% of the original light is avail-
able for the screen picture.

§ 853. Keeping the condenser cool. — One of the causes of the
condenser breakage is the stream of hot gases from the arc which
strikes the upper part of the condenser and heats it unequally.
This is specially troublesome when the lantern is tipped up at an
angle. To prevent this a thin sheet of glass (watch glass) or mica
may be used between the arc and the condenser. Glass is to be
preferred as it is more transparent than mica and has less defects
to cause shadows on the screen. The following data refer to two
sheets of mica new and in good condition.

Light absorbed Energy absorbed

Thin piece 25.3% 27.5%

Thick piece 36.0% ,35-°%

CH. XIV] EFFECT OF APERTURE IN PROJECTION 609

This shows a heavy loss in light, the absorption being non-
selective, that is the total energy transmitted is in proportion to
the light.

§ 854. Example of light and energy losses. — In fig. 342 is a
diagrammatic representation of the light and energy losses actually
found in a certain projection system. An arc light was used.
The light and energy from the arc striking the first surface of the
condenser were each called 100%.

After passing the first part of the condenser LI, there remains 82%
of the light and 54% of the energy.

After passing the condenser water-cell the light was reduced to
73% while there was left only 16% of the energy.

After leaving the second part of the condenser L2, there was 68%
of the light and 14% of the energy. Of this remaining 14% of the
energy, 57% is invisible and 43% is visible as light.

When used with a magic lantern the projection objective trans-
mits only 70% of the light reaching it. As 68% of the original
light reaches the objective, the screen image must be formed by
68 x 70 = 47.6% of the original light.

With the microscope further losses occur due to the presence of
the stage water-cell. The microscope objective lets through but a
small amount of the light incident upon it, the loss being greater
the higher the power of the objective. A 16 mm. objective, for
example, transmits 10% of the incident light. In the case investi-
gated only 10 x 60 = 6% of the light originally striking the first
lens of the condenser reached the screen. If a substage condenser
and an ocular are used the light for the screen image is still further
reduced.

EFFECT OF APERTURE

§ 855. Increasing the aperture of a perfect lens or a combina-
tion of lenses with undirected light has two effects.

i. It increases the definition, that is, the image shows finer
structures than does a lens of smaller aperture, i. e., it will show
more lines to the millimeter or inch. See under Abbe diffraction
theory § 910.

6 io EFFECT OF APERTURE IN PROJECTION [Cn. XIV

2. It lets through more light.

The effect of increasing the aperture of a lens when using
directed light as with the magic lantern depends somewhat upon
circumstances. If the directed light spreading out in the form of a
cone has a greater diameter than that of the lens, the larger the
lens up to the full diameter of the cone, the greater the amount of
light which gets through, and the brighter will be the screen image
just as with undirected light.

If however, directed light from a point in the object spreads out
over a cone which has a smaller diameter than the lens, then the
size of the lens is immaterial, for all the light which gets through
the lens, gets through a small part of its area, and the rest of the
lens is not used at all. Increasing the diameter of the lens will not
increase the brightness of the screen image.

Another method of looking at the problem is to suppose the
diameter of the lens fixed and that of the cone of directed light to
be increased in diameter, assuming the light source to have the
same intrinsic brilliancy, i. e., same brightness per square centi-
meter. Suppose we start with a very small source of light behind
the pinhole in the screen 5, fig. 343, This light will get through a
small part of the lens. Now increase the area of the light source L,
keeping everything else the same, the cone of light from 5, will for
a time all get through the lens and be collected at a point, hence the
image 7, of the pin hole will keep getting brighter until the cone of
light just fills the aperture of the lens. When this occurs a further

FIG. 343. SIZE OF THE LIGHT SOURCE AND BRILLIANCY OF THE SCREEN IMAGE.

Up to a certain point the larger the light source L, the greater will be the
amount of light from the specimen 5, which gets through the objective O, but
beyond this point increase in the size of the light source produces no further
increase in the intensity of the screen image as the light (u, v) passes outside
the objective.

CH. XIV]

EFFECT OF APERTURE IN PROJECTION

611

increase of the diameter of the cone of
light will do no good because the light
falls outside of the opening of the lens.
The conclusions from this are:

1. If there is a cone of directed

light from a given point it is of no ad- FIG. 344. THE "CLOSING AN-
, . , . ., , GLE" OF LIGHT FORMING A

vantage to use an objective with SCREEN IMAGE.

lenses of larger diameter than this The shaded portion shows
cone. the closing angle when an or-

TTT-.I . . , . ,. , dinarv magic lantern is used

2. With a given size objective, when with an ar° lamp as a sol?rce

the object is illuminated by directed of light; and the outside lines
v t, • • r , v show the closing angle when an

light an increase in area of the source extended source of light, like a
beyond that which will fill the aper- gas or acetylene flame, is used,
ture of the objective is of no advantage.

§ 856. Method of determining the aperture of the objective
which is used. — The simplest way is to look directly into the
objective when in use; of course, using a colored glass or smoky
mica to protect the eyes. Do not hold the head too close to the
objective. If the whole of the back lens of the objective is filled
with light the aperture is filled. If the bright light is only in the
center of the back lens the bright spot is the part of the aperture
which is used. It often occurs that different parts of the objective
are used by the light from different parts of the object. This can
be determined by looking at the objective and moving the head
from side to side.

FIG. 345.

THE CLOSING ANGLE OF LIGHT
TO FORM AN IMAGE.

In order that the images i and j, shall be equally bright with opaque pro-
jection, the closing angle of light from the objectives must be the same.
As the distance bj, is twice as great as ai, the diameter of the lens (b) must be
twice as great as that of (a) and its area must be four times as great.

612 EFFECT OF APERTURE IN PROJECTION [Cn. XIV

BRIGHTNESS OF THE SCREEN IMAGE

§ 857. The brightness of the image can be calculated in either
of two ways. —

1. The relative area of the object and image and the illumina-
tion of the object.

2. The intrinsic brilliancy of the source and the closing angle
of the rays forming the image.

The first case is more applicable to directed light where all of the
light illuminating the object gets through the objective. For
example, with a magic lantern, let the area of the slide be 50 sq. cm.,
7.1 x 7.1 cm. and the area of the screen 2x2 meters = 40,000 sq.
cm. or 800 times as much surface. Let the brightness of the slide
illumination be 48,000 meter candles. The illumination of the
screen will be -[ of this or 60 meter candles.

BOO

Actually only 70% of the light from the slide will get through the
objective, and the illumination will be 42 meter candles. For
ordinary reading with artificial light one needs an illumination of
from 30 to 50 meter candles.

The second case is most useful where the entire aperture of the
lens is filled with light, as with a large light source with the micro-
scope, and with opaque projection. Consider the same example as
above except that the object is a white opaque body illuminated
from the front. More data concerning the lens will be needed.
Let the lens be one of 14 cm. (sJ^in.) focus, 6.25 cm. (2^in.) diam-
eter, the size of the picture being two meters square as before ; and
as before the object illuminated with an intensity of 48,000 meter
candles. To secure the same magnification as before requires a
distance of 396 cm. (4 meters approximately) from the screen with
this focus objective. Suppose the objective is looked at from the
screen. Its entire opening will appear of the same brightness
(except for absorption) as if there were no glass present and the
illumination on the screen will be just the same as if the light
reaching it were from a piece of white paper having an area of 50
sq. cm. illuminated by 48,000 meter candles. Considered as a

48,000
source of light this paper disc would have a can die-power of

CH. XIV] EFFECT OF APERTURE IN PROJECTION 613

per sq. cm. (§ 85 ;a) 50 sq. cm. gives 76 candle-power. At a dis-
tance of 4 meters from the screen this gives ^ = 4.8 meter candles.
Counting the losses due to the lens as 30% the illumination of the
screen would be 3.16 meter candles. This is about a third of the
minimum illumination for projection in a perfectly dark room, and
about one-tenth of what would be required for good projection.

If the intrinsic brilliancy of the source is the same and the closing
angle is the same the illumination will be the same, thus, if the
screen is twice as distant and the objective has twice the diameter
the illumination would be the same (fig. 344-345).

In the above example no use has been made of the focal length of
the objective nor the magnification of the object, these having no
direct influence on the screen illumination. If a higher magnifica-
tion were desired a shorter focus objective would be substituted
and the object brought nearer to it. The apparent brightness of
the paper seen through the objective will not change if the paper
is moved closer to the objective. Therefore, if the objective has
the same diameter the illumination on the screen will be just as
before.

Another way of looking at the matter is this : with the shorter
focus objective a certain small area of the object will be spread
over a larger area on the screen, but bringing the object nearer the
face of the objective, more light from the small area of the object
will enter it. These two effects exactly counterbalance each other,
the increased light taken in by the objective being sufficient to
illuminate the larger area.

§ 857a. Formula for finding the candle-power of a surface illuminated at a
given intensity. — -Suppose a perfectly diffusing, perfectly white surface to be
illuminated at a given intensity, say the intensity of the incident illumination
is I meter candles, i. e., the incident light flux is I lumens per square meter.
The light falling on one square centimeter will be 1/10,000 lumens. This light
will be scattered in all directions so that the surface appears equally bright
when seen from any direction, but as the surface appears fore-shortened when
seen from any other direction than the perpendicular, more light will be
reflected perpendicular to the.'surface than in any other direction. The candle-
power of one square centimeter of this surface in any given direction can be
expressed as B cos 9, where the constant B, is the intrinsic brilliancy in candle-
power per square centimeter of the surface considered as a source of light, and
6, is the angle between the normal to the surface and the given direction.

614 EFFECT OF APERTURE IN PROJECTION [Cn. XIV

Consider a sphere of one meter radius having this lighted surface at its
center. The light received by one square meter of the surface of this sphere
will then be B cos e lumens. Only half of the sphere can receive light from
this opaque surface and the entire light received by this hemisphere will be :

sin0 cos9

7TB

Now if the reflecting surface is perfectly white there will be no light lost and
the entire light received by the hemisphere will equal the light incident upon
the reflecting surface, that is ir~B = 1/10,000 and B = i/io,oooir candle-
power per square centimeter. In the above example where the incident
illumination is 48,000 meter candles, the surface considered as a source of
light will have 48,000 candle power per square centimeter.

IO.OOOIT

This same formula will apply to the case of opaque projection (§ 2/4a) where
it is desired to determine the ratio of the light getting through the objective to
form the screen image and the light falling on the opaque surface, assuming that
this opaque surface is perfectly diffusing and perfectly white. In the case of
the objective, light over a certain zone of the hemisphere is used. If the angle
which the objective subtends with a point on the object taken as the center is
called 26, then the angle between the axis and the edge of the objective is e,
and the above formula will apply, i. e., the light flux striking the objective
from one square centimeter is 7rB/2 (i — cos 26). Also the total light flux
reflected from the surface over the entire hemisphere is irB, hence the ratio of
the light flux striking the objective to form the screen image to the light flux
received by the reflecting surface is i — cos 26. This takes no account of losses

due to reflection and absorption by the objective.

CH. XIV] EFFECT OF APERTURE IN PROJECTION

^\\ urn mated by

FIG. 346.

CANDLE-POWER OF A SURFACE ILLUMINATED AT
A GIVEN INTENSITY.

O A surface having an area of one square centimeter located at the center
of a hemisphere of one meter radius. This surface is illuminated at an inten-
sity of / meter candles. It receives and reflects 1/10,000 lumens. As a light
source it has in the direction OP, B candle-power.

OP Perpendicular to the surface.

b'a' ab A zone on the surface of the hemisphere. This zone is located at
an angular distance of 0, from the perpendicular to the surface. The angle
subtended by this zone from the center is d0, and its width ab, is d0 meters.
The radius aN, of the circle aa1, is sin 9 meters, and its circumference is 2ir sin 0.
The area of the zone is then 2 sin 0 dO.

The intensity of illumination of this zone is B cos 0, meter candles. The
light flux received by this zone is then (illumination x area) equal to
B cos 0 x 2 TT sin 0 d 0 = 2 IT B cos. 0 sin 6 d 0.

IMAGE FORMATION WITH THE MICROSCOPIC OBJECTIVE WITH
REFERENCE TO APERTURE

§ 858. Let a b, fig. 347, represent the face of the condenser
which is in such a position with respect to the objective that its
image s' t', is in focus on the screen. With high powers the speci-
men will be very close to the front of the objective.

The front lens or combination of the objective 0\, will form an
image a' b', of the condenser face which may or may not coincide
with the back lens 0%, of the objective as here shown.

Tracing the light from the condenser we see that all the light
from a, which gets through the front lens passes through a', and all
light from b, passes through b', and so on.

6i6

EFFECT OF APERTURE IN PROJECTION [Cn. XIV

The light from the point 5, spreads out over the angle xsy, which
equals angle asb. Light from s, which has come from a, can reach
the screen along the path s x a' s'. From 6, the light follows the
path syb's', and from the central parts of the condenser light will
go from 5 to s', along the paths which lie between a' and bf.

The result is that the light which goes to make up the point s',
of the screen image has come from the entire area of the circle a'b'.
That is, the circle a'b', is the diaphragm which limits the illum-
inated aperture of the objective.

Fn;. 347. IMAC.E FORMATION WITH A MICROSCOPIC OBJECTIVE.

The- shaded portion shows the cone of light which illuminates the point t, of
the specimen and which goes to make its image /', on the screen; all other
points are similarly lighted and similarly pass on to form the screen image.

a,b The last element of the condenser (see fig. 332).

5, / Two points of the specimen to be projected.

O,, O2 The front and back combinations of the objective.

iv x y z Points on the front lens of the objective.

a', b' Image of the condenser face.

s', t' The inverted screen image of the object s, t.

When used with a magic lantern, the projection objective transmits only
70% of the light reaching it. As only 68% of the original light reaches the
objective, the screen image must be formed by (68 x 70 = 47.6%) of the
original light.

With the microscope, however, only about 6% of the original light gets
through the objective and goes on to form the screen image (fig. 342). If a
substage condenser and an ocular are used, the light for the screen image is still
further reduced.

The illuminated aperture of the objective may be increased by :

1. Using a larger diameter condenser of the same focal length.

2. Using a shorter focus condenser of the same diameter.
Either method will increase the angle asb, and the diameter of

the disc a'b'.

CH. XIV] EFFECT OF APERTURE IN PROJECTION 617

The illuminated aperture might be decreased by using a large
iris diaphragm to cover part of the condenser face a b.

In the figure the aperture illuminated a'b', is less than the
diameter of the rear lens. If the size of the condenser were greatly
increased until its image was as large as the rear lens of the objec-
tive, the marginal ray from s, would move from sxa's' to szs'.
The entire aperture of the objective would be illuminated and no
more light would be used by a further increase in the size of the
condenser (Fig. 347).

§ 859. Image formation of a point not on the axis. — Light from
t, will spread out over the angle w t y, which equals angle a tb, will
pass through a' b', and be collected to a point t', on the screen.
This light will of course fill a cone of which the limiting rays are
t w b' t' and t y a' t' (Fig. 347).

§ 860. Illumination of the screen image. — Any single point on
the screen as s' or t', will be illuminated by light which has come
from the bright disc a' b'. The illumination will therefore depend
on the three factors, the brightness and area of the disc a 'b', and
its distance from the screen (Fig. 347).

The area of the disc can, of course, be no greater than the area of
the back lens of the objective, and is usually smaller. For this
reason the brightest projection in a given case is obtained when the
back lens of the objective appears to be entirely filled with light.

The brightness of this disc of light would, if it were not for light
losses, be exactly the same as that of the original source. This
follows from the fact that the brightness of an object remains the
same, except for light losses, when seen through a lens or a system
of lenses as when viewed directly. A lens can only change the
direction, not the intensity of light, or in other words it can only
change the apparent size of an object.

This being the case the screen brightness is limited not by the
candle-power of a source but by its intrinsic brilliancy (candle-
power per square centimeter). This assumes the image of the
light to have an area great enough to cover the front lens of the
objective, which is the case with most microscopic projection.

6i8 EFFECT OF APERTURE IN PROJECTION [Cn. XIV

The effect of light losses by reflection and by absorption is to
reduce the brilliancy of the bright disc a'b'. These losses are very
great, and as only a small amount of light is available anyway, that
is the reason we do not recommend the use of the substage con-
denser except in the special cases of high power demonstration, for
photography and for high power drawing, where fine details are of
more importance than brilliancy (see Ch. IX, X, § 401, 477).

A substage condenser will reduce the brilliancy of the disc to
70% of its former value, and our experience has been that the full
aperture of all but the highest power objectives (8, 6, 4, 2 mm.
equivalent focus § 8o8a) can be entirely filled without its use.

§ 861. Appearance when one looks into the objective. — If the
eye is held at 5' (a dark glass being of course held in front of the
eye or better yet held just before the front of the objective at s)
light will strike the pupil from all parts of the condenser image
a' b', the appearance being that of a bright disc of light.

The larger this disc, the greater the aperture of the objective
illuminated. With low powers the entire aperture will be illum-
inated by the use of the large condenser alone. With high powers
only the central part of the back lens will appear bright. When
the bright disc spreads over the entire back lens the aperture of
the objective is fully illuminated and no further increase of light
is possible with a given source.

As often happens, the back lens appears illuminated not with a
uniform bright disc but by a bright ring with a bright center
separated by a dark ring or crescent. This is due to the spherical
aberration of the condenser.

§ 862. Appearance when an amplifier or an ocular is used.—

An amplifier or an ocular will spread the light from the objective
over a larger area than before, of course decreasing the brightness
of the screen image. This effect could be foretold by looking
directly at the instrument from the screen for the bright disc of
light a'b' (fig. 347) will appear smaller when the ocular or amplifier
is in place.

§ 863. Limit of brightness with the projection microscope.—

The screen image with microscopic projection apparatus is not as

CH. XIV] EFFECT OF APERTURE IN PROJECTION

619

bright as could be obtained with the magic lantern. The reason
is the physical impossibility of crowding more than a limited
amount of light through the very small opening of a microscopic
objective. The objective can be illuminated so that light comes
from the entire area of the rear objective lens to form the screen
image. When this occurs this objective will act like a luminous
source having the same intrinsic brilliancy (except for losses) as the
original source, and having the area of the rear lens. The area of
this rear lens is fixed and cannot be increased. The intrinsic
brilliancy of a given source is fixed. Hence, only by using a
brighter source as changing from a lime light to the alternating
current arc, from the alternating current arc to the direct current
arc, or from the direct current arc to sunlight, or by reducing the
losses due to unnecessary complication of lenses and condensers
between the source and the screen, can the image brightness be
increased. When using sunlight one has reached the limit of
possibility for light brilliancy.

KOEHLER'S METHOD OF ILLUMINATING A SPECIMEN FOR

MICROSCOPIC PROJECTION

§ 864. The simple method of lighting a specimen for micro-
scopic projection by a condenser is, as stated above, to focus the
image of the crater upon the front of the objective (fig. 140). For
high powers this is practically the same as if the crater image were
focused upon the specimen.

With the Kcehler method a substage condenser is used. The
large condenser forms an image of the crater on the diaphragm of

FIG. 348. ORDINARY METHOD OF ILLUMINATING MICROSCOPIC SPECIMENS.

(See § 376, 833).

620

EFFECT OF APERTURE IN PROJECTION

[Cn. XIV

the substage condenser at d2 (fig. 170, 349). The effect of the
diaphragm at d\, in front of the large condenser, and at dz, where the
crater image is formed, is just opposite in the two cases. With the
usual arrangement (fig. 348) the diaphragm A, will limit the aper-
ture of the objective while the diaphragm D2, will limit the size of
the field illuminated. With the Kcehler arrangement, however,
(fig. 349) the diaphragm DI, limits the size of field illuminated, while
the diaphragm D2, limits the aperture of the objective used.

FIG. 349. KOEHLER'S METHOD OF ILLUMINATIM; MICROSCOPIC Si-i-x IMKNS.
L Light source.
C, Condenser.
Dl First diaphragm.
D3 Second diaphragm.

L' Image of light source on second diaphragm.
Ca Substage condenser.
5 Specimen.

D' Image -of diaphragm Dlt on the specimen.
0 Objective.
D\ L* Image of diaphragm D2 and light source.

§ 865. Advantages and disadvantages of Koehler method. —

The Kcehler method has the advantage that it enables an easy
control of the size of the field illuminated and of the aperture of the
objective used. If a larger field than necessary is illuminated,
there may be undu£ heating of the specimen and the best results
are obtained only when just the right objective aperture is used.
On the other hand, the use of a substage condenser precludes the
use of a cooling stage, (fig. 134), except a very thin form. It
limits the size of field which may be used, and transmits only 70%
of the incident light and reduces the general flexibility and ease of
handling the apparatus.

CHAPTER XV

SOME USES OF PROJECTION IN PHYSICS ; EXPERIMENTS

ILLUSTRATING NORMAL VISION AND SIMPLE,

REFRACTIVE EYE DEFECTS.

§ 875. Apparatus and Material for Chapter XV:

For polarized light, see § 879; For projection of spectra, see
§ 885; For photography, see § 908; For Abbe Diffraction Theory,
see § 909; For eye defects, see § 916.

§ 876. Historical development of experimental projection. —

Works of reference giving methods of projecting experiments.

In every book, and in every article on projection, directions and
hints are given. The following are especially full in directions, and
rich in suggestions : Dolbear. — The Art of Projecting; Fourtier et
Moltini . — Les Projections Scientifiques ; Hassack und Rosenberg. —
Projektions-apparate ; Lehmann. — Flussigekrystalle ; Trutat. -
Traite des Projections ; Tyndall. — Six lectures on light ; Wright. —
Light, a course of experimental optics chiefly with the lantern;
Wright. — Optical Projection.

INTRODUCTION

§ 877. Many physical and chemical experiments can be
exhibited to an entire class in a striking manner by the aid of
projection apparatus. Sometimes transparent objects are used,
and then again, as suggested by Dolbear, many experiments with
opaque objects show very clearly as shadows on the screen if they
are performed in the beam of the magic lantern.

Indeed, for exhibiting to a class or any large audience the varied
experiments necessary in physical and chemical work, all the pro-
jection apparatus described in this book and combinations of them
may be needed. By thoroughly mastering the principles of projec-
tion one can so adjust and combine the different pieces of appara-
tus that almost any phenomenon can be shown on the screen.

One can find many suggestions, and often detailed directions for
showing various experiments in the works referred to at the begin-
ning of this chapter.

621

622 PROJECTION EXPERIMENTS IN PHYSICS [Cn. XV

No directions for the ordinary experiments shown by projection
apparatus in every university course in physics and chemistry are
given here, but we thought it wise to include a few special projec-
tion experiments that have been found by us to be especially
instructive or difficult to perform by the means ordinarily used.

The experiments illustrating normal vision and the simpler re-
fractive eye defects are included because of the importance of
these defects and their prevalence with school children and stu-
dents and others doing close work; and because, with projection
apparatus, it is so easy to show in a striking manner what is
meant by the defects, and how certainly the defects can be cor-
rected by using the proper spectacles.

SOME SPECIAL EXPERIMENTS IN PHYSICS

§ 878. Kind of apparatus. — Most projection experiments in
physics are only shown occasionally, hence permanent apparatus
to demonstrate many physical phenomena is so costly as to be out
of the question. The apparatus here described consists in lenses,
objectives, prisms, clamps, arc lamps, etc., generally to be found in
any laboratory where such experiments would be shown. The
apparatus can either be clamped to rods or laid upon a horizontal
table. The former method has the advantage that one can project
in an inclined as well as in a horizontal direction ; the latter method
is easier to set up. When permanent apparatus is used the
principle is exactly the same, permanent instead of temporary
supports being used to hold the lenses.

An optical bench like that shown in fig. 158-159 is satisfactory
where the apparatus is set up on a table. When it must be held
at various angles some clamping arrangement is desirable.

EXPERIMENTS WITH POLARIZED LIGHT
§ 879. Apparatus.—

1. Right-angle arc light; Condenser; Water-cell;

2. Small Nicol prisms with openings of i to 2 cm. which are
mounted so that they can be rotated.

CH. XV]

EXPERIMENTS WITH POLARIZED LIGHT

623

3. Pile of glass plates to polarize light. Prefer-
ably thin sheets of plate glass, but a pile of lantern-
slide cover-glasses can be made to answer.

4. Plano-convex condensing lens, 5 to 10 cm. in
diameter and 20 to 30 cm. focus. This lens must be
free from strain.

5. Projection objective, preferably of large diam-
eter and short focus.

6. Two sets of lenses to give converging polarized
light. Two substage condensers of microscopes will
answer if free from strain.

7. Objective of short focus and large diameter.
A plano-convex lens will answer (see § 881).

8. Specimens. Pieces of mica, crystal sections,
plate of glass on which crystals have formed, an-
nealed and unannealed pieces of glass, clamp for
putting the glass under strain when in the field of
the lantern.

Many of the most beautiful experiments in optics
require the use of polarized light. The demonstra-
tion of this phenomenon is growing more difficult
owing to the increasing scarcity of the natural
mineral calcite, which is used to make the Nicol
prisms needed for polarizing and analyzing the
light. Clear pieces of calcite are getting so rare
that except for a few large Nicol prisms in private

FIG. 350.

PROJECTION WITH POLARIZED LIGHT, USING SMALL
NICOL PRISMS.

L Source of light, right-angle arc.

Ct Ordinary magic lantern condenser.

W Water-cell.

NI Nicol prism of I to 2 cm. opening (polarizer).

C2 Condenser free from strain which renders the polarized
light parallel or slightly converging.

5 Specimen.

0 Magic lantern objective.

N2 Nicol prism (analyzer) of i to 2 cm. opening.

624

EXPERIMENTS WITH POLARIZED LIGHT

[Cn. XV

collections and a few large crystals in museums, no prisms of
large openings (5 to 8 cm.) are now to be obtained.

§ 880. Use of small Nicol prisms. — A method will be described
for using Nicol prisms of small openings (i to 2 cm.). This
method consists in concentrating the light to small diameter in
those places where it must pass through the Nicol prisms.

Let L fig. 350 be the source of light, preferably a small right-
angle direct current arc. W, is a water-cell. It is imperative to
use a water-cell, otherwise the polarizing Nicol will be ruined.
The polarizing Nicol NI, is placed at the image of the
crater. Light spreading out from the farther side of this
prism is polarized. This Nicol is treated exactly
as if it were an original source of polarized light.
The second condenser Ca, which must be free

FIG. 351. LIGHT POLARIZED BY A PILE OF GLASS PLATES.
The light source.
Plano-convex condenser lens.
Condenser of long focus and free from strain.
The specimen.
Magic lantern objective.

N2 Nicol prism with I to 2 cm. opening (analyzer).

from strain, renders the light parallel or slightly convergent. The
specimen is at S. The objective O, is placed at such a distance
from the specimen that the image of the latter will be in focus on
the screen. The analyzing Nicol N2, is placed at the right in front
of the objective at the point where the rays cross, i. e., in the image
of NI, cast by the lenses C2 and 0.

When either the polarizing or analyzing Nicol NI or N2, is rotated,
two positions, 180° apart, will be found in which the screen is dark.
If when these positions are found, a piece of mica, for example, is
put in the field, it will change the plane of polarization and will give
on the screen the most beautiful colors.

CH. XV]

EXPERIMENTS WITH POLARIZED LIGHT

625

Another method of producing polarized light is to reflect the
light from a pile of glass plates (lantern-slide cover-glasses will
answer). At an angle of incidence of about 57° from the normal
as shown in figure 351, the light reflected from the glass surfaces
will be plane polarized. The specimen is placed at S, the objective
at O, and the analyzer N2, at the crossing of the rays as before
(fig. 351). The heating effect of the light reflected from the glass
surface is so small that a water-cell is unnecessary.

§ 881. Converging polarized light to show rings and brushes. —
If the polarized light passing through a crystal is a converging
cone, the most beautiful phenomena are shown. Fig. 352 shows
the apparatus used to project rings and brushes. A parallel beam
of polarized light obtained as before, strikes the lens system Li,
designed to bring parallel light to a focus in a strongly convergent
beam. 1/2, renders the light again nearly parallel. LI, is the usual
form for this work and L2, is a substage condenser from a micro-
scope, either form will give good results if free from strain. The
objective O, is a lens of short focus and large diameter. It need not
be a special projection objective. Three objectives were tried
which gave good results, (i) A photographic objective, focus
12 cm., diameter 2 cm. (2) A magnifying glass, focus 6 cm.,
diameter 4 cm. (3) A plano-convex lens, focus 5 cm., diameter
6 cm. The single plano-convex lens gave the best results except
that the figures were slightly distorted. The analyzing prism N2>
must have a medium sized opening, 15 to 20 mm. free aperture,
otherwise it will cut off part of the field.

FIG. 352. CONVERGING POLARIZED LIGHT TO SHOW RINGS AND BRUSHES.

L, is the usual form of a lens system designed to bring parallel light to a
strongly convergent beam.

L2 is a microscope condenser, to parallelize the light from Lt.

0 objective to converge the light beam upon the Nicol prism (N2), and to
focus the rings and.brushes on the screen.

N2 Nicol prism^with at least 1.5 to 2 cm. opening.

626 EXPERIMENTS WITH POLARIZED LIGHT [Cn. XV

§ 882. Detecting strain in lenses. — Substage condensers from
the microscope if free from strain are excellent for showing rings
and brushes. The glass of which they are made should be per-
fectly homogeneous and should have no effect at all on polarized
light. If the glass of which they are made is imperfectly annealed,
the lenses will affect polarized light. An achromatic condenser is
more likely to be free from strain than an ordinary Abbe condenser,
because it is likely to be made of better glass. Strain may be
tested for in each lens by holding it between crossed Nicols. A
strained condenser if put in position L2, will show a black cross on
a white ground when no crystal is used.

§ 883. Setting up the experiment. — The condensing lenses LI,
and L/2 can be clamped on a ring stand. The distance between
them is adjusted until the emerging light is practically parallel, the
objective O,is put in place so that the image of the lens L2, is in focus
on the screen, a Nicol prism N2, is put in the narrowest part of the
beam of light from the objective. When the analyzer N2, is turned
to give a dark field on the screen and a crystal section (mica, for
example) is placed in the converging light between the two con-
densers the field becomes illuminated with bright colors and
beautiful patterns.

The final adjustment of the apparatus is now made, the distance
between the condensers is adjusted until the screen has the most
uniform light. The objective O, is moved until the figures on the
screen are as sharp as possible; the analyzer may also require a
slight adjustment.

§ 884. Brightness. — The screen image with polarized light is
never very bright, hence a very dark room is needed. A screen
picture over one or two meters in diameter, (3 to 6 feet) should
never be attempted.

PROJECTION OF SPECTRA

§ 885. Apparatus. — Magic lantern.

Stand to hold the objective in proper position if the lantern
bed is not long enough.

Slit with adjustable blades.

CH. XV] PROJECTION OF SPECTRA 627

Glass prism, hollow prism to hold carbon bisulphide, direct-
vision prism.

Diffraction grating, ruled on speculum metal, or glass, or one of
the replica gratings, 6,000 to 8,000 lines to the centimeter,
(15,000 to 20,000 lines to the inch).

Glass cell to hold colored liquids.

Colored liquids, solutions of didymium nitrate, copper sulphate,
analine dyes, etc.

Colored glass, especially red and blue.

Arc lamp with vertical carbons.

Hollow carbons stuffed with salts as: lithium chloride, sodium
chloride, potassium chloride, calcium chloride, aluminum.

Flame arc electrodes, "Yellow" and "Brilliant white."

Metallic electrodes, iron, copper, aluminum, uranium oxide in
tin tube, rutile in tin tube or else luminous arc electrodes § 885a.

Screen coated with anthracene, 50 x 150 cm. to use instead of
a white screen to show ultra-violet.

APPARATUS FOR THE DEMONSTRATION OF ULTRA-
VIOLET LIGHT.

Quartz condensing lens and two plano-convex quartz lenses.
Source of ultra-violet light as :

Quartz mercury arc.

Arc lamp with carbons filled with various salts.

Arc lamp with carbons filled with metallic aluminum.

Arc lamp with brilliant white flame arc carbons.

Arc lamp with iron tube filled with uranium oxide.
Small card coated with anthracene to render the ultra-violet
visible.

Especially since the days of Newton, the exhibition of the
spectrum has been one of the most fascinating experiments in
physics. It is also one of the simplest of the "special projection"
experiments to perform.

§ 88sa. These tubes can be made by rolling strips of tinned iron into
tubes.

628 PROJECTION OF SPECTRA [Cn. XV

§ 886. Source of light. — In order to demonstrate an absorption
spectrum of a substance it is necessary to use a source of light
which has a continuous spectrum in order not to confuse the dis-
continuities of the light source with those caused by the absorbing
medium.

§ 887. Sunlight.— Sunlight while not having a perfectly con-
tinuous spectrum is near enough to it for most purposes. The im-
age of the sun should be focused on the slit as described for the
crater of the arc lamp (§ 376, 833-834).

§ 888. Carbon arc lamp. — In the visible parts of the spectrum
the carbon arc lamp will give a continuous spectrum, using pref-
erably the positive crater of the direct current arc, although alter-
nating current will answer.

§ 889. The uranium arc. — Using the vertical arc, the lower
electrode made of uranium oxide is connected to the positive
terminal as described in § 905. About 4 to 6 amperes should
be used and a fairly continuous spectrum will be obtained. This
continuous spectrum will extend well into the ultra-violet and can
be observed by using an anthracene screen (§ 899) .

§ 890. The Nernst lamp. — This can be used if necessary.
Focus one filament on the slit or use a very long focus lens (one of
100 cm. focus or longer) and use the filament as a line source of
light. The spectrum is continuous but not very bright.

The tungsten incandescent lamp. — One filament is focused on
the screen by the objective and a slit is placed over the side of the
bulb so that none of the other filaments show on the screen. This
single filament acts as a line source of light.

OPTICAL SYSTEM

§ 891. The optical system for the projection of spectra is
shown in fig. 353. First a condenser C, is put in front of the arc.
This condenser brings an image of the arc to a focus at the slit.
An objective is put at O, so as to focus an image of the slit
on a distant screen at i. When a prism is used to disperse
the light into a spectrum, it is placed at P and turned as shown.

CH. XV] PROJECTION OF SPECTRA 629

If a source is used which shows a line spectrum, this spec-
trum will be found approximately in focus at R V. By focus-
ing the objective and by rotating the prism the spectrum can be
sharply focused on the screen. The sharpness of the spectrum can
be increased by narrowing the slit or the brightness of the spectrum
can be increased by opening the slit. If the slit is adjustable in
width, judgment must be exercised to secure a spectrum which is
as sharp as the occasion requires and at the same time
sufficiently bright.

OPTICAL SYSTEM FOR THE PROJECTION
OF SPECTRA.

FIG. 353.

L Light source, arc lamp.

C Condenser.

Slit.

0 Objective.

1 Image of the slit focused on the wall before the
prism is in place.

R V Spectrum focused on the wall after placing the prism in the path of
the rays.

The distance from P to R V must be relatively much greater than here
shown, 2 meters (6 ft.) or more, in order to get a sharply denned spectrum.

Note that the red (R) is deviated less than the violet (7). Compare with
a grating spectrum, fig. 360.

§ 892. Slit. — The slit for projection should be a fairly large one ;
12 cm. square, with jaws 5 cm. long.

If one is limited by time or expense a very serviceable slit may be
made by soldering a couple of pieces of tinned iron on a larger piece
with a hole in it, or one of the pieces may be fastened so it can be
slid closer or farther from the other. The main point to be
observed is that the edges forming the slit opening must be per-
fectly smooth and straight so that when they are brought close
together there will be no unevenness in the light which gets
through.

Large slits with adjustable jaws may be obtained of dealers in
projection apparatus.

630

PROJECTION OF SPECTRA

[CH. XV

§ 893. Prism. — A glass prism may be used, but it is much
better to use a hollow prism filled with carbon bisulphide as this
liquid gives a much higher dispersion than glass, thus enabling
one to obtain a more extended spectrum than would be possible
with a glass prism.

Caution. — In using carbon bisulphide remember that it is very
volatile and its vapor is easily ignited. Hence this liquid should
not be poured or left in unstoppered vessels in the neighborhood of
the lighted arc. Also be sure that the hollow prism has no leaks
around the stopper or elsewhere.

i — :

— 1

aria

M.v

.bl.

FIG. 354. HOME-MADE SLITS FOR PROJECTION.
a Slit with stationary blades.
b Slit with one movable blade.
c Side view of b.

§ 894. Other prisms: gratings. — The 60° prism, either solid
or filled with liquid is usually the most available, but other forms
are often at hand.

The compound prism due to Rutherford (fig. 356), composed of a
dense flint glass prism of a large angle and two crown glass prisms
cemented to it, will give a much higher dispersion than will a single
prism of even very dense glass. Such a prism is used in practically
the same way as a simple prism.

With a direct-vision prism (fig. 357) the axis of the spectrum is not
turned to one side. Such a prism may be constructed of pieces of
different kinds of glass or it may be made by filling the hollow cells
of a prism with different liquids.

CH. XV]

PROJECTION OF SPECTRA

631

A convenient way of get-
ting a prism which has a de-
viation of but about 150°
while having a very good dis-
persion is to immerse a hol-
low 60° prism, filled with car-
bon bisulphide, in a cubical .

j- u £11 A -4.1. FIG- 355- HOLLOW GLASS

glass dish filled with water PRISM WITH COVER AND
(fip ?cR} STOPPER FOR CARBON

BISULPHIDE (CS2).

§ 895. A transmission

diffraction grating, either one ruled on glass or a
replica grating, held in front of the projection objec-
tive will give wide but rather faint spectra on the
screen.

If a grating with rather coarse lines, 100-200 to
the centimeter (250-500 lines to the inch) is used

FIG. 356. RUTHERFORD PRISM.

This prism consists of a flint glass prism F, with an angle
of about 90° and two crown glass prisms C C, cemented to it.
The combination as here shown has a prism angle of about
30° and has the same deviation as a 60° flint prism, but has a
much higher dispersion than could be obtained with a simple
prism of even dense flint glass.

F!G. 357. USE OF A DIRECT-VISION PRISM FOR THE

PROJECTION OF SPECTRA.
5 Slit.
O Objective.

Flint Flint prism to cause dispersion.
Crown, Crown, Prisms of crown glass to obviate the devi-
ation.

V R Spectrum.

632

PROJECTION OF SPECTRA

[Cn. XV

there will appear on the screen not only the
central image O, of the slit but also fainter
diffraction irr.ages i, 2, 3, on both sides of the
central one (fig. 357). These diffraction im-
ages are really short spectra. By using colored
glasses it can be shown that with red light the
images are farther apart than with green or
blue light.

FIG. 358. LIQUID PRISM OF GOOD DISPERSION

BUT SMALL DEVIATION.

It consists of a hollow prism filled with carbon bi-
sulphide (CS2) immersed in a glass box filled with
water.

§ 896. Grating spectra. — If a grating with 0 1

fine lines, 5,000 to 10,000 lines per centimeter
(12,500 to 25,000 lines to the inch), is used the
diffraction images are spread out farther and /
appear as extended spectra. In case the de-
tails of a spectrum are to be studied it is
necessary to turn the axis of the lantern to one
side as in the case where a prism is used. The

FIG- 359- USE OF A GRATING WITH COARSE LINES.
5 Slit.
O Objective.
G Grating.

o Primary image of slit formed without grating.
123,123 Diffraction images of the slit formed
by the grating. These images are short spectra.

CH. XV]

PROJECTION OF SPECTRA

633

diffraction spectra are not as
bright as are the spectra obtained
with prisms and cannot well be
used except to demonstrate that
such spectra can be produced.

Sometimes if a very high order
spectrum is to be shown the grating
can be held obliquely as in fig. 361.

§ 897. Use of reflection gratings. — In

case a reflection grating is to be used the
lantern is pointed directly away from the
screen and the reflecting grating held so as
to reflect the light back to the screen. The
objective is focused until the central image
is sharp on the screen and the spectra are
observed at both sides, or the grating may
be tipped so as to reflect the central image to
one side, when the spectra will appear in
the center of the screen.

In this work it is essential in order that the
comparatively faint spectra can be seen, to have
the room perfectly darkened ; the arc house per-
fectly light-tight and the lantern well enclosed to
avoid stray light.

Concave reflection gratings can be used by a
method similar to that for plane gratings, but in
order to have the central image sharply in focus
on the screen the objective must be closer to the

FIG. 360.

USE OF A GRATING WITH FINE LINES FOR THE

PROJECTION OF SPECTRA.
5 Slit.

0 Objective.
G Grating.

* Primary image of the slit.

1 V R Primary image of the slit (*'), and the first dif-
fraction image of the slit spread out into a spectrum (R V).

Note that with a grating, the red is deviated more than the violet. Compare
with the prism, fig. 353.

PROJECTION OF SPECTRA

[Cn. XV

FIG. 361. USE OF A GRAT-
ING FOR PROJECTING A
HIGH ORDER SPEC-

5 Slit.
O Objective.
G Grating.

« Image of the slit if
there is no grating in position.
R V Spectrum.
The grating is tilted as shown.

slit than for the plane
grating, that is, the light
from the objective must 7
be diverging instead of
slightly converging when
it strikes the grating (fig. 363).

§ 898. Direction of the light.— As
the direction of the light from a 60°
prism is oblique to the axis of the lan-
tern (fig. 353), it is necessary to turn
the entire projection apparatus to one
side so that the spectrum will strike
the screen.

§ 899. Screen, white and anthra-
cene.— A white screen such as is suit-
able for ordinary lantern projection

FIG. 362. USE OF A PLANE REFLECTING
DIFFRACTION GRATING FOR THE

PROJECTION OF SPECTRA.
i Primary image of the slit as it would be
reflected by a plane mirror.

R V Diffraction spectrum produced by
the reflecting grating.

These high order spectra are very faint.

CH. XV]

PROJECTION OF SPECTRA

635

will show the visible parts of the spectrum very well, that is, it
will show the red, green, and blue parts.

In order to illustrate the ultra-violet parts of the spectrum, use
a screen coated with anthracene. A suitable screen can be made

FIG. 363. USE OF A CONCAVE REFLECTING GRATING FOR THE PROJECTION

OF SPECTRA.
S Slit.

0 Objective.

G Concave reflecting grating.

1 V R Image of the slit (i) and of the spectrum (R V).

Note that in order to get a sharp image of the slit at i, it is necessary to have
diverging light strike the grating. The spectrum R V will be sharply denned
under these conditions.

by taking a piece of white cardboard 50 cm. x 75 cm. and painting it
with a suspension of anthracene in xylene. The anthracene used
is the ordinary commercial variety of resublimed anthracene.
Only enough xylene is used to make the mixture so it can be put on

636

PROJECTION OF SPECTRA

[CH. XV

the paper with a brush. This screen will show a brilliant green
fluorescence wherever ultra-violet light strikes it. When the
spectrum of an arc is projected upon such a screen, not only is the
visible red, green, and blue to be seen, but also beyond the blue end
is a vivid green fluorescence which indicates the presence of ultra-
violet light.

FIG. 364. ILLUMINATION OF THE SLIT FOR THE PROJECTION OF SPECTRA.

A The image of the arc focused to a small spot on the slit. The objective 0
is filled with light. The spectrum will be bright but a mere line.

B The image of the arc slightly out of focus giving a higher spectrum but
not so bright.

C The slit next to the condenser in the lantern-slide position. This gives
a relatively dim spectrum and illuminates a greater height of slit than is used
for the spectrum.

§ 900. Illumination of the slit. — Excellent results may be
obtained by focusing the arc on the slit by the condenser. This
gives an intense illumination but the spectrum is not very high, in
fact, it may be a mere line of color. To remedy this, the slit may
be brought closer to the condenser than the crater image. Thit
increases the height of the spectrum but reduces its intensity. Is

CH. XVI ABSORPTION SPECTRA 637

is a common practice in experiments with spectra to put the slit
close to the condenser as for a lantern slide, but this lessens the
brilliancy of the spectrum.

ABSORPTION SPECTRA

§ 901. The apparatus being arranged as above indicated to
project a continuous spectrum, all that remains is to insert the
absorbing medium between the light source and the screen, it
makes little difference where. The appearance of the spectrum
would be the same even if the absorbing medium were held between
the eye of the observer and the screen. As a practical matter it is
best to place the absorbing substance just in front of the slit. In
this position any slight lack of planeness of its surfaces will not
cause any interference with the optical system nor reduce the sharp-
ness of the spectrum on the screen. The specimen may cover the
entire slit, in which case the entire spectrum will show the absorp-
tion bands of the substance, or the specimen may be made to cover
part of the slit, in which case, part of the spectrum will be that of
the light source and part will show the absorption bands of the
specimen. The advantage of having this comparison spectrum
of white light is to bring out much more clearly any faint absorp-
tion of one end of the spectrum as with dilute copper sulphate or
with amber glass. Liquids may best be shown by placing them
in hollow glass boxes (fig. 365). Many variations of this method
and many fascinating experiments will soon suggest themselves to
the experimenter once the apparatus is set up.

§ 902. Suitable substances. — The following substances will
show interesting absorption bands:

Colored glasses. Red, blue, purple, canary-yellow.

Gelatines colored with solutions of ana-
line dyes, for example, methyl violet,
eosine (red ink), fuchsine.
Blood diluted with water.
Solutions of mineral salts, as cobalt ni-
trate in water, cobalt nitrate in alcohol or
FIG. 365. GLASS Box FOR concentrated HC1 ; potassium permanga-
ABSORPTION SPECTRA. nate.

638

EMISSION SPECTRA

[Cn. XV

Didymium salts, such as crude didymium nitrate or the pure
neodidymium, praseodidymium, erbinum, and other rare earth salts.

The following substances will show general absorption at one
end of the spectrum and should be shown in comparison with the
spectrum of white light :

Amber glass, green glass.

Copper sulphate, Ferric chloride, Nickel nitrate, Potassium
chromate and dichrorrate, Chrome alum.

FIG. 366. COMPARISON OF SPECTRA.
C Condenser.

Specimen This covers only a part of the slit.
Slit Longitudinal view of slit.

Through the rest of the slit passes white light or light traversing another
specimen. The two spectra then appear side by side.

EMISSION SPECTRA

§ 903. Besides the projection of absorption spectra, the optical
system as described above will serve also for the projection of
emission spectra. In this case the arc is both the specimen and
the source of light.

We will suppose that it is desired to project the spectrum of a
"yellow flame" carbon, this being about the easiest and most
satisfactory to begin with. The apparatus is set up as shown in
fig. 367. L is a vertical arc, the lower electrode of which is a yellow
flame arc carbon or a hollow carbon filled with a sodium, potassium
or other salt. The upper electrode is a carbon about 13 mm. (%
inch) in diameter. The carbon holder may be of the hand-feed
type or an automatic lamp may be used.

§ 904. Automatic lamp for use in projection of spectra.—

When using certain materials in the arc, the arc goes out frequently

CH. XV]

EMISSION SPECTRA

639

and it is desirable to have an auto-
matic machine to relight the arc
again instantly. A very conven-
ient device for this purpose is an
enclosed arc lamp mechanism for
shunt circuits. The wiring of the
arc will need to be slightly modi-
fied to adapt it to the heavy
currents (15 amps.) required.
This is done by connecting the
wires to the rheostat of the lamp
at A, B, C and D (fig. 367) and by
putting a germ an silver wire be-
tween E and F of the "series
magnet" so that this magnet will
not overheat and at the same time
will not lift the upper carbon
too suddenly.

§ 905. Current to use. — For
the projection of arc spectra the
current to use will depend upon
the substance in the arc. When
treated carbon electrodes are used,
either alternating or direct cur-

FIG. 367. AUTOMATIC ARC FOR THE
PROJECTION OF ARC SPECTRA.

The mechanism is that of an enclosed
shunt, direct current arc. In order to
get sufficient current the wire is connec-
ted to the resistor in the two points A
and B. The wire to the lifting magnet
is connected at the points D and E.
This gives three times the current for
which the arc was designed, i. e., about
1 5 to 1 8 amperes. The lifting solenoid
E F, must be shunted by a suitable
resistance easily found by experiment.
The clutch automatically lifts the upper
carbon — whenever current is flowing.
The lower carbon is stuffed with salts
and connected to the positive wire.

640 DEMONSTRATION OF ULTRA-VIOLET LIGHT [CH. XV

rent may be employed, but direct current is to be preferred.
About 1 5 amperes will give the best results. When direct current
is used the lower carbon, which contains the salt to be studied, is
made the positive.

With metallic electrodes or with metallic oxides contained in
sheet iron tubes, direct current only, can be used. About 6
amperes give the best results in this case. The metallic electrode
is made the lower, the upper electrode being carbon. Most
metallic electrodes will give different results when made the positive
than when made the negative terminal. The magnetite and
titanium ("Luminous arc") electrodes show the lines of iron and
titanium best when connected to the negative wire.

Uranium oxide contained in an iron tube will give a line spectrum
when connected to the negative wire but will give a very nearly
continuous spectrum when connected to the positive wire
(§ 88Sa).

DEMONSTRATION ON A SMALL SCALE; DEMONSTRATION OF
ULTRA-VIOLET LIGHT AND PHOTOGRAPHY

§ 906. Demonstration on a small scale.— Spectra may be
projected on a small scale for the observation of a few individuals
with the apparatus shown in fig. 368. This arrangement is similar
in every way to that for the projection of spectra on a large screen

FIG. 368. PROJECTION OF SPECTRA ON A

SMALL SCALE.
L Radiant.
C Condenser.
S Slit.

Oj Objective before the prism.
O2 Objective beyond the prism.
P Prism.
R V Spectrum.

For the best definition there must be an objective (02)
beyond the prism to focus the spectrum (R V).

CH. XV] DEMONSTRATION OF ULTRA-VIOLET LIGHT 641

except that the additional lens O2, brings the parallel rays of light of
each wave-length to a focus in the spectrum R V. The arrange-
ment in fig. 368, with the lens d, giving a converging beam, will
not give good results, and the second lens C>2, is required if any fine
details in the projected spectrum are to be shown. The two lenses
Oi and O2, should be achromatic. The two lenses from a symmetri-
cal photographic objective will give excellent results. Ordinary
spectacle lenses can be made to answer if no others are available.
The prism can be of any of the forms previously described. The
spectrum is received on either a white screen or one which is coated
with anthracene in order to show the ultra-violet.

FIG. 369. PROJECTION OF SPECTRA ON A
SMALL SCALE.

With this arrangement, where the distance from the
prism to the spectrum R V is relatively short, as here
shown, the definition will be poor: see fig. 368 for a better
method.

L Radiant (arc lamp with vertical carbons).

C Condenser.

5 Slit.

O Objective.

P Prism.

/ Image of the slit S when no prism is in place.

R V Spectrum projected by the prism.

§ 907. Projection of ultra-violet. — Ordinary glass prisms and
lenses if not noticeably yellow or green will transmit radiation in the
ultra violet to about .35^, which can be observed by the use of an
anthracene screen (§ 899). If the far ultra-violet spectrum is to be
demonstrated it is necessary to use a quartz system, that is, all
condensers, lenses, and prisms between the source and the screen
must be made of quartz, either quartz glass or quartz crystal.
The apparatus is arranged as in fig. 370.

The quartz prisms are usually made of two 30° prisms, as shown
in fig. 370, one of which is a right-hand crystal, the other a left-

642 DEMONSTRATION OF ULTRA-VIOLET LIGHT [Cn. XV

hand crystal. The space between the two prisms is filled with
glycerin.

Aside from the material of which the lenses and prisms are made
there is but one thing which is different from the case with a glass
system. By using quartz alone, no achromatic lenses are possible
and the spectrum instead of focusing in a line at right angles to the
axis of the beam, focuses along a line oblique to the axis. Thus,
the far ultra-violet, UV, focuses nearer lens O2, than does the visible
spectrum. Tilting the screen to the position indicated will enable
one to get all of the spectrum in focus at once.

The anthracene screen (§ 899) will enable one to demonstrate all
of the lines of arc spectra which would appear upon a photograph

FIG. 370. PROJECTION OF THE ULTRA-VIOLET
ON A SMALL SCALE.

L Light source.

C Quartz condenser.

5 Slit.

0, Plano-convex quartz lens.

P Quartz prism, preferably made of two
prisms r and / from right-and left-handed quartz
crystals cemented together with glycerin.

O2 Plano-convex quartz lens. Turn the convex sides of the lenses towards
the prism.

U V, V R Focus of the spectrum.

Anthracene screen, fluoresces to ultra-violet. Note its oblique position.

made by the use of a quartz spectrograph, the ultra-violet lines of
the aluminum arc at .217^ being easily seen. Demonstration of
fluorescence of other substances to ultra-violet may be shown by
substituting them for the anthracene.

The demonstration of the far ultra-violet on a large scale is
hardly possible owing to the small intensity of the light emitted in
this region.

CH. XV] PHOTOGRAPHY OF SPECTRA 643

USE IN PHOTOGRAPHY

§ 908. Apparatus. — Slit, prism, grating, symmetrical photo-
graphic objective, camera bellows, bromide paper, photographic
plate.

The systems described above for the demonstration of spectra on
a small scale (fig. 368, 371), may be employed for the photography
of spectra. Such a system can be used, for example, to determine
the wave-lengths of the radiation to which bromide paper is
sensitive. If the bromide paper is held firmly against a rigid sup-
port so that the spectrum of a right angle arc may fall upon it, the

FIG. 371. ARRANGEMENT FOR THE PHO-
TOGRAPHY OF SPECTRA.

S Slit.

Ol Front combination of a photographic
objective.

0,, Back combination of the objective.

P Prism between the two combinations
of the photographic objective.

R V The spectrum in focus on the photographic
plate.

Plate The sensitive photographic plate in the
camera.

paper will be found to be blackened where blue and ultra-violet
light struck it, but the red and green will show no action at all.
If dry plates, however, are used a more complete system of shield-
ing from the light will be required. The second lens O2, may be
held in a camera box as shown in figure 371. A more elaborate
system for making several exposures on the same plate is not here
described because, while good results may be obtained with such
apparatus by sufficient labor, it is more satisfactory to use one of
the regular spectrographic cameras.

644 ABBE DIFFRACTION THEORY [Cn. XV

DEMONSTRATION OF ABBE DIFFRACTION THEORY OF MICROSCOPIC

VISION

§ 909. Apparatus. — Condenser; Pinhole; Slit.

Convex lens of one meter focal length (spectacle lens of i diopter,
§ 356a).

Grating, photographic line screen (100 to 200 lines to the inch,)
fine wire gauze (100 mesh), fine bolting cloth.

Telescope. The eye-piece should be of high power.

Diaphragms to remove portions of the diffraction image.

FIG. 372. LENS SYSTEM AND ARRANGEMENT FOR SHOWING THE ABBE
DIFFRACTION THEORY OF IMAGE FORMATION.

L Right-angled arc lamp with small carbons (5 mm.)
C Condenser used temporarily for focusing.

C Grating with coarse lines. A halftoning, line screen or a fine wire net will
answer.

0 Spectacle lens of I diopter (i meter focus) for projecting the image of the
lamp.

DI Image of the arc lamp L, projected by the objective O.

When the grating (G) is in place, there is formed at this point a diffraction
pattern. Various shaped diaphragms placed at this point modify the screen
image of the grating at I.

1 Screen image of the grating (G).

The lines and numbers above and below indicate the approximate distances
between the different parts of the system which have been found to give satis-
factory results.

An interesting phenomenon connected with the Abbe Diffraction
Theory of image formation can be demonstrated by one of the
combinations described below. The simplest is shown in fig. 372.
Suppose the test object to be a diffraction grating with equidistant
lines such as a fine wire gauze or a line screen such as photo-
engravers use in making half tones.

§ 910. The Abbe diffraction theory. Image formation with
dkected light. — In microscopic work and with all transparency

CH. XV] ABBE DIFFRACTION THEORY 645

projection, objects are not self-luminous but are illuminated by a
narrow beam of directed light which, were no object present, would
pass through the center of the objective. When an object, a
diffraction grating for example, is illuminated with a narrow cone
of light, the light is spread out into a diffraction pattern. The
finer the details of the object, the larger will be the diffraction
pattern. The objective will unite the light scattered from the
object by diffraction just as it would light which was spread out by
reflection from a white surface. Now according to the Abbe
diffraction theory, the closeness with which the image will corre-
spond to the object will depend upon the completeness with which
the light from the entire diffraction pattern is collected to form the
image. If the entire diffraction pattern is not united to form the
image, but part of it is intercepted, the image will be that of such
an object as would produce a diffraction pattern like that part of
the diffraction pattern which is collected to form the image.

§ 911. Lens system for showing diffraction images. — The lens
system shown in figure 372 will show this phenomenon. The arc
lamp L, with 5 mm. carbons, three to five amperes direct current
used as a point source, is set up six to eight meters from the screen
at I. The condenser C is used temporarily to illuminate the
objective lens 0. This objective lens O, is of one meter focal
length (an ordinary convex spectacle lens of i diopter will answer).
It is placed 3M. (10 ft.) from the screen. The grating G, is placed
between the source L, and the objective O, so as to be in focus on the
screen at I . The condenser C is now removed. The image I, will
remain as before. At DI, would be found an image of the source
cast by the lens O, but it will be spread out into a diffraction pat-
tern by the grating.

If a vertical slit is placed at D I, so as to remove all but a vertical
line of images, the appearance will be of parallel horizontal lines.
A diagonal slit will give the appearance of diagonal lines, no vertical
or horizontal lines being seen. If a vertical rod is put in so as to
remove the central row of images, the diffraction pattern will be
that of a grating with fine vertical lines, twice as close together as
the coarse horizontal lines, and the image at I, will have heavy

646 ABBE DIFFRACTION THEORY [Cn. XV

horizontal lines and fine vertical lines very close together. Dia-
phragms cut from black paper of various shapes, will give many
curious and beautiful appearances at I. A small diaphragm
placed so as to remove all but the central image of the pattern, or
one of the lateral images, will allow light to fall on the screen but
no detail can be seen.

For lecture purposes, where one requires considerably more light
than for a small class demonstration, one can use a vertical slit
with a condenser as the source instead of the arc lamp (fig. 373).
See § 891 and § 900, figure 364. Use line gratings with the lines

FIG- 373- DEMONSTRATION OF ABBE DIFFRACTION THEORY TO A LARC.K

AUDIENCE, USING A SLIT.
Arc lamp.
Condenser.
Pinhole or Slit.
Ct Condenser, preferably an achromatic combination.
G Grating.

O Objective. The diffraction pattern is formed at the face of the objective.
Diaphragms are used at this point to modify the image.
7 Image of grating.

vertical. The phenomena shown are not as interesting as when
using a point source. By using slits or rods to intercept part of
the diffraction pattern, the image on the screen can be made to
appear as if it were of a grating having finer lines than the grating
which is actually used.

If this phenomenon is projected it will probably be desired also
to demonstrate it individually to a few of the observers. This may
be done by the use of a telescope t, (fig. 374). to observe the grating.
The eyepiece of the telescope should be of a high power. The
condenser C2 focuses the image of the pinhole just in front of the
telescope objective. When the grating g, is in place, the diffraction
images will appear on both sides of the pinhole image. If the
grating is viewed by the telescope it will appear normal but if part
of the diffraction pattern is stopped out by diaphragms, the grating

CH. XV] DARK GROUND ILLUMINATION 647

will appear changed as in the case of projection. The sharpness of
the pattern, and the intricacy of design are however much finer
than it is possible to project.

FIG. 374. DEMONSTRATION OF THE ABBE DIFFRACTION THEORY TO A SINGLE

OBSERVER.

L Arc.

C, Condenser.

S Pinhole or Slit.

C2 Condenser, preferably an achromatic combination.

G Grating.

T Telescope with high power eyepiece.

The telescope is focused on the grating and the diffraction pattern is focused
just in front of the telescope objective. By placing suitably shaped dia-
phragms at this point, the image as seen in the eyepiece will be modified.

DARK GROUND ILLUMINATION: METHOD OF STRIAE

§ 912. Many beautiful experiments in Physics and Chemistry
can be shown by what is best known as the Schliren-methode of
Toepler. This method will yield results almost as striking as those
obtained by polarized light.

See Wiedemann Annallen, CXXXI, p. 33.

The use of this method enables one to demonstrate any slight
lack in homogeneity of a medium which is sufficient to deviate a
beam of light.

To adapt this method to projection the following apparatus can
be used :

§ 913. Apparatus for the experiments with striae. —

(1) Magic lantern with the usual equipment of arc lamp,
projection objective, and the first element of the large condenser.

(2) A special condensing lens or combination. This need not
be of especially large diameter or short focus (5 cm. diameter, 20
cm. focus will answer), but it should be as free as possible from
spherical and chromatic aberration, and must have no scratches
and be kept perfectly clean.

648

DARK GROUND ILLUMINATION

ICH. XV

(3) Diaphragms to shut off the direct light of
the lantern. These may be simply sheets of tin.

(4) Glass cells with parallel faces.

§ 914. Method. — Light from the arc L, is ren-
dered nearly parallel by the lantern condenser Ci.
The diaphragm Di, cuts off the lower half of this
beam, the other half serving to illuminate the speci-
men S, in the glass cell. The distance between the
condenser and the specimen should be from 50 to
100 cm. (2 to 4 feet). Either before or after pas-
sing through the specimen S, (preferably before, as
in fig. 375) this light strikes the special condenser
Cz, which brings the diaphragm Di, to a focus at D*.
At this point is placed the diaphragm D2, which is so
arranged as to just cut off the remainder of the light,
its edge coming to the edge of the image of the dia-
phragm DI. The objective O, is focused to bring
the specimen S, to a sharp focus on the screen before
the diaphragm D2, is in place. With the apparatus
thus arranged the screen will be perfectly dark, all
light not intercepted by the first diaphragm being
stopped by the second. If, now, the liquid in the
cell S, is not quite homogeneous but is cordy, as
when glycerine and water are first mixed or when a
crystal of salt is dissolving, the image of DI, will not

FIG. 375. DARK GROUND ILLUMINATION; TOEPLER METHOD
OF STRIAE.

L Arc.

C, First part of the magic lantern condenser.

Dl Diaphragm.

C2 Condenser of long focus. It must be as perfect a lens
as can be found.

S Specimen, with slight inhomogeneity.

a An inhomogeneity in the specimen which deviates the
light.

D2 Diaphragm intercepting direct light from the lantern.

O Objective.

a1 Image of a.

Note that with the objective on the axis only the upper por-
tion of the objective is used.

CH. XV]

DARK GROUND ILLUMINATION

649

be quite sharp and some light will escape the edge
of the diaphragm D2, and reach the screen.

The result is very striking as even a slight inhomo-
geneity of the medium in the glass cell will deviate
light sufficiently to pass the second diaphragm and
thus be seen.

Suppose a slight cord of a substance of different
refractive index from its surroundings to exist at a1
in fig. 376. This cord will scatter the light. A ray
which would normally strike D2, and be intercep-
ted, will spread out in all directions. The part of
this light which strikes the objective will go to form
a screen image of the cord at a1 (fig. 376).

The sensitiveness of this method depends upon the
sharpness of the image of the diaphragm DI, and the
closeness of adjustment of D2, so as to encroach as
little as possible upon it. With a very sharp im-
age, it is possible to detect the minutest striae and
inhomogeneities in the specimen.

The image sharpness may be disturbed as much by
imperfections in the condensing lens C2, as by an in-
homogeneity of the specimen, hence these imperfec-
tions, if present, will show distinctly on the screen.
In fact, the method is as well designed to show the

FIG. 376.

DARK GROUND ILLUMINATION; TOEPLER
METHOD OF STRIAE.

L Arc.

Ct First part of the magic lantern condenser.

£>! Diaphragm.

C2 Condenser of long focus. It must be as perfect a lens
as can be found.

S Specimen with slight inhomogeneity.

a An inhomogeneity in the specimen which deviates the
light.

DI Diaphragm intercepting direct light from the lantern.

0 Objective.

a1 Image of a. Note that a small objective above the axis
is used. The dotted lines show the course of the rays which
are slightly deviated from their original path by the inhomo-
geneity of the specimen.

650

DARK GROUND ILLUMINATION

[Cn. XV

imperfections of the second condenser C2, as to show
the specimen. The difficulty can, of course, be les-
sened by drawing the condenser face C2, out of the focus
of the objective O. Dust, fingermarks, etc., will then
produce a general blur, rather than a distinct image.

§ 915. Foucault's method. — A slight modification
of the old method of Foucault for testing telescope ob~
jectives also gives good results. This method (fig. 377)
dispenses with the use of the first diaphragm Di, the
crater of the arc being the first diaphragm in this case.
Instead of the ordinary condenser, is substituted a lens
or set of lenses which are to form a very sharp image of
the crater. Just in front or behind the objective
(wherever the sharp image of the crater is formed) is a
diaphragm which just covers up the crater image. Such
a diaphragm may be made by fastening a round
piece of black paper to a piece of plate glass. The ob-
jective brings the specimen to a focus upon the screen
in the usual way. Any inhomogeneities in the speci-
men scatter the light so that some of it gets by the
central stop at D2. It is this scattered light which
serves to form the image on the screen at a1.

If the inhomogeneities of the specimen are great
enough, the specimen may be projected by the method
just described, (see fig. 377), except that instead of
using a central stop, the lens is provided with an iris
diaphragm, and the image of the arc is focused on

C
S
a
O

I),

FIG- 377- DARK GROUND ILLUMINATION,
FOUCAULT'S METHOD.

Arc.

Condenser of long focus, as perfect as can be found.
Specimen.

Inhomogeneity in the specimen which deviates the light.
Objective.

Central stop to intercept the direct light from the arc.
a1 Image of a, projected on the screen.
The dotted lines show the course of the rays deviated by an
inhomogeneity in the specimen which pass to one side of the cen-
tral stop and reach the screen.

CH. XV] DARK GROUND ILLUMINATION 651

the center of this opening. If, now, light is slightly deviated it will
not get through the small opening in the diaphragm and the
inhomogeneity will appear as a dark shadow on a light background.

EXPERIMENTS ILLUSTRATING NORMAL VISION AND SIMPLE,
REFRACTIVE EYE DEFECTS

§ 916. Apparatus needed for the demonstrations:

Suitable room for projection; White screen 70 to 100 centi-
meters (28 to 40 inches) square; Arc lamp and magic lantern con-
denser with lamp-house, fig. 378-379; Optical bench with a range
beyond the condenser of at least 40 cm. (16 in.) ; fig. 159, 378-379;
Lantern-slide carrier and lens support, fig. 159, 378-381; Metal
holder for four trial lenses, fig. 380; Oculists' trial lenses as shown
in fig. 382; Discs of tin or sheet -iron the size of trial lenses, and
with holes for the pupil and a stenopasic slit, fig. 399, A, B.;
Lantern slides (4) for illustrating accommodation, astigmatism
and anisometropia, myopia, etc., fig. 383, 391-392, 401, (§ 9i6ab).

§ 916a. The cost of the special apparatus needed for the demonstrations
in normal and defective vision: —

Metal lens holder for 4 trial lenses $2.25

Trial lenses in trial rings (14 at 20 cts.) 2.80

Double trial lenses for unlike eyes i .50

Lantern slides (4 at 35 cts.) i .40

$7-95

To this amount should be added the cost of the object and lens blocks and
the vertical pieces for carrying the lens holder and the slide-carrier.

The screen of white cardboard and the lengthening rods for the optical
bench are also extra, but all of these should not make a total outlay of over
$10.00 in addition to the magic lantern. As will be seen in the appendix, magic
lanterns cost all the way from $20 to $500.

The trial lenses, lens holder and double lenses may be obtained through a
local optician, or they can be got direct from a manufacturer of spectacles, etc.,
for example: Aloe & Co., St. Louis, Mo.; Bausch & Lomb Optical Co.,
Rochester, N. Y., New York City, Washington, D. C., Chicago, 111., San
Francisco, Cal.; Geneva Optical Co., Geneva, N. Y., and Chicago, 111.;
Hardy & Co., New York City, Chicago, 111., Denver, Col., Atlanta, Ga.,
Dallas, Tex.; Lloyd & Co., Boston, Mass.; E. B. Meyrowitz, New York City,
Minneapolis and St. Paul, Minn.; Williams Brown & Earle, and Joseph
Zentmayer, Philadelphia, Pa., and many others.

§ 916b. The authors feel greatly indebted to Dr. Melvin Dresbach and to
Dr. Albert C. Durand for suggestions and criticism in the preparation of the
manuscript for these experiments in normal and defective vision.

652

DEMONSTRATIONS OF NORMAL VISION

[Cn. XV

DEMONSTRATIONS REPRESENTING NORMAL AND DEFECTIVE

VISION

§ 917. Source of light. — For the most successful demonstrations
of vision and its refractive defects it is necessary to have a right-
angle arc lamp and a direct electric current. However, for all the
experiments except the one to show unequal refraction in the two
eyes, the other sources of light mentioned in this book can be used.
If the large sources are used it is desirable to have a shield with an

, Condenser

FIG.

378. PROJECTION APAPRATUS WITH THREE-LENS CONDENSER
OPTICAL BENCH FOR DEMONSTRATIONS REPRESENTING VISION.

Commencing at the left :

W1 Wl Supply wire to the knife-switch and from the switch through the
rheostat to the upper or positive carbon of the arc lamp.

W* W2 Supply wire to the knife-switch, and from the switch to the lower
carbon of the arc lamp.

L The source of light (crater of the upper carbon).

Lamp Block The block for supporting the arc lamp and by which it can be
moved back and forth on the optical bench.

Base Board The board on which are the tracks of the optical bench (see

%• 159)-

1 Condenser* The triple-lens condenser. The second element of the con-
denser (2) should have a focus of 30 to 40 centimeters (12 to 15 inches).

W The water-cell to absorb the radiant heat.

Obj. The object (lantern slide of radial lines, etc., fig. 383-393).

Lens The trial lens serving to project the image.

Block i The block supporting the condenser and water-cell.

Block 2 The block serving as a support for the lantern slide, and by means
of which the slide can be moved back and forth on the optical bench.

Block j The block supporting the lens carrier, and by means of which the
lens can be moved back and forth on the optical bench.

These experiments, with the accompanying explanation, have been com-
piled from lectures and demonstrations given by the senior author before the
Sixth District Branch of the Medical Society of the State of New York,
October, 1913; The Conference of Veterinarians at the New York State Vet-
erinary College, December, 1913; and before the Cornell University Summer
School, July, 1914.

CH. XV]

DEMONSTRATIONS OF NORMAL VISION

653

opening a little smaller than the trial lenses to put just beyond the
trial lens. This cuts off any stray light falling outside the lens.
If large sources only can be used, then it would be an advantage to
have lenses of greater diameter than the trial lenses so that all
the light could be utilized and thus give a brighter screen image.
The trial lenses answer admirably for the arc light, however.

§ 918. Centering along one axis. — As with the magic lantern
and the projection microscope, it is necessary to have all of the
elements of the projection outfit on one axis (§ 51-58).

Also as the light is liable to get out of the axis, it is a great advan-
tage to have fine adjustments on the arc lamp to bring it back in

FIG. 379. PROJECTION APPARATUS WITH TWO-LENS CONDENSER FOR
DEMONSTRATIONS REPRESENTING VISION.

Radiant. The arc lamp in the lamp-house. It has fine-adjustment screws
(L A, V A), and feeding screws for the carbons (F S) and the source of light in
the crater of the upper carbon (L).

Lamp House, V The lamp-house and its ventilator.

Rods The rods of this form of lantern. These serve as a kind of optical
bench along which the different parts can be moved. They should be long
enough to permit of a separation of the lens and the condenser of at least 40
cm. (16 in.).

Cond, i 2 The two lenses of the condenser. Lens 2 should be of relatively
long focus, 25 to 30 cm. (10 to 12 inches).

Object Block The block supporting the object, and by means of which the
object can be moved back and forth along the rods.

Lens Block The block supporting the lens. It can be moved back and
forth along the rods.

Image screen The white screen for the image. It is 5 meters (16 ft.) from
the projection lens.

J7 Centimeters The distance between the lens and the object for a 3 diopter
lens.

28 Centimeters The distance between the object and a 4 diopter lens.

Ind i, Ind 2 A white strip of cardboard to serve as an indicator so that the
spectators can see when the object is moved toward or from the lens.

654

DEMONSTRATIONS OF NORMAL VISION

[Cn. XV

position (fig. 3). The vertical boards holding the lantern slide and
the trial lens holder should have means of centering them accur-
ately. This is provided for by the U-shaped opening at the lower
B end of the uprights where the set-screw is inser-

ted to hold the uprights against the movable
blocks (fig. 159, 381).

FIG. 380. METAL LENS HOLDER FOR FOUR TRIAL LENSES.
(Half Natural Size)

A Side view of the lens holder showing the stem by
which it is held in place in the lens support (fig. 378).

B Face view of the lens holder showing the four grooves
for containing the lenses, and in which they can be rotated.

§ 919. Lenses, lens holder and white screen. — For the refrac-
tive part of the eye (cornea and crystalline lens), the trial lenses
used by oculists answer very well. To hold these and to permit
of their rotation it is necessary to have a metal lens holder (fig.
380). A lens holder with grooves for four lenses is very desirable.

To represent the retina of the eye there is needed a white screen
about one meter (3 feet) square. This screen is kept at the con-
stant distance of 5 meters (16 ft.) from the lens in all the experi-
ments.

§ 920. Demonstration of normal vision. — While men have
always known that the eyes were for seeing the things in external
nature, the knowledge that the eye acts like an optical instrument
and produces an inverted, real image upon the retina came only
when Kepler in 1604 demonstrated, in the clearest possible manner,

FIG. 381. SLIDE-CARRIER FOR THE TEST SLIDES.
This consists of:

An object block sliding on the optical bench or the rods
(fig. 378-379)-

A vertical board i cm. (y+ in.) thick with a U-shaped
opening in the lower end. A thumb-screw and washer
serve to hold the board in any position against the ob-
ject block.

The lantern-slide carrier consists of a thin board or a.
piece of cardboard with an opening of the proper size and
height (object) attached to the vertical board. The try-
square shaped piece (H H) is to hold the test slide (G SI)
in position. (See also fig. 159, b4).

"G"sr"

-i

1
1

Object

Board

RCH

Object Block

CH. XV]

DEMONSTRATIONS OF NORMAL VISION

655

Kepler, Retinal Image,

Accommo-
dation

1604

Presbyopia

Astigmia. 1800-1825

Effect of the Pupil

Hyperopia

Two Eyes Different

FIG. 382. OUTLINES OF THE TRIAL LENSES NEEDED FOR SIMPLE
EXPERIMENTS IN NORMAL AND DEFECTIVE VISION.

(About one-third size)

/ A 3 diopter convex lens (+3). For diopter see § 356a.

2 A3 and a 4 diopter convex lens (+3, +4).

j A 4 diopter convex lens (+4), and a i diopter concave lens ( — i).

4 A 2 diopter and a i diopter convex lens (+2, +i).

5 A3 diopter and a i diopter convex lens (+3, +!)•

6 A 4 diopter convex lens (+4), a 0.5 convex cylinder, (+0.5 cyl.) and a
0.5 concave cylinder, ( — 0.5 cyl.).

7 A 4 diopter convex lens (+4), and a 0.5 convex or concave cylinder
(0.5 cyl.).

656

DEMONSTRATIONS OF NORMAL VISION

[CH. XV

8 A half convex lens of 4 diopters, and a half convex lens of 3 diopters
(+4, +3) in the same trial frame.

A half convex lens of i diopter (-fi) and a half circle of plane glass (0), in
the same trial frame to serve as a correcting spectacle.*

that whenever an object is seen, there must be formed an image on
the retina, and that this image, following the laws of optics, must
be inverted. A few years later, (1619), Scheiner showed by actual
experiment with the eyes of animals that such an inverted, real
image is formed on the retina; and in the year 1625, he showed that
the same is true of the human eye.

1604

Kepler:

Retinal Image,

Inversion,
Accommodation.

FIG. 383. LANTERN SLIDE FOR THE EX-
PERIMENTS IN ACCOMMODATION.

FIG. 384 TRIAL LENS FOR KEP-
LER'S EXPERIMENTS.

For the demonstration of normal vision with the special projector
(fig. 378-379) are needed:

(1) A3 diopter convex, trial lens (fig. 384). For the meaning
of diopter, see § 356a.

(2) A lantern slide for object (fig. 383).

The screen representing the retina should be at a distance of
5 meters (16 ft.) from the lens and the lens should be 40 centimeters
(15.5 in.) from the condenser and the object 36 to 37 cm. distant
from the object (fig. 379).

CH. XV] DEMONSTRATIONS OF NORMAL VISION 657

The object should be right side up and face the condenser. If
the arc lamp is lighted there will be projected on the screen a sharp
image of the lantern slide. This image will be wrong side up as it
is in the eye. In order to have the image erect on the screen the
object must be wrong side up in the slide-carrier as in magic lantern
projection (fig. 8, § 35).

For the remainder of the experiments it is desirable to have the
screen image appear erect so that there may be no distraction from
the special points to be shown. It is worth while remembering
though, that when the image is wrong side up on the screen it will
be right side up in the eyes of the observers, and when it is right
side up on the screen it will be wrong side up in the eyes of the
observers. Objects appear right side up to a person only when the
image is wrong side up on his retina.

§ 921. Demonstration of the need of accommodation of the
eye for different distances of the object. — It was pointed out by
Kepler in his discussion of vision that the eye as an optical instru-
ment could have a sharp image on the retina only in one position
of the object, unless some change took place in the eye. Every-
one with normal eyes knows that objects at all distances from 10 to
15 centimeters up to infinity can be seen with equal clearness.
Kepler thought that the power to see objects at different distances
was due to the possibility of changing the relative position of the
crystalline lens and the retina by the elongation and shortening
of the eye-ball.

To demonstrate Kepler's hypothesis of accommodation there are
needed :

(1) A3 diopter, convex, trial lens (fig. 384).

(2) A lantern slide (fig. 383).

(3) A white cardboard screen about half a meter (15 to 20 in.)
square to hold in the hands.

If now the arc lamp is lighted and the lantern slide placed 36 to
37 centimeters from the lens a sharp image will be projected upon
the 5 meter screen. Now move the object to about 40 centimeters
from the lens; the image will not be clear, but much blurred. To
find the position of the sharp image, take the small white screen in

658 DEMONSTRATIONS OF NORMAL VISION [Cn. XV

the hands and hold it in the path of the light from the lens. It
will be found at a point between two and three meters from the
lens. This shows that if the object is farther from the lens, the
image will be nearer to it. Conversely, if the object is brought up
toward the lens the image will move farther off. Kepler thought
that following the changes in the position of the sharp image
with change in the object, that for a near object the eyeball elon-
gated to bring the retina in the most favorable position, and that
when the object was far off the eyeball shortened to bring the retina
up to the point where the sharp image was formed. Such a method
of accommodation for objects at different distances would be
effective, as everyone knows who uses a photographic camera, but
as is now known it is not the method used by the eye of the higher
animals and man.

§ 922. Demonstration of Schemer's theory of accommodation.

— Scheiner admitted that the method of accommodation proposed
by Kepler would be effective, but he thought that the eyeball
remained unchanged in shape, and the crystalline lens changed its
shape, being more convex for near objects and less convex for
distant objects. He put it thus: "The crystalline lens of the eye
is equal to many glass lenses."

There are needed for demonstrating Scheiner's theory :

(1) A convex, trial lens of 3 diopters (fig. 382, 385).

(2) A convex, trial lens of 4 diopters.

(3) A lantern slide of fig. 383.

Put the three diopter lens in the lens holder and light the arc
lamp. When the lens is 36 to 37 cm. from the lens a sharp image
will be projected on the 5 meter screen. Now move the object up
to 27 or 28 cm. from the lens. The _

image will be much blurred. Remove
the 3 diopter lens and put in its place
the 4 diopter lens. The screen image
will be sharp again. This shows that
if the crystalline can become more
and less convex, depending upon the

position of the object, the screen image TION EXPERIMENT.

CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 659

will be sharp without changing the position of the screen. And
this is now known to be what happens in the accommodation of
the eyes in the higher animals and in man. Furthermore, it
has been found that for near objects there must be a muscular
effort to make the crystalline more convex, while if the object is
distant, the eye forms a perfect image without effort.

REFRACTIVE EYE DEFECTS

§ 923. For a person with normal eyes it is almost impossible
to understand the difficulties under which one labors if the eyes are
defective. The difficulties become especially trying for those who
must do close work in the trades or in school work, and in exacting
professional work.

From the examination of tens of thousands of school children
in our own and other lands it is found that over 10% of them have
eye defects of some kind. And in a careful examination of 5,000
college students 45% to 50% had ocular defects of a kind that
made the use of spectacles desirable, and for many of them abso-
lutely necessary.

It is believed that if those with normal sight had anything like a
proper realization of the difficulties of those with eye defects every
effort would be made to give relief.

It is hoped that these demonstrations, which are so easily made
and show so strikingly the simpler refractive eye defects, will be of
service in helping to give an understanding of the facts and the
means for relief.

Great care has been exercised in selecting demonstrations which
shall show the common defects, and those of moderate severity,
not the unusually severe or rare. From the personal experience of
the senior author, it is known that the appearances shown for
presbyopia are not exaggerated; and friends with the other eye
defects have assured us that the appearances given in the demon-
strations are not uncommon.

§ 923a. For a discussion of the eye defects in school children see : Hermann
Cohn, Die Sehleistungen von 50,000 Breslauer Schulkindern, 1899; Dr. M.
Dresbach, Examinations of the Eyes of College Students, The Medical Record,
Aug. 3, 1912, also in the Educational Review, Dec., 1913. In Dr. Dresbach's
papers are many references to the work of others.

660 DEMONSTRATING REFRACTIVE EYE DEFECTS [Cn. XV

FIG. 386. TRIAL LENSES
FOR MYOPIA.

§ 924. Myopia, or short sight. — My-
opia is due to any condition in the eye
by which the image of distant objects is
formed in front of the retina. The retina
is too far away, hence the retinal image is
blurred. Persons with this eye defect
are able to get clear images only when

the object is quite close to the eyes.

As the effort of accommodation only aids in seeing near objects,
there is no way by which short sighted persons can see distant
objects clearly without the use of a telescope or of concave spec-
tacles.

For demonstrating myopia and its remedy are needed:

(1) A convex, trial lens of 4 diopters (fig. 386).

(2) A concave, trial lens of i diopter.

(3) A lantern slide of fig. 383.

The 4 diopter lens is to represent the refractive power of the
myopic eye. It is placed in the metal lens holder, and the object
is brought up to a point 27 to 28 centimeters from it. Then the
image will be sharp and clear on the 5 meter screen.

Move the slide back until the distance between it and the lens
is 36 to 37 centimeters. The image on the 5 meter screen will be
much blurred; it is too far off. One can prove this by taking the
white cardboard in the hands and finding the position of the sharp
image as in § 92 1 . Now to get a sharp image on the 5 meter screen
it is necessary to reduce the curvature of the 4 diopter lens. Do
this by adding the i diopter concave lens. This reduces the 4 to a
3 diopter convex lens, and now the image is sharp and clear on the
5 meter screen.

In the same way the short sighted
person can use concave spectacles which
will reduce the refractive power of the
cornea and the crystalline lens, and hence
the image will be formed farther away.
If the right spectacles are used, the image FJG ^ TRIAL LENSES
of distant objects will be clear and sharp FOR HYPEROPIA.

CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 66 1

on the retina. If the person wishes to look at near objects the eye
is accommodated to make the crystalline more convex and the
diverging rays from near objects are brought to a focus on the
retina as with persons having normal eyes and not using spectacles.

§ 925. Hyperopia, or long sight. — This eye defect is due to any
condition in which distant objects have their images formed
behind the retina. The retina is too close to the crystalline lens,
hence there is a blurred image formed on it even with parallel rays,
unless there is active accommodation and the crystalline lens is
made more convex. That is, with hyperopic eyes, no object can
be seen without effort.

For the demonstration of hyperopia there are required:

(1) A convex, trial lens of 2 diopters (fig. 387).

(2) A convex, trial lens of i diopter.

(3) A lantern slide of fig. 283.

The lens of 2 diopters is to represent the refractive power of the
hyperopic eye. Place the lens in the metal holder, and light the
arc lamp. Move the object near to and distant from the lens, and
no place within the range will be found where a clear image will be
formed on the 5 meter screen. The screen is too near the lens and
the sharp image is formed somewhere behind it. If the room is
long enough the place where the image is sharp can be located.

Now move the object until it is 36 to 37 centimeters from the
lens and add the i diopter convex lens. This will add its strength
and the refractive power will be equal to 3 diopters, and now the
image will be clear and sharp on the 5 meter screen.

With the proper convex spectacles, the long sighted person can
see distant objects without effort, then when he wishes to see near
objects clearly the crystalline is made more convex as with normal

eyes, thus making the entire range of
vision normal.

§ 926. Presbyopia, or old age sight. —

This comes gradually to every one with
advancing years, until finally, for most

people after 60 or 6s years of age, the
FIG. 388. TRIAL LENSES J ^ . .

FOR PRESBYOPIA crystalline lens has lost its elasticity so

662 DEMONSTRATING REFRACTIVE EYE DEFECTS [Cn. XV

90°

,06

FIG. 389. THE RADIAL LINES SHOWING THE SIZE OF THE IMAGE WITHOUT
SPECTACLES, AND WITH CONCAVE AND CONVEX SPECTACLES.

A shows the size of the image without spectacles.

B shows the diminished size of the image when a spectacle lens of — .5 diopter
is put with the 5.5 diopter lens. (This reduces the 5.5 to a 5 diopter lens).

C shows the increased size of the image when a spectacle lens of +.5 diopter
is added to the 5.5 diopter lens. (This increases the 5.5 diopter lens to 6
diopters).

The photographs were made by fixing the camera so that the objective and
sensitive plate were at a constant distance, then the image was focused by
moving the object farther off for B and nearer to the lens for C, just as in the
projection experiments (§ 922).

CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 663

that it cannot be made more convex no matter how great the effort
at accommodation. If the eyes were originally normal, the images
of distant objects are still clear, but the diverging rays of near
objects can no longer be brought to a focus on the retina, without
artificial aid.

For illustrating presbyopia there are needed :

(1) A convex trial lens of 3 diopters (fig. 388).

(2) A convex trial lens of i diopter.

(3) A lantern slide of fig. 383.

Place the 3 diopter lens in the metal holder and the lantern slide
in the slide-carrier, then move the slide up till it is 36 to 37 centi-
meters from the lens. Light the arc lamp and there will be a sharp
image on the screen. This represents the appearance for distant
objects.

Now move the object up to 27 or 28 centimeters from the lens to
represent a near object. The image on the screen is much blurred,
something as the ordinary print of a newspaper looks to an old
man without spectacles. Put the i diopter lens with the 3 diopter
lens making a refracting medium equal to 4 diopters, and the image
on the screen will become clear and sharp, just as the print of the
newspaper becomes clear and sharp to the old man when he puts
on the proper spectacles.

§ 927. Astigmatism, Astigmia, or unequal curvature of a
refracting surface. — This is a common defect in the eye, and is
found very frequently in the cornea. Roughly speaking an
astigmatic curve is like the bowl of a spoon or a hen's egg, the
curve being greater in one direction than in the direction at right
angles.

The greater curvature will, of course, bring rays of light to a
focus sooner than the lesser curvature ; and with such a refracting
surface not all points in a circle can be focused sharply in any posi-
tion. Usually objects at right angles can be sharply focused by
changing the position of the objects, bringing them nearer for the
greater curvature and moving them farther away for the lesser
curvature. With a radial disc like fig. 389, 391, 392, if the vertical
lines are sharp in one position, the horizontal lines will be sharp in

664 DEMONSTRATING REFRACTIVE EYE DEFECTS [CH. XV

a different position of the object. The intermediate lines cannot
be made perfectly sharp in any position.

With the eye when it is accommodated for vertical lines, the
horizontal lines will be blurred, and when the horizontal lines are
sharp and clear the vertical lines will be blurred. All the inter-
mediate lines will be more or less blurred all the time.

To correct astigmatism it is necessary to do away with the
inequality of the curvature of the refracting surface. This can be
done either by increasing the lesser curvature or by reducing the
greater curvature sufficiently to make the refracting surface
uniform.

To demonstrate astigmatism there are needed:

(1) A convex lens of 3 or of 4 diopters
(fig. 390).

(2) A convex cylindrical lens of 0.5
diopter.

(3) A concave cylindrical lens of 0.5
diopter.

(4) A lantern slide of the history of ^'
astigmatism (fig. 393).

(5) A lantern slide of the radial lines (fig. 391).

Put the 3 or the 4 diopter lens in the lens holder, and the lantern
slide of the radial lines (fig. 391) in the slide-carrier. Move the
slide until the radial lines are as sharp as possible on the screen.

Put with the projecting lens the 0.5 diopter convex cylinder and
turn it so that the axis of the cylinder is vertical. The horizontal
lines will remain sharp, and the vertical lines will be most blurred.

The addition of the 0.5 convex cylinder produced anunsymmet-
rically curved refracting surface. Along the axis of the cylinder
no change is made in the refractive power, hence light rays, from
points in the horizontal lines, which are in planes parallel with the
axis of the cylinder, are brought to a focus at the same distance as
if the cylinder were absent, but rays in any plane oblique to this
axis are affected by the curvature of the cylinder, and are
not brought to the same focus, hence only horizontal lines appear
sharp.

CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 665

FIG. 391-392. RADIAL LINES IN BLACK AND WHITE FOR DETERMINING THE
PRESENCE OF ASTIGMATISM.

A lantern slide of the radial lines is very desirable for the demonstrations on
astigmatism.

The black lines on a white ground have the advantage that the lantern slide
is less liable to break than the white lines on a black ground (§ 852). It will
be noticed that, by contrast, the central white circle seems lighter than the
white spaces between the radial lines. In like manner the central black circle
seems blacker than the black spaces between the radial lines. These are
optical illusions, for the white is uniform and so is the black.

Now place the concave cylinder in front of the convex cylinder
and make their axes parallel (fig. 390). All the lines will become
sharp again. This is because the concave and the convex cylinders
with their axes parallel just balance each other and then act like a
piece of plane glass. To compare the effect of astigmatism on
printed matter with its effect on the radial lines, remove the cylin-
ders and focus sharply the lantern slide of the history of astigma-
tism, (fig. 393). Now add the 0.5 diopter convex cylinder and
make the axis vertical. The horizontal lines in the print will be
sharp, but the others blurred (fig. 394-395). Now rotate the
cylinder until its axis is horizontal, and the vertical lines will be
sharp and clear (fig. 396-397). It is to be noted that with these
Gothic letters, it is easier to read the words when the vertical lines
are clear, because vertical lines preponderate.

666 DEMONSTRATING REFRACTIVE EYE DEFECTS [Cn. XV

As shown with the glass lenses it is possible to do away with
astigmatism by rendering the inequality of curvature uniform,
hence a person with astigmatism can be given normal vision by the
use of the proper spectacles.

§ 928. Correct position of spectacles. — It is extremely import-
ant that glasses to correct astigmatism should be correctly adjusted
to the eyes. The necessity of this can be strikingly shown by
making the axes of the two cylinders in the last experiment some-
what oblique . When the axes are

oblique the confusion is greater ASTIGMATISM
than when the correcting lens is
removed entirely. Not only must Thomas Young, 1800

the correct spectacle be used, but """TJ^awSSwSl" °f *"*
it must be correctly adjusted.

Pp. 39-40, astigmatism of the crystalline

Furthermore , it should be known lens-

that the axis Of astigmatism in P. 57. astigmatism of the cornea.

Correction by obliquity of the •p»ct«cU>.

the eye sometimes changes so

that the spectacles which gave DA o

perfect vision at one time would George B. Airy, 1825

not do SO at a later time. In Cambridge Phi.os. Trans. Vol. II (1827)

t 1 • j 1 Correction by obliquity of the »p«-etacl«i», but mo«t

such a case new spectacles with ,.„«„„ „„ ,„. „„ „, cyllBdriclll ,.„....

axes arranged to meet the changed FIG ^ THE DISCOVERERS OF As_

conditions in the eyes are neces- TIGMATISM AND THE MEANS OF

CORRECTING IT.
bcii y .

§ 929. The two foci of astigmatic lenses, and the correction of
astigmatism with cylinders having the same form (both convex
or both concave). — Use the same outfit as for § 927.

Focus sharply the image of the radial lines. Add the 0.5 diopter
convex cylinder and make its axis vertical. The horizontal lines
will be sharp. The vertical and intermediate lines will be blurred.
Now move the object up towards the astigmatic combination and
soon the vertical lines will become sharp and the horizontal and
intermediate lines dim. In this position the focus is for the original
projection lens (3 or 4 diopters) with the added curvature (0.5
diopter) of the cylinder. That is the greatest curvature is now
acting to focus the image of the vertical lines. With the horizontal
lines in focus, it was the least curvature which was acting.

CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 667

— ^: ^ t - onfl
--:-*5

:-.-:- -i-^ — -.--• — — -i
= r»-a Si.i — -_. EC

FIG. 394, 395. FIGURES SHOWING THE APPEARANCE OF THE RADIAL LINES
AND OF PRINTED MATTER WHEN AN ASTIGMATIC LENS is IN Focus FOR
HORIZONTAL LINES.

IK r

fc,

ASTIGMATISM

Ihnmnri Younu, MUM)

riiiiiinii|iliii'.(l 1 1 nii'iiM iinnh lit Mm

ll.ivnl 'in. l-ly. 111(11

II -III H~ll||ll."ll«lll >.l MIX 1-lynlHlllllH

II. Ally, IIU1!)

Mllllli- I'llllMX IlKIIH Vl.l M III '

FIG. 396, 397. FIGURES SHOWING THE APPEARANCE OF THE RADIAL LINES
AND PRINTED MATTER WHEN THE ASTIGMATIC LENS is IN Focus FOR

VERTICAL LINES.

Figures 394-397 were made by adding a +.5 cylindrical lens to a +5.5
diopter photographic objective, and the cylinder was placed with its axis
vertical in fig. 394-395, and horizontal in fig. 396 and 397; no change was
made in the focus of the objective. The aperture of the objective was F/i6
when the photograph was made.

As shown in figures 395-397, Gothic print seen through an astigmatic lens is
clearer when the axis is such that the vertical lines are in focus. This is
because the vertical lines are more numerous than the horizontal lines.

668 DEMONSTRATING REFRACTIVE EYE DEFECTS [Cn. XV

With the vertical lines in focus, add another convex cylinder of
0.5 diopter and arrange the axes of the two cylinders at right
angles (cross the cylinders). All the lines will now be sharp, for
the added convex cylinder increases the curvature where it was
lacking, and thus gives the combination a symmetrical curvature.
It is to be noted that when convex cylinders are crossed in this way
they add to the original lens the dioptry of the cylinders. In this
case 0.5 diopter, and the image is increased in size (fig. 389 C).

Two concave cylinders can be used in the same way, but with
concave cylinders the entire system is reduced in dioptry the
amount of the cylinders. In this case it would reduce the dioptry
half a diopter and hence the image would be smaller (fig.
389 B).

§ 930. Correction of astigmatism by the obliquity of the
spectacles. — It was pointed out by Young (1800), that astigmatism
might be corrected by making the spectacles sufficiently oblique to
neutralize the defect. This can be demonstrated very strikingly
as follows :

Use the same outfit as in § 927. Make the image of the radial
lines sharp on the screen and add the +0.5 diopter cylinder with
the axis vertical (fig. 390). Now put a convex lens of i diopter in
front of the cylinder and focus for the lines parallel to the axis of
the cylinder (vertical in this case). Tip the convex lens up or
down, i. e., across the axis of the cylinder, and when the right
obliquity is reached the lines will all be sharp. This is because the
tipped lens introduces the curvature lacking in the cylinder. This
can be shown by removing the cylinder and the horizontal lines will
be sharp showing that the vertical meridian is unchanged but the
horizontal meridian has been increased in curvature.

Use the same cylinder but a concave lens of i diopter instead
of the convex lens ; focus the combination until the horizontal lines
are sharp, then rotate the concave lens sidewise (i. e., parallel with
the axis of the cylinder), and when at the right obliquity the radial
lines will all be sharp. This is because the oblique, concave lens
neutralizes the greater curvature of the +0.5 cylinder. In a word,
the oblique position of the spectacle makes it act like a cylinder in

CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 669

addition to its magnifying or reducing power. Acting as a
cylinder it follows the law of the cylinder as given in § 929.

If one keeps in mind the effect of oblique lenses it will help to
appreciate the necessity of having the spectacles properly adjusted.

§ 931. Effect of the aperture of the pupil in vision. — As a

general statement it may be said that the larger the aperture of the
pupil the more brilliant will be the image as more light is admitted.
On the other hand the larger the pupil the more strongly do eye
defects deteriorate the retinal image.

When the aperture of the pupil is small, only a small part of the
refracting surface produces the image, and consequently any
defects of curvature are minimized ; but the small aperture makes
the image less brilliant as only a limited amount of light goes to
form it, and furthermore it requires muscular effort to contract the
iris to make the pupil small. With a small pupil, objects can be
seen clearly only when they are in a brilliant light, hence eye
defects cannot be compensated for in a dimly lighted place by
closing the pupil.

For demonstrating the effect of the pupillary aperture there are
needed:

FIG. 398. TRIAL LEN- FIG. 399. Discs WITH PUPILS OF LARGE AND SMALL

SES TO SHOW THE APERTURE; STENOPyEIC SLIT. (% Size)

EFFECT OF THE A Black metal discs of the size of trial lenses, one

UPIL- with a pupillary aperture of 2.5 mm. and the other

of 7.5*mm.

B Stenopaeic slit.

(1) A 3 or 4 diopter convex projection trial lens (fig. 398).

(2) A 0.5 diopter concave or convex cylinder.

(3) Two black discs the size of trial lenses and with apertures,
one of 2.5 mm., and one of 7.5 mm.

(4) A lantern slide of the radial lines.

(5) A black disc with a stenopasic slit.

6 70 DEMONSTRATING REFRACTIVE EYE DEFECTS [Cn. XV

Put the 3 or 4 diopter lens in place in the metal holder, and the
lantern slide of the radial lines (fig. 391) in the slide-carrier.
Light the lamp and focus the slide by moving it toward or from
the projection lens. Now introduce the 0.5 cylinder. Only the
horizontal lines will be sharp with the axis vertical or only the verti-
cal lines if the axis is horizontal. Put in front of the projection
lens the black disc with an aperture of 7.5 mm. The image will
be much improved. Remove this and put in place the disc with a
pupil of 2.5 mm. If now the light is well centered the entire circle
of the radial lines will be fairly good. The image will be rather
dim, however.

Remove the small pupil and put in place the stenopaeic slit (fig.
39pB). Place the slit parallel with the axis of the cylinder and the
lines will all appear sharp. This is because the slit allows the
light to pass only along a line, thus eliminating most of the disturb-
ing rays from the unequal curvature. People with astigmatism
can partly overcome the trouble by narrowing the pupil and partly
closing the eye-lids so that objects are seen through a slit something
as in the experiment (§ 93 la).

§ 932. Anisometropia or unlike refraction in the two eyes.—

This is not a rare defect. One eye may be normal and one astig-
matic, one with myopia and the other long sighted or normal, etc.
Where the two eyes are different, the efforts to get a correct image
are greatly hampered, for an accommodation which would give a
correct image in one eye will make the image of the other eye more
confused.

When the differences in the two eyes are considerable, the image
of one eye is discarded, or the poor eye is turned aside (squinted)
to get it out of the way, and one gets along with monocular
vision.

To make this demonstration in the most perfect manner there
should be two lanterns side by side, each projecting an image at the

§ 93 la. Preparation of the pupils and slit. — These are easily made by
cutting out pieces of thin tin or other metal the size of the trial lenses and
boring the holes and cutting the slit. Metal is recommended because the im-
age of the crater must be focused on the pupil or slit, and paper or wood would
be burned by the absorbed energy (§ 852.)

CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 671

same time. If, however, direct current is available the demon-
stration is successful with one projector.
There are needed :

(1) A trial frame with half the lens of 4 diopters and half of
3 diopters (fig. 400).

(2) A trial frame with half the lens of i diopter and the other
half of plane glass.

(3) A lantern slide of fig. 401.

For the demonstration the arc lamp and the lens should be so
related that the image of the source of light is rather large as shown

Left Right

Eye Eye

FIG. 401. LANTERN SLIDE FOR THE
FIG. 400. DOUBLE TRIAL DEMONSTRATION OF UNLIKE REFRAC-

LENSES FOR UNLIKE RE- TION IN THE Two EYES-

FRACTION IN THE TWO
EYES.

by the concentric circles in fig. 400. The light must be accurately
centered also.

Put the lens in the metal holder and the special lantern slide in
its carrier and move the slide up to a point 27 to 28 centimeters
from the lens. The image of the right eye (4 diopter lens) will be
sharp, and that of the left eye will be blurred. Now pull the slide
back to a distance of 36-37 centimeters from the lens and the left
eye image will be sharp and the right eye blurred. This is com-
parable to a defect of myopia in one eye and hyperopia in the
other — one eye is short sighted and one long sighted. Put the
slide back in position for the 4 diopter lens so that the right eye
will be in focus. Now put in front of the lens the correcting lens
of i diopter for the left half. This will make both sides of the lens
4 diopters and both images will be sharp as in normal vision (fig.
402, A.B.).

It will be seen that the blurred, left-eye image (fig. 402 A) is
smaller than the sharp right-eye image. This is because the 3

672 DEMONSTRATING REFRACTIVE EYE DEFECTS [CH. XV

diopter lens gives a smaller image than the 4 diopter lens* (see also
fig. 389 B. C.).

Loft
Eye

Right
Eye

Left
Eye

Right
Eye

FIG. 402. APPEARANCE OF THE SCREEN IMAGE WITH UNLIKE REFRACTION
IN THE Two EYES AND WITH LIKE REFRACTION.

A Image of a 3 diopter left eye and a 4 diopter right eye; the 4 diopter
eye is in focus.

B Image when a i diopter convex spectacle is added to the 3 diopter left
eye, making it like the right eye.

§ 932a. In the experiment showing anisometropia each half of the double
lens projects both images, but when the light is properly centered and in the
correct position to give the large illumination on the lens (fig. 400), each half
lens projects a much more brilliant image of its own side, hence the fainter
image of the opposite side is overwhelmed and overlaid so that only one image
shows on each side. If the light is not in a good position, both images show
and that spoils the effect.

This demonstration with two half lenses was fully successful only when a
right-angle, direct current arc lamp was used as a source of light.

By using a I diopter concave lens to reduce the 4 diopter half lens to a 3
diopter power, the right eye image can be made sharp when the lantern slide
is in position to make the left eye image sharp, and the right eye image blurred.
It is a little more satisfactory to work with the 4 diopter lens, however, and to
add the I diopter convex lens to the 3 diopter left lens.

BRIEF HISTORICAL SUMMARY

In dealing with the historical development of projection three forms of
apparatus must be considered:

I. NATURAL CAMERA OBSCURA

The formation of images in a dark place, the light from the brilliantly illum-
inated objects or scenes being admitted through a small opening, is a perfectly
natural phenomenon and entirely independent of man's invention or control.
This is represented by images of the sky with its clouds and the brilliant scenes
of nature pictured on the walls of caves facing the scenes, and the images of
the sun admitted through chinks between the leaves, etc.

In rooms of man's construction such images are often seen if light enters
through a chance hole in the right position. General Waterhouse, from his
own observation, says it is a common occurrence in the bungalows of India,
and the writers have often seen the same in America.

*It was our intention when this work was undertaken to include a somewhat
extended account of the discoveries and inventions relating to vision, including
spectacles, general optics, and optical instruments, especially the telescope, the
microscope and projection apparatus of all kinds. As the book has already
exceeded its limit in size, this extended account must wait for a special work.
We have thought it best, however, to add a brief summary of the more per-
tinent points, and a historical bibliography which will put those interested on
track of the special and early sources of information.

Our appreciation is great for the aid we have received from many sources.
First of all to the Library of Cornell University for its magnificent collection
of works bearing on the history of science, for the purchase of rare and costly
works, and for the trouble taken to borrow from other libraries, rare works for
our use. Among the other libraries drawn upon we mention in the first place
that of the Surgeon General's Office in Washington, D. C. Those of Columbia,
Chicago, Harvard and the University of Pennsylvania also loaned us many
works.

Among the individuals who gave us special aid are:

Professor George L. Burr, for securing the portrait of Scheiner, (fig. 407).

Professor E. Lavasseur of the College of France who supplied the photograph
for the portrait of Marey (fig. 412).

Mr. Augustus J. Loos of Philadelphia for securing information concerning
the Langenheim brothers who were the first to make photographic lantern
slides by the albumen process (1850).

Mr. Edward Pennock of Philadelphia for putting us in communication with
Mr. C. W. Briggs of that city. Mr. Briggs gave us much valuable information
concerning his father, Dr. Daniel H. Briggs, who made the first photographic
lantern slides by the collodion process (1851-1852).

Effie Alberta Read, Ph.D., M.D., for looking up references and verifying
quotations in the libraries of Washington, D. C.

Theodore Stanton for aid in securing the photograph of Marey, (fig. 412).

And finally to Dr. A. C. White of the Cornell University Library for transla-
tions from the Greek and Latin works of the old writers, in which some of the
earliest information on our subject is to be found.

673

674 OPTIC PROJECTION

II. ARTIFICIAL CAMERA OBSCURA

No one knows who first designedly arranged a darkened room with a white
wall or screen oh one side, and on the other a small opening facing some object
or scene that could be brightly illuminated. All we know is that the earliest
accounts of the pictures in a dark place are in connection with the explanation
of some other phenomenon, and not to show that such pictures were possible.
It was also recognized in the first statements, as in the works of Aristotle and
of Euclid, that as light rays extend in straight lines, that those from an object
must cross in passing through a small hole, and hence the images beyond the
hole in the dark place must be inverted, the top being below and the right being
left.

According to Wiedemann and Werner, the Arabians, Iban Al Haitem (1039
A.D.), and Levi Ben Gersen (1321-1344), gave descriptions which clearly
belong to the camera obscura. However, that may be, we have the illustrated
manuscripts of Leonardo da Vinci, which not only describe the phenomena
of the camera obscura, but give pictures which are unmistakable. The pic-
tures and descriptions are in connection with his explanation of vision. As
Leonardo died in 1519, these manuscripts arc of an earlier date, probably before
1500 A.D. (See especially folio 8 of Ms. D.)

Also in the accounts of eclipses, etc., of the astronomers Reinhold, Frisius
and Moestlin, they very clearly describe and give figures of the arrangement of
the dark room pictures (1540-1545) ; and in the quaint old volume of Cardanus
(De Subtilitate, 1550), there is a very graphic description of the means of
getting dark room pictures and of their appearance. Baptista Porta, in 1558,
in his Natural Magic, also gives a good description. Porta is credited in the
popular mind with the invention of the camera obscura, but as seen from the
above, it is a natural thing, and man had got camera pictures by design before
Porta was born. The Natural Magic of Porta was very popular in its day,
and was translated from the Latin into most modern languages, hence it is
intelligible that people thought him the inventor, as he gave credit to no one,
and gave out that many of the things had never been known before. To credit
him with the discovery of the marvelous things he describes would be like
making the modern magazine writer the inventor or discoverer of the wonder-
ful things he describes. In justice to Porta, it must be said that he states in
the preface to his book that he has consulted all libraries, and has visited many
skillful artisans to find out all the secrets.

It may be stated in passing, that the name "Camera Obscura" was not used
by Porta, nor the others mentioned above. They used expressions like these:
cubiculum obscurum, cubiculum tenebricosum, conclave obscurum, locus
obscurus, etc. The first occurrence of the name "Camera Obscura" found by
us is in the Paralipomena of Kepler, (1604), p. 209 of the original, p. 261 in
the Opera Omnia, vol. ii. Kepler also uses the expression, "camera clausa,"
vol. ii, p. 160.

BRIEF HISTORICAL SUMMARY

675

While mirrors had been used in the camera obscura for changing the position
or causing the images to appear erect, so far as known at present, no one used a
projection lens in the aperture of the dark room until 1568. In that year was
published the work on perspective by Daniel Barbaro, and on p. 192, Ch. V, he
directs that to make the image more brilliant, a convex spectacle glass be put
in the aperture, and that a white paper screen be moved back and forth until
the picture shows most clearly, then it can be traced. From this time onward
a projection objective has always been used, except for experiments, such as
with pin-hole photographic cameras, etc.

In the camera obscura considered above, the observers were in the room
where the picture was formed. For a small, movable camera, something like
the photographic cameras of the present, where the observer is outside the
camera box, the first description found by us is the one of Robert Boyle, and
dates from 1669. He called it a "A Portable Darkened Room," and says that
it had already been exhibited to many friends several years before the paper
was written.

FIG. 403. WALGENSTEN'S MAGIC LANTERN (1665).
(From Milliet de Chales, Mundus s. Cursus Mathematicus, 1674, vol. ii, p. 666)

Here is a naked light with a reflector behind it. There is no condenser.
The object is put in the proper inverted position before the objective, and the
image appears erect and enlarged on the screen.

III. PROJECTION INSTRUMENTS

The third form of projection apparatus consists of a relatively small instru-
ment in which a small object is brilliantly illuminated, and the light from it
extends out through a projection lens or objective and forms a relatively large
image on a white wall or screen in a dark place.

The third form is the converse or conjugate so to speak of the camera obscura
where the object is large and the image small.

676

OPTIC PROJECTION

Projection instruments of the third class can be properly divided into three
groups: i, the Magic Lantern; 2, the Projection Microscope, and 3, the
Moving Picture Machine.

1. The Magic Lantern

It is not certainly known who first produced a workable magic lantern.
The first figure and description we have found is the one of a Danish mathe-
matician (Walgensten). The figure and description occur in the mathematical
treatise of Milliet de Chales (1674), where it states that "in the year 1665 there
came to Lyons a learned Dane well versed in dioptrics. Among other things
he exhibited a magic lantern. . . In the first place the greater the distance

eamcTcaeijaamanimumadje^fcnbendis iis inventiones Lucernamfc
termt, Quos mier primus fu,t Thomas 7s 71 a nob.s dcfcnptam . in meliorcni
waieenttenius Danus , haud mfimz notz formara reduxit, quam &pofteamaeno
Mathemaucus , qoi rccolcns mcas m dc- ; fuo lucro dircrfis in Italia prmcipibosvcn-

FIG. 404. THE MAGIC LANTERN OF KIRCHER. ' Its

(From the Ars Lucis et Umbrae, 1671, p. 768)

The lamp is a naked flame with a concave reflector behind it. The lantern
slide is a long strip with many pictures which can be shown one after the other.
The lantern slide appears at the wrong end of the projection objective,
making it difficult to see how any image could be projected. At the bottom
of the picture is a part of the text in which the better form of Walgensten 's
lantern is conceded.

BRIEF HISTORICAL SUMMARY

677

of the wall upon which the image was exhibited the larger was the image. . .
In the third place, the little image in the lantern was inverted in order to
exhibit its figure erect upon the opposite wall. If the object was removed there
appeared only a circle of light" (vol. ii, p. 655; 2d ed., vol. iii, p. 680).

Figure 403 is a facsimile of the lantern of Walgensten which he exhibited at
Lyons in 1665. A glance at it will show any one that it is in all essential par-
ticulars like the modern magic lantern. Indeed such lanterns are much in
vogue for Christmas presents at the present time, differing only in having a
kerosene lamp with a chimney instead of the naked flame as shown in the
original.

Kircher himself in the second edition of his work, (Ars Magna Lucis et
Umbrae, 1671, p. 768-769), claims that the lantern of the Dane is merely a

. 1 81

FIG. 405. MOLYNEUX'S MAGIC LANTERN WITH A CONDENSING LENS BEFORE

THE OBJECT.

(From Molyneux's Dioptrica Nova, 1692}

This is the first picture of a magic lantern with a condensing lens that we
have found.

slight modification of the one described by him, but he admits that Walgen-
sten's instrument is in better form and has many pictures on a single slide
painted in transparent colors that can be shown one after the other.

Kircher figures his magic lantern, which is here reproduced in facsimile (fig.
404) . As pointed out by Neuhauss, it is difficult to see how a picture could be
projected by the arrangement shown by Kircher. The text describes the lan-
tern as here shown, so both text and figure agree. In Kircher's lantern as
figured and described by himself, the object is put at the wrong end of the
projection objective; or if the tube and glass shown represent a condenser,
which he does not claim, then in that case there is no projection objective. In
either case no image could be projected.

678 OPTIC PROJECTION

So far as the evidence goes then, it was not Kircher, but Walgensten who
exhibited the first workable magic lantern, and the date was 1665.

FIG. 406. JOHANNES KEPLER, 1571-1630.
(From the Library of Original Sources Vol. V)

Astronomer. Father of Modern Dioptrics, Keplerian Telescope and Micro-
scope. The Amplifier and the Telo-Photo Combination. Inverted Retinal
Image.

BRIEF HISTORICAL SUMMARY

679

Projection Microscope

As pointed out in the text (p. 221), the projection microscope is only a magic
lantern with a relatively short focus projection objective. The screen image is
therefore correspondingly larger than with the magic lantern.

The first magic lantern described (1665) was recognized as a kind of micro-
scope by Milliet de Chales, for he says in the description: "microscopium

FIG. 407. CHRISTOPHORO SCHEINER, 1573-1650.

Astronomer and Inventor
(From the biography by Anton von Braunmuehl, 1891)

Projection Apparatus for Drawing Sun Spots. Demonstrated the Retinal
Image. Said the Crystalline Lens of the Eye is Equal to Many Glass Lenses.
Invented the Pantograph.

habes in hujusmodi machina," vol. ii, p. 667. A few years later (1685), Zahn
in his work on all kinds of optical instruments (Oculus Artificialis) , says on p.
255, "Lucerna magica est species microscopii." Both also point out that this
kind of a microscope is preferable to the ordinary one as many can see at the
same time.

68o OPTIC PROJECTION

If any individual should be mentioned in connection with the projection
microscope, it is Kepler, for in his Dioptrics, 161 1, he showed the advantage of
adding an amplifier in projection, and also a second convex lens (ocular), to
magnify the real image of the objective, and also at the same time to render it
erect. See Opera Omnia, vol. ii, pp. 549-550, 555.

3. Moving Pictures

Moving picture projection is like micro-projection when no ocular is used.
The screen distance is usually rather great and the many slightly differing
pictures are changed so rapidly that the successive screen images seem to fuse
together and thus give the appearance of motion.

The first step in getting moving pictures was an investigation of persistence
of vision by momentary glimpses of similar moving objects. The men inves-
tigating the matter were all physicists, and the results of their observations
were given in scientific papers. See in the bibliography papers by Fara-
day, Plateau, Horner and Stampfer. The paper on the magic disc by Plateau
was dated Jan., 1833, and the paper of Horner on the da3daleum (zoetrope)
was dated 1834, as was also the paper of Stampfer on the magic disc. Both
the magic disc (fig. 408) and the zoetrope (fig. 409) give the appearance of
movement with great satisfaction.

As the instruments were for one or at most for very few observers, the magic
lantern was called in to give screen images so that many could see at the same
time. The magic lantern was used successfully by Uchatius in 1853. He
used several (as many as 12) slightly differing transparencies, each transparency
having its own projection objective. The objectives were all directed toward
the same point on the screen, hence the images all appeared in the same place.
A lime light and condenser were attached to a crank, and moved from picture
to picture in rapid succession, and the projected images gave the appearance
of movement as perfectly as did the magic disc.

It was also natural that the new art of photography should be called upon
to depict the various phases of a moving body for use in place of the drawings
which had been previously used; this was suggested by Plateau about 1848.
In 1870 Heyl realized this possibility by arranging a series of photographic
transparencies of posed motion, and projecting them on the screen. The
transparencies were arranged on the edge of a large disc, and by the step by
step movement of the disc the successive transparencies were brought in the
axis of the magic lantern. To prevent the blur while the pictures were changed ,
a two wing shutter was used to cut off the view. This method of projecting
was very successful and required only one projection objective, consequently
the number of pictures was limited only by the practicable size of the rotating
disc.

Up to 1872 the pictures used were either drawings or photographic trans-
parencies of posed movements, not photographs of movement in continuous
change as at present.

BRIEF HISTORICAL SUMMARY 68 1

From 1872 onward there have been three epoch making periods in reaching
approximate perfection in moving pictures.

The first period is represented by the work of Eadweard Muybridge, who
first made successful analyses of rapid movement in 1872-1881. In 1879 he
arranged the successive stages of a movement on a glass disc and projected the

FIG. 408. PLATEAU'S MAGIC Disc (PHENAKISTOSCOPE).
(From the Correspondance Mathematique et Physique, Tome VII, 1832)
Notches were cut around the edge as indicated by the dark terminations of
the radii. A pin is put in the center, the figures turned toward a well lighted
mirror, and the disc rotated. By the momentary glimpses through the radial
slits the figure seems to go through the movements of the dance. The back
of the disc should be black, and the figures show better if the outlines are made
heavier than in the picture.

682

OPTIC PROJECTION

same by means of a magic lantern, and synthesi^ed or recombined the move-
ment on the screen as he had previously done in the zoetrope. From 1883-
1885, under the auspices of the University of Pennsylvania, over one hundred
thousand (100,000) pictures of movements of men and all kinds of animals were
made. These were published in several folio volumes in 1887.

FIG. 409. THE D^EDALEUM OF HORNKR (ZOETROPE).

(From Marey, Movement, 1895)

In this instrument figures or photographs can be arranged in a band around
the inside of the cylinder, or, as in this, case models of a moving animal can be
arranged in order. When the instrument is revolved the images or models
seem to perform their natural movements of walking, flying, etc.

The second period is represented by the making of the gelatino-bromide
process of photography practical by Maddox in 1871, and by making this
process exceedingly rapid by heating or boiling the emulsion (Bennett and
others in 1878 and later).

The third epoch making period was inaugurated by the Rev. Hannibal
Goodwin when he worked out a practical method of making a solution and then
a film of transparent, tough, flexible cellulose which was unaffected by the
chemicals and liquids used in photography.

His application for a patent was filed in 1887, and the patent granted in
1898, and the validity of the patent finally confirmed by the United States

BRIEF HISTORICAL SUMMARY

683

District Court of New York in 1913, and this decision confirmed by the United
States Circuit Court of Appeals of New York in 1914. (See in the Bibliog-
raphy).

Muybridge's first pictures were made by the wet collodion process, but his
Philadelphia work was done with the new, rapid gelatino-bromide plates. He
used many cameras, sometimes 24 in a row to get different phases of a motion,
and sometimes the cameras were arranged in groups to get the movement
simultaneously from different points of view.

In 1 88 r he gave demonstrations of his pictures in Europe, and projected
the synthesis on the screen with the lantern, the first demonstrations being in
the physiological lecture room of Marey, the French master of investigating

FIG. 410. THE MOVING PICTURE PROJECTOR or UCHATIUS.

(From the Sitz. Berichte d. k. Akad. Wiss., z. Wien. Math. Nafur. CL, Vol. X,

1853)

This shows some of the pictures with the individual objectives directed to
the same point. The lime light and condenser and the crank for moving them
from picture to picture are also shown.

animal movement by the graphic method. From that time on Marey took
hold of the photographic method for the analysis and synthesis of animal
motion with the greatest enthusiasm. Instead of the batten' of cameras used
by Muybridge, he adopted the system of the French astronomer, Janssen,
using a single camera and objective, but taking many pictures on a single plate.
In 1887, he used the roller films on paper, and immediately that they were
available, the celluloid films devised by Goodwin. In this way pictures could
be made in a long series. Not only did Marey use the ribbon films but he
devised a special camera for doing so, and a projector for showing the ribbon
pictures on the screen.

684

OPTIC PROJECTION

FIG. 411. EADWEARD MUYBRIDGE, 1830-1904.

(From Animals in Motion, 1899)

Photographic Analysis and Synthesis of Animal Motion, Commencing in
1872.

BRIEF HISTORICAL SUMMARY

685

FIG. 412. JULES ETIENNE MAREY, 1830-1910.

College de France
(From a photograph furnished by Professor E. Lavasseur, College de France)

Graphic Method in Physiology; Photographic Analysis and Synthesis of
Animal Motion, 1881-1910.

686 OPTIC PROJECTION

In perfecting cameras to make ribbon pictures, and projectors for exhibiting
ribbon transparencies of these pictures on the screen, many inventors have
taken part. Among these should be mentioned Marey and his assistant,
Demney, and the Lumieres in France; Green and Evans, Donisthrope and
Crofts in England; Jenkins and Edison in America. These were among the
first to work out practical apparatus that made moving pictures possible and
practical. For the present perfection of cameras, films, and projectors, and
the general methods employed, the number of manufacturers and inventors is
legion.

The first light used was sunlight, and that remains the most brilliant of all.
Animal and vegetable oils were burned in lamps without a chimney (fig. 403-
405), and very recently mineral oil (kerosene) has been used in lamps with a
chimney (fig. 65-67).

FIG. 413. DAVY'S CARPON ARC.

(From Davy's Collected Works, vol. iv, pi. Hi, fig. 17)

See p. 1 10 of vol. iv for a discussion of the carbon arc. The carbons are
horizontal, and the arc arches upward hence the name arc.

The lime light, the most brilliant after sunlight and the arc light, came in
with the discovery by Hare in 1802 that the oxyhydrogen flame when blown
against lime, etc., gave a dazzling light. This was applied to projection by
Birkbeck in 1824 for the magic lantern; and in the same year by Woodward for
the phantasmagoria. (Goring and Pritchard's Micrographia, pp. 170-171;
also the Microscopical Journal and Structural Record, Vol. I, 1841). This
light is still much used for all forms of projection. For the oxygen ether lime
light, see Ives, in the Bibliography.

The electric light. This most satisfactory and powerful artificial light yet
devised, was first shown by Humphrey Davy in Sept., 1800, and recorded in
Nicholson's Journal of October in that year (See Cantor Lectures of Silvanus
P. Thompson on the arc light, Journal of the Royal Society of Arts, Oct. 25,
1895, and fig. 413 for Davy's carbon arc). According to the same lecturer,
W. E. Straite devised the first automatic electric lamp in 1846.

The first arc lamps wore for direct current. As it was not desirable to have
the carbons burn off unequally with the Jablochoff lamp where the carbons
were parallel and close together, alternating currents were used (1877). (S. P.

BRIEF HISTORICAL SUMMARY 687

Thompson, p. 953-954) . While this works well for general lighting, it is shown
in the preceding pages (553-566) that alternating current is far inferior to
direct current for projection purposes.

At first the carbons were both horizontal (fig. 413), then they were made
vertical, and later at various angles of inclination. In order to keep the crater
of the positive carbon constantly in the optic axis, Mr. Albert T. Thompson of
Boston manufactured and used, especially for projection purposes, an arc lamp
in which the carbons are at right angles, the positive carbon being horizontal
and hence constantly in line with the axis of the projection instrument. This
was in 1894.* From that time onward the advantage of this position has
become more and more appreciated, and the superiority for projection purposes
is shown graphically in the curve given in Chapter XIII (fig. 302).

The following is the statement of Mr. Albert T. Thompson concerning the
90° arrangement of the carbons in an arc lamp for the magic lantern:

BOSTON, Dec. 6, 1907.

"Replying to your valued communication of the 2d, I will state that I first
manufactured the 90° arc lamps in 1894 and a careful search of all arc lamp
and stereopticon catalogs published about that period, fails to show arc lamps
of the 90° construction.

"I did not patent the lamp, for at that time there was no demand for them,
and of course it was difficult to look into the future and realize that in a few
years thousands and thousands would be sold.

"The facts to the best of my knowledge and belief were never published in
any scientific journal.

Yours very truly,

A. T. THOMPSON."

SOME MANUFACTURERS AND DEALERS IN OPTICAL
AND PROJECTION APPARATUS AND SUPPLIES

Within recent years there has been great improvement in projection appara-
tus and all the necessary accessories, and many optical manufacturing houses
have taken hold of the work in earnest, so that now one can find in the open
market practically everything required at reasonable prices. Furthermore, if
special apparatus or combinations are desired, or if a person has notions of his
own, it is not difficult to obtain the optical and electrical apparatus needed of
the manufacturers, and only a small amount of special construction will be
needed to adapt the apparatus to the special individual or the special purpose.

It is hoped that the special apparatus described in this volume, for example,
the projection microscope and the projection outfit for showing normal and
defective vision, and for some special demonstrations in physics, will give sug-
gestions which will open the way for those who do not find the apparatus in
the open market quite suitable" to their needs.

As models of instruments are constantly changing and new forms are being
produced, the authors advise that any one desirous of installing projection
apparatus of any kind should get the catalogues of several manufacturers and
select that which best suits his needs and means. It may be stated in passing,
that the most expensive apparatus is not necessarily the best adapted for a
given case. Often apparatus of moderate price is easier to manage and more
effective. Naturally the manufacturers prefer to install an expensive outfit,
but if the needs are clearly stated, and the sum available, the manufacturer
will give most excellent advice as to the outfit required.

The dealers in lantern slides have a system of rental by which one can get for
a moderate fee a set of slides to illustrate some special or general subject. Of
course, slides of any number or grouping can also be purchased, but often a
special lecture on a country or a period can be greatly helped by a good selec-
tion of lantern slides, the use of which will cost but a small sum.

For those interested in moving picture cameras, the development of exposed
films, etc., the advertising pages of the Moving Picture World will give the
names of the firms who can give the information or the help needed.

The following list of manufacturers and dealers is arranged alphabetically,
and from our experience with them we know that they try to be of real service
to their customers. Of course there are many others who are equally reliable;
and new manufacturers and dealers are constantly coming into the field. One
can get on track of them by consulting the advertising pages of standard
periodicals as: Science, the Scientific American, the Moving Picture World,
Journals in Electrical and Illuminating Engineering.

688

MANUFACTURERS AND DEALERS 689

After each name in this list are given the text figure or figures taken from
the publications of the given manufacturer, or the section (§) in which the
apparatus or material is considered :

American Theater [Curtain and Supply Company, 105 North Main Street,
St. Louis, Mo. Radium, gold fiber screens, § 629.

The Bausch & Lomb Optical Company, Rochester, New York. Photographic
objectives, microscopes, projection apparatus, spectacles and all laboratory
supplies, §916, fig. 17, 33-34, 70, 100-101, 104-107, 123, 131, 136, 169-173,

2OO-2OI, 223-224.

R. & J. Beck, Limited, 68, Cornhill, London, England. Microscopes, photo-
graphic objectives, projection apparatus.

John A. Brashear Company, Limited, Pittsburg, N. S., Pa. Optical, physical,
astrophysical and astronomical instruments, including heliostats. Ch. VI.

Chas. Beseler Company, no East 23d St., New York. Projection apparatus
and lantern slides, fig. 55, § 598. Small automatic arc lamps.

C. W. Briggs, 628 Callowhill Street, Philadelphia, Pa. Magic lantern slide
manufacturer. Mr. C. W. Briggs is the son of Dr. Daniel H. Briggs who
made the first photographic lantern slides by the collodion process. The
beautiful lantern slides made by the son are made by the same collodion
process used by the father before 1855. Ch. VIII.

Brown & Sharpe Manufacturing Co., Providence, Rhode Island. Manu-
facturers of fine tools. See their wire gauge, p. 502.

Century Manufacturing Company, 272 West Genesee St., Buffalo, N. Y.
Manufacturers of "Sanitary paint" including "Artists' Scenic White" for
image screens, § 6250.

Conrad Lantern Slide and Projection Company, 4028 Jackson Boulevard,
Chicago, 111. Lantern slides for science teachers and lecturers. Ch. VIII.

Detroit Engine Works, Detroit, Michigan. Direct current electric lighting
outfits for projection and moving pictures. Kerosene engines for power
§682.

Detroit Motor Car Supply Company, Detroit, Michigan. Sandow moving
picture electric light plant, using a Sandow kerosene stationary engine, §682.

Dolby & Company, 3613 Woodland Ave., Philadelphia, Pa. Importer and
dealer in microscopes and optical apparatus, lantern slides, laboratory
supplies.

Eastman Kodak Co., Rochester, New York. Photographic outfits and sup-
plies, including moving picture film, fig. 119, § 3330, 451.

Edison^Manufacturing Company, Orange, N. J. Moving picture machines
and films, the home kinetoscope, etc., etc., fig. 63, 221, 224, 233-236.

Education Department, Division of Visual Instruction, State of New York,
Albanv, N. Y. Many series of lantern slides for use throughout the state,
(Ch. VIII).

Enterprise Optical Manufacturing Co., 564-572 West Randolph St., Chicago,
111. Moving picture machines, Calcium gas outfit etc., fig. 56.

The Ernon Camera Shop, 18 West 27th St., New York. Moving picture
camera.

Folmer & Schwing Manufacturing Co., Manufacturers of enlarging, reducing
and tilting cameras. With Eastman Kodak Co., Rochester, N. Y., fig. 119.

690 OPTIC PROJECTION

Foos Gas Engine Co., Springfield, Ohio. Oil, gas and gasoline engines for
supplying the power for a private electric lighting plant and for projection,
8 683.

Fort Wayne Electric Works of the General Electric Co. Compensarc instead
of a rheostat, § 736.

R. Fuess, Steglitz bei Berlin, Germany. Optical instruments, projection

apparatus, heliostats, etc., fig. 79, 84.
General Electric Company, Schenectady, New York. Electric apparatus of

all kinds, generator sets, mercury arc rectifiers, mazda concentrated filament

lamps, and nitrogen lamps for the magic lantern, etc., fig. 258-264, § 754.
General Film Company, 200 Fifth Ave., New York. Educational films for

the moving picture machine, Ch. XI.

General Specialty Company, St. Louis, Mo. Indirect and semi-indirect
lighting fixtures, § 606.

J. H. Centner Co., Newburgh, N. Y. Mirroroide screens § 629.

Gregory Electric Company, 16 & Lincoln Sts., Chicago, 111. Electric supplies,
generators, motors, etc., Ch. XIII.

Gundlach-Manhattan Optical Co., Rochester, New York. Photographic
objectives and cameras, microscopes, projection objectives for moving pic-
tures, etc., fig. 229.

J. H. Halberg, 36 East 23d St., New York. Moving picture machines and
supplies of all kinds.

Hartford Machine Screw Company, Hartford, Conn., fig. 161.

Harvey Hubbell, Inc., Bridgeport, Conn. Manufacturers of Machinery, tools,
Electrical specialties, fig. 48-50, 268-269.

P. Keller & Co., Successors to J. B. Colt Co., 465 Greenwich St., New York.
Projection apparatus and accessories, fig. 36.

Kleine Optical Company, 166 North State Street, Chicago, 111. Motion pic-
ture apparatus and supplies; theater supplies.

Max Kohl, Chemnitz, Germany. Projection apparatus and accessories,
chemical and physical apparatus, fig. 68, 81.

F. Koristka, Milano, 2 Via G. Revere. Italy. Microscopes and projection
apparatus, fig. 181.

Ward Leonard Electric Company, Bronxville, N. Y. Rheostats, circuit
breakers, theater dimmers, etc., fig. 147, 183, 186-187, § 723.

List of Electrical Fittings. Published by the National Board of Fire Under-
writers, 135 William St., N. Y., § 691. Manufacturers of standard fittings
and supplies.

Ernst Leitz, Wetzlar, Germany, 30 East i8th St., New York. Microscopes,
photographic objectives, projection apparatus, fig. 41, 96, 123, 163, 202-205.

T. H. McAllister Company, 49 Nassau St.. New York. Projection apparatus
and lantern slides, fig. 89.

Mclntosh Stereopticon Company, 35 and 37 Randolph Sts., Chicago, 111.
Projection apparatus and lantern slides, fig. 66, 166.

Motion Picture Camera Company, 5 West I4th St., New York. Cameras
and projectors for moving pictures.

Motion Picture Screen Company, Shelbyville, Indiana. Mirror screens, § 629,
6291, 630.

MANUFACTURERS AND DEALERS 691

National Electric Supply Company, Chicago, 111. Rheostats, etc., fig. 138,

193, 196.
National X-Ray Reflector Company, 236 Jackson Boulevard, Chicago, 111.

Eye-Comfort Illumination from concealed sources, fig. 237, § 606.
New York State Education Department, Division of Visual Instruction,

Albany, N. Y. Lantern slides for use throughout the state, Ch. VIII.
Newton & Co., 3 Fleet St., London, England. Projection apparatus and

lantern slides, fig. 67.
Edward Pennock, 3609 Woodland Ave., Philadelphia, Pa. Microscopes and

supplies, photographic objectives and cameras. Lantern slides, etc.
Pennsylvania Flexible Metallic Tubing Company, Broad & Race Sts., Phila-
delphia, Pa. See fig. 60.
Phantoscope Manufacturing Company, Washington, D. C. Motion picture

cameras and motion picture projectors of C. F. Jenkins for the house lighting

system, § 598.

Picture Theater Equipment Company, 21 East I4th St., New York.
Nicholas Power Company, 90 Gold Street, New York. Manufacturer of

Powers Cameragraph, electrical appliances for motion picture machines.

New dissolving stereopticon, "bill-splitter" current ballast, § 736, fig.

222-223, 227, 232.
Prest-O-Lite Company, Indianapolis, Indiana. Compressed acetylene, see

fig. 71-

C. Reichert, Optische Werke, Vienna, Austria. Projection apparatus, micro-
scopes, etc., fig. 43-44, 54-

Ross, Limited, 3 North Side, Clapham Common, London, England. Projec-
tion apparatus, microscopes, etc.
Franz Schmidt & Haensch, Berlin, Germany. Projection apparatus, etc., etc.,

fig. 57, 69.
Alfred L. Simpson, 131 West 132 St., New York. Simpson's solar screen for

receiving the projected image of the magic lantern, moving picture machine,

etc., § 629.
Slingerland Lantern Slides. Lantern slides, plain and colored of insects, birds,

trees, fruits and other nature-study subjects. Manufactured by Mrs. Mark

V. Slingerland, Ithaca, N. Y.
Spencer Lens Company, Buffalo, New York. Microscopes, photographic

objectives, projection apparatus and accessories. Laboratory supplies,

fig. 38, 108-111, 130, 149, 174, 198-199.
L. S. Starrett Co., Athol, Mass. Starrett Tools, fig. 160.
C. H. Stoelting Co., 121 North Green St., Chicago, 111. Projection apparatus,

laboratory apparatus and supplies of all kinds, fig. 16, 75, 102-103, 167.
The Chas. A. Strelinger Co., Detroit, Michigan. The Brush electric lighting

set. This consists of an engine for gas, gasoline or kerosene and a proper

dynamo for direct current, § 683.
Arthur H. Thomas Company, 1200 Walnut St., Philadelphia, Pa. Dealer and

importer in microscopes and other optical apparatus and all laboratory sup-
plies.
A. T. Thompson & Company, 15 Tremont Place, Boston, Mass. Projection

apparatus of all kinds. Inventor of the right-angle arc lamp, fig. 97, 168,

1 86.

692 OPTIC PROJECTION

Underwood & Underwood, 3 and 5 West igth Street, New York. Magic lan-
terns, lantern slides showing tours of the world.

Valentine & Company, 456 Fourth Ave., New York. Valspar varnish for
making glass boxes, etc., § 3940.

Voigtlander & Sohn, A. G., Optical Works, Braunschweig, Germany. Micro-
scopes, photographic objectives and cameras, projection apparatus, fig. 124,
142.

W. Watson & Sons, 313 High Holborn, London, England. Microscopes,
Projection apparatus and accessories.

Westinghouse Electric Manufacturing Company, Pittsburg, Pa. Rectifiers,
transformers, balance coils, motion picture, motor-generator set, etc., etc.,
§ 68 1, 723, 736, 739.

Weston Electric Instrument Company, Newark, N. J. Voltmeters and
ammeters, etc., fig. 133, 145, 272-273, § 662, 664, 666, 700, 7020.

Whyte Whitman Company, 36 East 23d St., New York. Moving picture
cameras.

Williams, Brown & Earl, 918 Chestnut Street, Philadelphia, Pa. Microscopes
and accessories, Laboratory supplies, Projection apparatus and moving
picture machines and lantern slides, fig. 32, 52, 59, 72-73, 98-99, 164-165,
§ 598.

Carl Zeiss Optischc Werkstaette, Jena, Germany. All kinds of optical appara-
tus; Microscopes and projection apparatus, fig. 95, 123, 128-129, 15&> 217-
219.

Joseph Zentmayer, manufacturing optician, microscopes, spectacles and
lenses of all descriptions etc., 226-228 South I5th St., Philadelphia, Pa.

See also the list of spectacle manufacturers, p. 651.

I. BIBLIOGRAPHY

Arrhenius, Svante August. Lehrbuch der kosmischen Physik. 1026 p. 304

fig. 3 Plates. Leipzig, 1903. Price 40 marks.
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Printing and Pub. Co., Ltd., Salisbury Court, Fleet St., London, E. C.,

1902. Price I2s, 6d.
Ball, Sir Robert Stawell. Elements of Astronomy, New ed. 459 p. 136 fig.

Longmans, Green & Co., London, New York, 15 East i6th St., 1886.

Price $2.00.
Barnard, J. Edwin. Practical Photo-micrography. 322 p. 79 fig. 10

Plates. E. Arnold, London, 1911. Price, 155.
Barrows, William Edward. Electrical illuminating engineering. 212 p. 135

fig. McGraw Pub. Co., New York, 1908. Price $2.00.
Bayley, R. Child. The Complete Photographer. 410 p. 65 Plates. 32 fig.

McClure, Phillips & Co., New York, 1907. Price $3.00.
Bayley, R. Child. Modern Magic Lanterns, a Guide to the Management of

the Optical Lantern for the use of entertainers, lecturers, photographers,

teachers and others, 110 + 15 p. 73 fig. L. Upcott Gill, London;

Charles Scribner's Sons, 153-157, Fifth Ave., New York. Price $.50.
Beck, Conrad, and Andrews, Herbert. Photographic Lenses. 7th edition

completely revised. 287 p. 163 fig. 44 plates. R. & J. Beck, Limited.

68 Cornhill, London, England. Price i shilling. Full discussion of modern

objectives for photography and for projection.
Colin, Hermann (Ludwig). Die Sehleistungen von 50,000 Breslauer Schulkin-

dern, nebst Anleitung zu ahnlichen Untersuchungen fur Aerzte und

Lehrer. Schlesische Buchdruckerei, Kunst-und Verlags Anstalt v. S.

Schottlaender. Breslau, 1899. 148 p. 26 Plates. 5 fig. Price 3M,

Geb. 4 M.
Cole, Aaron Hodgman. Manual of Biological Projection and anesthesia of

Animals. 200 p. 28 fig. Neeves Stationery Co., 543 W. 63d St., Chicago

111., 1907. Price $1.50.
Cyclopedia of Motion Picture Work. 2 vols. Vol. I. Stereopticon, motion

head projecting machine, talking pictures. 206 p. 119 fig., many plates.

List of moving picture makers. Vol. II. Photography, motography, photo-
plays, motion picture theater. 311 p. no fig., many plates. Published

by the American School of Correspondence. Chicago, 111., 1911.
Dolbear, A. E. The Art of Projecting; a manual of experimentation in

physics, chemistry, and natural history with the port-lumiere and magic

lantern. 158 p. 112 fig. Lee & Shepard, Boston and New York, 1877.

Price $2.00.

Donaldson, Leonard. The cinematograph and natural science, the achieve-
ments and possibilities of cinematography as an aid to scientific research.

120 p. 23 fig. Ganes Ltd., 85 Shaftesbury Ave., London, 1912. Price

2s, 6d.
Donders, Franciscus Cornelis. On the anomalies of accommodation and

refraction of the eye, with a preliminary essay on physiological dioptrics.

Translated by W. D. Moore, xviii -f i + 635 p. 175 fig. New Syden-

ham Society, (Publications 22). London, 1864.

693

694 OPTIC PROJECTION

Fourtier, H. La pratique des projections. 2v.ini. i, 146 p., 66 fig. ii, 142
p., 67 fig. Gauthier-Villars et Fils, 55 Quai des Grandes- Augustus, Paris,
1892-93. Price, 4 fr.

Fourtier, H., et Molteni, A. Les projections scientifiques; etude des
appareils, accessoires et manipulations diverses pour 1'enseignement
scientifique par les projections. 292 p., 113 fig. A. Molteni, 44 Rue du
Chateau-d' Eau, Gauthier-Villars & Fils, 55 Quai des Grandes Augustus.
Paris, 1894. Price 3 fr., soc.

Gould, George M. The New Ophthalmology and its relation to General
Medicine, Biology and Sociology, Congress of Arts and Science, Universal
Exposition, St. Louis, 1904, Vol. VI, pp. 422-445. Discusses among other
things the relation of general health and truancy, etc., of children to defec-
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Halberg, J. H. Motion picture electricity. 299 p., 125 fig. Published by
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Hassack, Karl, and Karl Rosenberg. Die Projektionsapparate, Laternbilder
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Helmholtz, Hermann (Ludwig Ferdinand), von. Handbuch der physiologischen
Optik. 3d ed. 564 p. 81 text fig., 6 plates, 1910. Leopold Voss, Ham-
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Price 14 M, geb. 16 M. II Die Lehre von den Gesichtsempfindungen.
viii -f 391 + 17 p. 80 fig. 3 Plates. Price 16 M, geb. 18 M. Ill Die
Lehre von den Gesichtswahrnehmungen 564 p. 81 text fig., 6 plates,
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Hep worth, Cecil M. Animated Photography, the A B C of the Cinemato-
graph. Hazell, Watson & Viney, Ltd. i, Creed Lane, Ludgate Hill, London
E. C. 1900. 128 p. 31 fig. Price $.50. Amateur Photographer's
Library, No. 14.

Hepworth, Thomas Cradock. The Book of the lantern ; a guide to the work-
ing of the optical or magic lantern, with directions for making and colouring
lantern pictures. 278 p. 75 fig. Wyman & Sons, Great Queen St.,
Lincoln 's-Inn Fields, London, W. C. 1888. Price 33, 6d.

Hopkins, Albert A. Magic, stage illusions and scientific diversions including
trick photography. 556 p., 400 fig. Munn & Co., Scientific American
Office, New York. 1 898. Price $2.50. 1 2 pages of references to books on
magic.

Jenkins, C. F., and O. B. Depue. Handbook for motion pictures and stereopti-
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D. C., 1908. Price $2.50.

Index Catalogue of the Library of the Surgeon General's Office of the United
States Army. Government Printing Office, Washington, D. C. First
series, vol. i-xvi, 1880-1895. Second series, vol. i-xviii + 1896-1913 +.
Book and periodical literature; subjects and authors in one continuous
alphabetical list. Full lists of current literature.

Journal of the Royal Microscopical Society. 1878 +. Published by the
Society at 20 Hanover Square, London, W. England. 6 numbers per
year; subscription, price 37 shillings 6 d.

Lambert, Rev. F. C. Lantern Slide Making. 140 p. 27 fig. Hazell,
Watson & Viney, Ltd., 52 Long Acre. London, W. C. 1907. Price, $.50.
The Amateur Photographer's Library, No. 22.

BIBLIOGRAPHY 695

Leiss, C. Die Optischen Instrumente der Firma R. Fuess, deren Beschreibung,
Justierung und Anwendung. Wilhelm Engelmann, Leipzig, 1899. xiv +
387 + i p. 3 Plates. 233 fig. Price II M., geb. 12 M.

Lummer, O., and Silvanus, Thompson. Contributions to photographic optics.
*35 P-» 55 %• MacMillan & Co., New York, 1900. Price $1.75. Dis-
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"National Electrical Code." Rules and requirements of the National Board
of Fire Underwriters for Electric Wiring and Apparatus as recommended
by the National Fire Protection Association, Ed. of 1913 or later. Fur-
nished by the National Board of Fire Underwriters, 135 William St., N. Y.
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Nichols, E. L. The Outlines of Physics. Pp. 452; 414 figures. The Mac-
millan Co. New York, 1897. Price $1.40.

Nicholas Power Company. Hints to [Moving Picture] Operators. 17 figures,
96 pages. Published by the Nicholas Power Co., 88-90 Gold St., N. Y.
1914.

Norris, Henry H. An introduction to the study of Electrical Engineering.
Ithaca, N. Y., 1912. 224 pages, 26 plates, 167 figures. Price ,$1.50.

Norton, C. Goodwin. The Lantern and How to Use It. Hazell, Watson &
Viney, Ltd. i, Creed Lane, Ludgate Hill, London, E. C., 1901. 152 p.
74 fig. Price $.50. The Amateur Photographer's Library, No. 10.

Nutting, Perley Oilman. Outlines of applied optics. P. Blakiston's Son &
Co., 1012 Walnut St., Philadelphia, 1912. ix + 234p. 73 fig. Price $2.00.

The Moving Picture World. The Chalmers Publishing Co., 17 Madison Ave.
New York City. Published weekly. Subscription price $3.00.

Pringle, Andrew. The Optical Lantern for Instruction and Amusement.
149 p. 72 fig. Hampton & Co., 13 Cursitor St. London, E. C., 1899.
Price 43, 6d.

Richardson, F. H. Motion Picture Handbook, a Guide for Managers and
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Scientific American, 1845 -f- and Scientific American Supplement, 1876 -f-
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Talbot, Frederick A. Moving pictures, how they are made and worked.
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Tennant, John A., Ed. Lantern Slides. 51 p., 8 fig. Tennant & Ward, 122
East 25th St., New York, The Photo-Miniature, Vol. I, No. 9, 1899.
Price $.25.

Tennant, John A. Coloring Lantern Slides. 48 p., II fig. Tennant and
Ward, 122 East 25th St.,-New York, The Photo-Miniature, Vol. 7, No. 83,
1907. Price, $.25.

Trutat, Eugene. Trait6 general des projections. 391 -f- 276 p. 185 + 137

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Price 7 fr., 500.
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Lehrzwecke in Schule und Haus, sowie fur Lichtbildvortrage, etc. 360 p.

112 fig. 26 Plates. Otto Nemnich, Leipzig, 1911. Price 5 M.

696 OPTIC PROJECTION

Wistar Institute's Style Brief. A style brief, giving the typographic arrange-
ment and methods to be followed in the preparation of manuscripts and
drawings for publication in the Journals published by the Wistar Institute.
Sent free to authors by the Wistar Institute, 36th and Woodland Ave.
Philadelphia, Pa.

Wright, Lewis. Light; a course of experimental optics, chiefly with the lan-
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Price, $2.00.

Wright, Lewis. Optical projection; a treatise on the use of the lantern in
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Zeitschrift fur wissenschaftliche Mikroskopie und fur mikroskopische Technik.
1884 +. Verlag von S. Hirzel, Leipzig, Germany. Published quarterly.
Subscription to foreign countries, 21.60 marks.

Zeitschrift fur Instrumentenkunde, herausgegeben unter mitwirkung der
physikalisch-technischen Reichsanstalt. Verlag von Julias Springer,
Berlin, 1881+. 12 numbers per year; subscription, 24 marks.

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Adams, George, 1750-1795. Essays on the microscope, containing a descrip-
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697

698 OPTIC PROJECTION

Boyle, Honourable Robert. "Of the systematical or cosmical qualities of
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Goodwin, Rev. Hannibal. United States patent No. 610,861 for a film sup-
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is the fundamental patent covering the production of films or ribbons of
cellulose for taking the place of glass and paper to serve as the backing for
the sensitive coating. It is practically unaffected by the liquids and
chemicals used in photography. See the opinion of Judge Hazel in the
United States District Court, of New York, Aug. 14, 1913, Federal Reporter
Vol. 207, pp. 351-362 in the case of Goodwin Film and Camera Co. versus
Eastman Kodak Co., deciding that the patent is valid. See also the
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tube for the convex objective and concave ocular.

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Hop wood, Henry V. Living Pictures: their History, Photoproduction and
Practical Working, with A Digest of British Patents and Annotated
Bibliography. 275 + xxvii, p. 242 fig. The Optician & Photographic
Trades Review, 123-125 Fleet St., London, E. C. 1899. Price, #1.25.

Horner, W. E. On the properties of the Daedaleum, a new instrument of
optical illusions. Philos. Mag., 1834, vol. iv, pp. 36-41. The Dsedaleum
is a hollow cylinder with slits around the edge and pictures in various phases
of movement on the inside. It is revolved on the long axis of the cylinder
and gives the same appearance as the magic disc of Plateau. It is now
called a zoetrope.

Ives, Fred E. The Ether-oxygen Lime Light, Journal of the Franklin Insti-
tute, Vol. 125, 1888. pp. 28-31. Also vol. 129, 1890, pp. 230-234.
Report of a committee of the Institute on the Ether-oxygen portable
lantern, (see Ch. IV, above).

Janssen. Presentation du revolver photographique. Built, soc franc,
photog. vol. xxii (1876, p. 100).

Jenkins, C. F. Picture Ribbons. An exposition of the methods and apparatus
employed in the manufactxtre of the picture ribbons used in projecting
lanterns to give the appearance of objects in motion. 56 p., many plates
and cuts unnumbered. Published by the author. Washington, D. C.,
1897. Discussion of the origin and development of moving pictures.

Kepler, Johannes. Opera Omnia, Vol. II, Ad Vitellionem Paralipomena.
(De modo visionis et humorum oculi usu). 1604 pp. 226-269, JI fig-
Correct dioptrics of the eye here given, and also the explanation of the effect
of convex and concave spectacles. Dioptrica. Demonstratio eorum quae
visui et visibilibus propter conspicilla non ita pridem inventa accidunt.
pp. 519-567, 35 fig. 1611. The amplifier, real images, and erect images.
The Keplerian microscope (modern microscope).

Kircher, Athanasius. Ars Magna lucis et umbrae in decem libros digesta. 2d
edition. Hermanni Scheus, Amsterdami, 1671. ist ed. Romaa, 1646.
ist ed. 40 -f 935 + 17 p., 2d ed., 30 + 810 + 9 p. About 656 fig. 34
Plates. The magic lantern is described in the second, but not in the first
edition.

Libri, Guillaume. Histoire des mathematiques en Italic depuis la renaissance
des lettres jusqu' a la fin du dix-septieme siecle. 4 vol. 8°. Chez Jules
Rennard et Cie Libraires, Paris, 1838. In Vol. IV, pp. 303-314 there is
discussed the invention of the camera obscura. Refers to Leonardo da
Vinci. Thinks Porta reported what had been known a long time.

700 OPTICAL PROJECTION

Langenheim, W. Catalogue of Langenheim's colored photographic magic
lantern pictures. W. Langenheim, 722 Chestnut St., Philadelphia, 1861.
First edition, 1850. The Langenheims were the first to make photographic
lantern slides. They used the albumen dry process, and exhibited their
slides at the London World's Fair in 1851. Art Journal of London, April,
1851, p. 106, Athenaeum, June, 1851, p. 631.

Lenses, Their History, Theory and Manufacture. Bausch & Lomb Optical
Co., Rochester, N. Y., 1906. 47 p., 34 fig.

Marey, E. J. Photo- chronographie. Comptes-Rendus Acad. de Sciences,
cvii, (1888), pp. 607, 643, 677. Description of camera with the band form
of sensitive surface for photography of moving objects.

Marey, Etienne Jules. La Chronophotographie. Nouvelle methode pour
analyser le mouvements dans les sciences physiques et naturelles. Revue
generates des sciences pures et appliqu6es. Vol. II, 15 Nov., 1891, pp.
689-719. The text is accompanied by many figures including the way the
ribbons are actuated in the chronograph camera. There are given pictures
showing the movements of men and animals including insects and some
other invertebrates. Some microscopic objects with their changing shapes
are also shown. Important for the history of the moving picture.

Marey, Etienne Jules. Director of the Physiological Station. Movement.
International Scientific Series (No. 73). 318 p., 200 fig. D. Appleton &
Co., New York, 1895.

Marey, Etienne Jules. The history of Chronophotography. Annual Report
of the Smithsonian Institution for 1901. pp. 317-340; 42 .fig. IX pi.
vSee also his work. Movement, N. Y., 1895.

Matas, Rudolph, M.D. The cinematograph as an aid to medical education
and research. A lecture illustrated by moving pictures of ultramicroscopic
life in the blood and tissues, and of surgical operations. Precedential ad-
dress. Transactions of the Southern Surgical and Gynecological Associa-
tion, 1912. 27 pages. 4 plates. A bibliography of 50 publications given,
with special reference to those in medicine and surgery.

Mayall, John, Jr. Cantor Lectures on the Microscope delivered before the
Royal .Society for the encouragement of arts, manufactures and commerce,
Five lectures, Nov., Dec., 1885, 97 p., 103 fig., and two additional lectures
in 1888, 18 pp., 26 fig. Published by the Society at John Street, Adelphi,
London, W. C., England. Price, 2 shillings 6d, and i shilling.

Melloni, M. Memoir on the free transmission of radiant heat through differ-
ent solid and liquid bodies. Scientific Memoirs, Vol. I. Longman, Brown,
Green and Longmans, 1837. 39 p. This paper shows the superior absorb-
ing power of water for radiant heat. See also Ernest Nichols.

Milliet de Chales, Claude Francois. Cursus seu mundus mathematicus; mine
primum in lucem prodit. Ex officina Anissoniana, Lugduni, [Lyons], 1674.
3v., folio. The second edition is dated 1690 and has four folio volumes.
Fig. 403 is given on p. 666 of vol. ii in the first edition, and on p. 697 of vol.
iii of the second edition.

Molyneux, William. Dioptrica nova, a treatise of Dioptricks, in two parts,
wherein the various effects and appearances of spheric glasses, both convex
and concave, single and combined in telescopes and microscopes, together
with the usefulness in many concerns of human life are explained. By
William Molyneux, of Dublin, Esq. Fellow of the Royal Society. Pre-
sented to the R. S., 1690, printed 1692. Much history and translations of
many Latin extracts. The first figure of a magic lantern with condenser
lens, see fig. 404.

HISTORICAL BIBLIOGRAPHY 701

Montucla, J. F. Histoire des mathe'matiques. New edition in 4 quarto vols.
Edited by J. de la Lande. 1802. The progress of optics during the i8th
century is given in vol. iii, pp. 427-605.

Muybridge, E. Animal locomotion. The Muybridge work at the University
of Pennsylvania, the method and the result. Printed for the University by
J. B. Lippincott Company, Philadelphia, 1888. 136 pages, many text
figures and diagrams.

Muybridge, Eadweard. Animals in Motion. An electro-photographic
investigation of consecutive phases of animal progressive movements.
Commenced, 1872, completed, 1885. Folio, 264 p., many hundred figures
reproduced from original photographs. Portrait of the author as frontis-
piece. Chapman & Hall, LD. London, 1899. In the preface is given a
historical summary of the author's work in analyzing and synthesizing
animal movement, and in an introduction a brief statement of the views
of writers on animal locomotion from the earliest times; also diagrams and
descriptions of the methods used by the author.

Muybridge, Eadweard. Born 1830, died 1904. For biographical account see
the Dictionary of National Biography, 2d Supplement, Vol. II, pp. 668-669.
The Macmillan Co., New York, 1912.

Nichols, Ernest F. A study of the transmission spectra of certain substances
in the infra-red. Physical Review, Vol. I (1893) pp. 1-18. Alum does
not improve the absorbing power of water.

Pansier, P. Histoire des Lunettes par le Docteur P. Pansier, d' Avignon.
T37 P-. r9 fig. 6 plates. Price, 3f soc. An excellent history of spectacles
with many references to original sources and numerous figures and plates
showing the various forms of spectacles at different periods.

Petri, R. J. Das Mikroskop von seinen Anfangen bis zur jetzigen Vervollkom-
mung fur all Freunde dieses Instruments. 248 p., 191 fig. 2 plates. R.
Schoetz, Berlin, Germany, 1896. Price, 10 marks.

Plateau, Jean Ant. Fr. Sur un nouveau genre d'illusions d'optiques. Cor-
respondance mathe'matique et physique de 1'observatoire de Bruxelles,
Publie'e par A. Quetelet. Vol. VII, 1832, pp. 365-368. One plate. This
paper is dated by the author, Bruxelles, le 20 janvrier 1833. (See fig. 408)

Poggendorff, J. C. Gesichte der Physik, Vorlesungen gehalten an der Univer-
sitat zu Berlin. 937 p., 40 fig. Verlag von Johann Ambrosius Barth,
Leipzig, Germany, 1879. Price 16.80 marks.

Porta, Giovanni Baptista. Magiae Naturalis sive de miraculis rerum natural-
ium, Naples, 1558. 2d ed., 1589. English translation of the second edi-
tion. London, 1658.

Priestley, Joseph. The history and present state of discoveries relating to
vision, light and colours. 812 pp., 23 pi. and a bibliography of 288 works
bearing on the subjects treated. London, 1772.

Ptolemaeus, C. L'Ottica di Claudio Tolemeo. The optics of Ptolemy trans-
lated from the Arabic into Latin in the I2th century. Edited by Gilberto
Govi. Torino, 1885. Here is stated very clearly the method of refraction
from rare to dense and the reverse.

Scheiner (Christophorus) , 1575-1650 S. J. Oculus, hoc est: fundamentum
opticum, in quo ex accurata oculi anatome, abstrusarum experientiarum
sedula pervestigatione, ex invisis specierum visibilium tarn everso quam
erecto situ spectaculis, necnon solidis rationum momentis radius visualis

702 OPTIC PROJECTION

eruitur; sua visioni in oculo sedes decernitur; anguli visorii ingenium
aperitur, etc. 5 p. 1., 254 pp., i pi., sm. 4°. Aeniponti, apud D. Agricolam,
1619.

Scheiner, Christophorus. Rosa Ursina sive Sol, ex admirando facularum et
macularum suarum phenomena varius. A Christophoro Scheiner,
Germano sueuo e societate Jesu. Ad Paulum Jordanum II, ursinum
Bracciani ducem. 784 folio pages, many plates. Apud Andream Phaeum
Typographum Ducalem. Bracciani, 1626-1630.

Smith, Robert, LL.D. A Compleat system of opticks. pp. 458 -f 171 of
remarks. 63 plates in the text, 20 plates in the remarks. Cambridge,
England, 1738.

Stampfer, S. Uebef die optischen Phaenomene welche durch die stroboskop-
ischen Scheiben hervorgebracht werden. Koeniglich-Kaiserliches poly-
tech. Institut. Wien. Jahrbucher, Vol. XVIII, 1834, p. 237-. Describes a
magic disc like Plateau's.

Thompson, Silvanus P. The arc light. Cantor lectures delivered before the
Royal Society of Arts, 1895. Jan. 14, the physics of the arc, pp. 943-960.
Carbon arc by Davy, Sept., 1800. Jan. 21, the optics of the arc, pp. 961-
976. Jan. 28, the mechanism of arc lamps, 980-^991. It is stated, p. 981,
that W. E. Staite devised an automatic lamp in 1846.

Uchatius, Franz. Apparat zur Darstellung beweglicher Bilder an der Wand.
Sitzungsberichte der kaiserlichen Akademie der Wissenschaften. Math-
Natur. Classe., Vol. X, Wien. 1853, pp. 482-484, one plate. Describes a
method of projecting moving pictures drawn on glass by means of a lime
light and condenser moving from picture to picture. Each picture was
fixed in position and had its own projection objective; the axis of each
objective pointed to the same place and the pictures all appeared in the
same position.

Vinci, Leonardo da. Les manuscrits de Leonard de Vinci. Manuscrits A-M
de la Bibliotheque de L'Institut, publics en fac-similes phototypiques avec
transcriptions litterales, traductions franchises, avant-propos ct tables
me'thodique par M. Charles Ravaisson-Mollien. 6 folio volumes, Maison
Quentin, 7, Rue Saint-Benoit. Paris, 1881-1890. Price, 900 francs.

Waterhouse, J. Notes on the early history of the camera obscura. The
Photographic Journal, including the transactions of the Royal Photographic
Society of Great Britain, Vol. XXV, May 31, 1901, pp. 270-290. This is
the best statement of the case found. Many extracts from original sources
are given. See also the last edition of the Encyclopedia Britannica under
Camera obscura, written by General Waterhouse.

Waterhouse, J. Notes on early tele-dioptric lens-systems, and the genesis of
Telephotography. The Photographic Journal, including the transactions
of the Royal Photographic Society of Great Britain, Vol. XLIL, Jan. 31,
1902, pp. 4-2 1 , one pi. This paper gives a good account of the introduction
of the combination of a convex and concave lenses for projection, t. e.,
the use of an amplifier.

Wiedemann, Eilhard. Ueber die Erfindung der Camera Obscura. Verhand-
lungen der Deutschen Physikalischen Gesellschaft. 28 February, No. 4,
1910, pp. 177-182, 1 fig. Wiedemann speaks of the camera obscura of Ibn al
Haitem about 1039, and of the description of this by Kamal al Din, 1300.

Werner, Otto. Zur Physik Leonardo da Vincis. Inaugural — Dissertation zur
Erlangung der Doktorwuerde der hohen philosophischen Facultat der
Friedrich-Alexanders-Universitat Erlangen, June, 1910. 179 p, 103 fig.

HISTORICAL BIBLIOGRAPHY 703

Werner discusses the claims of Da Vinci, (1500), and of Levi ben Gerson,

1321-1344. Wiedemann (which see) refers hack to Ibn al Haitem, about

1039.
Young, Thomas. On the Mechanism of the Eye. Read before the R. S.,

Nov. 27, 1800. In the transactions of the R. S., 1801, pp. 23-88. On pp.

39-40 he describes astigmatism and shows that it can be corrected by making

the spectacles oblique, p. 43. See also Airy. On pp. 57-58 is described a

decisive experiment to show that the accommodation of the eye is due to a

change in the crystalline lens.
Wilde, Dr. Emil. Geschichte der Optik vom Ursprunge dieser Wissenschaft

bis auf die gegenwartige Zeit. 2 parts. I, 3,52 p., 3 p., II, 407, p., 4 pi.

Rucker und Puchler, Berlin. Part I, 1838; Part II, 1843.
Zahn, Joannes. Oculus artificialis teledioptricus, sive telescopium nora

methode explicatum ac comprismis e triplici fundament o physico seu

naturali, mathematica dioptrico et mechanico, seu practice stabilitum;

opus curiosum theorico-practicum magna rerum varietate adornatum.

2d ed. 50 + 645 + 15 P- over 600 fig. Johannis Christophorilochneri,

Norimbergae, 1702.

INDEX

PAGE

Abbe diffraction theory 644-647

apparatus for 644

demonstrating to an audience
and to an individual . . . 646-647

grating needed for 644

light for 646

Abbe substage condenser

273, 280, 626

Abbott, the Sun 139

Aberration, chromatic and

spherical 580-583

maximum and minimum. 582, 589

Absolute temperature 547

Absorption spectra 637-638

comparison spectrum with

637-638

glass box for use with 637

substances for 637-638

Accommodation in vision

651, 656-659

Kepler's theory of , . 657

lantern slide for experiments 656

necessity for 657

produced by muscular effort 659

Scheiner's theory 658

Acetylene, flame and lamp. .127-130
amount in prestolite tank. . . 129

position of lamp 128

summary 135

Achromatic and aplanatic com-
binations 581-583

substage condenser 272, 626

Adam's Essays 142

Adapters for small carbons in

large lamp 80

Advertising Magic Lanterns .... 435

a-k on microscope slide 361, 387

Alcohol, burner 130

ethyl, methyl and denatured

supply 130

Alcohol lamp 131

lighting 131

precautions, with 133

putting out 133

summary for use 137

Alco-radiant 130

Alternating current 475

advantages 475

PAGE

amperage 484

arc 542

arc, form of lamp 68

ballast with 544

carbons for 87, 552

cycle 486

disadvantages with arc lamp

68, 475

frequency 486

how produced 475

magic lantern with 68-77, 87

micro-projection with 284

power factor with 485

radiant efficiency of arc. .568-569
summary for use with the

magic lantern 76

units of 484

voltage 484

watts with 485

wiring 69, 512

Ambronn's Handbuch, Astrono-

mische Instrumente 138

American, Institute of Electrical

Engineers 500

Journal of Science 157

lantern slides 200, 201

Microscopical Society 383

Ammeter 479, 510

alternating current 73

connections 480-482

direct current n, 481

does not register 52, 510

external shunt 479

in projection 236, 481

need in micro-projection .... 236

precautions for use 481

self-contained shunt 479

soft-cored 510

to measure current 479

testing polarity with 509, 510

Amperage 476, 484

alternating current 484

carrying capacity of copper

wires 500

cords and cables 501

for different forms of projec-
tion 500

found by Ohm's law 521-522

706

OPTIC PROJECTION

found by wattmeter 483

varying with inductor 532

Ampere 476

Amperes and volts in transform-
er 533

Amplifier 229-600

actual magnification and im-
age 257

image formation with 600

magnification due to 600

on projection microscope .... 238

Anatomical Record 289, 320

Anatomische Gesellschaft 363

Angle, closing of light 61 1

brilliancy of image depend-
ent on 611-613

visual 210

Anisometropia, unlike refraction

in the two eyes 670-672

Anthony & Co 210

appearance of the screen

image 672

source of light for demonstrat-
ing . 670-672

Anthracene screen for ultra-
violet 635

Aperture, 172, 233, 274, 278, 406, 601,
609-611, 616

and light losses 601

effect of 609, 610-611

image formation and 615

of condenser, to increase. ... 616
of microscope objective. .615-616
of projection objective, in
opaque and transparency

projection 172

plate of moving picture ma-
chine 406

relation of condenser and

objective 274

standard for moving pictures 406

substage condenser 278

substage condenser to objec-
tive 233

with directed light 610

Apparatus, adapted to frequency 495
arrangement with sun pro-
jection 286

blackened 242

block and its construction 291-294

block, guide for 292

block, size and weight 29 1

home-made 287-296

Apparatus for, drawing and

photography 319

drawing with the microscope 354
electric currents and arc lamps 474

lantern slide making 200

lantern with alternating cur-
rent 68

lantern with direct current . . 9

house lighting system 78-99

lime light 100

oil, gas, acetylene, alcohol. . . 1 19
measuring radiant efficiency 567

micro-projection ". 221

moving pictures 390

opaque projection 166

photography 319, 643

polarized light 622

preparation of lantern slides 200

projection microscope 221

rooms and screens 439

small current arc lamps 78

sockets and stems 292

spectra 627

sunlight 138

ultra-violet light 627-641

vision experiments 651-672

Apochromatic objectives. . . 583, 600
compensation oculars for .... 600

Arc 535

alternating current 549

carbon 537, 539

cause of light from 546

characteristics of 486

construction of 535

direct current 1 1, 536

electric 535

electrical behavior of direct

current 536

figure of, 507, 537-538, 540-543.

545, 54s

inclined. . . .543, 545, 54s

lamp for spectra 628, 639

length and potential 538

length and voltage 53*

parts of 536 537

parts as source of light 547

right-angled figures of

507, 538, 540-542
stream and bright violet lines 546

vertical 537

voltage 476, 536, 538, 544

with 10 and 20 amperes. 249-540

INDEX

707

Arc lamp 12, 15, 61, 401, 536

alternating current 68, 72, 74

automatic 12, 61, 83, 328, 364, 512

ballast for 539, 544

choke coil for. ... 88, 352, 532, 544
direct current

candle-power 553-565

carbons for 87,551

current needed . . . .24, 248, 328

drawing with 328, 341

for spectra 628, 639

for micro-projection 236

for vision experiments.. 65 1-672
fine adjustments for. . .12, 72, 237

going out of 48

hand-feed 12, 62

house circuit 80

inductor for 88, 352, 544

installation of 500-505

intrinsic brilliancy of crater . . 564

lighting 21, 24, 88

"Lilliput" or baby form 81

managing 24, 414

material for installation 502

opaque projection with 182

polarity test 84, 506-511

position of carbons in

V 49, 50-51, 61, 70, 72, 550, 553

rheostat for

ii, 70, 83, 521, 539, 544

small 81, 82

small automatic 83, 364

small carbons for 341

small with clock-work 364

small with drawing 340

special dynamo for 486

starting 23

summary for small 97

testing polarity 506-512

three- wire, automatic 238, 263, 512

three-wire, supply 514

turning off 88

with transformer 532-534

wiring 15, 69, 496-505, 513

wiring when far from main

.supply 513

wiring for large currents ... 513

Arrhenius 138

Asbestos-patch gloves 21, 74

Astigmatism, astigmia or un-
equal curvature 663-669

change with age 666

correct position of spectacles
for .666

correction by cylindric lenses

665-666
correction by obliquity of

spectacles 665

correction by stenopaeic slit

669-670

demonstration of 663-668

discovery of by Young and

Airy 666

radial lines for 662-665

Atmospheric pressure 103

Attachment plugs . . . .86-87, 502-504

Auditorium for projection . . 395, 439

Automatic arc lamp, 12, 61, 83, 286,

328,334, 336,364, 512,639

Bausch and Lomb form 512

Ewon's 548

Leitz 364

Nernst 92

Reichert's 83

Thompson's 334

Axis of lenses 576-581

principal, and secondary . 576-579
Ayrton, Mrs 539

Balancing devices 521, 532

Ballast, alternating current

69-70, 521, 532, 544

direct current u, 521, 539

moving pictures 399

need with arc 539, 544

Nernst lamp 92

position of 512-513

small arc 83

Ball's Astronomy 139

Ball-pointed pen on unvarnished

glass 187, 207, 304, 306

Balopticon,

diagrams 188, 189

Edison Moving Picture at-
tachment for 405

large for opaque objects 192

Nicholas Power, moving pic-
ture attachment 404

home 184

Balopticon, convertible. 187, 304, 306

Universal 191, 307

Barbara 675

Barrel, rheostat 526

Baseboard, for home-made ap-
paratus 288-296

fixing track to 290

708

OPTIC PROJECTION

Bausch & Lomb Optical Com-
pany, cuts loaned by, 35, 60, 90,
91, 127, 184, 187-189, 191-192,
304, 306-308, 356, 358, 404-405
ref. in text, 80, 190, 286, 459,
512

Bayley 9, 217

Bench, home-made optical 288

Beseler, (Chas. Beseler Co.) .... 94

Blackboard, lighting 448-452

Black stain for tables 289

Blackened, apparatus. . . .7, 242-246

objectives 246

Block with lead sheets 292

Blocks for apparatus 291

Blondel, intrinsic brilliancy de-
termination 564-566

Blood corpuscles, demonstration

of 227

"Blow" said of a fuse 83

Blow-through jet 105

Botanical Gazette 289

Botanische Zeitschrift 289

Boyle's law and camera. . 104, 129,675

Box to cover switch 517

Break in circuit 514

Briggs* lantern slides 673

Brilliancy, of screen image 610

intrinsic. . .564-566, 613-615, 619
limit of, with projection micro-
scope 618

reduction by substage con-
denser, amplifier and ocular 61 8

sunlight most brilliant 619

British lantern slide 200, 203

British photographic society .... 217
Brown & Sharp's wire gauge .... 502
Bulletin of the Bureau of Stand-
ards 547, 566

Bunsen burner 125

Burner, blow-through type 105

lime light 105

mixed jet 107

roars or hisses 115

Burning out a fuse 519

Burning point or focus 578

Burr, Geo. L 673

Bushings for small carbons in
large lamp 80

Camera, adjustable back 212

and plate-holder 382

enlarging, reducing, copying 210
hinged baseboard 211

landscape drawing 166, 167

lantern slide making 209, 213

printing by projection 378

spectrographic 643

tilting 212

vertical 382

Camera obscura, history . . . 673-575

Candle-power, and current 559

current and size carbons

87, 248, 551-552
direct current inclined carbons 556

formula for finding 613

of arc lamps 553-565

from kilowats 558

lime light, relative 100

petroleum lamp 125

power consumption 561-562

rectifier 558-559

rectifier

inclined carbons 557

right-angled carbons. . . . 558

sun 138

variation with current . . . 553-556
watts, alternating current,

inclined carbons 557

right-angled carbons 557

right-angled carbons 556

Capacity of gas cylinders 104, 128-129

Capacity meter 103

Carbon arc lamp for spectra. ... 628

Carbons, adjusting 252

alternating and direct cur-
rent with 340

arc with 10 and 20 amperes . . 249
arrangement of 543, 545, 548,
inclined; right-angled, 342, 507,
538, 540-542

arrangement in micro-pro-
jection 284

bad position 50

composition and movement. 72
converging, with alternating

current 564-565

cored and solid 250

correct position 49

efficiency of different ar-
rangements .560-561

electrodes 539

feeding with alternating cur-
rent 74

feeding on house circuit 342

feeding with small arc 87

house system 341

image on screen 251

INDEX

709

inclined 61, 70

incorrect position 51

not near enough 47, 48

observing with moving pic-
tures 403, 414-415

polarity right and wrong. 506-5 12
position, 72, 284, 507, 538, 540-

548,
preparation for exhibition 250, 433

distribution of light 565

size 87, 415, 551-552

size for moving pictures 415

current, candle-power. . . 248

house circuit 341

small with alternating current 342

small arc lamp 79, 87

small with direct current. . . . 342

solid and cored 250

terminals 12, 539

too short 47

why small on house circuit ... 341

Cardboard screen 80, 458

Cardanus 674

Carrying capacity of electric

cords, how calculated 501

and insulation 500

flexible cables 501

of copper wire 500

Caution for lime light 107

Celluloid, films 431

inflammability of 431

Center of lenses 576, 577

of lens face, how to find .... 40, 41

Centering 23, 39

alternating current lantern . . 73
apparatus for vision experi-
ments 653, 671

heliostat 150

mazda lamp 91

mechanical 40

micro-projection. . . .246, 251, 285
moving picture machine. 4 10-4 12

Nernst lamp 93

objective 247

objective hood 246

objective, vertical 45

optical test, condenser and

objective 44

perpendicular to axis 41

radiant and condenser 41

substage condenser 280

troubles if incorrect 53

Centimeter rule 27, 318, 371

Cerium iron gas lighter 106

Chadburn 173, 180, 183, 190

Chadwick, Hepworth and Wright 173

Chamot, Dr. E. M 207

Chemical polarity indicators. ... 510

Chevalier 322

Choke-coils 88, 352, 532, 544

Chromatic and spherical aberra-
tion 580,583

Circuit, electric 496

breakers 518

open and closed by switches

514-518

with break 514

with one ground 497

with two grounds 498

Clock-shaft of heliostat, parallel

with earth's axis 147, 160

Cloth screens 457

Cohn, eye defects in school child-
ren 659

Color, used in moving pictures . . 392

Colored moving pictures 392

Coloring lantern slides 217

Combined apparatus 356

Combined projection, 175, 176, 180,
182, 184, 186, 189, 193, 194, 295,
297, 300
avoiding contrast of images . . 177

two complete outfits 297

Comparison spectrum 637-638

Compensation ocular 232

Concave lens for parallelizing

light 337

Condenser 14, 587

Abbe substage 273, 626

aperture of, to increase, or

diminish 616-617

achromatic substage 272, 626

aperture of substage 278

centering substage 280

diaphragm opening 337

distance from objective, in

microprojection 247

distance from radiant 41

end to be next the radiant

58, 62, 66

ends reversed 58, 62, 66

first element of 65, 588

focus of second element for

the objective 65, 590-591

focus too long 55

focus too short 55

for demonstrations in vision

652-653

710

OPTIC PROJECTION

for lantern slides 202

for sunlight 139, 161

lamp with special 343

lens breaking 57

light for different positions of

lamp 42

lighting with main condenser

only 234

lighting opaque objects with 189
micro-projection with. . .237, 273

mounting of 58, 62

moving picture 403

and objective, proportions ... 43

opaque projection with 175

out of center 53

parabolic for micro-projec-
tion . 588

protecting by sheet mica

or glass 608

reduction of brilliancy by. . . 618
second element of, and the

objective 65, 588, 590

second element for micro-
projection. .. .274-278, 591

size for drawings 331

size for lantern slides 202

spherical aberration of 585

substage 232, 347

substage, high power 271

substage, K6hler method with 278

sunlight 139, 161

three-lens 13, 652

two-lens 10, 653

with spherical aberration 585
without spherical aberra-
tion 584

types of 587

triple 588

two lens 588

Conditions for good micro-pro-
jection 309

Connections, acetylene 129

ammeter 480-482

attachment cap of separable

plug 86-87,503-504

electric supply 502, 513

gas lamp 126

lime light 106

to the switch 504

voltmeter 478-482

wattmeter 482

Connectors, metallic 106

separable extension 87

Converging light, parallelizing. . 273

Cornell University 147, 673

Crater, increase in size with cur-
rent 248, 249, 564

temperature of 546-548

Crystalline lens of the eye, change
in shape for accommodation . . 658
in hyperopia and myopia . 660-661
rigid in presbyopia. . .661-662

Cuff and Adams 141

Current, alternating 475

amount compared with direct

73, 553-565

at anodes, oscillogram of .... 495
appearance of arc. . .342, 542, 545

and candle-power 553-565

connections of ammeter to

measure 482

controlled by resistance . . 542-544

direct 1 1 , 474

direct for arc with magic lan-
tern 24

for drawing 338

for spectra 628, 640

for experiments in vision 651-672

for micro-projection 248

increase in heat by increase

of current 248

increases size of crater 248

insulation for large 513

lack of 48

lamps for small 78

magic lantern with alternating

68-77

micro-projection with alter-
nating 284

moving pictures with alter-
nating 398, 401, 566

opaque projection 182

oscillogram of delivered and

supplied 493

potential drop at arc 536-539

precaution for heavy 186

rectifier 489-492

screen distance and 73

size carbons and candle-
power 248, 55I-552

unit of, the ampere 476

wiring for large 513

Curtains for darkening room .... 446
Curves of light reflection .... 460-465
Cycle with alternating current . . 486

INDEX

711

Dark ground illumination . . . 647-650

apparatus required 647

for demonstrating inhomo-
genieties in liquids or trans-
parent solids 648-650

Foucault's method 650

method of striae 647

Toepler's "Schlieren-Methode"

647-650

Darkening room, method 446

Darkness, avoiding 112

in opaque projection 175

of room for sunlight 143

relative for different projec-
tion 443-446

Davids (Thaddeus Davids Co.) 207

Davy's Carbon Arc 686

Daylight vision 175

Decentering, effects of 53

Decoration of projection room

440-441

Defective vision 659-672

apparatus required for. . .651-655
condenser for demonstrations

652-653

demonstrations of 659-672

lantern slides for. . . .656-666, 671

source of light for 670

trial lenses for 651-655

Delineascopes

63, 193-196, 310, 312, 355
Descartes sine law for refraction 576

Deschanel's Physics 212

Diaphragm, effect on cone of

light 279

of substage condenser, effect
with Koehler method, and
with main condenser me-
thod of illumination 620

Diffraction, gratings 631, 644

images 632

pattern 644-645

Diopter, definition 229

in spectacle lenses 655

Direct current apparatus 9

arc, radiant efficiency com-
pared with alternating

69, 474-475, 553-565

production of 474

summary for magic lantern . 64-67

units 475-478

use of 474

Dirt on lenses 55

Distortion, how to avoid in draw-
ing 338

of image 338

Distribution of light, different
forms of arc 562

Diverging light, to parallelize. . . 274

Dolbear, Art of projecting

122, 138, 174, 211, 621

Double-pole switch. . . 12, 70, 515-517

Drawing apparatus 319-389

attachment for ordinary ma-
gic lantern 327

booth 321

direct current arc lamp for ... 328

distance, varying for 335

fastening letter to 375

from lantern slides 326

high power 335

horizontal surface with magic

lantern 331

with opaque lantern 332

house lighting system for .... 339

landscapes 166

large objects with low powers 333

lettering the 374

microscope for 344, 385

microscope and lamp at right

angles 348

microscope in a dark room . . . 346
microscope without ocular or

substage condenser 334

models 357

objectives, 16-8 mm 335

oculars for 337

opaque lantern 332

outfit 350

house system 343

with inductor 352

(Koristka) 323

with microscope 343~354

with small currents 343

photographic camera for. . . . 332
projection apparatus for. ... 319
projection apparatus, early

forms 372

projection microscope for. . . 333

publication of 373

radiants other than the arc . . 329
range of objects with projec-
tion microscope 333

room and curtains 321

room, special 320

shelf 325

on projection table 326

712

OPTIC PROJECTION

size condenser to use 331

size and lettering 376

small 330

small arc lamp, for 340

summary 388

surface horizontal 322

surface vertical 32 1

table 324

with mirror attached . 323-325
with movable shelf . . . 325-326

tracing 374

troubles in 384

varnished slides for 205

wall diagrams 329

Dresbach, Dr. Melvin 651

papers on eye defects 659

Durand, Dr. Albert C 651

Dust to show light rays 247

Dynamo 474

connected directly to arc lamp

486-488

special for arc lamp 486

wattmeter to measure power
delivered by 482

Eastman Kodak Co. ...212, 215, 319

Eclipse voltmeters and ammeters 510

Ecliptic 163

Edinger 345

apparatus, erect images with 373

large apparatus 360-362

outfit for small arc lamp and

ordinary microscope 363

vertical apparatus 360

Edison, Thomas A., figures loaned
by and text references

113, 400, 406, 418, 429, 435, 686

"Effects" with multiple lanterns 34
with single lanterns 36

Efficiencies of current and car-
bon arrangement 560-562

Electric, apparatus, list of. .474, 499

arc 535-538

circuit 496

fan for drawing room 320

flashlight 14

measurements 475-486

supply, connections to ... 502-505
units 475-478

Electrical behavior of arc . . . 536, 539
World 553

Electrodes, of carbon 539

and potential drop at arc 536-539
for spectra 627, 638

stuffed with various sub-
stances 638, 639

Electromotive force, unit of .... 476

Emission spectra 638-643

arrangement of the electrodes

for 640

automatic arc lamp for ... 638-639

current to use for 639

different appearances with
positive and with negative

electrodes 640

electrodes for 640

yellow-flame carbons for. ... 638

Enclosed knife switch 517

Energy losses in projection . . 603-609
comparison of in water, etc . . 607

example of 609

in the condenser 604

in the projection objective . . 609

in sheet mica 608

in the specimen 607

table of 607

Energy, required for projecting

moving pictures 57O-571

with inductor and rheostat . . 533

Energy transmission 606

table of absorption and trans-
mission 607

English Mechanic 221

English Photographic Club 29

Enlargements with projection

apparatus 378

Enterprise Mfg. Co 101

Episcope for drawing 332

Equivalent focus 580

Erect images, in drawing .... 359-373
with two projection lenses 368

Ether saturator 113

Ewon's automatic lamp 548

Exhibition, with, alcohol light. . 132
alternating current lantern . . 73
direct current magic lantern 19

high powers 271

lime light no

mazda lamp 92

moving pictures 433~435

Nernst lamp 95

opaque lantern 179

projection microscope 270

room lighting, 19, 52, 66, 96-99,
1 12, 120, 143, 175, 235, 303, 314,
320, 395, 441-443, 626, 633
with small arc lamp 90

INDEX

713

Exhibition with sunlight 161

Experiments, on flicker 423-427

with projection in physics 621-651

with polarized light 622-626

with vision, normal and defec-
tive 651-672

Explosion with lime light 107

Exposing dry plates directly to

the projected image 381

for printing by various lights 214
with projection apparatus. . . 380
Eye, accommodation for distance

657-658

as a part of projection appara-
tus 4

demonstration of normal vi-
sion 654

inverted image on the retina

655-656

prevalence of defects 659

refractive defects 659-672

two eyes unlike 670

Fan, electric in drawing room . . 320
Faraday and moving pictures . . . 680
Field, need of large, in micro-
projection 254

and objectives 254, 255

Filament, position of (mazda

lamp) 91

Filar micrometer ocular 234

Film, burnt in concentrated light 432

celluloid 431-432

direction of motion 419-420

effect of opacity on energy 571

inspection of 427-428

invented by Goodwin 682

lantern slides 215

magazine 407

security from fire by . . 43 1-433

mender 429

position in machine 415

splicing 428-429

threading 417-418

Finimeter 103

Fire, danger from 431

escapes 443

-proofiing curtains 321

Fire-shutter 420, 431-433

automatic 420

Firetrap, security or 431-433

Fire Underwriters 498-505

regulations 399

Fish, Dr. P. A 289

Flexible metallic tubing 106

Flicker 423-427

curve 424

experiments on 426

formula of 426

position of shutter to prevent

422-423

table on speed 427

theory of 426-427

Fluorite in apochromatic objec-
tives 583

Flux of light in projection . . . 172, 614

Focus, or burning point 578

condenser and objective. .587-592

conjugate 579

principal focus 578-582

principal, how to obtain 579

Focusing 28, 30

device on the microscope. ... 241
device with the magic lantern

objective 28

image on screen 28

for photographing 379

of screen image for micro-
projection 256

Folmer & Schwing 212

"Fool-proof" 7

Formula, for absolute and centi-
grade temperature ....... 547

amount of gas in a cylinder
of acetylene, hydrogen or

oxygen 104, 129

black table stain 289

determining visibility 227

dioptry and focus of lenses 229-230
electric quantities, Ohm's law

483-485, 501-502, 522
flicker with moving pictures 426
intrinsic brilliancy of the sun

I.38-I39

light flux passing an objective

172,613

magnification 260-262

making lantern slides direct

2OI, 2O5-2O8

size of screen and focus of

projection objective. . .467-470
sizing and painting screens . . 456

starch paste 375

table black 289

Foucault, method of detecting
inhomogeneities by dark
ground illumination . . . 650-65 1

OPTIC PROJECTION

Foucault, Recueil des Travaux

Scientifiques 138

Fourtier 9, 202

et Moltini 621

Frame for darkening window

margins 445~447

for retouching slides 29, 203

French Congress of Photography 200

French lantern slides 200

Frequency, of alternating cur-
rents 486

apparatus adapted to 495

Fuess, R 56, 138, 150, 159

Fuse blocks, location of 520

Fuses 399.519

"blowing" of 519

burned out 47, 519-520

capacity 520

and circuit breakers 518-519

location and installation .... 519

on each line 519-520

on house circuit 83, 520

replacement of 520-52 1

wattmeter 519-520

Gas, amount in cylinders

103-104, 129

lamps 125

lighter 106

management 127

reflector 126

summary 136

Gases, proportion of in lime light 108

Gauges, pressure 102, 103, 129

Gelatin for lantern slides 205

General Electric Co 553

cuts loaned by 489-495

Generator 474

shunt 487

Gcntner (J. H. Gentner Co.) .... 459

German lantern slides 200

"Ghost" from reflections 245

Glass plates for polarizing light . . 624
tinted in combined projection 177

unvarnished 207

Glassine ink for writing on glass 207
Gloves, asbestos-patch . . . .21,22, 109

Glower of Nernst lamp 92-93

Golgi method 240

Goodwin, inventor of photo-
graphic film 682-683

Goring and Pritchard 283, 454

Gothic type for drawings 377

Gratings for spectra. . . .627, 632-635

s 'Gravesande . 140, 146

Ground of electric current

48, 497-498

Ground-glass to diffuse light 380

screen, transmission. 46 1-462, 465
Guide for making lantern slides . 207
Guides for apparatus blocks .... 292

Guil pastils 101

Gundlach-Manhattan Optical Co. 412

Hand-feed arc lamp 12, 15, 62

lamp for alternating current - 68

lamp for small currents 286

Hartford Screw Co 296

Hassock and Rosenberg 9, 621

Heat, getting rid of, in rheostat 523

with small currents 350

Heating unequal on condenser . . 58

Heliostat 139

clock-driven 145

for east window 144

for south window 143

for west window 145

hand-regulated 140

how to set clock-shaft 147

kinds of 140

lens and prism 157, 159

mirror parallel to clock-shaft

154, 155

setting up 154

one-mirror 145, 146, 148, 150

setting up 149

positions of mirror 144, 156

principle of 151

southern hemisphere 156

setting up 158

two-mirror 145, 152, 153

arranging fixed mirror. . . 154
arranging movable mirror 152

Hepworth, C. M 391

Heyl, H. R 680

Hepworth, T. C 9

Historical Summary 673-687

Hitchcock, Prof. Romyn 265

Holland, translucent screens in . . 462

Home kinetoscope 435

Home-made optical bench . . . 288-296

projection apparatus 4, 287

rheostats 525~53°

Home projectors 435

Hood, on objective 245

showing light centered and

not 246

Hooke's joint and rod 82, 329

INDEX

715

Hopwood, H. V 391

Horizontal objects 32, 268

Homer's Zoetrope 680, 682

House system for projection

microscope 285

fusing 83

lamp for 78

Hubbell (Hubbell Inc.)

86-87, 503-505

Huygenian ocular 230, 233, 280

Hydrogen, cylinders 103

substitute for 1 13

Hyperopia or long sight 66 1

Iban Al Haitem and the camera

obscura 674

Illumination, dark ground. . .647-650
Foucault's method of . . . .650-651

Toepler's method of 650-651

Illumination, flashes per second

at which flicker disappears 424

with high powers 599-601

with lower powers 598

with magic lanterns 16-18,584-587
with moving pictures

411-415, 593-598

with projection microscope

247-256, 271, 278, 284, 286, 287,

328, 335, 346, 598-601, 617-620

Illuminating gas for lime light ... 112

for magic lantern 125-127

Image, brightness of . . 18-19, 612-617

carbons 251

.condenser with spherical aber-
ration 585

condenser without spherical

aberration 584

connection with aperture .... 615

correct 26-27, 371, 387

dim and brilliant in combined

projection 177

erect 26-27, r9O, 359. 363-365-
367/369,371,373,387
with opaque lantern . . 363-365
with translucent screen

26, 362

with two lenses 368-369

formation with, the magic

lantern 584

effect of an extended source

of light 586

moving pictures 59i~598

point not on the axis 617

projection microscope. . .598-601

Image projection microscope with

amplifier or ocular. . . .600-601

a microscope objective 616

hazy 308

inverted 26-28, 359-371, 584

for micro-projection 470

for different microscope tubes 243

for moving pictures 468

for opaque projection 181

for screen distances 466

lantern slides for 464

sharpness of 28-30, 256

size, found by calculation. . . 261
found by measurement

257-260
stereoscopic with the magic

lantern 37~39

troubles with screen images

52-57,301,308

Incandescent lamp 14

after rheostat 17

before rheostat 13

testing for voltage 46

tracing pictures 374

Inclined carbons 61, 70, 543-550

Indirect room lighting 441-443

Inductor 352-532

with arc lamp 352, 532

varying amperage with 532

wiring. 352, 532

Infra-red radiation 566

Ink for unvarnished slides 207

Inks and pens for varnished slides

206-207
Insulation with large currents

5I3-5H

for long distance wiring 513

of wires 497-498

regulating amperage 501

Intensity of light, in different

directions 563

Intrinsic brilliancy, of crater of

arc 564

of the light source

138, 564, 612-615

of the sun 138-139

Iodized starch polarity indicator 510
Iris diaphragm, effect on cone of

light 279, 620

Italian lantern slides 201

Ives, Fred E. — Ether oxygen lime
light, history and new ap-
paratus 686

716

OPTIC PROJECTION

Janssen astronomer, pictures of
the transit of Venus with a
photographic revolver in-
vented by him in 1873 (see
Marey, Hist. Chronopho-

tography) 683

Jablochoff, arc lamp with paral-
lel carbons (see Silvanus P.

Thompson) 686

Jenkins, C. F 391, 686

Journal of Applied Microscopy. 289
Journal Royal Microscopical So-
ciety 221

Journal Royal Society of Arts 200, 686

Keller (P. Keller & Co.) 61

Kepler. . .321, 361, 368, 656-657, 678
accommodation for distance 657
hypothesis of accommodation 657
inverted retinal image. . .656-657

Kilowatt 477

Kilowatt-hour 477

Kilowatts for candle-power 558

Kinemacolor moving pictures. . . 392

Kinetophone 393

Kinetoscopc 400

mechanism 418

Kingsbury, Dr. F. B 324, 327

Kircher, A. magic lantern . . .676-677

Knife switch 515, 517

Knife switch enclosed 517

Knife switch vertical 516

Kohl, Max 126, 153

Kdhler method, substage con-
denser 278, 305, 619-620

Koristka's drawing apparatus . . . 323

Labelling lantern slides 218

Lambert, F. C 200

Lamp arc, 12, 74, 78, 80, 182, 236, 286
328, 339, 343, 401, 506, 536, 549,
628, 639, 652, 670

Lamp, for general lighting 442

for small currents 78, 286

gas 125

incandescent 14

lime light . 105

on house system 78

with special condenser for

house system 78, 343

Lamp-house 14, 72

window in 14, 72

Landscape drawing, camera for 166

Langenheim 673

Lantern, college bench 301

combined 176, 180, 182

multiple 34

testing 20

Lanternists, work of early . . . 1 19-120
Lantern slides, actual size of

opening 202

American 201

arranging 30

black ground 213

British, spotted 203

coated with gelatin for writ-
ing 205

collecting at close of exhibi-
tion 30

coloring 217

condenser to illuminate 204

confusion of size 202

correct position in carrier .... 26

developing 209

for showing c\ c defects

656, 665, 666, 671
directions for making direct 201
directions for "spotting". .26, 216

duplication 19

film 215

for experiments in vision. 65 1-672
for showing eye defects . . =5 56, 665,
666, 671
frame for retouching, 203

with guide lines 206, 207

hand-made 205

in individual carrier 31, 32

labeling 26, 218

making 200

mounting 216

negatives as 211

on mica or gelatin 208

opaque 56

order of 19

photographic 208

possible images of 27

possible positions of American 28
possible positions of British .. 28

printing with camera 213

printing from negative 208

rapid preparation 215

sizes 200

size of condenser required for 202

size of image 464

size of print on 208, 464

size of screen for 464

smoked glass 208

INDEX

717

Lantern, square 29

"spotting" or marking

19, 201, 216

standard 202

standard American 25, 200

storing 218

summary 220

troubles in making 219

typewritten 215

unvarnished, ink and pen for 207

varnishing 205

Lantern slide carrier for delinea-

scope 28, 63, 196

carrier for experiments in

vision 651-654

permanent 31-32

push-through form 23

ways of making 203

writing them with a pen . . 204-208
Lathe bed type of projection ap-
paratus 262, 289

Latitude and heliostat 160

Lavasseur, Professor E 673, 685

Law of magnification and reduc-
tion 357,369

Law of reflection 140, 572-574

Law of refraction 576

Lecturer, suggestions to on magic

lantern exhibition 19

Lecture rooms 439-454

Lecture room with gallery 448

with raised seats 449-451

screen at side 450-45 1

transluctent screen 453

Lehmann, fluid crystals, etc. ... 621

Leiss, C 138

Leitz, Ernst, 79, 178, 225, 299, 360,

362, 363, 364,
Lens, aberration, chromatic. . . . 583

spherical ; . . . 580-586

how corrected 581

achromatic 583

aplanatic 581

axis of, principal and second-
ary 576-579

center of curvature 577

center, optic 576-577

concave, convex and menis-
cus 577

concave to make light cone

parallel 273,276

convex to make light cone
parallel 274-276

Lens, crystalline of the eye 657

change of shape in accom-
modation 658

definition of 576

dioptry of 229-230

equivalent focus of 580

focus of, principal and con-
jugate 578-582

focus, equivalent 580

holder for trial lenses. .651-654

meniscus 577

optic center of 576-577

principal focus of 578

principal focus, how found

579-58o
principal and secondary axes

576-579

radius of curvature of 577

secondary and principal axes

of 576-58o

support for trial lenses. . .651-654
unit of strength (diopter) .... 229
Lenses, cylindrical for eye defects 664
detecting strain in by polar-
zed light 626

for experiments in vision. 65 1-672

forms of 577

position in the triple and in
the double lens condenser

10, 13, 591-592

Oculists' trial 651

Leonardo da Vinci 674

Lettering, drawings 374

fastening the letters 375

size for drawings 376

Letters, a-k on specimen. . . .361, 387

on tissue paper 375

white on black ground 375

Levi Ben Gersen 674

Light from alternating current

arc 565

absorption by lenses and ob-
ject v 601-602, 620

amount from sun 138

amount with petroleum lamp 125
for demonstrations in vision

652, 670

from arc, cause of 546

from arc direct and alter-
nating current 554

of arc dependent on tempera-
ture 547

avoiding stray 380

in center of condenser face ... 44

7i8

OPTIC PROJECTION

Light, centered on objective

face 44

centering by objective hood 246
closing angle for screen

image 611

direct on screen 52

distribution of intensity . . 562-563
distribution in semi-diffuse

reflection 459

distribution from white screen

462-463

early sources 1 19, 686

energy, proportion of total

radiation 566, 569

flux 172, 613, 614

flux in opaque projection .... 172
from converging carbons . 564-565

from right-angled arc 564

in exhibition room. .112, 161, 441

increasing with sunlight 161

insufficient 51

on screen in

on screen, insufficient 301

parallelizing 273, 276, 335

projection with feeble 120

red near exits 443

reflection with various screens

249, 250

relative amount with trans-
parent and opaque projec-
tion 171

shield beyond objective .... 246
shield on the window frame

445-447

sun most brilliant. ... 138, 619
sources, arc lamp, oil, gas,
acetylens, lime and mazda
lamps ; sunlight
11, 79, 100, 119, 125, 127, 138

sources for spectra 627-628

source, parts of 537, 547

size and brilliancy of

screen image 610

stray, how to cut off 266

stray, vertical microscope. . . 268
stray, effect on screen image

443-444
turning on and off .20, 88, 1 10, 162

unequal on screen 308

weak for micro-projection ... 287

Light losses by, 601

absorption by lenses and

object 601-602, 619

polarization 603

reflection from lens surfaces

601-602
shutter in moving pictures . . . 603

sheet mica 608-609

small condenser or objective

601-602

substage condenser 618-620

Lighting, blackboard, 448-449

entire opaque object 187

indirect method 442

Lilliput lamp 81

Limes, for lime light 104

Lime, arranging 109

cracks in 115

pitting of 109

putting in place 108

rotating 109

warming 108

Lime light 100

burner 102

connectors 108

lamp for 107

lighting 106

management 106

micro-projection by 287

oxygen generator and ether

saturator 113

oxygen and illuminating gas 112

regulating flame 108

shield in

snaps out 114

starting 106, 1 10

summary 117

troubles 114

turning out no

Litmus polarity indicator 510

Long sight or hyperopia 66 1

Loos, A. J 673

Loss of light 601-609

Loss of energy 603-609

Lumen 613

McAllister 167

Mclntosh Bat. and Opt. Co. ... 123
Mclntosh Stereopticon Co. . .62, 301

Maddox 682

Magic disc 680

Magic lantern, 9, 68, 78, 100, 119, 138,
414, 584-587

acetylene 127

addition of micro-projection 309

advertizing form 435

American forms 59

INDEX

719

Magic Lantern, centering 39

Condenser with and without
spherical aberration . . 584-587

for small drawings 330

history 676-678

image formation with 584

inversion of the image 584

light source extended and a

point 586-589

simplest with arc and 2-lens

condenser 10

standard for projection

apparatus 9

wiring for 15, 71, 82, 504, 512

attachment for drawing 327

automatic lamp, inclined car-
bons 61

optical bench and ordinary

microscope 262

drawing accessories 329, 330

drawing surface horizontal. . 331

hinged baseboard 450

inclined carbons 70, 543-548

light too far off 54

light too near 54

radiant below the axis 53

lamp with microscope. . 328, 336

large .source 120

lathe-bed form 3 -lens con-
denser, water-cell 60

lime light 100, 101

mantle, gas lamp 125

mazda lamp 90, 9 1

microscope 265, 365

moving pictures 587, 591

Nernst lamp 92, 95

opaque objects 173

ordinary miscrocope 263

petroleum lamp 121, 122

small arc 85, 86, 90

small current 80

sunlight 138, 139

three-lens condenser 13

two-lens condenser 10

two-lens condenser, hand-fed

lamp 62

with direct current 9

with rods and microscope. . . 264
Magnification, with,

micro-projection 225, 257

opaque projection 181

opaque lantern 356

wall diagrams 355

Magnification, various objective

and screen distances . . . .257-261
Magnesium oxide as standard

screen 462

Magic lantern with weak lights 119
amplifiers and oculars

258, 262, 598-601
Magnification

181, 228, 257-262, 357, 369

actual 257-261

calculated 261

due to the amplifier 262, 600

due to the ocular 232, 262

how calculated 261

how found 260, 353

law for 357-369

varying 351

Management of apparatus for

projection, 24, 73, 92, 106, 123,
127, 414, 433

Mantle, gas lamp 125

inverted 126

position 125

upright 126

Masks, how to employ 253

kind and color 253

for microscopic slides 252

Masked sections and slides. .253, 270
Marcy's petroleum lamp .... 121-122

Marey 685-686

Mayer, Alfred M 138, 157, 159

Mazda lamp 90

summary for 98

wiring of 90

Measurement of electric quanti-
ties 478

Mechanism of cameragraph 410

for moving the film 406

moving picture machine

400, 402, 416

Mercury arc rectifier 490-491

Metallic screens 458-459

Meter, unit of length 318

candles 612-615

number for reading 612

Method of striae 647-650

Metric rule 27, 318, 371

system 318

Micrometer ocular 232, 233

Micrometer ocular, filar 234

Micrometers by photography . . . 353

Micro-planar 224, 225

Micro-projection, 221, 223, 236, 262,
267, 284, 286-287, 296, 313, 344,
385, 598,615-620

720

OPTIC PROJECTION

Micro-projection, addition to the

magic lantern 307-309

advantages 223

alternating current 284

apparatus 221, 319

centering 246

college, bench lantern 301

condenser for 237

conditions for good 309

direct current arc lamp 236

illumination for 236, 248, 271, 284,
286-287, 305, 328, 335, 339, 598,
616-620

house-circuit 285

lime light 287

objectives 224-226

ordinary microscope 262

outfit for photography . .382, 386

room 235

screen 235

size of image 470

size of screen 470

size of specimens 224, 255

summary 313

sunlight 286

troubles with 301

weak lights for 287

Microscope with amplifier. . .238, 600
for drawing on horizontal

surface 344

drawing with inclined 345

focusing device 241

house circuit drawing outfit . . 343
getting light through with

concave mirror 347

magic lantern lamp and con-
denser 336

objectives, objects to project

with 269

photo-micrography 385

plane mirror 34^

position for drawing 345

projection and drawing 275

projection with ordinary. 262, 263

projection with vertical 267

slide, masks for 252

specimens, projection of 269

tube 241

size and image 243

vertical with high powers. ... 283

vertical, stray light 268

without ocular or substage

condenser 334

Microsummar 225

Microtessar 225~579

Milliet de Chales 676, 679

Mirror 574

at end of clock-shaft (helios-

tat) 152,153

concave 573

convex 575

reflection 572-573

attached to drawing table ... 324

concave, as reflector 127

concave, without substage

condenser 349

45 degree with vertical objec-
tive 46, 267

to illuminate opaque object . . 1 75
light through microscope with

concave 347

with plane 346

on drawing table 324

parallel to clock-shaft (helios-

tat) • 154

position at equator, for helio-

stat I58

position at poles for a helicstat 1 58
position and time of day with

the heliostat 144

screen 459

silvered on face with dim light 331

to get erect image 190, 361

to reflect image to drawing

surface 322> 337

Mirroroide 459

Misframe 434

Mixed jet 105, 107

Models, drawing for 357

Moler, Geo. S 218, 219

Molyneux 677

Motion of picture film 419

Motion Picture Screen Co 459

Motor-generator set 489

Mounting, condenser 17, 62

lantern slides 216

low power objectives 241

Movement of lamp at its limit . . 47
intermittent with camera-
graph 41?

Moving pictures,

390, 409, 435, 468, 570, 591, ooo

apparatus for 39°

apparatus, getting in correct

position 412

development of the art ... 391 , 680
and education 394

INDEX

721

Moving Pictures, energy required

for projection 570

at home 435

history 680-686

illumination and optics

409, 591, 593-598

image formation 594~598

in science 394~395

size of image and screen 468

summary and troubles. . .436, 438

Moving picture and magic lan-
tern projection 587, 591

machine. . .400-408, 410-418, 435

inside shutter for 406-407

installing 407

mechanism of 415-417

objective for 406, 412

operating room for 396-397

operator for 390

optics of 409, 591

phonograph with 393

position in the operating

room 396

principle of 416

Moving Picture World

101, 207, 390-391, 397, 4«6

Multiple lanterns, composition of 34

dissolving views 35

effects with 35

use of 34

wiring for 34

Municipal regulations for wiring 499

Muybridge 393, 681-684

Myopia or short sight 660

Nash, A. E., painting screens. . . 456
National Board of Fire Under-
writers 499

National Electric Code

499-500, 502, 515, 528

National X-Ray Reflector Co ... 442

Negative carbon, 12-13, 222> 25O,

507-508, 537-538, 547-549, 551-

552, 640

Negative photographic for lan-
tern slide 208, 211

making with projection ap-
paratus 384

Nelson 274

Nernst lamp 92, 93, 94

automatic 92

for drawing 323

for spectra 628

summary 98

Nernst Lamp, wiring alternating

current 93

wiring direct current 93

Neuhauss 9, 202, 677

Newton and chromatic aberra-
tion . 583

composition of white light. . . 391
experiments with the

spectrum 6?7

& Co., lanterns and lantern

slides 124

Nicholas Power Co.,

391, 402, 404, 410, 417

Nichols, physics 521

Nicol prism 623

for analyzer 623-624

polarizer 623-624

Nicol prism spectacles 37

Normal vision, projection experi-
ments 652-659

apparatus for demonstrating

651-655

trial lenses for 651-655

lantern slides for 656

source of light for 652

Norris, electrical engineering . . 522

Norton 9

Nose-piece for objectives. . . .249, 250

Obliquity, avoidance of 41

Observation window in lamp-
house 72

Oculars, 230-234, 280, 337, 366, 369-
372, 600

compensation 232, 600

designation of 231

drawing, for 337

eye-lens of 366, 600-601

field lens of 366, 600

filar micrometer 234

image formation when used

366, 601

Huygenian 230

magnification by. . .232, 258-262

designation 231

for drawing 337

filar mictometer 234

micrometer 232, 233

position of image and course

of light 366, 601

projection .230, 232, 280, 366, 601

722

OPTIC PROJECTION

Oculist's trial lenses, 651, 655

where to be obtained 651

Ohm 477, 521

Ohmage 477

Ohmage by Ohm's law 521-522

Ohm's law 521

Oil lamp 121

Object, position on stage 257

for projection, objectives. . . . 268
projection when horizontal. . 268
size for opaque projection ... 179

visibility of 227

to draw with projection

microscope 333

erect image when vertical. . . 190

horizontal, image erect 190

how to light 335

illumination with main con-
denser 234

large, drawing with low

powers 333

for opaque projections 1 79

position with different ob-
jectives 256

Objective 18, 224, 226, 281, 609

aperture for opaque and

transparent objects 172

apparatus at rear of room . . . 464
apparatus not at rear of room

464, 468
brilliancy of the screen image 613

centering 247

demonstrations with high

powers 271

diaphragm of 380

distance from condenser 43, 247

drawing 335

focus and brightness of

screen 613

focus for room and screen . . 467

for different sized fields 254

formula for getting proper

focus 467

with hood for 245

immersion 225, 349

low power, mounting 241

magic lantern

18, 464, 597, 601, 609
microscope construction .... 616
microscope and image forma-
tion with 616

moving picture 406

painting black 246

Objective, photographic, con-
struction 225

for photography 384

for photographic printing. . . 379
proportioned to condenser 43, 277

revolving nose-piece for 249

screen limited in size by 469

shield for 30-31, 246, 293

to use with substage condens-
er 281

various fields 255

various objects 268-269

Old age sight or presbyopia 66 1

Opaque lantern for drawing. . . . 332
drawing on horizontal surface 332

one source 173

two sources 173

with sunlight 174

Opaque objects, bright images of 174

lighting entire 187

magnification and size of

image 181

projection, 166, 168, 183, 184, 187,
188, 191, 192, 196

screen and distance 181

size and kind of objects 179

summary 198

troubles 195

versus transparency. 1 68, 169, 170
Operating room, for moving pic-
tures 396-398, 453

construction (M. P. World) . . 397

permanent 396

temporary 396

Operation of moving picture

machine 420

Operator, competent 390

Optical bench 288-289, 294

deceptions 36

system for spectra 628, 641

Optics of moving pictures. . .409, 591

of projection, general 572

Orange G for staining masks .... 253
Oscillogram, anode currents. . . . 495
current supplied and de-
livered 493

potential and electromotive

force 494

rectifier 493~495

Oxodium 113

Oxone 113

Oxygen, cylinder 101

INDEX

Oxygen-ether 114

generator 113

use of 113

tank 102

Oxy-hydrogen flame 100

Oxylithe 113

Paint, amount for cloth screen . . 456
for blackening apparatus .... 245

for projection room 440

Painted cloth screen 457, 460

Painted wall screen 454

Paper, holding while printing. . . 379

Paper for masks 253

Paper for printing with projec-
tion 379

Paper screen 458

Parallel light, how to obtain. ... 273

Parallel, rheostats in 531

Parallelizing light 276, 335

Patching a film 428

Pen for slides •. 206, 207

Pennock, Edward 673

Pennsylvania Metallic Tubing

Co 108

Petroleum, amount to use 125

lamp 121, 123, 124

lamp, amount of oil 125

lamp, candle-power 125

chimney 122

condenser position 125

management 123

lamp for photo-micrography 386

position of flame 125

reflector 122

smoking 133

summary 136

Phenakistoscope or magic disc of

Plateau 680-681

Photographic booth 321

camera for drawing 332

enlargements with projec-
tion 378

lantern slides 200, 208

objectives for prints 379

for projection 224

objects to project with .... 268

paper with projection 379

room 320

Society, British 217

Photography apparatus 319

with projection apparatus

319,381

with Quartz spectrograph . . . 642

Photography, with spectra 643

summary 388

troubles 384

Photo-micrographic camera. . . . 382

microscope 385

vertical camera 384

Photo-Mineature Series 200

Physical Review 567-569

Physics, special projection ex-
periments in 622

Plaster Paris screen 454

Plate, exposing directly 381

Plateau and moving pictures . . . 680

Pointer for lecturer 271

Polarity determination 506-5 1 1

by arc lamp 506

with direct current. .496, 506-511

direct current ammeter 506

direct current voltmeter. . . . 508

incorrect 51, 507

indicating on wires 51, 508

indicators, chemical 510

Nernst lamp 95

right and wrong in arc lamp

51,507

small arc 84

test 51, 506-511

testing by ammeter 506

testing by voltmeter 508

Polarized attachment plugs. .87, 504

light 622-626

brightness of screen image 626
condensers and objectives

for 625

dark room needed for experi-
ments 626

detecting strain in lenses by . 626
projection experiments with

622-626

setting up the experiments. . 626
showing rings and brushes

with 625

small Nicol prism for 624

specimens for 623

use of piles of glass plates . 623-624

use of small prisms 624

wall socket 86, 503

Popular Science Monthly . . .321, 521
Porta, Baptista and the camera

obscura 674

Porte lumiere 140, 144

installation 140

with microscope 142

operation 143

724

OPTIC PROJECTION

Porte lumere, setting up 142

for south window 143

for west window 145

Porter, T. C 424-426

Position of Ballast 512-513

condenser and objective. . .43, 251
of filament (mazda lamp) ... 91
of moving picture machine . . 396

of object on stage 257

of object with various ob-
jectives 256

of switch 515-518

Post-card projector 183

Potato polarity indicator 511

Potential drop and arc length 536-539

and current 536

and electrodes 536

Potential, oscillogram, electro-
motive force supplied 494

Potter, A. O 177

Power at arc, wattmeter connec-
tions 482

Power, Nicholas Company

391,402,404,410,417
Power consumption, candle-
power 559, 561

factor 485

Precautions, alcohol lamp 133

ammeter, wiring 481-482

heavy currents 186, 513

lime light 116

in polarity tests 506

for wiring 498, 513

Precision Machine Co 406

Presbyopia, old age sight. . . .661-662

Pressure gauges 102, 103

gauge, acetylene tank 128

of one atmosphere 103

reducing valve 102

Prestolite tanks 127, 128, 129

Principal axis 577~58i

focus and how obtained . . 578-579

Print, size for lantern slides 208

Printing, artificial light 214

on paper or a dry plate,

directly 381

exposure for 380

how to hold paper 379

lantern slides 209

lantern slides with camera. . . 213

paper to use 379

Prints, enlarged from negatives. 378

white on black 213

Prism, 37, 46, 190, 196, 322, 337, 344,
368, 629-632, 640-643

erecting 368

direct vision 627, 631

drawing 322, 337, 344

glass 627, 629-630

hollow for liquids. . .627, 631-632

polarizing 622-626

quartz for ultra-violet 641

right-angled with vertical

objective 46

Rutherford's 630-631

spectacles of 37

Prism, Nicol for polarizing or

analyzing light. . .622, 624, 626
Proceedings of the American
Association for the Ad-
vancement of Science ..211,321
Proceedings of the International

Electrical Congress 566

of the American Microscop-
ical Society 320

of the Royal Society 426

Projection apparatus, with mir-
ror on lecture table. . . .451-452

outside the room 453

for photographic enlarge-
ments 378

rules for construction 6

aid to drawing 319

apparatus, best position in

room 446

early use with drawing. . . . 372

form for drawing 321

home-made 287

similarity of all 221

from three standpoints 3

universal 178, 185, 191

of human face 1 70

horizontal objects

33, 185, 190, 268
illumination in high power. . . 271

in physics 621-651

material and apparatus for

622, 626-627, 643, 644, 651

of spectra 626-642

special experiments in 622

with polarized light 622-626

with ultra-violet light

627, 641-642

microscope, 178, 221, 222, 271,
282, 299, 300, 303, 309, 322, 598
60 1
microscope with amplifier

238, 366, 599-600

drawing with 333

erect images with 367-373

INDEX

725

Projection apparatus, exhibition 270

on house circuit 234, 285

illumination for, 237, 247, 251,
271-282, 328, 335, 347, 598, 601

limitations 223-224

on market 296

solar 142

microscope 221, 234, 249, 271

use 249

defective vision demonstra-
tions 659-672

moving pictures

409, 420, 433, 593

microscopic specimens 269

multiple lanterns 34

normal vision 652-659

objective 18, 224, 464

shield for 292

objectives, micro-projection

224-226, 244, 263
objective for moving pictures

406, 412

ocular 230-234, 280, 366, 600

optics of 572

position of carbons for
49-50, 249, 342, 415, 507, 537-550
room, form and darkness . 440-444

specimens for 252, 268

table 287

with drawing shelf 325

with vertical microscope. . . . 267

with weak lights 120

Projectors for home use 435

Projectoscope combination. .185-186

universal 302

Proportion of gases for lime light 108

Pseudoscopic effect 39

"Push-through" slide carrier. ... 23
Pupil for eye experiments . . . 65 1 , 669

effect of size in vision 669

Ptolemy, phenomena of refrac-
tion 576

Quartz optical system for ultra

violet 641

spectrograph for ultra violet 642
system for spectra of ultra

violet 641

Queen & Co 166

Radial lines for showing eye

defects 662-667

Radiant or light source, 1 1, 68, 78, 90,

92, IOO, 113, 119, 121, 125, 127,

130, 138, 553

Radiant, arrangement and cen-
tering 39-46, 56

distance from condenser. ... 41

for drawing 328, 329

Radiant efficiency 567

and mirror. 189

energy for moving pictures
with alternating current. . 570

direct current 571

for opaque projection 1 74

position and illumination of

condenser 42

position of in opaque projec-
tion 174

tilted 188-196

Radiation, light energy of. . .566, 569

visible and invisible 566

Ramsden's Circle or disc 230

type ocular 234

Reactor 532

Read, E. A 673

Rectifier for alternating current 489

Red lights near exits 443

Reducing valve for gas cylinders 102

Reflection 572

irregular 572~573

law of 140, 572

losses by 175, 602

metallic surfaced screens. . 458

mirror 573

regular 572

semi-regular 573~574

of various screens. .460, 463, 465
visibility of reflected beam. . 573

Reflector, acetylene lamp 127

gas lamp 126

in opaque projection 175

petroleum lamp 122

Reflectoscope 180

Refraction 575~576

at plane surfaces 575

at curved surfaces 575

air to glass 575

air to water 575

density and refraction 576

index of 576

law of sines 576

Ptolemy's laws of refraction 576
Snell and Descartes law of

sines 576

Refractive eye defects 659-672

Regulations, fire underwriters,

for wiring 499~5O5

Reichert 82, 83, 93

726

OPTIC PROJECTION

Reinhold, use of camera obscura

in eclipses 674

Requirements for good projec-
tion

7, 18, 222-223, 313-317, 433
for projection with weak

lights 120

Resistance, to control current . . . 539
how to get amount needed ... 521

unit of 477

Retouching frame 29, 203

Rewinding moving picture film . . 430

Reynolds 374

Rheostat 521-531

adjustable 263, 326, 523-524

alternating current 69, 532

barrel, saltwater 525-526

calibration of 525

direct current 1 1 , 521

energy used 533

fixed 523

heat developed in 523

home-made 526-530

installing 84, 339, 504, 524

in parallel 531

on 1 10 volt line 542

range of 263, 326, 523-524

salt water 525-526

series 531

tin strips 529-53°

water cooled 527-528

wiring 504, 524

Richardson's Motion Picture
Handbook . *..i ^

390-391,397,420,439

Riley, Dr.. 327, 331

Ripolin white paint 264

Roaring of alcohol lamp 134

Roaring of lime light 115

Rods for optical bench 290

Roller screens 456

Rooms, for projection 439

Room, apparatus on lecture table

451-452

darkening 443

darkness for opaque projec-
tion 175

for drawing and photography 321

for micro-projection 235

for moving pictures 395

form for projection 440

light in projection 441, 443

limiting size of screen 469

Room, size with sunlight 162

tint and decoration 440

troubles with 471

Salt and water polarity indicator 511
water rheostat 525-526

Scale, diagrams with opaque lan-
tern 356

wall diagrams 355

Scheiner 361 , 679

demonstration of retinal

image 656

theory of accommodation 658

Oculus 368

Science 2 1 1 , 327

Scientific American 221

Schmidt & Haensch 102, 126

Screens 454-466

anthracene 627, 635-642

Screen, in alcove 443

cloth 457

dark spots on 309

distance 464-47 1

distance, for high powers. ... 282
distance, micro-projection

225,282
for opaque projection. . . 181, 454

for high powers 272, 282

hinged to meet axial ray 449

image, actual size 257

with amplifiers and oculars 258
perfection and brilliancy

18,612-617
image, not photographically

sharp 256

with polarized light 626

size of 464

spectra 627, 635, 642

stereoscopic 37

with vision experiments 654

not light enough 301

with metallic surface. . . .458-463

micro-projection 235, 454

moving pictures 395, 454

painted cloth 455

painted on wall 454

paper 45$

plaster of Paris 454

qualities of goods. . . 454

reflection 460-465

reflection, semi-diffuse 459

reflection, white . . 246, 460, 462

INDEX

727

Screen, roller 456

size 464

size dependent on distance. . 466

size for lantern slides 464

size limited by room 469

size for micro-projection. . . . 470
size for moving pictures. .395, 468

size for petroleum lamp 125

size for sunlight 162

tipped to meet axial ray. . 449, 452

translucent 461

travelling 458

troubles 471

white washed wall -455

Screws and nuts, thumb 296

Search-light, use in opaque pro-
jection 187

Sections, masked 253

Separable, attachments . . 87, 503-504

extension connector 87

wall receptacles 86, 503

Serial sections, masked 253

Series, rheostats in 531

Shadow projection with the lan-
tern 62 1

Shadows, alternating current

lantern 75

avoidance of 190

on screen 53

Shedd , Dr. on history of Ohm's law 521

Shellac for sizing 456

Shield, asbestos paper for 266

beyond objective 246

for drawing in dark room .... 351
for drawing in light room .... 351

objective 31, in, 292

on condenser tube 349

to hold objective 293

Shock, electric, if on damp floor 59

Short sight or myopia 660

Short circuit 47, 496

Shunt with ammeter 479

generator 487-488

adaptibility for arc lamp . 487-488

connected to arc lamp 488

Shutter, automatic fire 420

Shutters for darkening room .... 446
on moving picture machine

406-408, 410, 680
early use, by Heyl and Muy-

bridge 680-682

best position 422

on moving picture machine
inside 407

Shutters, number of wings 425

outside 408, 410

setting or "timing" 422

speed to prevent flicker . .423-427
Size of carbons for house system

87-88, 285, 341

of condenser for drawings. . . 331
of drawing, how to obtain . . . 329
of screen

125, 162, 395, 464, 468-470
Sizing, and amount for cloth

screen 456

Slide box 271

cabinet 219

carrier, individual 31,32

lantern 23

on moving picture machine. . 404

push-through form 23

masks and masking. 2 52-2 53, 387
thickness, and position of sub-
stage condenser 281, 337

tray 270

Slip-slides 36

Slit for spectra 626, 629-630

for Abbe diffraction experi-
ments 644

home-made slit 630

illumination of slit 636

Smoke to show the path of light

rays 247, 582

Snap switch 515, 518

Snell, mathematical law of re-
fraction 576

Sockets for apparatus 292

Socket, railing flange and 293

separable attachments. . . .87, 504

wall 86,503

Sodium peroxide 113

Sodium tungstate for fireproof-
ing cloth 321

Source of light for projection

n, 68, 78, 90, 92, 100, 113, 119,
I2i, 125, 127, 130, 138, 553
of light in demonstrations in
normal and defective vision

652, 670
Specimen, effect of heat on. .252, 607

for high powers 283

for high power drawing 338

for projection 224, 252

Spectacle makers for supplying

trial lenses 651

Spectra 626-643

728

OFTIC PROJECTION

Spectra, absorption and substan-
ces for 637-638

apparatus and material for, 626,643

arc lamp for 639

chemicals to use with 627, 637, 640

comparison spectra 638

current to use for emission

spectra 639

electrodes and stuffed elect-
rodes 627, 638

emission spectra 638-640

gratings for 627, 632-635

illumination of the slit for. . . 636

optical system for 628

optical quartz system for. 62 7-641

photography of 643

prisms for 627, 631-632, 642

quartz optical system. . . .627, 641
screens, white and anthracene

for 627,635, 642

slit for 626, 629-630

substance to use for absorp-
tion spectra 637-638

for emission spectra 640

ultra-violet 641-642

Spectrograph 642

Spectrographic camera 643

Specimen, stage for 239-240

Spectacles for correcting eye de-
fects 659-672

prism and colored for stereo-
scopic screen images 37~3^

Spectrum, showing visible radia-
tion 566

Speed of shutter to prevent flick-
er .422-427

of moving picture machine. . 420

Spencer Lens Co., 28, 63, 8 1, 190. 193-

196, 233,241,271,309-312,354,

355

Spherical and chromatic aberra-
tion 580-583

Splicing moving picture film .... 428

Spots on screen 309

Spotting lantern slides

19, 26, 29, 201, 203, 216

Stage cooling device 239, 240

Stage, mechanical 239, 241, 242

in micro-projection 239-240

water-cell 239, 240, 608

Stain for tables 289

Stampfer's magic disc 680

Standard, aperture 571

Standard, material for installing

arc lamp 502

Stanley, Geo. C 375

Stanton, Thecd >re 673

Starch polarity indicator 510

Starrett (L. S. Starrett Co.) 295

Stenopaeic slit 651-670

Stereoscopic screen images. . . .37-38

Stimson Hall 155

C. H. Stoelting Co. .33, 141, 185, 302

Storing lantern slides 218

Straite, inventor of automatic arc

lamp ooo

Stray light, avoiding 380

Striae, method of demonstrating

647-650

Style Brief, Wistar Institute. ... 374
Substage condenser

232,271,347,619,626

achromatic 282, 626

aperture of 278

centering 280

and concave mirror 348

Kohler method with 278, 619

objectives with 281

plane mirror with 348

condenser, position 337

position depending on slide. . 281
position with different ob-
jectives 282

position with parallelizing lens 277
reduction of brillaincy by

235, 618-620

Summaries of the different chap-
ters: I, 64; II, 76; III, 97;
IV, 117; V, 135; VI, 164;
VII, 198; VIII, 220; IX, 313;
X, 388; XI, 438; XII, 472
Summary, alternating current. . 76

direct current lanterns 64

drawing and photography ... 388

heliostats and sunlight 164

house circuit 97

lantern-slide making 220

lime light 117

micro-projection 313

moving pictures 438

oil, acetylene, alcohol 135

opaque projection 198

rooms and screens 472

sunlight 164

weak lights 135

Sunlight for opaque projection. . 174

INDEX

729

Sunlight, for spectra 628

troubles with 162

Support for magic lantern 80

Sun, apparent positions 163

intrinsic brilliancy 138, 613

with projection microscope. . 286
temperature 547

Sunlight, apparatus for 138

condenser for 161

for micro-projection 286

for spectra 628

Switch for opening and closing

the electric circuit 514-518

closed and open 515

double pole 12, 70

double pole double throw

knife switch 297

enclosed 517

end of knife switch next sup-
ply 516-517

for all the wires of a circuit .. 514

installation of 515

knife switch 515

lamp or table switch

12, 15,71,238,263.288

open and closed 515

position of 516-517

snap switch 515

table or lamp switch

12, 15,71,238

Table for drawing with attached

mirror 323-324

for projection 287

for projection with drawing

shelf 325

stain for 289

stand for moving picture

machine 401

switch for opening and clos-
ing electric circuit

12, 15,71,238

Talbot, F. A 391, 439

Temperature, absolute 547

of crater, of arc lamp 546

of sun 547

Tent of cloth for drawing 350

for drawing 1 66

Terminals, carbon 12, 539

Tests of polarity 506-51 1

Theory of flicker 426

Thompson, A. T. & Co

80, 180, 191, 286, 303, 334, 687
Silvanus P . History of arc lamp 686

Thorium discs 101

Three-lens condenser. . . .61, 587-592
Three-wire automatic lamp . . 238, 263

Thumb screws and nuts 296

Tin for rheostat 530

Tinted glass in combined projec-
tion 177

Tint for projection room 440

Toepler's method of striae . . . 647-650

"Schlieren-Methode" 647

Tracing pictures 374

Track 290

fixing to baseboard 290

for optical bench 290

Transformer 533-535

with arc lamp 534

wiring 532

Translucent screen. . . .453, 461-462
Transparency and opaque pro-
jection 168-170

Troubles in the various chapters:

I, 46-59; II, 74-75; HI, 96;
IV, 114-116; V, i33-!34;
VI, 162-163; VII, 195-197;
VIII, 219; IX, 301-309; X,
384-388; XI, 436-437: XII,

471

alternating current 74

direct current magic lantern 46
drawing and photography . . . 384

house circuit 96

lime light 114

making lantern slides 219

micro-projection 301

moving pictures 436

opaque projections 195

rooms and screens 471

sunlight 162

weak lights, oil, gas, acety-
lene 133

Trutat 202, 621

Tube of miscroscope 241

Tubing, metallic 108

for optical bench 290

Tungsten lamp for spectra 628

Twilight vision 121, 175, 444

use with weak lights 121

Two-lens condenser 62, 587-591

Tyndall 621

Uchatius, moving picture projec-
tion (1853) 680,683

Ultra-violet, anthracene screen

for 627, 635, 642

730

OPTIC PROJECTION

Ultra-violet, quartz optical sys-
tem for 641-642

radiation 567, 641-642

projection of 640- 642

United States Geological Survey 147
Units of alternating current . 484-486

Units of direct current 475-483

Units of electricity. . 475, 484

University of Pennsylvania and

animal movement 682

Unpacking moving picture ma-
chine 401

Uranium arc for spectra 628

when in the positive, and
when in the negative elec-
trode 640

Valspar varnish for water cells

205, 264

Varnishing lantern slides 205

Vertical microscope for high

powers 283

Vertical knife switch 516

Vertical projection 33

Violet lines in arc stream 546

Visibility of objects 227

Vision, daylight and twilight

121, 175, 444
Vision, experiments with normal

651-659

defective 659-672

persistence of in moving pic-
tures

Visual angle 228

Voigtlander und Sohn 226, 250

Volt and voltage 476, 484

Voltage, alternating current .... 484

by Ohm's law 521

intermediate 544

of line and arc 544

of line, voltmeter connections 475

not present in the line 46

Voltmeter, connection for arc

voltage 476

connection to circuit 478

direct current 478

for line voltage 475

for polarity testing 508

soft cored 510

Walgensten. 3, 675-677

Wall diagrams 329, 355

Wall receptacle 86, 503

Water-cell, 13, 15, 18, 57, 63, 71, 161,
194, 222-223, 237, 239-240,
264-266, 276-277, 281-282,
287, 299, 301, 309, 355, 365,
421, 432, 571, 592, 604-609
absorption of radiant heat not
affected by temperature . . . 606

effect of alum, etc. in 607

efficiency 568-569

light and energy absorbed by

604-607

in micro-projection 237

the microscope stage in 608

how to make 264

need in moving pictures . .421, 432
with ordinary microscope. . . 266
position in micro-projec-
tion 239, 240

shadow from 57

stage 239-240

sunlight 161

sun and micro-projection .... 287
with two-lens condenser. . 365, 421

Water cooled rheostat 527

Waterhouse, Gen. J 673

Watt, and wattmeter. 477, 482-483

Watts, alternating current 485

equal volts times amperes

477, 521

Wattmeter 482-483

amperage by 483

connecting 482

power at arc by 482

power from dynamo 483

White, Dr. A. C 673

White-washed wall screen 455

Wiedemann's Annalen 647

Williams, Brown & Earle

59, 91, 107, 130, 131, 182, 183, 300

Wimmer 9, 202

Winding and rewinding moving

picture film 430

Widow in lamp -house

72, 267, 402-403
Window shades for darkening

the room 445-447

Wires for the electric circuit . 496-505

how to connect 503

insulation of 497-498

wrong polarity 51, 506-511

Wiring the arc lamp, 10, 13, 15, 71,
84, 263, 324, 339-340. 399, 496-
535, 639

INDEX

731

depending on amperage 496 small arc lamp 82, 84

for large currents 513 Wistar Institute 320, 374

for automatic lamp 512 Woodward, use of lime light for

for moving pictures 399 projection (1824) 686

for multiple lanterns 34, 297 Wright, Lewis

for a distant supply 513 9, 105, 207, 243, 245, 256, 282

inductor and transformer. . . 532

mazda lamp 90 Zahn, early projection apparatus 679

municipal regulations for .... 499 Zeiss, 176, 181, 187, 221, 225, 231,

Nernst lamp alternating cur- 232, 250, 272, 281, 382, 385, 386

rent 93 Zeitschrift fur Instr 221

Nernst lamp, direct current. . 94 Zeitschrift fur wissenschaftliche

on three- wire supply 514 Mikroskopie 221

regulations for 498-501 Zirconium discs. .' 100

rheostat 504 Zoetrope and magic disc .... 680-682

186

DUE DATE:

' SAMUE

JAN 2 9 1992

Fines increase
50tf per day
effective
September 3,

Gage, Simon Henry
Optic projection

P&A Sci.

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Full text of "Optic projection : principles, installation and use of the magic lantern, projection microscope, reflecting lantern, moving picture machine, fully illustrated with plates and with over 400 text-figures"

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