COLLEGE "OF "DENTISTRY

OF CALIFORNIA j

OPTICAL PROJECTION

OPTICAL PROJECTION

A TREATISE ON THE

USE OF THE LANTERN

IN EXHIBITION AND SCIENTIFIC DEMONSTRATION

BY

LEWIS WEIGHT

AUTHOR OF ' LIGHT : A COURSE OF EXPERIMENTAL OPTICS '

WITH 243 ILLUSTRATIONS

FOURTH EDITION
FIFTH IMPRESSION

REVISED BY RUSSELL S. WRIGHT

LONGMANS, GKEEN, AND CO,

39 PATERNOSTER ROW, LONDON
NEW YORK AND BOMBAY

1906

•R

All rights reserved

PREFACE

A.BOUT the year 1851 there was placed at my disposal a
Lantern, of a character very unusual (at that time) for
any boy of my age to possess a share in ; in fact it was
one of Carpenter and Westley's * Phantasmagoria '
lanterns, unrivalled at that period of transparent sheets
and sperm -oil. The result was that Optical Projection, in
its various forms, has been with me more or less a hobby
ever since ; less followed for some years during which
pursuits of a more open-air character,1 for sufficiently
serious reasons, engaged more of my attention, but
never abandoned, and again and again returned to with
renewed interest. Mere slides of course came first, but
I soon discovered for myself that things could be pro-
jected as well as pictures ; and for a long time past I
have found much relaxation from my literary work, in
reducing various optical and other physical experiments
to conditions which enable an audience to behold them
upon a screen, and in devising contrivances for making
the beautiful phenomena of Polarised Light more spec-
tacularly imposing. Since the publication of a little

1 Poultry-breeding.

iv OPTICAL PROJECTION

work treating Physical Optics from this point of view,
ample evidence has been furnished that others have felt
strongly the same fascination as myself in this class of
experiments.

The preparation of the following pages, as a treatise
upon the more general use and manipulation of the
Lantern through its entire range, in a manner that may
suggest proper arrangements even where want of space
or my own lack of detailed experience may prevent specific
treatment, was not spontaneous on my part, but waa
suggested to me by Mr. Herbert C. Newton. During
years past I have spent very many hours associated
with him — many of them actually in his company-
engaged in contriving, perfecting, testing, or adjusting
scientific optical apparatus ; often for myself, but more
frequently for the use of others, and especially of
colleges and public institutions. He was good enough
to suggest on many occasions that a practical treatise
from my pen, and embodying the results of my experience,
would be of service ; in fact he has urged the task upon
me with some persistence. Whether or not I have been
able to add anything of value to such handbooks as were
previously obtainable, must be left to the reader's judg-
ment. But that is the real origin of this work; and
it is best to state it frankly, because it will naturally
account for such description as will be found hereafter
of apparatus — microscopic and polariscopic in particular
—which was worked out to the best of my ability with
a primary view to being constructed by the firm which
Mr. Newton represents, from whom I have received

PREFACE v

much kindness and assiduous care in my own personal
requirements of this kind. Such apparatus, to express
it briefly, is described here not as their apparatus, but
as being in greater or less degree my apparatus, by
contrivance, or selection, or modification, or personal
connection with it in some way or other, even when not
(as frequently the case) planned in the first instance for
my own use. ,

But while it is best to state this quite simply, it will
I trust be found that, beyond what thus became
unavoidable as forming part of that very experience
which is the basis of the whole, the subject has been
treated so as to be of most use to all, and without
any prejudice to other optical workshops of well-known
character with which I do not happen to have been
brought into the same personal contact. Except a very
few articles or details which may be patented in various
quarters, and which are well known to the trade and
readily obtainable from the various manufacturers, the
apparatus and arrangements here described are free to
all. It is perfectly open to any reader to have them
constructed by whom he chooses, with any improvements
he can suggest ; while it is equally open to any optician
to construct them as excellently, and sell them as
cheaply — or the other way if he prefers — as he possibly
can.

I need only add to this explanation, that in the
following pages the first person singular has been pur-
posely adopted, as more simple and less really egotistic in
a book of such necessarily strong individuality, than any

vi OPTICAL PROJECTION

other style would have been ; and that the frequent use of
italics is not for emphasis in the usual sense, but chiefly
as the easiest way of marking points it is desirable to
distinguish with some clearness.

I have to thank Messrs. Macmillan & Co. for permis-
sion to use the greater part of the illustrations to my
previous work, entitled ' Light : a Course of Experimental
Optics,' and also the publishers of Professor Weinhold's
' Physikalische Demonstrationen ' (Leipzig) and Dr.
Stein's ' Die Optische Projektionskunst ' (Halle) for many
illustrations from those works. Most of the others are
original ; but a few have also been taken from Ganot's
' Elements of Physics,' Prof. Forbes's ' Lectures on
Electricity,' some articles by Mr. G. M. Hopkins in the
' Scientific American,' and Prof. Dolbear's * Art of Pro-
jecting' (Boston). The very few experiments taken or
adapted from the latter work, are owing to the fact that
it is written with especial reference to projections with
the heliostat, which is almost useless in this country.
Many such arrangements I have found unsuitable for
ordinary lantern use ; but where sunlight is available,
Prof. Dolbear's treatise may be consulted with advan-
tage.

My grateful acknowledgments are finally due to my
old friend and correspondent, the Eev. P. E. Sleeman,
for reading the proofs of the last twelve experimental
chapters, during which process several suggestions and
additions of value have also found their way into those
pages.
LONDON : October 31, 1890.

PBEFACE

TO

THE FOUKTH EDITION

THE third edition of this book being exhausted, it was
thought advisable to make some revision and to write a
fresh Appendix, with a view to bringing it up to date.

The Author was accordingly preparing to take the
work in hand, and had already discussed with me as to the
main points of the alterations, when, on December 16,
1905, his energetic and useful life was cut short by a
fatal railway accident at Saltford, in Somerset.

Many and widespread have been the expressions of
sympathy with his family in their bereavement. From
literally all over the world have come letters testifying
to the respect in which he was held and to the loss
which workers in many branches of science feel at his
death.

To me, his son and the companion and sharer in
much of his experimental work, the loss is not only that of
a revered and honoured father, but also that of a guide
and adviser in much of my own scientific work.

a

viii OPTICAL PROJECTION

It is under these sad circumstances that I have
taken up the pen in order that, as nearly as possible,
the fourth edition should be presented to the public as
it would have been had the Author been spared to
revise it.

So far as possible I have preferred to leave my
father's original work untouched, a few paragraphs
slightly altered and a footnote here and there comprise
all the alterations that have been made, and the bulk
of the work has consisted in writing a new and fairly
voluminous Appendix describing many of the more
modern optical instruments. In doing so I have
followed, so far as possible, my father's plan of including
no instruments or experiments that I cannot speak of
from practical experience.

With hardly an exception every paragraph in the
new Appendix has been written from my own personal
knowledge. In a work of this class, dealing as it does
with the easiest and most convenient way of handling
apparatus, to do otherwise would be dangerous and
unwise, and this must be my apology if omission has
been made of any new appliances which should have
been included.

KUSSELL S. WEIGHT.

June 1906.

CONTENTS

C HAPTER

PAG a

L

1

II.

THE PARTS OF A LANTERN

. . 13

III.

THE EADIANT

. 34

IV.

THE LIME-LIGHT

. . 42

V.

PREPARATION OF GASES

. 73

VI.

COMPRESSED GASES

, . 82

VII.

THE OXY-ETHER AND OXY-CARBON LlGHTS .

. 91

VIII.

LANTERNS AND THEIR MANIPULATION

. . 101

IX.

SCREENS AND OTHER LANTERN ACCESSORIES .

. 124

X.

SLIDES, CARRIERS, AND EFFECTS

. . 136

XI.

ACCESSORY INSTRUMENTS .

145

XII.

APPARATUS FOR SCIENTIFIC DEMONSTRATION

. . 153

XIII.

THE PROJECTION MICROSCOPE

. 179

XIV.

DEMONSTRATIONS OF APPARATUS, AND IN

ME-

CHANICAL AND MOLECULAR PHYSICS .

. . 212

XV.

PHYSIOLOGICAL DEMONSTRATION

. 230

XVI.

. 241

XVII.

SOUND

. 247

XVIII. LIGHT : EEFLECTION, EEFRACTION, DISPERSION,

AND COLOUR 274

XIX. THE SPECTRUM 304

XX. -INTERFERENCE OF LIGHT 321

XXI. LANTERN POLARISING APPARATUS .... 338
XXII. POLARISED LIGHT 349

XXIII. HEAT 385

XXIV. MAGNETISM AND ELECTRICITY 394

XXV. SCIENTIFIC DIAGRAMS 411

APPENDIX ...,..,.. 417
INDEX ..... . 441

NOTE

For the most recent information respecting
various matters connected with Optical Projection,
see Appendix at the end of the book.

OPTICAL PEOJEGTION

CHAPTER I

ON PROJECTION

1. Definition. — Projection, in the optical sense and in
this book, is the production of a picture or image — usually a
magnified image — upon some kind of screen or visible plane,
generally by means of a lens. The image may be that of a
picture, or of some solid object or piece of apparatus. It is
manifest that the excellence of the projection — that is, the
brightness and distinctness of the picture on the screen —
must depend upon many practical conditions, which are to
be treated of in these pages. But first of all it is essential
that the nature of the problem itself be really understood, if
the best result is to be obtained with the apparatus at our
disposal.

2. Image not formed by the Lens. — The reader would
probably gather from most of the treatises upon optics, that
the image on the screen which he has to produce is formed by
the lens ; but that is not true, except in a very secondary sense.
The first and cardinal truth for an intelligently successful
demonstrator to grasp, is the fact that all power of really
forming ' an image resides in those rays of light themselves
by which any object is visible to us.

Starting with, and here taking for granted without need-

2 OPTICAL PROJECTION

less discussion, the almost self-evident fact that objects are
visible by means of rays of light which they either emit of
themselves (as in a gas -flame) or reflect back from some other
luminous source (as in an object lighted up by the gas-flame), all
we need assume here is that these rays are sent out in straight
lines, in all directions which are open in space, continuing to
travel in straight lines so long as they traverse the same
medium, as the air, for instance. This is a familiar fact of
experience, as shown by sunbeams or rays from the lantern.
Now it is the fact that every ray which thus proceeds from
any point of any object, really forms an image of that point
of the object upon any surface on which it falls; and it
should be clearly understood that if the mere bare rays of
light, by themselves alone, had not this power of forming
images which is here affirmed, all the lenses in the world
could never do it. Not being very self-evident, this fact
should be realised by experiment.

3. All Rays form Images. — As the reader will possess a
lantern of some sort, this will afford the readiest demonstra-
tion. Place a slide in the stage, choosing one which has
some well-marked and large features, and is tolerably trans-
parent in the rest, and throw the image on the screen as
usual ; such as is seen every day, and which is supposed to be
formed by the lens in front of the lantern. We now take
that lens out, and there certainly appears to be no image upon
the screen, though the rays from the illuminated slide stream
out to it in plenty, and the screen is lighted up well enough,
and there may be signs of colour if it is a coloured slide.
But let us consider a moment. The rays of light can go now
from every point of the slide alike, to any one point on the
screen ; therefore what we see now at any point on the screen
will be the total of these superposed images, of all the points
in the slide, from which no one stands out particularly. But
we can stop all that easily. Cover over the empty brass front
with a sheet of tinfoil, and in the centre of this prick a hole

ON PROJECTION

with a large pin. It is different directly. Because all the rays
are straight lines, only the small bundle of rays which the
pinhole allows to pass from one point of the slide, can now
get to any one point on the screen, and no others can get
there. Simply to secure this is all that has been done ; but
now it is quite another story, and it will be seen that the bare
rays of light, without any lens, do form an image of the slide
plainly enough.

It will be found that the light will have to be drawn back
somewhat from its usual position in the lantern to get the
best effect, and especially to make the edges of the picture
visible clearly. The reason for this will appear in § 6.

It will be under-
stood at once how
the rays crossing at
the pinhole, as in
fig. 1, go from the
bottom of the slide to
the top of the screen,
and from the right to
the left, so that the
image must be in-
verted, and we have to place the slide upside down in the lan-
tern to make the image come right. And it will also be seen
how and why the relative size of the image on the screen
depends upon the ratio between the distance of the object, o,
from the pinhole, A (fig. 1), and that of the screen-image, I,
from the same point.

The image before us is but dim, because so few rays can
pass through the pinhole to form it.1 Prick four more holes
at equal distances, half-an-inch out from the central hole.

i With a mixing jet, an ordinary landscape slide can be made clearly
visible on a 12-feet disc, and with a blow-through jet 8 or 9 feet. With oil-
lamps a somewhat smaller disc must suffice if the image is to be seen dis-

B2

FIG. 1

4 OPTICAL PROJECTION

With each we get another image, and it will be seen how they
confuse each other ; while if we stop all the others each one
is distinct, and each of the last four is outside, on the screen,
that from the central hole. It will now be understood
instantly, that if we could bend in the rays which form the
four outside images, so that they would fall exactly on the
same spot as the central image, we should have again but
one and a distinct image, but five times as bright as from one
pinhole.

4. Use of the Focussing Lens. — To do this is the sole
operation of the focussing lens, in a lantern or any other form
of projecting apparatus. We can bend rays of light easily,
by sending them at an angle through the surface of some
other medium of greater (or less) density than the air ; and
the greater the angle at which the ray strikes the denser
medium, the more it is bent ; the amount of ' refraction ' at
lifferent angles being connected by a simple law which need

not be discussed here.
We only need to remem-
ber, that on entering the
denser medium the ray
is bent in towards the
perpendicular, and in
leaving the denser me-
dium away from it. If,
then, we have a piece
of glass with inclined
faces, called a prism
(fig. 2), to whose faces the dotted lines N.I and N E are perpen-
diculars or * normals,' 1 the ray s i will be bent towards the
normal, to the path i E, and on leaving the prism will be
similarly bent away from E N to E R. Thus it is permanently
bent in, or rofracted, towards the thick side of the prism.

1 All angles in optics are reckoned from the perpendiculars or normals,
not from the surfaces themselves.

ON PROJECTION 5

A convex or focussing lens must act in the same way,
because it is a piece (or combination of pieces) of glass
formed so as to be thickest in the middle, the faces gradually
becoming more and more inclined till they go off to an edge.
It is a circular prism, of gradually-increasing inclination
towards its edges. If we place such a lens in a beam of
parallel rays, as from the sun, it is easy to see what must
happen. The centre ray, striking the glass perpendicularly,
proceeds straight on unrefracted. The next outer rays,
meeting the glass at a small angle, are a little bent in towards

the centre or thickest

part. Then rays farther

out from the centre are —

more, bent in, because

they strike the surfaces

at a greater angle ; and FlG<3

so the whole beam of

rays meet practically in one point, F (fig. 3), which is the

' focus ' for parallel rays, or ' principal focus.' This is the

well-known phenomenon of a burning-glass. v

Take the converse, however ; if the rays diverge from the
luminous point F at this focus of the lens, they are refracted
just the same, being converted into parallel rays. Suppose,
however, that the rays diverge from a point, /, farther away

FlQ. 4

from the lens than its principal focus, F (fig. 4). What must
happen then is, obviously, that all the rays diverging from
the point / will be bent in so as to meet again in some other
point, Fa, on the other side of the lens, which point must have

6 OPTICAL PROJECTION

some fixed distance related to the other distance, according to
the refractive power of the glass. It is accordingly called the
conjugate focus of the point/.

Here we have exactly what we wanted to get — a brighter
image. To make it more clear, take both the slide and the
condensers also out of the lantern, leaving only the perforated
tinfoil with its five holes, and let the object be the light in
the lantern itself, because its images will be both brighter, and
will stand out obviously apart upon the screen. Now take a
lens ! in the hand, and hold it close to the tinfoil. At
once the outside images will be bent in towards the centre
one, and a position will be readily found in which all are
exactly made to fall on one spot. Finding that position, and
placing the lens there, is ' focussing ' the lens. This image
is obviously five times as bright as the original one, and if
now the lens can be supported in that position, we may prick
any number of holes, and break away the tinfoil altogether,
and it makes no difference except to further brighten the image ;
because in the same way all the images formed by every ray
are bent in to the same point, and unite their brightness .in
one.

This is the only action of a projecting lens. People talk,
and even write, as if it formed the image, or inverted the
image ; but it does nothing of the kind. The rays themselves
form the images ; and their crossing at the place where the
lens is, inverts the image ; and all the lens does is to bend
into one spot the rays (forming images) which fall upon its
surface, and so to combine countless faint and jumbled
images into one bright and clear one. That is the true
nature of Optical Projection.

This much being experimentally realised, several very
important principles at once become clear. First of all, it

1 This lens must be of somewhat longer focus than the distance from the
light to the tinfoil, or the light must be pushed up to the front so as to malca
ihat distance less than the lens focus.

ON PROJECTION 7

will be seen that for any given distance of the lens from
the slide or object, there can be but one distance from the
screen which will properly unite all the ray-images in one.
Hence, if we want to produce images under various conditions,
or of various sizes, it becomes necessary to have several
focussing lenses. Further consideration and experiment
will establish the fact that, with a fixed object and a fixed
screen, there are two positions in which the lens will pro-
duce an image, the focal distances being the same in each
case, only reversed. The lens may be near the object, and
produce an enlarged image ; or nearer the screen, and produce
a diminished image. We have the first case when a photo-
graphic lens is used in a lantern : the second case when the
same lens is used in a camera. In almost all cases it is the
first kind of focussing which is used in projection.

Secondly, it is plain that the distinctness of the projection
will depend upon the perfection with which the lens bends in
all the ray-images to precisely one spot. Unfortunately, it
will be found that a simple lens, made of one piece of glass,
does not perform its office perfectly. The different colours —
be they evident, or only as existing in white light — are not
refracted alike, the blue rays being bent in to a shorter focus
than the red ones : this is called the ' chromatic ' aberration.
And the rays striking the edges of the lens are also bent in to
rather a shorter focus than those passing through its centre :
this is called its ' spherical ' aberration. Also, the outer parts
of the image tend to a shorter focus than the centre part :
another form of spherical aberration. Where great distinct-
ness of image is necessary, as in a lantern for exhibiting views
or diagrams, we have to correct these faults by constructing
compound lenses made of different kinds of glass, with or
without air-spaces between. We may also use a diaphragm
or limiting stop, cutting off the worst of the marginal rays,
which is often done in lanterns. But in most experimental
projections, where only broad features have to be made visible,

8 OPTICAL PROJECTION

simple lenses answer every purpose, and are preferable because
they stop less light ; some being lost at every piece of glass
by reflection from its surfaces, and by absorption in its
substance.

For, in the third place, it will be manifest that the brightness
of a projection must depend upon utilising in our final image
a sufficient number of luminous rays. If we regard the rays
proceeding from an object as equally luminous in every
direction, the number of rays collected must obviously depend
solely upon the size of the lens, and its distance from the
object when focally adjusted : the brightness of the image will,
however, further depend upon the screen-distance. By a very
simple and obvious law, the light falling on a lens must
diminish as the square of its distance from the object; and that
on the screen, in the inverse ratio of the surface of the lens
and that of the superficies covered by the rays on the screen ;
both depending, as we have already seen, upon the focus of the
lens.

Hence comes the utility of large lenses, which are highly
advantageous for many physical experiments in projection, as
we shall see. But it by no means follows that a large lens
will give more luminous results in every case ; for the result
depends upon collecting an adequate number of luminous rays,
and it is by no means to be taken for granted that the rays
emitted from an object are equally luminous in all directions :
it may rather be the case that all which are luminous enough
to afford us much help towards our final image, are confined
to a very limited range. To understand this is the last essential
point in the problem of projection.

5. Use of a Condenser. — We have already had a proof and
example of this. When we first removed the lens and pricked
a hole in the tinfoil, the margin of the image of the slide was not
illuminated, and we had to draw back the light considerably from
its usual position before we could make it so. The slide itself
was amply illuminated, we know, for with the lens the image

ON PROJECTION g

was all that could be desired. The fact must be, therefore,
that the more luminous rays from the edge of the slide, while
they fell upon and got through the lens, did not get through
the pinhole until we drew back the light.

Let us investigate this further. With the slide in the
lantern, leave the lens in its place ; but this time remove the
condensers instead. The slide itself is now very nearly as
brightly illuminated as before ; but only the centre of the
picture is at all bright upon the screen, and we have again
the dark margin, all the more striking by contrast with the
bright centre. Considering this, we begin to understand the
state of the case. Our slide is very greatly transparent.
(It is not wholly so, or there could be no picture of it, nor

FIG. 5

could we see it.) Now, any imperfectly transparent object
scatters light falling upon it, and in this way sends out some
rays in all directions open in space. But the greater part of
the luminous rays which strike on it do go straight on from
the light in the lantern, or continue diverging ; and as regards
far the greater part of the luminous rays, therefore, the case
stands as in fig. 5. Those passing from the radiant, B, through
the outer part of the slide, s, pass outside the lens, L, and
give no light to the final image ; the lens only picks up from
the margin of the slide the few comparatively dim ' scattered '
rays. On the other hand, a central cone of bright direct rays
does get to the lens, L, and forms a good image of the central
portion of the slide.

The properties of a lens already indicated, suggested long

to OPTICAL PROJECTION

ago the remedy for this, in the shape of a * condenser ' — viz., a
lens used in this case merely to bend in the illuminating rays,
till they are brought to strike within a given area, or as nearly
so as possible. Taking the same case as before, suppose we
place immediately after the slide, s, a large convex lens, 0
(fig. 6), which more than covers it. If this lens is of suitable
focus, the outer part of the luminous cone, which before
diverged uselessly in the directions B A, R B, is bent in so as
to pass through the focussing lens, L. The consequence is
that there is now a bright image of the whole of the slide.

This is not the best position for the condenser, because it
both impairs the sharpness of the image (which has in a

FIG. 6

manner to be focussed through it) and it leaves the slide so
near and exposed to the heat of the light. But it is men-
tioned first, because it actually was the position first given to
the condenser, and still used in those ' toy ' lanterns which
are copied by toy-makers from generation to generation ; and
still more, because it shows us that the common view of the
function of a condenser is a mistake. It is usually stated in
books upon this subject that this function is to ' condense '
the greatest number of luminous rays upon the slide. That
is not so at all ; this object could be obtained, as in the above
arrangement, by placing the slide itself near enough to the
source of light. It is not to condense rays upon the slide,
but to converge the luminous rays so that they shall pass

ON PROJECTION

ii

through the focussing lens ; and to understand this is most
necessary to any successful projection which may be the least
out of the beaten track. We shall see hereafter that, for
want of understanding this, even slides and diagrams are
often badly shown, and unnecessary expense is often incurred.
This understood, however, it is obvious that a much
better arrangement in several respects must be to interpose
the condensing lens between the slide or object and the light,
as in fig. 7. The otherwise wasted divergent rays E A and E B
are still bent in so as to pass through the focussing lens, L ;
but a large part of the heat is borne by the lens c, and the
slide or apparatus so far shielded ; and the focussing lens, L,

,8

FIG. 7

has no distorting medium between it and the object. This,
therefore, is the arrangement now always used in instruments
of a serious kind.

6. Management of the Rays. — We now understand exactly
why the margin of our slide in the first experiment was not
illuminated upon the screen, until the light was drawn back
from its usual position. There was a condenser, and the rays
were sufficiently converged to pass through the lens — say a
circle of two inches diameter — in the usual manner. But
they were not sufficiently converged for any appreciable rays
passing through the margin of the slide, to pass through the
small central pinhole. By drawing back the light, they were

12 OPTICAL PROJECTION

made more convergent so as to do this. We thus learn that
for a successful projection, especially of apparatus, the position
of the condenser relative to both the light and the object, and
to the focussing lens, is almost as important as that of the
latter ; and may have to be varied materially, according to
any of these circumstances. All the parts of the optical
system must be so arranged that (1) as many rays as possible
may be made to pass through or fall upon the slide or object ;
and (2) also pass through the projecting lens.

But it is further necessary that the rays should not be
more converged than will cause them to pass through the lens.
For lastly, we can hardly help seeing that the rays proceeding
in fig. 7 from the slide s to the projecting lens L, must be
considered in a double character. From the focussing or
projecting point of view, they must be regarded as bundles of
image-forming rays diverging from every point of the slide s
to the surface of the lens L. On the other hand, from the
illuminating point of view, we must regard them as converging
bundles of rays proceeding from the condenser c to the lens L.
It is natural to suppose that this dual character might lead to
some want of sharpness or definition in the focussed image.
In ordinary lantern arrangements, and in exhibiting slides,
this is not sensibly the case, because any ray from each point
in the slide, to any point on the lens, forms an image on the
same spot of the screen ; and thus the luminous rays are
identical with the chief image-forming rays. But in movable
projecting arrangements, the matter is of the greatest im-
portance. Thus, the light might be so drawn back from the
condensers as for all the rays to pass through the slide, and
then actually converge and cross before entering the focussing
lens.1 All the light would then still pass through the lens,
and would reach the screen in some form or other ; but such
violent crossing would blurr the image and spoil the even

1 I have seen this mistake made repeatedly in physical demonstration,
especially with the electric light.

ON PROJECTION 13

illumination of the ' disc ' (or lighted area) on the screen.
And in microscopic projection, where definition in the image
is so much more severely tested, it often becomes necessary
practically to ' focus ' the light itself upon the object, in order
that the rays diverging again from that focus as from a new
point, and the image-forming rays from the object itself, may
practically coincide.

Such are the elementary principles and conditions of
Optical Projection. Simple as they may seem, for want solely
of a due understanding of them, many demonstrators never
half develop the powers of their apparatus. This does not
apply only to so-called itinerant lecturers ; for I have repeatedly
seen polarisation and other physical phenomena projected,
with the most elaborate electric -light apparatus, at what are
considered the very head-quarters and principal arenas of
scientific exposition, in a manner inferior to what I had been
accustomed to obtain with only oxy-hydrogen illumination.
These principles are the essential key to the whole of what
follows; and both excellence of apparatus, and success in
using it, depend upon their being thoroughly grasped, in the
first place by the optician, and in the second place by the
operator who uses the apparatus which the other has con-
structed.

CHAPTER II

THE PABTS OF A LANTEEN

A LANTEEN is an optical apparatus so arranged, with all its
parts approximately fixed in their places, that pictures or
apparatus can be exhibited on a screen with the least and most
convenient manipulation. In this place we will consider only
the exhibition lantern, for the projection of slides or diagrams,
leaving other apparatus for separate consideration in a
chapter devoted to experimental lanterns.

M OPTICAL PROJECTION

7. Parts of a lantern.— The diagram (fig. 8) of one of the
simplest forms of lantern, once general, but now generally

confined to mere toy instruments
conveniently represents the es-
sential parts, which we will con-
sider singly. These are (1) the
light L, which may or may not
be supplemented by a reflecting
mirror M. This, as it is the
original source from which all
illumination is derived, while
the word * light ' may need to
be used in other senses, it will
Fia* 8 be convenient to distinguish as

the radiant ; and as it forms no part of the lantern itself, and
various kinds of radiants are often employed in turn in the same
lantern (as when an experiment is worked out in a small way
with a lamp, and afterwards publicly performed by the oxy-
hydrogen light), we will postpone it for treatment separately,
and pass on here to other details. These are (2) the lantern-
body B, with its chimney or heat-vent. (3) The condenser c.
(4) The stage for slides or diagrams, s. (5) The focussing lens,
objective, or power, p. All improvements in lanterns relate to
one or other of these parts.

8. The Body. — This has two purposes : 1, to support and
keep in due relation the other parts ; and 2, to prevent any
light not utilised in the projection from scattering about the
room and impairing the effect. Japanned tin or sheet iron is
the simplest and cheapest material, and when economy is an
object, will really perform as well as anything else, provided
the optical parts of the apparatus are equally good. Thus, a
tin bi-unial will do all that the most expensive body can ; or
an experimental lantern made in this cheap way for a science
school, will come short in no part of the demonstration, and
may be within reach when a more expensive one would not

THE PARTS OF A LANTERN 15

be so. But besides the want of appearance, there is the ob-
jection to a simple metal body that it becomes very hot, and
may manifest this fact very suddenly and unpleasantly when
any manipulation is necessary. For this reason all the better
lantern bodies are made of wood, lined with an iron or tin
casing supported at a small distance from the wood, so as to
keep the outer body cool.

The shape of a single lantern body may vary a great deal,
and has some connection with the radiant employed. With
Argand lamps (either gas or oil), which require a tall glass
chimney, a body is still employed (almost of necessity) rather
tall in proportion; and the earlier lime-light lanterns were
similarly made, from custom. But the introduction of tho
Sciopticon form of lamp caused a revolution in the form of
body also. There being no wick chimney, the tall chimney of
the flame chamber was attached to the lamp itself, and the
body of the lantern was lowered. All lanterns of the edge-
wick class are now made with low bodies ; and the same are
found ample for the lime-light.

The wood of the body should be perfectly plain and sound
in grain, old and well-seasoned. Any ' ornamentation * is out
of place, as will be remarked upon again.

Where exhibitions may have to be made in various places,
it is of some importance to have a door on both sides of the
lantern. Most people work from the right side (as the lantern
faces the screen), but this is not always possible, especially
with experiments. In all lanterns used with the lime-light,
each door should be furnished with a sight-hole glazed with
dark blue glass, through which the state of the lime can be
examined without dazzling the eye. For a lantern used only
with paraffin oil, sight-holes are useless, the flame being
examined through a sight-hole at the back of the lamp.

The back of the body also demands a word. For Argands,
it is closed in entirely. For paraffin lamps it needs little con-
sideration, the lamp itself being closed in, with the exception

16 OPTICAL PROJECTION

of the small blue sight-hole. For the lime-light, there ig
usually a perpendicular slot sufficient for the passage back and
forward of the tray-pin which carries the jets (figured in
Chapter IV.) and enlarged at the bottom for the jet itself, and
for the tray. Usually little light comes through these open-
ings ; but with a powerful light it is unpleasant, and mars
the effect. When this is found to be the case, two small brass
eyes should be screwed into the back, near the top corners,
into which drop the two ends of a wire bent as in fig. 9. On
this semicircular wire is hung, by small
rings or a broad hem, a curtain of black
cloth, which quite stops the stray light,
while allowing the jets to be got at
readily.

The top and chimney also claim a
word. Paraffin lanterns have open tops,
but, if also used for lime-light, a cover
mus^ be provided. Argands need a tall
chimney ; but for lime-light lanterns such
have quite gone out, and a short cowl on a bulged top is
generally used, as shown in the tri-unial figured on page 119.
If this has to be packed inside the lantern, it is very dan-
gerous to the condensers, unless carefully wrapped in cloth ;
therefore, if the lantern-box will not contain it in situ, the
cowl should be so tapered that it will drop upside down into
its flange or socket, which can also be reversed on the top of
the lantern. It is very much better to make the top (always
of sheet iron) flat, and the top of its quite shallow cowl
also flat. This saves space and manipulation to begin with,
but has a more important advantage. In experimental work
it is often necessary to warm fluids and objects, and the flat
heated top offers a convenient means for doing this. Even
in slide exhibition, every exhibitor knows the difficulty en-
countered from ' dew ' upon his slides on a cold and foggy
night. Such a flat top offers to him, also, a handy means of

THE PARTS OF A LANTERN 17

warming his slides. If the plate itself is too hot to lay them
upon, they can be readily supported on three small slabs of
wood or cork. Opticians have long been behindhand in this
respect, and such a low flat top has only to be known to be
universally approved of.

In the United States, lanterns are sometimes made with
no rigid body at all. One such which was purchased there,
and used in this country by the late Mr. E. A. Proctor for his
lectures, had for its base a light metal quadrangle supported
on four short pillars at the corners. The front alone, carrying
the lenses, was hinged to this, and a light body fitted on the
base behind like the tilt of a waggon. Another common
American plan (shown hereafter in Chapter XII.), is to hinge
the front to the base in the same way as just described, to add
two light pillars behind, on which and the front is supported
a sheet-iron top, and to form the sides of black cloth curtains.
The arrangement may occasionally be useful for its lightness,
and for packing into a small space ; but so far I have met
with none who liked it in this country, in comparison with the
more solid English bodies.

9. The Condenser. — The use of this has been explained.
A single lens is, however, only used in toy lanterns. The
object is to take up as wide an angular pencil of rays from
the radiant as possible, and send them through the objective.
A single lens, to do this, must be of such thickness as to lose
much light by absorption ; and moreover the chromatic and
spherical aberration would be excessive. The use of two or
more lenses accomplishes the object with a moderate thick-
ness of glass, and also allows a large part of the aberrations
to be corrected.

Fig. 10 shows the principal forms of double condensers
which have been used. Two double convex lenses, c, were
used by Messrs. Carpenter & Westley in the early phantas-
magoria lanterns, and for many years afterwards, till very
lately, in the electric lanterns of M. Duboscq. They aro

C

18 OPTICAL PROJECTION

abandoned now, because the aberrations are very imperfectly
corrected, and there is loss oi light by reflection at the edges,
the curve increasing the angle of incidence. The two
meniscus lenses of D were better, but never came into general
use, being superseded by F, a meniscus and double convex,
with the meniscus towards the radiant. This is known as
the Herschel condenser, being in general form modelled on a
burning-glass designed by Sir John Herschel, and was long
reputed to be free from spherical aberration, though it was
afterwards discovered that this supposition was due to an
error in calculation. As a burning-glass, however, such a
pair does give exceedingly good results ; and, correspondingly,
is excellently adapted for converting the light from a luminous

Fro. 10

point into a parallel beam. The light from a lantern has
however to be converged, except in optical experiments ; and
therefore this condenser is not so superior for lantern use ;
but it does well for lanterns used with the lime-light alone,
and is largely employed for such ; for larger radiants, like
lamps, it is not well adapted. Its best form is that devised
by Gravett, shown at a, where the meniscus next the light is
made rather smaller, and the second lens is of flatter curve
on the inner face. Optically, this is a very good condenser
for the lime-light, its principal defect being that the bulging
front prevents slides being brought up close to the margin,
and hence somewhat diminishes the size which can be illumi-
nated by a condenser of given diameter.

THE PARTS OF A LANTERN 19

The condenser at E, consisting of two plano-convex lenses,
was also used by Herschel, was adopted by Mr. Marcy for
his Sciopticon lamp, and is the best form for all large and
hot radiants. Such lamps have to be placed at a greater
distance from the condensers to avoid cracking them, and are
generally used with short focus objectives ; the two cones of
light being thus more alike in angle, two such lenses represent
pretty well the optical conditions. When properly modified,
this form is equally adapted for the lime-light, and is now the
most usual condenser found in good lanterns.

10. Correction of the Condenser.— For oil lanterns the
plain form shown at E, of two similar lenses, cannot be
improved. Optically, it would be better that the lens next
the lamp should be rather deeper in curve, but the great heat
makes this undesirable. With the lime-light it is different,
and in considering the very best form, it is well to understand
the principles upon which the correction of aberrations
depends. This has been very familiarly explained in the
diagram, fig. 11, by Mr.
Eobert Bow. Here D E re-
presents the upper half of
a plano-convex lens, the
faint line h e the outline
of a double convex lens in
contact with it, and de a
meniscus lens of the same
focus. Consider now the
different effect of these two

latter upon the spherical aberrations, due to the fact that
the marginal rays are brought to a shorter or closer focus
than the central rays. It is quite plain that the focus F of
the central rays, A A, will be almost exactly the same in
the two arrangements — we may practically consider that a
fixed point. But with the marginal rays, B B, it is different.
Tracing the top one, B D, let us suppose that the lens h re-

02

20 OPTICAL PROJECTION

fracts it to /2: then the distance between F and /2 repre-
sents the aberration of the D h combination. But, owing
to the curvature, away from the lens D, of the meniscus d,
the marginal ray passes through d nearer the centre than
through h, and consequently its second refraction by such a
lens is less on that account ; the same ray also passes through
the meniscus at a less angle of incidence, which in another
way also reduces the second refraction. Consequently, the
marginal focus is lengthened, and the aberration is reduced
to the distance from P to /.

The main factor in this correction is the bending away
from each other at the margins of the two lenses, which is
obtained equally in the double piano form, and explains its

superiority to the two double
convex lenses, c, fig. 10. But
it will be evident that the other
condition, of ' minimum devia-
tion ' at the margin, is only ap-
proximated to when the curves
or thicknesses of the lenses are
in some proportion to the foci on
each side of the condenser (i.e.
the position of the radiant, and

the position of its image on the other side of the condenser).
Hence, for a lime-light condenser, the lens next the radiant
should be of considerably deeper curve, the two lenses taking
the form of fig. 12 rather than of E in fig. 10. Then the
spherical aberration F/ will also be comparatively smaller.
A thicker lens, however, is more in danger of cracking from
the heat ; therefore, as it will be obvious that a somewhat
smaller diameter at d d will collect all the bundle of diverging
rays which can reach the second lens, D, this fact should
be taken advantage of, in order to reduce its thickness while
keeping the deeper curve (see fig. 13).

All things considered, I regard this as practically the best

THE PARTS OF A LANTERN 21

model for a lantern condenser. Some opticians — including
Mr. Dallmeyer — prefer to make the lens next the slide a
' crossed ' lens ; but practically there is no advantage in this,
and it (slightly) diminishes the size of the slide which a given
condenser will cover, as already indicated.

There remains to consider the chromatic aberration ; for
of course the margin of a lens acts precisely like a prism,
and makes the different colours diverge into a spectrum, as is
easily seen by experiment. Now these diverging rays from
the first lens, falling on the second lens, are by it more or less
converged, like any other rays diverging at the same angle.
The amount of this convergence is greater, the farther the
rays are allowed to proceed before convergence by the second
lens ; the conjugate foci are altered in relation to each other, as
in other cases. Hence there is one particular distance at which
the differently- coloured rays dispersed by the first lens are
made approximately parallel by the second lens for a given posi-
tion of the radiant ; and at this distance between the lenses,
the condenser becomes very nearly achromatic, only the nar-
rowest line of colour being visible at the extreme edge of the
illuminated disc. The exact posi-
tion is a matter of experiment or
calculation for a given glass ; but
practically, in a condenser of the
kind here described, the clear dis-
tance between the lenses is usually
between a quarter and three-
eighths of an inch for a four-inch
condenser. Such a condenser is
shown in fig. 13, and will work " pm 13

exceedingly well through a wide

range of foci. For constant long-focus work — say with objec-
tives over ten inches focus — the second lens may be a ' crossed'
lens, with the deepest curve insid.e, but for all-round work, two
planes will be found best, as the slide may be brought close

2i OPTICAL PROJECTION

up. For much long-focus work, an extra condenser (also of
longer focus), gives the best result.

For all usual exhibition purposes the ordinary double-piano,
with the lenses in contact, answers very well ; and it is only
for sharp detail in the best class of photographic slides, or
when it is desirable to utilise the field as far as possible up to
the very edge of the disc— as in square or cushion slides —
that such optical refinements acquire importance.

11, Practical Points in Condensers. — Condensers are
usually made of crown glass, density about 1*516. This
answers perfectly for ordinary purposes ; but for the highest
glass of modern exhibitions, which have to cover large screens
of twenty to thirty feet diameter, sometimes at great distances,
it is objectionable, as the green colour absorbs light which
can ill be spared in such circumstances. Chance's ' optical '
flints, of course, leave nothing to be desired in this respect,
but condensers so made are very expensive ; in some cases,
however, they are worth while, as the glass is both more
colourless, and the lenses are thinner for the same focus. A
more common flint would answer practical purposes, and it is
desirable that some colourless glass should be introduced if
possible, rather than green crown. This need not make them
so very much more expensive ; for while ' optical ' flint must
be homogeneous, and is usually wanted dense, for a condenser,
a perceptible amount of stria is of little practical importance,
and density is not required, only colourlessness. It is utterly
useless to pay the cost of optical perfection in a condenser
for any ordinary purposes. A perceptible bubble in the lens
next the slide, however, would be a defect of importance,
probably showing as a black spot. It is very likely that many
purchasers would reject a colourless lens with perceptible striae,
rather than a crown lens which showed none ; nevertheless
there can be no question that the first would be the better
condenser, unless the striae were excessive.

The point most commercial condensers chiefly fail in,

THE PARTS OF A LANTERN 23

however, is that the lenses are not ground thin at the edges ;
nearly all lantern -makers being far too careless in this respect.
Not only is a needlessly thick lens much more likely to crack,
and more absorbent of light, but it is distinctly worse in optical
performance. The lenses should be ground to as nearly
knife-edges as possible, being only just edged down for fixing
into their cells.

Both lenses should be mounted so loosely in their brass
cells, that they can be turned round with the fingers, else they
may crack merely from expansion when heated, and they often
become hotter than the hand can bear. Holes must be
pierced in the margin of the cell, to allow of the escape of the
aqueous vapour which always forms when the lantern is first
lit. Only the back lens ever cracks from heat alone. If the
crack be irregular, the lens must be replaced ; but it often
happens that the crack is quite straight across a diameter, in
which case the cracked lens will answer perfectly well if the
crack is arranged perpendicularly, and the operator may feel
certain that his lens will never crack again.

12. Size of Condensers. — The best general size of con-
densers is 4 inches diameter next the slide. The standard
size of slides is 8J inches square, and 4 inches will cover
1 cushion ' slides on such square of glass. Of course a
diameter of 3^ inches is ample for the usual circular slides,
which give a disc of 3 inches diameter. If it is desired to
cover square slides of three inches, as the corners are never
absolutely square, it is better to have 4| inches diameter than
4| inches, because practically only a certain angle of light
can be taken up from the radiant, and the larger surface this
is spread over, the more it is diluted, so that for the usual
size of slides, less light is passed through them by a large
condenser than a small one. Theoretically any angle might
be taken up by a condenser, but practically it is limited ; first
by the thickness of the glass, which would both crack, and lose
as much light by absorption as was gained in angle ; and also

24 OPTICAL PROJECTION

by such loss from reflection at the edges of the first lens, owing
to the high angle of incidence, that the edge of the disc would
be less illuminated than the centre. Practically the angular
pencil is thus limited to somewhere from 65° to 70° in lanterns
for exhibiting slides.

18. Triple Condensers. — Condensers of three and even four
lenses have been used, especially in America. Theoretically,
they admit of more perfect correction for aberrations, and a
larger angle of light ; but this is complicated by the number
of extra reflecting surfaces. I have perfectly satisfied myself,
that for exhibition lanterns they afford no gain whatever.
For large condensers, as five inches and over, and where the
light is to be condensed upon a small surface, as for the
projection microscope, they are of advantage, and allow us to
use a pencil of 90°. Such a condenser will be described in
connection with the instrument just named.

14, The Slide-stage. — Little need be said here about this.
For a single lantern only used to exhibit simple slides and
diagrams, it does not matter much how it is constructed.
When, however, experiments may have to be made in the
stage — as in a chemical tank, and for some ' effects ' with
slides — as the ascent of a balloon, it is important that the stage
be open at the top, with the exception of the pillars needful to
carry the objective mount. Also, for anything like general
work, it is important that while the spring pressure-plate
allows the slide to be inserted easily, for which purpose the
edges should be carefully turned back to a smooth curve, the
slide should be held firmly when in place. If it be not so, the
working of any mechanism, as the handle of a chromatrope,
may move the slide about in a very unpleasant manner. Both
these requirements should be attended to, even in a cheap
lantern of japanned tin.

15. The Objective. — This is the lens directly employed in
the projection. A simple lens is never now used except in toy
lanterns, and for physical experiments, the latter for reasons

THE PARTS OF A LANTERN 2$

which have been already mentioned in § 4. When hand-
painted slides only were in use with oil lamps, two simple
meniscus lenses combined gave fair results, because there was
no detail sharp and colourless enough to manifest optical
defects ; but photographic slides and diagrams make con-
spicuous either chromatic fringes, or distortion of figure.
Practically, achromatic lenses only are now used in lanterns.
They may be of four kinds.

(a) The simple achromatic lens, or lens composed of one
convex crown c, corrected by a concave of flint F, usually of
the plano-convex total form, as shown at A, fig. 14. TLislens

FIG. 14.— Single Achromatic Objectives

is employed with its convex surface towards the slide s. If
the plane face is turned to the slide, the definition is sharper
in the centre of the field, but falls off rapidly at the margin ;
and the image is also formed on a hollow curved surface
instead of on a plane. By reversing the lens a little is sacri-
ficed at the centre, but the picture is better and more uniform
over the rest. A lens cannot, however, be made of this con-
struction of short focus, say 4J to 7 inches, which will give a
good image.

It may be well to explain here the principal errors which
have to be corrected in the objective. As regards a condenser,
it has already been seen that the most noticeable result of
spherical aberration is to bring the marginal rays to a shorter

*6 OPTICAL PROJECTION

focus than the central rays. But when we use a simple lens
to form an image, there are other results from this. Eeferring
to fig. 15, it will be seen that the image mm of the arrow M M
is really brought to its focus upon a concave surface instead of

FIG. 15. — Spherical Aberration

a plane ; or conversely, the diagram or picture m rn must be
drawn upon a concave surface to produce a flat image MM, an
expedient actually employed in early solar microscopes.

FIG. 16.— Distortions of Image

The curved image maybe more or less ' flattened,' as it is
called, by adopting a meniscus form for the lens, as shown in
fig. 14 by the various achromatics B, c, E. But we are now
confronted with another result of aberration, in the form of the

THE PARTS OF A LANTERN 27

image. In fig. 16 let s represent the slide, a true image of
which we wish to produce. Then B represents what is called
the * barrel ' distortion, the usual distortion of a single lens.
If we correct this merely for flatness of field, which is the
easiest and most obvious error to correct, we usually get the
figure ewer-corrected, producing the ' hour-glass ' distortion
denoted by H. Either distortion, if perceptible, is simply
intolerable in architectural subjects, lines of type, or diagrams
which may contain straight lines or circles.

The correction of these various errors in lenses of moderate
focus is a task of no little difficulty. The chromatic correc
tion is a comparatively simple affair, needing simply a certain
proportion, depending on the dispersions of the glasses,
between the convexity of the crown and concavity of the flint ;
and the object of various ' figures ' for the curves, as in A, B,
c, E, (fig. 14) is to correct the spherical aberrations. The
forms given in fig. 14 have all been at one time or other used
for photographic purposes. They all need a stop or diaphragm
on the side farthest from the slide, and c is probably the
best of them ; but no single achromatic lens is capable of
perfect correction for anything like short foci.

(b) Double or triple achromatic lenses. — With foci of ten
inches and over, however, the spherical aberration is much
less, and these lenses then perform very well, and are in
common use for long-focus work. Two or even three of such
long-focus lenses combined, make better short-focus lenses
than single achromatics of such short focus ; and hence it is
very common to furnish a lantern with three achromatic lenses
of graduated long foci, ranging from nine or ten up to eighteen
or twenty inches, which by combining different pairs, or the
whole three, will give fair results throughout the whole range.
This result will depend upon the quality and figure of course,
for of these ' triple sets,' as they are called, there are both
bad, middling, and good. The only way to be sure is to have
a trial, which a good optician will always afford.

28 OPTICAL PROJECTION

(c) Triplet achromatics have also been used. D in fig. 14
shows a triplet devised by Dallmeyer which is said to answer
very well. But a more usual form, made sometimes in France,
is that shown in fig. 17, a concave of flint being used between

two convex lenses as in the preceding, but the
whole lens assuming a double convex form.
Some of these lenses perform exceedingly well.
They were used a great deal by Mr. Dancer for
his lanterns, and I possess a pair of them,
6-inches focus, whose performance can hardly
be distinguished from that of the best of the
construction next to be described. They appear
to me to combine flatness of field, evenness and
sharpness of definition, and ortho-symmetry of
image, in a greater degree than any other single lenses, and
I think it is to be desired that more attention should be
directed to this construction for long-focus work. My pair of
6-inch lenses have a clear diameter of If inch, and require a
1-inch stop placed about 2^ inches in front to produce their
best effect. In this position the stop cuts off scarcely any
rays of serious importance, and the image of a slide of printed
matter is exceedingly good. For lenses of 9-inches focus and
upwards, no stop whatever would be required, and such lenses
would be much cheaper, and pass more light than double
combinations.

(d) Double combination lenses are, however, most used in
the best lanterns for short and moderate foci, ranging, say,
from 4J to 8 inches. The type always employed is that well-
known as the Petzval combination, shown in fig. 18, and
in many lanterns quarter-plate photographic lenses are used
for the shorter foci, and half-plate lenses for the longer. The
pick of a dozen or so of such lenses will leave little to be
desired; but generally a lot as imported from France by
opticians is very unequal in excellence, and most of these
lenses require a stop at s, which loses a great deal of light.

THE PARTS OF A LANTERN 29

These photographic lenses have however the advantage, that
very often by unscrewing the double lens at the back, and
substituting for it the front lens reversed, or with its convex
side to the slide, a very good long-focus single lens is obtained
of the (a) kind.

The best lantern-makers have lately, taking this photo-
graphic lens as a basis, worked out by screen tests improved
curves for lantern use only. Such are sent out in the highest
class lanterns. These lenses reverse the two single lenses F, c,
of the Petzval system (fig. 18), need no stop whatever, and give

FIG. 18— Double Combination

magnificent definition ; but, as a rule, they are not corrected
for photography, and the single lens can rarely be used satis-
factorily. It only remains to combine these points with the
other excellent qualities already attained ; and there is little
doubt that the growing use of enlarging lanterns, or adapta-
tion of the lantern to photography direct, with the aid of the new
optical glasses now made in Germany, will enable this crown-
ing perfection to be obtained, at least in all lenses of 6-inches
focus and upwards. With lenses of 4^ inches focus, used
chiefly for paraffin oil lanterns in small rooms, to exhibit
through a transparent sheet, a perfect image of a slide 3 inches

30 OPTICAL PROJECTION

diameter is a task of tremendous optical difficulty, and the
wonder is that such approximation to it has been attained.

16. Focus and Diameter of Objectives. — The proper focus
of an objective of course depends upon the size of disc which is
required to be covered by a slide at a given distance, and the
range of foci suitable in a set of objectives, will depend upon
the range of work, or variety in size of rooms, it is intended
to provide for. This is dealt with in detail in Chapter VIII.
Here it is only necessary to mention the matter in connection
with the diameter of objectives. We have seen that brilliance
of image depends upon sending the rays collected on the

FIG. 19. — Scattering of Rays

object through the objective, and we know that only from a
radiant point could the rays be alike converged into a given
area at different distances from the condenser. With, say, a
luminous spot of f -inch diameter, the light cannot be con-
verged save into an image, proportionate in size to the
conjugate focus of convergence, as shown in fig. 19. Hence,
while all the light can easily be converged into a small lens
at a few inches from the slide, at twelve inches it is another
matter : the body of rays must spread out more, and require
a larger lens to utilise them in the image. Besides this, a
certain amount of the rays are irregularly ' scattered ' by the

THE PARTS OF A LANTERN 31

slide itself, and these scattered rays also diverge more with
the greater distance. This is the chief reason why more light
is required in working a long way from the screen ; though
light is also perceptibly absorbed by the atmosphere, especially
if it is at all a damp night.

Hence it is an important question, how far this loss of
light from long focus must be provided for. Some makers
recommend several sets of double -combination objectives of
very large diameter, at a cost of 9Z. to 10£. apiece ! After
investigating the matter carefully, both in theory and practice,
I have not the slightest hesitation in saying that such costly
lenses are a sheer waste of money, of absolutely no benefit to
anyone but the seller of the apparatus. Assuming the back
conjugate focus of the condenser to be 4 inches (i.e. about
8 inches from the back of the condenser) and the radiant
to be |-inch in diameter, nearly all the effective rays can be
condensed into an objective of 2-inches diameter, up to 9
inches from the face of the front lens of the condenser.
This gives a focus for a double combination of, say, 10 inches,
within which there is no need of greater diameter, and no
benefit from it. Beyond that a 2-inch lens will begin to lose
light, but 2J-inches diameter will carry the same result up to
13 inches ; and it is only when we reach distances of 15 inches
and upwards that 3-inch lenses are of any real advantage.
For such long foci, however, the ' double combination ' is quite
unnecessary, as with care in selection, single achromatics can
be obtained, which will give quite as good images with far less
weight and expense ; and perfection should rather be sought
in improving this class of lenses, especially in the triplet form
described a page or two back ; which will also pass more light
than the double form at these long ranges.

A ' double-combination ' lens of average focus — say 6 to
8 inches — and of most excellent quality, such as will give
perfect definition without any stop, is now obtainable for so
moderate a sum as II, 10s., including the rack-work mount.

32 OPTICAL PROJECTION

I have seen such lenses most carefully tested against the most
expensive of similar focus, without any perceptible difference
being discernible.

17. Testing Lenses. — It is no use to ' test ' a lens upon an
ordinary slide, for anything beyond definition and flatness of
field. These points can be seen by the image of any good
photograph, but the ' figure ' may be quite distorted. The
very best test is a cushion- slide covered all over, to the corners,
by lines of type sharply photographed, or by black lines ruled
in squares. A lens which gives an image of this sensibly
alike in focus all over, and the lines or squares straight,
especially towards the corners of the cushion, has every
needful quality for projection, though the photographer must,
of course, look for his special requirements in addition.

18. The Objective Mount, — A lantern only meant to be
used at one focus needs no consideration in this respect ; into
the nozzle of the lantern will slide stiffly a tube of the proper
length, into the end of which will screw a rack-and-pinion
mount carrying the objective. Practically, all such lanterns
are made alike. The travelling exhibitor, however, often needs
a very wide range of focus, perhaps from 4J inches up to 20
inches, or more. To give him this, three methods have been
adopted.

The first is to fit the front of the lantern with what are
termed telescopic draws, as shown in the tri-unial lantern on
p. 119, into the front of which the objective rack-mount
screws. No plan has been more usual than this ; and if the
draws are of first-class workmanship, and fit tightly, it works
well for a greater or less time. It is, however, very difficult
to pull such draws out, and at least one leading lantern
optician has found it desirable to insert a pair of strong metal
handles into the front draw, in order to give more strength to
the pull. Sooner or later, however, the draws are apt to wear
a little loose, and then the front end sinks out of the true
optic axis under the weight of the objective.

THE PARTS OF A LANTERN 33

A second plan consists in fitting separate draws, each,
with rack-and-pinion. This is known as the ' triple- rack '
front, and the racks remove the difficulty in pulling the front
out for use. Of the rest I am not so sure, my experience of
racked tubes in general being, that with regular use stiff ones
either wear loose, or * grind ' and set fast. I have, however,
had no actual experience of these fronts themselves, and have
seen a letter which, after four seasons' use, speaks of them
in very high terms.

The third plan, which I regard as the only good one, is
rigid, simple, and the cheapest and lightest of all. It consists
in somewhat lengthening the lantern nozzle A and tube B
sliding into it, so as to give there alone an adjustment of

Pia. 20.— Lantern Front

three inches. Into the outer end of the tube B screws a
diminishing screw- collar or adapter c, into which again, for
all moderate foci, is screwed the rack-work mount D. This
racked mount does not, however, carry the objective directly,
but is only a casing, into which the different objectives,
mounted in simple smooth tubes, are fitted to slide. This
gives another sliding adjustment of three inches. Thus the
original mount alone, even if the shortest and longest focus
are double combinations, will give a range of focus from six
to twelve inches, which will cover the needs of the great mass
of exhibitors ; with a single achromatic lens it practically
becomes over thirteen inches. If, however, very long focus
is required, the adapter c is removed, and in its place is
screwed a lengthening adapter (fig. 21) of any required

D

34 OPTICAL PROJECTION

length, which is best as a single tube, but may be a double
sliding one as shown in the figure. In the first case the whole
.7 ^ is perfectly rigid, and per-

fectly simple ; and lengthening
tubes may either be ordered
with the original apparatus,

r~ or, if suddenly called for by

FIG 21— Ada ter some extraordinary occasion,

could be fitted in all large

towns in a few hours, by any working optician worthy of the
name.

This method of construction is being gradually adopted
by most of the lantern-makers who really have much experi-
ence with the instrument.

CHAPTER III

THE RADIANT

THE qualities desired in the radiant are brilliance and
\\hiteness of light, and that this light be concentrated into as
small a space as possible. The perfect radiant would be an
intensely luminous point, which would give the most equal
illumination and the best definition. The radiants used in
practice by no means come up to this ideal. They comprise
(1) fatty oil lamps, (2) petroleum oil lamps, (8) gas-burners,
(4) the lime-light in its various forms, and (5) the electric
light.

19. Fatty Oil Lamps.— Except in small toy lanterns, it
is seldom we now find these lamps employed ; but for many
years they were the only radiants used at all, even in public
exhibitions of dissolving views. With hand-painted trans-
parent slides they did good work, too ; giving usually discs of
eight feet diameter on transparent screens. They will not

THE RAD

give such a disc with modern phoi

are still used to some extent, on accoul

freedom from risk of fire. Only such

lanterns for the navy ; and they are usefuf

where things become dry like tinder. Some

can also be generally obtained when nothing else

hence these lamps are sent out to missionaries.

Such lamps are of two kinds : viz. with the cistern
beneath, as part of the lamp, and with a separate cistern ;
but the wick arrangement is, with little modification, similar
in both. This is what is known as a ' solarised ' Argand,
external and internal metal cones concentrating an upward
current of air upon both inside and outside of the wick, so as
to give intensity to the light. The oil may be either sperm,
or colza, or olive, and greater whiteness and solidity of flame
is generally obtained if camphor is dissolved
in it. Fluid oil can be used alike in either
form of lamp ; but when the cistern is beneath,
so as to keep hot while burning, either tallow
or solid paraffin will also burn excellently if
first melted. The last— solid paraffin so
melted — I think gives the best light of all, but
it may not burn equally well in all lamps.
Such solid fatty matter cannot of course be
used in cistern lamps, like fig. 22, which are
now most usual. The glass chimneys of these
lamps must be pretty tall, and are tapered
towards the top.

There is little difficulty in getting a light of about twenty-
eight candles with these lamps ; and as the flame is trans
parent, this can be increased to about thirty-five candles by
using a reflector, which is always done. This must not be
parabolic, but circular, the centre of curvature being the flame
itself ; the rays are then reflected back in the same path, and
go to the condenser just as if emitted by the radiant. The

D2

FIG. 22

36 OPTICAL PROJECTION

power of a reflector entirely depends upon this, and upon the
reflector taking up enough rays to illuminate to the edge of
the condenser.

To get a good light, the glass chimney and the reflector
(best of silvered glass) must be polished bright. A rather
loose or woolly wick must be used (the hard wicks used in
petroleum Argands will not answer for fatty oil), and, if new,
carefully dried before a hot fire, or in the oven, before use.
Several hours must be allowed for the oil to soak up in a new
wick, and always the lamp is better for warming before the
fire, previous to use. Lastly, after trimming anew wick, light
the lamp and let it burn several minutes ; then blow it out.
It can now be trimmed really clean and smooth, as it could
not be before. When finally lit, let it burn a minute or so
moderately, and then turn up as high as possible without
smoking.

20. Petroleum Oil Lamps. — Paraffin or petroleum oil
could be, and was, burnt in some of the preceding lamps, but
there was little gained by it, and the first real advance in
illumination was made by Mr. L. Marcy, of Philadelphia, in
the lamp (and lantern) introduced into England by Mr.
Woodbury under the name of the Sciopticon. In this lamp
two flat wicks, about two inches wide, were placed with their
edges towards the condenser, outer flat cone-pieces directing
the air upon the flames, and driving them very close together.
The heat of each flame intensifies its neighbour, and the result
is a very brilliant light, very ' solid ' also, owing to the depth
of wick behind it. The Sciopticon two-wick lamp was found
equal to 60 or even 70 candles, and gave a really good disc of
9 feet with photographic slides. Its chief fault is, that the
space between the wicks generally manifests itself as a rather
darker streak up the centre of the screen.

Partly to avoid this, and partly to get more light, a third
wick was soon suggested. This was stated by Mr. Marcy to
be inferior to the double wick ; but general experience has not

THE RADIANT

37

borne this out, and three-wicked lamps, giving a light of 80 to
90 candles, are of all forms the most popular. Four and even
five wicks have since been introduced by different makers,
with slight variations in the arrangement of the wicks, some
placing four wicks parallel, II 1 1, and others in W form. By
these means a light of even 110. candles has been reached,
and for some purposes these lamps are useful ; but the heat
is intense, and the light does not increase in the proportion
of oil burnt. Also, with every wick the difficulty increases of
getting the lamp to burn
steadily ; and the three -
wick is generally pre-
ferred, unless the utmost
light is really necessary.
A three-wick lamp is
shown in fig. 23. Mr.
Marcy's general plan has

been practically followed filHJ^Kfflf fBX1 W JU' B
in all, the wicks A (here
shown parallel) being
placed in the middle of a
rather large flame-cham-
ber, roughly shaped like a
cylinder, with its end
to the condenser. A tall
chimney (usually made in two lengths) fits on over the open-
ing in the top, to ensure a good draught. The ends of the
flame chamber are closed by a glass in front and a reflector
behind with a coloured sight hole in the centre. Each wick
has its own milled head as shown at B, and at the other end
of the rectangular cistern beneath is a screw cap for filling
with oil.1

1 A few alterations in the construction of oil lamps have been made since
the above paragraph was written. For details the reader is referred to the
appendix at the end of fche book, — B. S. W

*T)3

FIG. 23.— Refulgent Lamp

38 OPTICAL PROJECTION

With more light the heat was greatly increased, and for
some time it was found very difficult to prevent constant
cracking of the glasses in front of the flame-chamber. In the
Sciopticon, this glass was removed to a good distance and the
heat was less : but the triple lamps were constantly cracking,
until the introduction of what were for many years known as
the Newton patents. The first of these consisted in making
apertures in the front of the top of the flame-chamber, between
the glass and the chimney. So far from letting hot air out,
as many supposed, the strong draught drew cold air down,
and kept the glass cool enough to prevent cracking, except in
rare instances. Mr. Newton's later improvement was, however,
the most effectual, and consisted in heating the originally flat
squares of crown glass in iron tubes, till they took the curve
of the tube, and then annealing them. This allows the glass
to spring or curl with the heat, and I believe no such glass
has been known to crack, whereas no annealing was found to
absolutely prevent the breakage of flat glasses.

To ensure a good light with these lamps, of whatever
make, the following directions must be attended to. The
wicks should in most lamps be put in new about seven inches
long. Each wick must be fed in and pressed a little, while
the milled head belonging to it is turned backwards, or to the
left, when the ratchet will catch the wick and draw it down
into place. As in the preceding case, the wicks must always
be carefully dried. Trim off as evenly as possible with sharp
scissors. The next thing is to unscrew the cap at the front
end of the cistern, and fill this with oil. I mention this,
because I have received actual proof that persons have
positively lit these lamps and expected them to burn with
no oil at all ! The quantity is generally about three quarters
of a pint for three wicks, and a pint for four ; but the cistern
should be filled nearly up. The common trash often sold as
1 paraffin oil ' should not be used ; much of it is not safe in any
lamp, and none of it gives a good light ; what are known as

THE RADIANT 39

best 'refined' or best 'crystal' oils at the shops, or the higher-
priced ' safety ' oils, are what must be employed.

The least quantity spilled will of course cause un-
pleasant smell. The top or flame-chamber may be lifted up
by the hinge, as shown in the figure, and the wicks lighted so
soon as they are saturated with oil, not before : it is better if
this be done inside the lantern, as the chimney (drawn out
to its full length) can then be at once placed upon its seat.
In most lanterns the lamp is fitted by the makers to push in
from the back as far as it will go, but a little room to adjust
to and fro is an advantage. A most important point is that
the lamp should be thus lighted, and the flames turned up to
about an inch only in height, about ten minutes before it is
really wanted : without this precaution a steady flame cannot
be had, and countless failures are traceable to neglect of it.
When the whole lamp has thus become hot, the outer flames
may be gently turned up first, then the centre one or pair, so
raising the flames slowly till four or five inches in height, with
the centre rather the highest. If either flame smokes at the
top of the chimney, it must be slightly turned down again ;
and there will be trouble from unsteadiness in this way un-
less that preliminary burning be attended to which I have
described. Broadly speaking, the rule is to keep the flames as
high as they can go without smoking, the outer ones rather
the lowest ; and if gradually warmed and adjusted as here
directed, they will remain steady with little trouble.

After use once, the wicks should be smoothed before lighting
again, by rubbing off the superfluous charred part with the
finger or a bit of linen. Only occasionally may a little
trimming with scissors be necessary, and a set of wicks will
last a good while. After every time of using, all superfluous
oil must be poured back into the tin, or it will gradually
diffuse itself all over the surfaces of the lamp, and it will be
impossible to avoid offensive smell. Of course the glass in
front of the flame-chamber, which is loose for removal in all
lamps, will always be kept clean.

40 OPTICAL PROJECTION

21. Gas Burners. — A great deal of class demonstration may
be done, and views well exhibited on a disc of seven feet
diameter, with an Argand gas-burner ; and it is a very great
convenience thus to be able to get to work, or to decide the
details of a projection, with only a flexible tube from some
gas-supply (for supply see § 27). A Silber, or Sugg's London
Argand, will give a light of about twenty candles with
London, and twenty- eight with cannel gas. This may be in-
creased to about thirty and thirty- eight candles by the use of
an adjusted reflector, as described under Oil Lamps, § 19. A
Welsbach incandescent burner also gives pretty good results,
but is awkward on account of the great depth of the burner
beneath the radiant portion. It is rather superior to an
Argand in equality of intensity, an Argand flame being most
luminous at the edges, where the flame is seen edgeways. For
this latter reason an Argand burner is no use in microscopic
experiments, even on a small scale, there being two extra-
luminous edges to the flame, an inch apart.

22. The Lime Light. — The radiant in most common use
for public exhibitions will be the subject of separate chapters ;
but a few words may be added about

23. The Electric Light.— Of this three forms are available.
The arc, the ordinary incandescent filament, and the Nernst
lamp.

The Arc, Lamp. — This is the most powerful of all illumi-
nants for lantern work, and the radiant being extremely
small and concentrated, the greatest perfection of optical
projection is obtainable by means of it.

Up to a few years ago the use of such lamps was chiefly
confined to large instruments such as described in Chapter XII.,
and the lamps themselves were large and expensive ; but the
tremendous developments in electric lighting have caused a
corresponding increase in the application of the electric arc
to lantern work, and the result has been the introduction of
small arc lamps suitable for any lantern of ordinary size, and

THE RADIANT 41

in lecture halls and institutions all over the country such
electric lanterns are now to be found. A fuller description
of these lamps and their management will be found in the
appendix at the end of the book.

The Incandescent Filament Lamp. — This has for some
years been applied to the ordinary slide lantern, and where
the electric current is laid on is a very convenient and handy
radiant. The ordinary loop filament diffuses the radiating
part too much, and is also too feebly luminous, but the
Edison and Swan Company have brought out a lamp in
which the carbon is bent into a close grating, covering a
space of about half an inch square, which is perfectly suitable
for the purpose. This lamp is supplied complete with
reflector, etc., with a fitting to clamp on to any limelight
tray, for about forty- two shillings, and has been consider-
ably used where current heavy enough for an arc lamp is
not available. The current required is from three to six
amperes, and a light of 50 to 100 candle-power is obtainable.
Of late years, however, these lamps have been practically
superseded by

The Nernst Lamp.— This is really also an incandescent
lamp the filament taking the form of one, two, or three
straight rods and the vacuum bulbs being entirely dispensed
with.

The light does not compare with the electric arc as the
size of the radiant is too large for really critical definition,
but these lamps do mark a very great improvement on the
above-mentioned incandescent lamps; and as they can be
connected to any ordinary lamp socket in connection
with the ordinary house -lighting mains, and can be used
with alternating or continuous current indifferently, they
are being extensively employed for ordinary lantern
demonstrations.1

1 For fuller details of Nernst lamps the reader is referred to the
Appendix.— B. S. W.

42 OPTICAL PROJECTION

CHAPTER IV

THE LIME-LIGHT

IN one or other of its various forms, the lime-light is far more
largely employed than any other for serious lantern-work of
all kinds, and is only likely to be superseded by such advances
in electric lighting as may make an electric current avail-
able in public buildings generally, without burdening the
lecturer with the provision of generating apparatus. It is
cheap, easily managed, and perfectly free from danger when
its principles and methods are understood.

Till very recently, it was the almost universal practice to
use the oxygen from a gas-bag under pressure, and this
method is and will be still so general, that it is convenient to
begin with it.

24. Gas-Bags. — These are of three kinds. A thin kind,
made of jeanet treated with india-rubber solution, has scarcely
any wear, and even for low pressure such are bad economy.
Much better are those usually known as ' best black twill,' and
generally used. It pays best to purchase these for all forms
of the low-pressure jet; and if of thick make, which can be
judged by the thickness at the edges, they are pretty durable
even under heavy weights. But the best of all are made of
sheet india-rubber, cemented between two fabrics. These are
the most expensive, but wear so long, that for anything like
hard wear they are far the most economical in the end. As
india-rubber ' perishes ' in time, however, for only occasional
use it may be better to use the cheaper black twill, and when
necessary procure another.

When a bag leaks, so long as the leaks are circumscribed
and definite, it may be patched so as to last a considerable
time longer. At india-rubber shops, a shilling tin can be
purchased of ' india-rubber solution,' which is principally the

THE LIME-LIGHT 43

gum dissolved in naphtha. Having procured a piece of thin
waterproof — the kind of which nursing aprons are made — and
cut a piece to cover the leaky place with a good margin, the
solution is smeared all over one side of it, and over the space
on the bag it is to cover. It must be left for half an hour to
two hours, until it has become very sticky or ' tacky,' when
the patch is applied, all air worked smoothly out, and the
patch then left under a heavy weight, with a soft cloth
collected into a pad between, to equalise and apply the
pressure. The leak can generally be found by the smell, if
the bag is filled with house-gas and pressure applied.

Definite leaks can be stopped in this way, especially such
as may be caused by mischievous boys sticking pins into a
bag ; but when they get numerous it is a sign that the india-
rubber is worn out generally.

The cubic content of a wedge-shaped bag is of course
found by multiplying its three dimensions and taking half the
product ; but the bulging of the bag when full, will add from
one to two feet more. A size 3x2x2 feet will keep all
ordinary jets going in a bi-unial lantern for two hours, but is
not sufficient for the powerful jets presently described. To
be certain there is plenty of gas is a very great comfort ; and
to be uncertain, very much the other thing.

Gas should never be kept in a bag any length of time.
Oxygen acts rapidly upon the material ; but independently of
this, no gas can remain pure, owing to that wonderful process
of diffusion through the material which physicists call osmosis.
A bag filled with perfectly pure oxygen, will contain a portion
of air in some hours' time, and any bag tight in the morning
is perceptibly slackened by night. Supposing gas to be left
over one night, to any amount, there is no reason, if con-
venient, why it should not be kept, and merely filled up for
the next night ; but beyond this gas should never be kept, or
the light will be perceptibly affected.

keep best in a moderate temperature, both cold

44 OPTICAL PROJECTION

heat being injurious. I believe they wear out chiefly by the
opening and shutting in turn at the creases, where leaks
nearly always first occur. Hence they keep better, if con-
venient, laid flat between their boards, than if folded up,
which makes extra creases to wear out. If they have to be
folded to go on a shelf, as is usually the case, they should be
folded once only as loosely as possible, the sides being tucked
properly in. The taps should be left open. Whenever bags
are stiif with cold, they should be softened by gentle warmth
before use ; the creases will rapidly wear through in a har-
dened bag.

The very large and heavy taps used by some makers are
to be avoided, as are nozzles with a projecting rim on the end.
A moderate nozzle, with fluting all round, either the same
diameter or gently increasing back from the tip, is best. Most
taps are, however, smaller than they should be in the bore.
Few are more than j- inch in aperture, and they manifestly
ought to be the same as the rubber tubing. This is a point
really needing the attention of manufacturers.

Lock taps are made, to prevent accidental or wanton
turning of the tap in transit. It would be much simpler to
have a plain lever on the plug, instead of a thumb-piece, the
lever being set parallel to the nozzle when the tap is closed.
Then it could be simply bound round with the nozzle by some
string, and would be safe. If there is no way of securing the
tap, it is best to take a piece of rubber tubing which strains
tightly on the nozzle, double over the end and tie tightly, so
as to make an outside stopper, which is slipped on. Such
precautions are only needed, of course, when full bags have
to be taken about.

With a bag full of oxygen, the next step is to force this
out with some regularity of pressure, to do which the bag is
placed between a pair of pressure boards, the top one of
which is loaded with weights. In all forms of the lime-light
with only one bag filled with oxygen, pretty much the same

THE LIME-LIGHT 45

pattern is used, as shown in fig. 26, which speaks for itself.
It is only necessary to say, in case a pair of boards is home-
made, that the pair must not come closer at the hinges than
half-an-inch apart, to
allow for the thick-
ness of the bag when
flattened. If this is
not attended to, the
boards will not shut,
or use the last por-
tion of the gas. In FlQ> 26_Bag in use
case of an emergency,

a bag has been ere now laid on the floor, and a black-board,
or the ends of a couple of reversed forms, hinged down to
the floor by two pairs of staples or hooks driven into floor
and board respectively.

25. Tubing. — The best soft grey or red vulcanised tubing
should be used; common harsh grey, or cheap red, spoils
very soon. Tubing with spiral wire inside must be scrupu-
lously avoided ; the wire soon rusts, breaks, and chokes and
perforates the tubing. There should be a clear ^ bore for
oxy-calcium jets, and not less than f for high-pressure jets.
The ordinary ' stout ' thickness answers, but successive coilings
after use generally twist and kink it up after a bit, especially
the red. For anything like regular use, therefore, the cheapest
in the end is good soft tubing of best quality and double thick-
ness. Fig. 27 is an exact section of what I use.
It is more than double the price of the common
tube, owing to the weight of material ; but
there is immense wear in it, it never kinks,
and it may be slipped on a nozzle in any posi-
tion without choking the bore at a bend. 27

Occasional extra long lengths of tubing are
joined up by slipping the shorter pieces over the ends of a
few inches of brass tube. All such pipes and nozzles should

46 OPTICAL PROJECTION

make a tight fit with the tubing ; but if there is any doubt
about this, small strong vulcanised rings (of which the Ian -
ternist should always have some in his box) will make all
secure. It is a better plan, however, for a travelling operator
to carry some yards of cornpo gas-pipe for bringing up distant
supplies of gas.

26. The Oxy-Spirit Jet.— All forms of the lime-light in
which oxygen alone is used under pressure, are generally called
the * oxy-calcium ' light. Of this there are two main forms.
The first is where a fine jet of oxygen is blown through the
flame of some volatile fluid, usually methylated spirit. This
form of the lime-light was invented by Lieut. Drurnmond in
1826, and was formerly largely used, but is now chiefly em-
ployed in country
villages where gas
is still unprocur-
able. The present
usual form of this
jet is shown in fig.
28, where a cistern
A B outside the lan-
tern, adjustable on
a rod by a clamp -

FIG. 28.— Spirit Jet J

screw P, feeds the

alcohol to a small circular wick, through the flame of which,
almost non-luminous in itself, the oxygen is blown by a nipple
c, pierced with a very small bore, on to a cylinder of lime D,
which can be rotated on a spindle E, and adjusted at different
distances from the flame. G is the stopcock to control the
oxygen from o. A very good light, equal to a disc of twelve
to fifteen feet in diameter, can be obtained in this way, pro-
vided the jet be adjusted as described (see § 28) a little farther
on. If the cistern is arranged, as it generally is, outside,
where it cannot be dangerously heated, no possible accident
can occur with this jet except by upsetting the fluid. All the

surplus should be invariably poured bad
is closed.

Mr. S. Highley used to insist on the
ing a double flat wick turned edgeways to the
or cut nearly to the slant of the latter, and the
separated, so that the oxygen blew up a little slanting trougn*
of wick. I was never convinced of the benefit of this, and the
ordinary jet is good enough for any of the places where it is
ever used.

I do, however, believe that gain follows the adoption of
Lieut. Drummond's original form of lime. He always used a
small spherical ball, about f inch in diameter, supported on
a platinum wire. Heat is wasted on a large mass, which is
utilised in incandescence when concentrated on a smaller frag-
ment ; and the small balls will bear very well for the requisite
time the action of this weakest form of the lime-light.

What are called soft limes give the best light, and with
average jets this may be taken as equal to about 120 standard
candles. It is rarely less than 100, and I have known it coaxed
to nearly 150 candles.

For the use of this jet with two lanterns, see Dissolving
Views, § 56.

27. The Oxy-gas Jet— This is often called the ' oxy-cal-
cium 'jet, and also the ' safety ' jet, or the ' blow- through '
jet. None of these names are distinctive, and I prefer that
chosen, as indicating that
oxygen is used with * gas '
from the ordinary meter
supply.

The flow of house gas
must be pretty free, and
it seldom answers simply

to slip the end of a rubber tube over the nipple of a common
burner. In many halls there is a nozzle provided somewhere,
specially for such purposes. If not, the nipple of a burner

FiG. 29.— Gas Nozzles

48 OPTICAL PROTECTION

is unscrewed with a pair of gas-pliers, when a gas-nozzle of
one or the other pattern shown in fig. 29 (each of them being
provided in both standard sizes of screw-thread) can be screwed
in or on in a moment, and will give a good supply, with a
nice descending start for the rubber-tube. A gas-flame being
provided in this way, a jet of oxygen is then blown either
through its centre, or across it. This jet must, therefore,
have two gas-tubes, and be controlled by two taps.

The oxy-gas jet is used in four forms, the principles of
which are shown in fig. 30. That marked A is most usual, a
small stream of oxygen being blown through the centre of the
flame from an open tube of gas, which it will be convenient
to call ' hydrogen ' henceforth. No explosive mixture can

n
Fm. 30.— Oxy-gas Jets

take place with this jet, but when the oxygen has been turned
off for a little time, the hydrogen may creep down the oxygen
orifice a little, and cause a slight ' snap,' if the oxygen is
turned on again suddenly. Very gradual turning on of the
oxygen will avoid this, or a very small quantity of oxygen left
on will do so.

The B form is the same, except that the oxygen terminates
£ inch or more below the top edge of the hydrogen tube.
Hence there is a little better mixture, and rather more light.
This form is, however, a little more apt to ' snap ' out the light,
if, after being off some time, it is turned on suddenly. A small
supply of oxygen left on, or an oxygen « bye-pass ' (see § 58),
is almost necessary to avoid this with certainty. The general
form of both these jets is shown in fig. 31, but the lime-turn-

THE LIME-LIGHT 49

ing movements described under the mixed jet are often ap-
plied to them.

The c form is less common. Here the hydrogen flame
comes from an open perpendicular nozzle, and the oxygen is
blown across it. This form of jet gives larger wings of hy-
drogen flame outside the jet which plays upon the lime, and
consequently heats the lantern more ; but it is impossible to
' snap ' it, as the hydrogen cannot get down the oxygen tube.

In the D form, a dome with a smaller orifice contracts the
hydrogen flame, through which the oxygen is blown. The

FIG. 31. — Oxy-gas Jet

consequence is a better mixture, a smaller flame, and when
properly adjusted a better light ; but this form, like A and B,
is liable to ' snap ' unless the oxygen has a bye-pass. Perhaps
it can hardly be called absolutely a ' safety ' jet, as it mani-
festly approaches somewhat to the ' mixed ' character. It is
barely conceivable that under some conditions a mixture
might occur sufficient to cause a violent snap ; but I never
heard of such an accident occurring. This is probably the
form that an experienced operator would prefer to use, for
its more contracted flame and better light. The best form
for a novice is c.

* E

50 OPTICAL PROJECTION

28. Adjusting Oxy-gas Jets, — These forms of jet will
easily give 200 candles, and with care have reached 250
candles, and more. In many cases, however, they never give
what they might for want of a proper adjustment, which is a
matter of more nicety with them than with mixed-gas jets.
The same remark applies to oxy-spirit jets, and the adjustment
is practically the same in both cases.

The most general error is to use too much weight on the
bag. It seems considered the correct tiling to put on a 56-lb.
weight in any case, but in many instances 28 Ibs., or only a
few Ibs. beyond, will give a better light, and the too fast rush
of the oxygen through the gently-emitted hydrogen actually
cools the lime. The distance of the lime from the jet is also
all-important. These points cannot be adjusted in the lantern,
as every change in position there affects the optical adjustments
also, and thus disguises the result as regards brilliance. The
jet should therefore be taken out of the lantern, and placed
so that its naked illuminating effect upon the screen or a wall
can be seen. Then experiments should be deliberately made :
first as to oxygen, and then as to the distance of the lime
from the orifices. This will probably not differ much either
way from half an inch ; but it will be found that an eighth of
an inch will make a great difference. At each change in
distance, however-, 3t may be found that a different flow of
the gases gives a better light at that distance ; and it has to
be ascertained what distance, properly adjusted by tap and
pressure for its proper flow of gas, gives the best light. That
is the adjustment for this particular jet, and, once made, is
made once for all. It is, therefore, worth while to take
trouble over it.

What are sold as ' soft ' limes are best for oxy-gas as well
as spirit jets, but ' excelsiors ' do also very well. The lime is
little acted upon by the comparatively moderate heat, and a
lime-spindle which can be turned by a milled-head underneath,
as in fig. 81, will answer if economy is desired. But any of

THE LI ME- LIGHT 51

the lime- turning movements presently described can be
applied to them.

The jets next treated of can, with care, be also used as
oxy-gas jets, as by-and-by mentioned.

29. The Mixed-gas Jet— This term is often abbreviated
into ' mixed ' jet, and signifies that the two gases are actually
mixed in proper proportions before combustion at the orifice ;
for in this jet there is but one, to which both gases in a mixed
state are conveyed. It gives the most powerful and whitest
light, in a smaller space ; and is easily reduced to any degree
desired ; and for the same amount of light, uses less gas. It
is, therefore, the form used in first-class lantern exhibitions,
and gives far the most brilliant results in optical and physical
experiments, while for high microscopic powers it is indis-
pensable. There is a widespread notion that it is ' liable to
explosion,' which has been partly fostered by the errors of
opticians, and their mistaken so-called ' safety ' arrangements.
But the conditions of safety are now well understood ; and
while it is not a jet to be trusted — if, indeed, any jet ought to
be — to the manipulation of clodhoppers, schoolboys, or absolute
novices, with those who understand it the mixed jet is the
most simple to manage and easy to control of any, and
perfectly safe.

Both gases have to be used under approximately equal
pressure, and the first arrangement employed was to mix pure
hydrogen and oxygen, in the proper combining proportions,
in one vessel, expelling the mixed gas through a minute
orifice. It is often stated that this method gives the most
brilliant light. That is a total mistake. No light was ever
obtained in those days nearly equal to what is obtained now ;
while the mistake led to a series of explosions which it was
impossible to prevent with any certainty, and which gave the
mixed-jet a bad character. Mr. Newman used a nipple of
one-eightieth of an inch bore with success for some time ;
but directly he changed it for one slightly larger— about

K2

5-2 OPTICAL PROJECTION

one-sixtieth of an inch — his reservoir burst like a bomb.
Hemming packed a tube with fine wires, on Davy's principle;
and a series of layers of fine wire gauze was also tried ;
finally the gases were bubbled through a water-chamber.
Sooner or later the gases thus mixed exploded through them
all.

As a matter of fact, the gases are, not burnt in a jet in
their combining proportions, when properly adjusted by the
taps for the most brilliant light ; nor is the available heat
developed what was once supposed. The heat-energy liberated
by the combination of H2 and 0 into water, can be calculated
easily ; but no sooner is water formed, than a considerable
portion of that energy is absorbed in again dissociating it.
A certain amount of mechanical current, conveying the gases
to a certain distance away from the orifice, may and does,
therefore, increase the effective heat, as does a certain amount
of external free hydrogen flame, which aids in the same
object of carrying the water away in vapour.

Even now it is almost impossible to burn either pure
hydrogen or carburetted hydrogen (house-gas), and oxygen,
in their combining proportions, without explosion, though
with modern apparatus this latter will be only the * snap ' of
a small and harmless explosion in the jet-chamber itself.
Take any mixed jet, and gradually turn on the oxygen ;
almost invariably, just as the surplus flame of the hydrogen
is absorbed, the jet will snap and go out. Before this, the
light will have diminished considerably ; and it will be found
that the most brilliant light is always obtained with a con-
siderable surplus of the hydrogen. That surplus is greater
as the bore of the nipple is larger ; and those manuals are
wrong which state that the gases are burnt ' in equal volumes.'
With large orifices, nearly ten feet of house-gas may be
required for eight feet of oxygen, and it will be found that at
the best light there is always a loose flame of hydrogen
playing about the lime. With this increase of hydrogen from

THE LIME-LIGHT 53

the larger bore, the distance of the lime must also be in-
creased, to get the best result ; so that whereas a small bore
may do best with the lime almost close to the metal, a large
bore may require a clear space of an eighth of an inch (or a
quarter of an inch measured in the oblique line of the jet)
from the centre of the orifice to the lime.

The first step to safety was therefore to store the two gases
in separate bags, when the quantities could be adjusted
according to the light itself. The mixture ,at the back of the
nipple became, while the light was better, far less explosive ;
so that a moderate speed of exit would prevent a ' pass-back '
of the flame. Explosions at once became rarer, and confined
to accidents or carelessness. For instance, it was usual in
those expensive days of chemicals, to keep unused gas in the
bags ; and if any bag should be filled up with the wrong gas,
it is easy to fancy the result. Even recently a fatal explo-
sion at a theatre was traced to this cause. A mistake of this
kind is inexcusable ; and if the reader cannot trust himself
never to make such, he should not meddle with oxygen in
bags at all. The two bags may be of different colours, or, if
both are black, the tap of the hydrogen-bag should be
coloured, or a large H and 0 should be painted on the bags.

Another danger was found from the liability to unequal
pressure on two bags, as it was usual to place each in a pair
of pressure-boards like fig. 26. Fire-irons and fenders were
often used as weights, or a lad might be asked to sit upon the
top board. Hence there was a danger of the gas under
greatest pressure, sent to the nipple faster than it could
escape, being driven back so as to gradually mix with the
other gas under less pressure. Every alteration of the weight
on a bag was thus a precarious experiment, and occasional
explosions were practical proofs of the necessity to start with
the bags about equally full, and under equal pressure, and to
keep them so to the end. For the pressure of gas differs a
great deal as the bag gradually discharges, and it is of im-

54 OPTICAL PROJECTION

portance that the reader should understand this, and the
terms in which pressure is described.

Pressure is stated as so many • inches ' ; meaning of
water. This is measured very simply by what is called a
U-tube, shown in fig. 32. A glass tube is bent into U form,
deep enough to measure any pressure likely to
be met with — say eighteen inches from the
top to the bend. One end is corked, with a
bit of tube passing through the cork on which
can be stretched a vulcanised tube from the
bag ; and a scale of inches is drawn between
the legs of the U, measured from a zero-line
across the centre. Water is poured in at the
open end till it stands level at the zero-line.
Then the tap from the bag is opened, and the
pressure sends down the water in that arm of
the U, driving it up the other by an equal
amount ; and the difference of levels is the
pressure of the gas. House-gas in London is generally rather
under two inches, i.e. it drives the water down 1 inch on one
side, and up the other. A gas-bag of 36 x 24 x 24 size, under
1 cwt. pressure, when drum-tight, may have almost any pres-
sure, as the tension of the bag is added ; but when this is
gone off after a few minutes, the pressure is usually about
9 inches ; when the bag is about three-quarters empty, this
is generally gone down to about 4^ or 5 inches.

It will show the kind of danger to be guarded against, to
describe the only explosion — a very slight one— which ever
happened to myself, almost at the beginning of many years'
use of the mixed jet. I was using up half a bag of oxygen
in determining a ticklish experiment, and had filled the
coal-gas bag considerably too full in proportion, so that it
was quite one third full when the oxygen was nearly ex-
hausted. Absorbed in the experiment, I did not notice that
the board was practically ' down ' upon the flattened oxygen

THE LIME-LIGHT 55

bag, and unconsciously kept balancing the decrease of oxygen
by turning off the hydrogen tap, to keep at its best the waning
light, when — bang ! There was an explosion almost as loud
as a pistol-shot in the nearly empty oxygen bag, rolling off
the weights on to the floor, and tearing the boards apart at
the hinges. There being hardly any oxygen pressure to resist,
the hydrogen had gradually forced itself back into the last few
cubic inches of oxygen ; and as there was hardly any back-
pressure at the nipple to resist a * pass-back,' the result was
the explosion. Owing to the small quantity in the bag, no
other damage was done, the bag itself not being injured, and
being often used afterwards; it expanded enough without
rupture, only the hinges giving way from the suddenness of
the shock.

Lesson. — Never work a bag down quite empty with the
mixed jet. Observe how low the boards come down when
empty, and always leave off while there is an inch or two to
spare. Also, so soon as you have to be frequently turning off
the tap of one of the gases at the jei to restore a fast de-
creasing light, it is a proof that the pressure is diminishing
rapidly in the other bag ; if you go on, mixture may occur,
and it is time to stop. With decently competent management
in other respects, this is the grand rule of safety, but I have
never seen it stated in any lantern manual.

80. Double Pressure-Boards. — To resume practical details,
the last step to safety was the abolition of all difference in
weights by the use of double pressure-boards as shown in
fig. 33. By this arrangement one bag is placed over the
other, under the same weights, and it no longer matters what
these weights are, so long as they are sufficient to keep up
pressure.

In making these boards, the hinged ends of the two
outside ones must be wider apart. The two bags are not
placed in contact, because full bags would slip ; but either a
middle board, or a piece of stiff sail-cloth is connected to the

56 OPTICAL PROJECTION

hinged end, and extends between the boards as shown. They
are often made on what is called the * skeleton ' plan, for
lightness in travelling ; a mere framework being covered either
with a trellis of thin hoop-iron, or with a sacking laced up
as old-fashioned bedsteads used to be. Solid boards, however,
need not be much heavier, are much cheaper, and for home
or at an institution are to be preferred ; in the latter case they
should be made with a flat board between. When a piece of
canvas is used to separate the bags, it is best to pass a buckled
strap round the back of the bags from top to bottom, to pre-

FIG. 33. — Double Boards

vent them slipping backwards till the bags have flattened out
a little, after which there is no tendency to this. A pair with
a board between needs no straps.

It is necessary to raise the front or hinged ends of the
boards considerably wliile the bags are full, or the top will
be too steep for the weights to press upon. From this arises
a risk, not of explosion, but of a vexatious accident which
has been known to occur. The middle board must be nearly
level with full bags, as in fig. 34, and it will be seen that in
this position the weights w are supported well at c within the

THE LIME-LIGHT

57

base-line A B. But when the bags have worked down as in
fig. 85, the weights are farther back in proportion, and so
far as the back edge of the bottom board is concerned, over-
Jiang the base. If this is not provided for, the weights may
therefore tip up the whole concern backwards, and roll off,
when there will be sudden darkness and a serious fright,
though no further accident could happen unless there had
been mixture in one of the bags ; if there had, of course the
removal of pressure would produce an immediate explosion.
I never heard of this latter happening ; but of the other I
have, and it is unpleasant enough.

The boards must therefore be let down on the floor when

A q

G A

FIG.

Fio. 35

the bags are about half emptied, or else the ' base ' must be
so lengthened behind the back of the bottom board, as to
guard against all danger. It is quite easy to do either.
Solid boards are generally made without any separate base,
for simplicity, a support being hinged to the front at A, fig. 36,
which may be a pair of light legs with a connecting strip.
Then a stretcher-bar, s, may be hinged near the back end of
the lower board at B, resting on the cross-piece connecting
the two legs ; and if a hole in the end of the stretcher drops
over a wire pin projecting from the cross-piece, at c, it is
impossible for the legs to slip from under the boards. When

58 OPTICAL PROJECTION

half empty, the stretcher is lifted off the pin with the toe,
and the front end of the board being gently lifted (there is
hardly any weight on this end) the legs can be pushed back
underneath in the same way, and the front gently lowered on
to the ground.

Skeleton boards are usually made with a base as in figs. 33
and 37, the legs or support being hinged to the front of this
at B, and also to a point some distance back at A, on the under
side of the bottom board. When the front is elevated, this
arrangement brings the back of the bottom board forward to
c, where it is kept from slipping back by iron pegs slipped
into a hole at each side. The base extending back to D, pro-
vides security against an overturn ; when half empty the pegs

w

FIG. 36 FIG. 37

are withdrawn by an assistant, and the boards gently lowered
and slid back.

To get a good light (and it is also an element of safety)
fair pressure must be used. On a small screen a sufficient
light may often be had with 56 Ibs., adding another towards
the close. But for a large disc, a pair of bags 36 x 24 x 24
will require two \ cwts. to begin, and a third to finish with ;
and larger bags — say 42x32, for larger orifices, will need
three such weights to begin, and a fourth to finish. If gas
is no object, even more may be used with gain to the light,
but not in proportion. I believe the profitable limit to He,
with proper jets, somewhere between 12 and 15 inches
pressure. Of course, if jets are used with a lot of so-called

THE LIME-LIGHT 59

' safety packing,' it is impossible to say what pressure may be
needed to drive the gas through them.

31. Safety arrangements. — This brings us to the most
important point. The one, the sufficient, and the only security
for safety with the mixed jet, is the certainty of pure unmixed
gases in the bags to start with, and next, a good and fairly
equal pressure on both bags, with sufficiently free vent at the
nipple of the jet to make sure the gases shall always be flow-
ing outwards. This is simple, easily understood, and, when
understood, easily carried out. No other safety arrangement
can be depended upon, and if it is depended upon, to the pre-
judice of proper management in these respects, may be even
worse than useless. Self-acting valves which allowed passage
outwards, but not back, were soon invented, and are still sold.
I am not prepared to say that they are inoperative, for there
is evidence that, experimentally, they have prevented ex-
plosions which otherwise must have occurred. But such
arranged * experiments ' rarely represent the conditions of
unforeseen accidents ; and such occurred more than once in
spite of these valves. They also obstruct the flow of gas,
which is one safeguard. But the real mischief is, that people
who use these things usually depend upon them ; and that
means real danger. Packings of gauze in the jet are absolutely
useless, if the gas behind is in a really explosive condition.
The packing of granulated pumice presently described is
indeed a real stop to explosion, so long as it is in order ; but
it checks pressure materially, and a few ' snaps ' in front of it
may so disarrange and alter it by repeated shocks, as to render
it useless. With the double pressure-boards, a cool and in-
telligent operator ensures that explosive conditions cannot
occur, and that alone is true safety.

32. Forms of the Mixed Jet. — This brings us to the forma
procurable of the mixed gas jet, its faults and difficulties,
and the construction which will give the best results. Daniel's
form, one of the first made, resembled the blow-through jet D

60 OPTICAL PROJECTION

of fig. 80, except that the outer orifice was contracted to a
small aperture. It gave a fair light for those days, and is
used now sometimes, but the mixture almost at the very
orifice generally causes a whistling when much light is at-
tempted. The jet next introduced, and still very often seen,
had a plain lime-spindle like fig. 81, but the two gas-tubes
were conducted side by side into a chamber, packed with
layers of gauze. This gauze is always getting rusty and
obstructed, to the detriment of the light, and actual increase
of the danger of mixing behind ; and it is unpleasant for both
operator and audience to be obliged to be continually opening
the lantern door to turn the lime round with the fingers.

. 38.— Common Mixed Tap

For under the greater heat and stronger blast of the mixed jet,
even * hard ' limes are rapidly burnt into holes, or ' pitted ' as
it is called ; and a fresh surface must be exposed to it every
few minutes, or the blast may be reflected back from the
concavity on to the condenser, and crack the lens. Hence a
cogged lime-turning movement was speedily introduced,
resembling fig. 88, by which the lime can be rotated from
the back of the lantern. This is the common trade jet
of the present day, and will answer fairly well for small
apertures, say up to 1 mm. diameter, if the gauze is removed,
or only one piece left in the chamber. The mixing-chamber
usually found is however too small for a really powerful light ;
and as the circular plate which supports the lime is simply

THE LIME-LTGHT 61

rotated by the cog-movement on a screwed pin, It not only
has a tendency to stick fast, but it scarcely rises with an entire
revolution, and the movement has to be continued for many
rapid revolutions to bring a fresh ring of the lime surface into
play. Hence a good ' blow- through,' or oxy-gas jet, will often
equal the light of such a mixed jet. The performance can be
greatly improved by clearing out the gauze to leave a free
passage, and attending to the jet as presently described. But
more than 350 candles can scarcely be got from such jets, and
with this gain in power the inconvenience of adjusting the
lime becomes much greater, as it has to be oftener made.
A good lime-turning movement should raise the lime
sufficiently, at every revolution, to bring the required new
zone before the jet.

What is called the ' improved ' arrangement amongst
London opticians answers fairly well, the cog wheel being
made to turn a barrel with a square hole, through which the
squared lower end of the lime spindle slides freely, whilst a
treble-threaded screw on the upper part works in the brass
frame of the combination. The mixing-chamber into which
the gas-tubes deliver the gases is also rather larger in these
jets, which can often be made to give a very good light,
especially by taking out the gauze. If they do not, or if more
light is desired, or if, as is often the case, they whistle or roar
under good pressure, they must be attended to, and it is always
worth while to spend a little trouble over a well-made jet. The
tap-plugs should be taken out to see that they are not choked
by tallow, as is sometimes the case. Whistling or roaring is
generally due to some roughness in the nipple, or end of the
tube bearing it. To remove this the nipple should be taken
off, and a watchmaker's broach twirled round in it, which will
smooth out the bore itself ; and any roughness in the larger
bore below, or at the end of the tube, should also be cleaned
down with a tapered steel rimer. Filially a steel needle should
be taken, rather smaller than the bore, and twirled well between

OPTICAL PROJECTION

coarse emery cloth pinched upon it with finger and thumb ;
this will surround the needle with circular scratches, making
it in fact into a fine round file, to be carefully used inside the
bore, by which the latter can be made as smooth as glass.
This wi 11 in most cases stop the noise. If more light is wanted,
we must burn more gas to get it, and the bore may be enlarged
with the broach to any size desired, before polishing, if the
jet will allow, for it will be found that a certain chamber will
only allow of a certain size of bore without noise.

In making systematic experiments as to the light possible
from the oxy-hydrogen jet, I found, therefore, that it was

essential to employ a larger
chamber if it was desired
to use a really large bore.
And with any jet which has
a large chamber, let us
suppose it is a circular ca-
vity J inch in diameter and
the same in depth, a large
bore may be used. Very
likely, however, after all
has been done to clear and
polish the nipple, there
may still be noise, and this is generally due to eddies in the
gases caused by the form of the chamber. This will probably
be as in fig. 39, and the sharp corners at A B cause the noise.
By tapering off these, especially at B, as shown by the same
letters in fig. 40, silence will generally be procured, and this
is, therefore, the chamber I have adopted for my mixed gas-
jets. With it there is no difficulty in employing nipples TV of
an inch in bore, beyond which there appears little, if any,
gain in light. Such a chamber and nipple will easily reach
700 candles with the photometer, and over 800 has been re-
corded.

The outside of the nipple, however, also claims attention.

FIG. 39

FIG. 40

THE LIME-LIGHT 63

With most of the jets commonly sold it is fair too thick and
clumsy at the tip. Such a thick end positively obstructs the
light from the lime, and by preventing the smooth access of
air to the outside of the flame, it further diminishes the power
of the jet. The nipple should, therefore, be carefully tapered
down to a thin edge. Small bores up to -^ inch can be
tipped with platinum, though this is not necessary ; but with
large bores, unless the platinum is carried over \ an inch
down, the brass will be melted away from it. For large bores
solid plain brass nipples are therefore best. It is well to have
at least two nipples of different sizes, and they should screw
on so nicely as to be gas-tight without cement, for interchange
when desired.

After all possible pains, a powerful jet such as above de-
scribed, will sometimes ' sing.' In that case it can usually be
quieted by dropping a wire ring
on the bottom of the chamber,
laying on that a disc of perfo-
rated zinc cut to fit the chamber,
and keeping that down in place
by another ring, cut so as to
spring tightly. But the best
arrangement of all I have found
to consist in a few alternations
of thin discs pierced as fig. 41,

separated by rings, as devised by Mr. Lancaster. With a
chamber properly tapered at top and bottom, as shown, this is
quiet under any reasonable pressure, and gives aperfect mixture
and a powerful light, which can sometimes be pushed as high
as 1,000 candles. After exhaustive trial of many experimental
chambers most carefully made for me by Messrs. Newton, this
is the form I have finally adopted as the best.

It must be clearly understood that the object of this
packing is not ' safety,' which is never ensured by such means,
but solely a quiet and thorough mixture of gas in the chamber.

64 OPTICAL PROJECTION

If the empty chamber will work quietly, it is nearly as good.
Either is a true safety arrangement, because if by any careless-
ness an explosive mixture did occur, it would occur first in the
chamber, which would * snap ' the jet out at once.

One other pattern of jet deserves mention, i.e. the
' Injector ' jet. This is essentially a mixed-gas jet, the coal
gas being supplied from the main. With care, any ordinary
mixed jet can be used in this way, and the oxygen being
supplied from a compressed cylinder with a good regulator,
such a method of working is perfectly safe. Without very
great care in adjusting the oxygen tap, however, the jet is
liable to snap out, and in any case the light obtained is very
little if at all better than with the blow-through form.

With the ' Injector ' jet, however, the oxygen blast sucks
more coal gas out of the mains, and thereby the necessary
pressure is obtained. The greater the pressure of oxygen
used, the more coal gas is drawn out to combine with it, and
hence it is hardly possible to ' snap ' these jets.

The light comes about midway between that given by the
- mixed gas and by the blow-through forms. It is sometimes
advertised as being equal in power to the mixed, but I have
never found it so in practice.

To get the best light a higher pressure of oxygen is re-
quired than is given by the usual pattern Beard's Regulator, and
Mr. Beard constructs a special regulator for use with this jet.
The best lime-turning movement, in my opinion, for prac-
tical work, of all that have come under my notice, is that
shown in fig. 43, which was originally devised by Mr. J.
Place. The long steel spindle has a longitudinal groove
cut, which slides up or down over a feather in the cog-wheel ;
and there is also a long-pitched spiral groove which gives
the required motion. To this Mr. Newton has added a
small wheel on the manual rod, in which notches are cut at
proper intervals. A spring detent bears against these notches,
so that the exact spot at which the line should be held is

THE LTME-IJGHT 65

felt by an almost imperceptible click. The whole lime is thus
traversed in one continuous spiral from top to bottom, each
halting-spot on the surface being exactly marked out. The
whole jet is shown in proportion, with the further addition of
a ' cut-off ' arrangement devised by Mr. A. Pringle, in fig. 65,
p. 118.

Mr. E. G. Wood has also constructed a jet with a click or
check mechanism, so arranged, that when a second revolution
of the lime takes place, the places where the jet impinges are
half-way between those of the previous revolution. In this

Fi<5. 43.— Lime-turning arrangement

way less perpendicular motion of the lime becomes necessary.
There is another screw motion, with a separate handle, for
adjusting the distance of the lime from the jet. The jet
itself is packed with gauze, and the tube and nipple are bent
like an elbow, which Mr. Wood considers to produce a better
mixture of the gases. As will be seen, my experiments — and
they have been long and many — have led me to a different
conclusion on this point, and moreover such nipples cannot
conveniently be employed of different sizes, which is very
desirable. The lime-turning movement is, however, ingenious
and good, and this jet would doubtless be more widely used

F

66 OPTICAL PROJECTION

were it not for its cost, which is more than double that of the
preceding.

Mr. Wood has further applied to his jet a shield or jacket
of copper surrounding the lime at a little distance, except a
space cut out in front for the play of the jet and for radiation,
which he strongly recommends as increasing the light (by
confining the heat), keeping the lantern cool, and avoiding
cracking of the lenses. There does appear to me a perceptible
gain in the two latter points ; but I am unable to trace the
least gain in light, and the copper hinders the state of the
lime from being seen through the side of the lantern. The
shield can be applied to any jet by such as desire it.

33. Attachment and Fixing of Jets. — The figures already
given show the principal methods of adjusting and fixing jets in
the lantern. Far the most common is a tray of thin sheet-iron
or stout tin, as shown in fig. 28, from the back end of which
stands up a steel rod, over which the socket of the jet slips
and can be secured in any position by a screw. The edges of
the tray slide backwards and forwards in guides, so as to draw
back the jet to any distance from the condensers. The sliding
adjustment has to be rather loose, and with the view of
obtaining more accurate motion in the line of the optic axis,
many opticians mount the rod on a wooden board of
mahogany, as in fig. 81, and I have also seen a jet mounted
to slide along tubes. I find that wood, however carefully
seasoned, is very uncertain in its behaviour in any but a single
or bottom lantern, owing to the heat ; hence, metal plates
with dove-tailed edges are better. My own opinion is, how-
ever, that the slight amount of freedom or ' play ' in the
ordinary tray is, for ordinary work, an advantage, facilitating
ready adjustment of the light.

I also prefer myself, for common exhibition work, to have
no fixing of the jet whatever, beyond one screw to pinch on
the rod. Many think differently, and various opticians make
their jets with a fork-piece projecting sideways from the front

THE LIME-LIGHT 67

end. Then, by having a perpendicular screw fixed in the board
or tray, on which two milled flange nuts screw up or down,
embracing the fork between them, the front part of the jet
can be easily fixed in any position, either perpendicularly or
sideways. This can be applied for five shillings to any jet.
For those who prefer it, opticians have also fitted jets like a
compound slide-rest, with vertical and two horizontal screw
movements. These are useful for optical or microscopical
work, but for plain lantern-work worse than useless. The
clamp-screw of the jet is not, however, put in the right place
by most opticians. It is best near the bottom of the socket,
instead of near the top, as usual, where the very act of tighten-
ing the screw alters the adjustment. At the bottom, tighten-
ing the screw has not this effect.

I have never found much trouble in fastening the jet with
a single screw in the socket. But if there is any, it can be
effectually overcome by an expedient pointed
out years ago by Mr. Samuel Highley, of
filing a flat (fig. 44) on the opposite side of
the rod to that on which the screw acts.
Just the sharp edges of the flat must, how-
ever, be filed off slightly, or they will score
the brass socket and hinder adjustment.

34. Inclination of the Nipple. — For slides the nipple
should make an angle of 40° to 45° with surface of the lime.
For optical work, or for the microscope, where the jet has to
be brought nearer the condensers, about 35° is better, or the
nipple will cast a shadow on the lower part of the condenser.
Both to avoid this, and also because there is a perceptible gain
in light, the supporting rod, and consequently the lime, should
be inclined forwards about 20° from the perpendicular. This
both brings more luminous rays to bear, and takes the nipple
gtill more out of the way, and the gain of light is very
perceptible.

It will be seen that there is a great deal to be attended to

F2

68 OPTICAL PROJECTION

before we can obtain the greatest illumination the lime-light
is capable of affording. These hints are the result of long
and sedulous attempts in that direction, with a view especially
to microscopic projection. There will remain still what
scientific observers term 'personal equation.' With the
very same apparatus, two men will not get precisely equal
results. I have always found that if unusually fatigued, I
could never myself get the same light, by a perceptible
quantity. A steady and delicate hand in adjusting the taps
has much to do with this, and there are differences in this
respect between different people.

35. Limes. — While very soft ' chalk ' limes give rather a
better light with oxy-calcium jets, the very hardest are
needed for the mixed. ' Nottingham ' and ' Excelsior ' limes
are both well known. Both vary somewhat in quality ; the
best of both are excellent, but the Excelsiors are apt to
crack across under powerful jets. The other does not, but
only lately has hard lime like ' Nottingham ' (which really
I believe comes from Ireland) been obtainable turned true,
and this for some work is essential.

36. Substitutes for Limes.— Many attempts have been
made to avoid the turning necessitated by the pitting of the
lime. Magnesium oxide made into a paste with water, dried,
and then exposed to gradually increasing heat, is said by
Dr. Eoux to stand the jet for hours ; but he cannot have
used jets of much power, for I find no superiority whatever
over lime, though the light is very fair. A mixture of
calcium sulphate and magnesium oxide, which has been
recommended, is no harder, but I have sometimes thought
either this or the former might behave better if silicated
by mixing with solution of some silicate instead of water.

The only material hitherto used which really stands the jet
is zirconium oxide. This was stated many years ago by
M. Du Motay to be 'the most luminous' as well as most
refractory of all substances, and the statement has been

THE LIME-LIGHT 69

repeated by subsequent writers. Were it only true, all diffi-
culties would be solved ; but I am sorry to have to dispel
such expectations, after frequent and exhaustive experiments,
as regards any zirconia hitherto commercially obtainable.
My own hopes were high after reading the opinion of the late
Dr. Draper of New York,1 that zirconia equalled lime in
' intrinsic brilliancy ; ' but I can only come to the conclusion
that neither he nor Continental physicists who have so praised
it, ever really knew what a good light is, as I understand it.
I have tested three different zirconia samples of English
manufacture, and three of Continental (including one from
Schuckardt of Gorlitz, stated to be prepared especially for the
lantern). The result was the same in all — the light was
distinctly reddish in colour, and far inferior to that of a
lime cylinder with the same jet. Mr. H. G. Madan was kind
enough to test the matter photometrically, with the result
that the best of these zirconias only gave a light of 1 : 2*88
compared with lime. I have since ascertained that Du
Motay's own pencils really gave about the same results.

Zirconia does decidedly better when used in very thin
discs, instead of the jet playing on the end of a cylinder;
for it is most extraordinary for its non-conducting power,
and the most powerful jet will only illuminate a small surface
and for a small depth in the pure material. The best
substitutes for limes I have been able to obtain by pur-
chase are a mixture of zirconia with a small quantity of
some other substance— probably alumina — prepared under
the superintendence of Dr. Linnemann by Smith and Hamsch,
of 4, Stallschreiber-Strasse, Berlin, in discs about | in. in
diameter and -^ in. thick, held in platinum capsules mounted
at the end of a brass rod. The light is not equal to that of
lime, but is very fair, and nearly white. The discs will stand
a great deal of work, and are very convenient for many pur-

1 Dr. Draper's own process for purifying zirconia, and his observations on
its use, will be found in the American Journal of Science, xiv. 208.

70 OPTICAL PROJECTION

poses. They are sold in tlie capsules at 10s. each. For
long experiments I have found them very acceptable, in
spite of their inferiority in illuminating power.

Only very recently, in fact since the above paragraphs
(which have been modified in consequence) were actually in
type in a more pronounced form, my hopes have been revived.
Observing in a paper l by Mr. G. H. Bailey, D.Sc., of Owens
College, that his purest zirconias prepared for other purposes,
were distinguished by luminosity and a slight bluish tinge in
the incandescence, I communicated with him, and he was so
kind as to send me three separate samples, weighing 12, 15,
and 18 grains respectively. There were difficulties in the
manipulation, chiefly from shrinkage when first ignited, which
have prevented me (by premature breaking up) from as yet
ascertaining the full incandescence of these samples ; but I
found them all absolutely free from any trace of silica * glaze,'
and satisfied myself completely that the light was in truth
perfectly white (if anything slightly bluish) and far superior
in quantity to that of any previous zirconias I had obtained.
I also proved unmistakably that the incandescence of this
material was much superior when the small thin disc is only
supported by a thin ring of platinum, to that when the sample
was mounted in one of the above-named capsules emptied for
the occasion. Such discs, about ^-inch in diameter (more is
useless) will each require about 1 gramme or 15 grains of
zirconia, previously calcined in the heat of the OH flame
itself to complete the shrinkage.

Provided therefore that samples of zirconia of approxi-
mately equal purity to these can be supplied at any price
commercially practicable, the material does hold out hopes.
The real difficulty is to entirely get rid of the silica, and these
specimens alone, of all I have had, showed no trace of glaze
after heating. As already stated, my own experiments are
not yet completed, and it remains still to be ascertained how

* See Proc. Royal Society, xlvi. 74.

THE LI ME- LIGHT 71

far it may be possible to supply such material ; but as the
subject has occupied my own attention for years, and has also
interested others, I have thought it better to give, however
briefly and imperfectly, the very latest information I have
regarding it.

37. Management. — We will now suppose the two bags
full, and all apparatus ready at hand. The first step is to
place the bags in the boards, and it is best to place the oxygen
at the bottom. This is the direct opposite of the usual
direction, but I am certain of its correctness. It ensures a
shade more weight (that of the top bag and board) on the
oxygen, and the result is, that if any change in adjustment
of the gases does take place unnoticed, it will probably be
in the direction of too much oxygen, which will snap the jet,
put the light out, and so give notice. Then the weights are
placed on, and if necessary tied on, the vulcanised tubes con-
nected with the jet-nozzles (dissolving and cut-off taps will
be described in Chapter VIII.), and both taps of the jet turned
off. Generally the two taps will be different colours, or one
will have a hole through the thumb handle ; if not, care
should be taken that the same side be always used for the
oxygen, in order that the hand may always go to the proper
tap by instinct.

A lime should now be placed upon the pin, first clearing
out the hole from dust by turning a match round in it,
and wiping off all loose pow-
der by a good rub with the
tissue-paper in which it
was wrapped. For handling
limes, and especially for re- no. 45.— Lime-tongs

moving a used-up or cracked

one, the lime- tongs shown in fig. 45, made by bending a strip
of sheet- brass, will be found very convenient.

Both bag-taps are now turned on, and the hydrogen jet-
tap may be turned on a little and lighted, giving flame enough

7* OPTICAL PROJECTION

to play gently rather more than up the lime, which is to be
slowly turned round a minute or so to warm through. Then
turn on rather more hydrogen — say about half — and a very
little oxygen ; just enough to slightly diminish the flame, and
increase the heat, so that the lime is heated red-hot easily
The whole lime is again carefully subjected to this heat
which is a great security against cracking. More oxygen can
now be turned on ; then as much hydrogen as it is intended
to use (full, if the extreme light be desired), and finally the
oxygen is turned on until the best light is obtained. It will
be found that the merest hair's-breadth of adjustment will
make a difference, and the taps should, therefore, always be
kept nicely fitted, and lubricated with a very little tallow
(unsalted) or sperm oil. A powerful jet generally « roars ' till
the oxygen is adjusted, but quiets when the two are in pro-
portion.

The best light is not obtained till the jet has played upon
the same spot for half a minute or so ; after that it remains
very steady for a couple of minutes. Then, as the cavity
deepens, it deteriorates ; and if kept too long on a deep pit, the
flame may be reflected back again, and crack the condenser.
Some limes will stand many minutes ; and when a box is
found very good, it is well to keep it for particular occasions.

With the bags in proportion — that is, hydrogen about five
volumes to four of oxygen — and equally full, the taps will
scarcely need any subsequent alteration ; but if the light does
go down a little, the necessary adjustment must be made
from time to time. At the close, or if anything goes wrong,
let the first thing be to turn off the oxygen ; and cultivate from
the first a fixed, invariable habit of doing this. Remember,
also, when the bags are half empty, to let down the boards
as already described.

At the close of all, see that the jet is out and the bag-
taps turned off before removing the rubber supply-pipes from
the lantern nipples ; else possibly the light might remain long

PREPARATION OF GASES 73

enough to ignite the stream of gas, which for a moment
would probably be directed towards the lantern. I once
knew this happen, and should such a flame reach an oxygen
bag with any quantity of gas in it, the result would probably
be a combustion of the bag itself so furious as to be almost
explosive, and highly dangerous.

With care the mixed jet can easily be used with only
oxygen in a bag, and gas from the main, instead of employing
an oxy-gas jet. In this case the bag must be lightly weighted
— only fifty- six pounds being placed on. I often use the jet
thus in experiments ; but it should only be thus used by ex-
perienced persons.

CHAPTER V

PBEPARATION OF GASES

88. Hydrogen. — It is unadvisable to make this gas. It
has been often stated to give a better light, but careful
experiments made at the old Polytechnic proved that this
was a delusion, even with similar-sized nipples. No difference
could be perceived; and double the quantity of oxygen is
needed, besides the expense of the hydrogen. A bag of gas
can easily be sent by rail, if necessary ; or a bottle of com-
pressed coal-gas, at the present low prices, will be less trouble
and cheaper than making pure hydrogen, besides saving half
the oxygen. There are also the oxy-spirit jet, and the oxy-
ether and oxy-carbon lights (Chapter VII.) to fall back upon.
There are however a few out-of-the-way places where it may
be necessary to use hydrogen for lack of other materials.

A glass bottle may be used as a generator, but a leaden one
is more usual. It is furnished with a movable cap, bearing
a long tube reaching nearly to the bottom and with a funnel
at the top, and a delivery-tube coming from the cap ; the
general arrangement resembling an ordinary wash- bottle, or

74 OPTICAL PROJECTION

as in fig. 49, which would answer very well with a funnel at
the top of the long tube. Care must be taken that the top
is air-tight. Into the retort is introduced about half a pound
of granulated or scrap zinc, and through the funnel is then
poured about a pint of dilute sulphuric acid ; about one part
of acid to six or seven of water. This should be previously
mixed, to avoid heat in the retort. The gas as it comes over
is passed through one wash-bottle (§ 42) about half full of
water, as in washing oxygen ; but great care must be taken
that all atmospheric air is allowed to escape. To ascertain
this, a light is applied to the gas, which should take fire ; but
to avoid explosion, a piece of rather fine wire gauze should
be bent into a kind of loose cap, and held over the end of the
delivery tube. When the gas inflames quietly, the bag is
connected up as usual, turning on the tap at the same
moment. If the evolution of gas fails before the bag is full,
a little more acid may be poured down through the funnel :
or if done very gradually the acid may be added from the
first in this way. Any metallic zinc left may be preserved
for the next occasion. In turning off the bag-tap when full,
the very next moment remove the delivery-tube ; but be
careful it does not discharge near any light.

Should zinc not be obtainable, clean iron filings, or small
nails, may be used instead and in the same way.

39. Oxygen. — Till lately oxygen gas was universally pre-
pared by every habitual user for his own requirements, and
so much is still made from chlorate of potash that it is
necessary to describe the best methods of managing that
process. It has been stigmatised as dangerous, but is not
at all so when properly conducted. There are certain possible
dangers, it is true ; but, guarding against these, the operator
may proceed with perfect confidence.

The only practical method of preparing individual supplies
of oxygen consists in heating over a furnace a mixture of
potassic chlorate and manganese black oxide ; or in lieu of

PREPARATION OF GASES 75

the latter, which only acts mechanically, some use fine
washed sand, or even red oxide of iron, to avoid a possible
danger lurking in the black colour of the manganese, which
has been known to be adulterated with powdered charcoal.
Should this be the case, the carbon would probably ignite
with the heat, and, burning in the evolving oxygen with
explosive violence, produce what may be called a carbon ex-
plosion in the retort. A similar explosion once occurred when
the manganese had become, not adulterated, but accidentally
mixed, with antimony sulphide. From a known source, no
dread need be felt ; but in case of doubt a drachm of the
mixture should be placed in a dry test-tube and heated over
a Bunsen burner. It will always crackle, and a few tiny sparks
may be seen ; but if large, and bright sparks appear, or anything
like a small explosion be heard, it should be rejected.

Any sand or iron oxide, if these are substituted, should
be known to be perfectly cleaned. Of the chlorate itself, the
common commercial quality is quite pure enough ; but for
similar reasons, every parcel purchased should be spread out
upon a large sheet of the Times, and any bits of wood or
straw carefully picked out, should any such be visible.

40. The Oxygen Retort.— The mixture— of which imme-
diately— must be placed in a retort, of which three kinds are
sold. First, a copper one, the dearest to buy and the worst
to wear of any. Secondly, a conical flat-bottomed one of
sheet iron (fig. 46), which is perhaps the most usual, and fairly
durable. It should have a safety-tube at the top, in which a
cork can be placed, as well as the deli very- tube. This latter
should be of a good diameter, as a certain quantity of the
powdered manganese is carried up into it by the rapidly evolv-
ing gas, and a small tube might choke, when too great a rush
of gas might produce a sort of mild confined pressure explosion.
The cork in the safety-tube will sufficiently guard against this,
blowing out if necessary and letting the gas escape ; and there
is no fear if the delivery-tube is examined after every operation,

76 OPTICAL PROJECTION

and, if necessary, an iron rod passed right through it, and
any manganese that may have collected cleared out. By at-
tending to this, I used for years delivery-tubes composed of
half-inch gas-pipe, but three-quarters would be a better size,
and even this should be examined.

FiG. 46. Oxygen Retorts FIG. 47.

The third kind of retort is of cast iron, made of various
shapes, such as fig. 47. I have also known a small Papiii
digester fitted as one. Cast iron is most durable of all, and
such are usually fitted with very large delivery-tubes ; but the
greater thickness of metal requires more heat.

41. The Mixture and its Use. — Such a retort, with the
requisite quantity of mixture, is placed on either a fire, or,
what is far better, a Fletcher's ring gas-burner, whose flame
can be regulated according to the evolution of gas. The
mixture often advised, of two parts chlorate to one of man-
ganese, is pretty steady in evolution of gas, but apt to carry
much manganese into the delivery-tube. Four parts of
chlorate to one is much faster, but requires watching, or the
gas will come with a rush and blow out the cork, wasting the
oxygen. Till the gas begins to come, the flame may be full
on, but directly it begins to come pretty fast, must be turned
down considerably, or this * rush ' will ensue. That is the

PREPARATION OF GASES 77

difficulty; as with these proportions, if the gas is 'rushing,'
and the flame be lowered, the flow does not diminish for a
minute or two. Hence the probable behaviour must be anti-
cipated, as it were. This needs experience, and the beginner
should make his gas cautiously till he has gained it, which
will only take him rather more time to fill his bag. Steadi-
ness of action can, however, be attained by further adding to
the mixture common salt, for which hint I am indebted to
Mr. E. Holland. Take, say,

Chlorate of potash • . . 2 Ibs.
Manganese oxide .... f Ib.
Salt ...... 6 ounces.

In this mixture the chlorate is to be powdered as well as
the other ingredients, and the evolution of gas will answer
almost instantly to the lowering of the flame, and thus be
under perfect control. This is therefore the mixture I recom-
mend : chlorate, 8 parts by weight ; manganese, 2 parts ; salt
1^ parts.

When chlorate is used with manganese alone, it must
not be powdered, but used in the rough crystals ; and 1 Ib. of
the chlorate — the only active ingredient — must be allowed
for 4 to 4 \ cubic feet of oxygen. Powdered chlorate, without
salt, is liable to cause violent rushes. It may also be noted
that finely granulated manganese is better than powdered, as
not choking the delivery-tube at all ; but it is seldom easy to
procure, and the powder need cause no inconvenience.

Using the necessary caution, it often happens that when
the charge is about half exhausted the evolution of gas stops
for a while, and even the full flame fails to start it again for
some minutes. It will resume, however, and during this
second stage there is little need of caution for fear of a rush ;
the full heat is generally needed to the end. For its thorough
manageability, however, I strongly recommend the salt mix-
ture.

?8 OPTICAL PROJECTION

42. Purifying the Gas, — Oxygen thus prepare! from
commercial chlorate is not fit to pass direct into the bag.
It is hot, and must be cooled ; it will contain a great deal of
free chlorine, and must be purified, else both bag and brass-
work will be rapidly destroyed. And it is very seldom
properly purified. Properly purified and dried, new jets and
taps should be used half-a-dozen times without any sign of

FIG. 48.— Arrangement of Purifier

speak

green corrosion being visible on the bright brass. I
from experience.

The hot gas is passed through at least one cooler and
purifier, as shown in fig. 48, connecting the delivery-tube of
the retort by a piece of vulcanised tubing with the pipe which
goes nearly to the bottom of the purifier, and the tube which
leaves the top of the purifier, with the gas-bag or the next
purifier. Where only one purification is attempted, the whole
arrangement is shown in fig. 48, the gas bubbling up through

PREPARATION OF GASES 79

the fluid. The tube passing into the purifier should go nearly
to the bottom of it, and about two inches from the bottom
end should be perforated with six or eight holes about ^ inch
diameter, that the gas may enter the water in small streams.
Care must be taken to make no mistake about the proper
connections, as shown in the figure.

Many people pass the gas simply once, through water, in
this way. This will cool it but not purify it ; and it also
carries it damp into the bag, to the latter's rapid destruction.
To absorb the chlorine, sodic carbonate (common washing
soda) or potassic carbonate (salt of tartar) may be dissolved
in the water, and answers fairly well ; but the way to per-
fectly neutralise it is to use caustic soda. With this, the same
solution may be used several times, if at home ; the quantity
is not very material, using, say, a couple of sticks of the caustic
to a Winchester quart bottle. One washing is not, howevei\
enough ; two purifiers are necessary to get really pure gas,
connecting the delivery-tube from the first with the entering
pipe of the second.

Formerly sheet-metal purifiers were used ; but it ia
advisable, if not necessary, to see just how fast the gas is
coming over, by which the heat under the retort is to be
regulated, and hence glass bottles have long been usual.
These are generally made as in fig. 48, with screwed caps,
through which the entry and delivery-tubes passed. But
there are two far better methods. The first is to employ
vulcanised ' Woolff ' caps, through whose nipples the brass
pipes are passed, and which simply stretch over the open
mouths of wide-necked Winchester quarts. These will
also act as safety- vents for too great a rush of gas, allowing
either some leakage, or blowing off altogether if it were
necessary. But the best and handiest fittings of all are made
as in fig. 49. Here B is a brass tube about one inch in
diameter, closed in at the top, into which closed top the brass
entry-pipe (A) is brazed. Into the side, at (c) the delivery nipple,

8o

OPTICAL PROJECTION

of brass pipe, is also brazed. Over the open bottom end i<?
stretched half of a piece of stout vulcanised rubber tube, (DD),
about two inches long. The other end of this rubber stretches
tightly over the ordinary size of any narrow-necked bottle,
using when at home Winchester quart size. When from
home, however, ordinary wine or ale bottles can be used, and
thus only the small fittings need be carried about.

When only two bottles are used, the second should only
be half full, in order that as little spray as possible may pass
away with the gas. Still the latter
will be damp, and should be further
dried ; and for years I have used, after
the second purifier, a piece of large
glass tube, corked at the bottom, and
over the top of which stretches another
of the fittings shown in fig. 49. The
entry pipe goes nearly to the cork, and
most of the aqueous vapour is there
finally deposited.

This is what is meant by properly
D purified and dried oxygen gas. If the
two bottles and drying-tube are ar-
ranged on a little shelf, or in a box,
with their connections ready made,
such a really efficient apparatus gives
no trouble whatever, beyond renewing
the alkaline solution now and then.
Should the third or drying chamber
not be employed, the gas can be
tolerably dried by placing the gas-bag

which is being filled upon a table, while the rest of the
apparatus is below on the floor ; a great deal of the moisture
will then be deposited in the tubing connecting the bag.

Such being the detail of the process, a few words will be
useful as to its general conduct. It is convenient to make a

\

FIG. 49

PREPARATION OF GASES 81

quantity of mixture, and keep it in some metal vessel ; a
circular tin with hinged cover, such as is sold for biscuits,
answers admirably. It is also convenient to keep in the tin
an iron gardener's trowel to shovel up the mixture, an old
tin canister cut to such a size as will exactly hold the charge
found sufficient to fill the bag, and a tin funnel with a large
spout. Starting with a retort both clean and dry, it takes
but a moment to shovel up enough mixture to fill the canister,
unscrew the head of the retort, funnel in the charge, roughly
level it by giving the retort a shake, replace the top, and
screw it home; a properly-made retort needs no washer or
other packing. The retort is then placed on the ring-burner,
which itself is controlled by a tap, the delivery-tube connected
with the entry-pipe of the first purifier by half a yard or
more of vulcanised tubing, all the purifiers properly connected
as described, and another length of vulcanised tubing attached
to the final delivery-nozzle of the last piece of purifying
apparatus, the other end being placed loose where it can be
instantly slipped on the nozzle of the gas-bag. The Win-
chester quarts should be full nearly to the shoulder, unless
the second is left half full as before indicated, for lack of a
drier. Then light the burner. Meantime, the bag has been
flattened and rolled, so as to expel the contained air, and the
tap then closed. In a minute or two bubbles will begin to
ascend in the purifiers, but these at first are merely heated
air, driven over by expansion. Soon they come more steadily,
and then a match should be held ready near the mouth of
the last rubber tube. When the operator tninks about as
much air is driven over as filled the vessels, he should strike
a match, and when well caught blow it out, holding the red
coal in the stream of gas from the end of the tube.1 If
oxygen is beginning to come over, the coal will brighten

1 Care should be taken to hold the tube horizontally, and the match in
front and not over it. A case has been reported in which the burning end of
Ihe match fell in and ignited the robber, resulting in a fierce combustion.

* G

82 OPTICAL PROJECTION

again into a blaze, in which case the match is thrown safely
away, the end of the tube slipped over the nozzle of the bag,
and the bag- tap instantly turned fully open. Then the speed
of the gas must be noted, and the rest of the preparation
proceeds as described.

The bags should be filled tightly, unless for immediate
use. When full, the tap is turned off, and the tubing
detached the very moment afterwards. The next step is to
detach the first piece of tubing from the delivery-pipe of the
retort, and this must be done before the heat under it is
diminished. The reason is, that if the flame were lowered
first, for any appreciable time, the cooling would reduce the
pressure, and so suck back a portion of fluid from the purifier.
Such might possibly result in a steam explosion, which has
been known to happen from such a cause, but of which there
is no business to be any risk at all. When the retort has
cooled down a bit, the top should be unscrewed, water poured
in, and the contents washed out, adding one or two waters
till there is no blackness left to speak of. It saves much
time and trouble to wash it out at once, while warm, and the
residue corrodes the iron if left in. When washed out, the
retort should also be dried out, which is readily done on
the hot plate of a kitchen stove.

CHAPTER VI

COMPRESSED GASES

FOE years oxygen and hydrogen had been used for the lime-
light compressed in iron cylinders furnished with screw valves to
their nozzles, the gas feeding the jet by its own pressure until
this was gradually reduced to that of the atmosphere. Such

Should a full bag ever be tested in this way, and the match fall in, the result
would be such rapid combustion as to be tantamount to an explosion. There
should pot be the slightest risk run of such an accident.

COMPRESSED GASES 83

a cylinder cost little more than a bag, while the gas in it did
not deteriorate by osmosis, and might be kept for months.
But these bottles were very heavy, and the charge of Sd. per
natural cubic foot for each gas was prohibitory as regards
general use with the mixed jet, amounting to 13s. £d. for
20 feet of gas, against about 2s. Gd. to an exhibitor who
made his own oxygen, and filled the other bag from the
nearest gas-pipe. The gradually diminishing pressure also
necessitated a constant turning on of more at the screw
valves ; so that to work a mixed jet required a skilled hand
to manage the light alone, and dissolving was very difficult,
the least mal- adjustment of the dissolver generally causing
such accumulation of pressure behind as to blow off the
rubber tubes. The use of cylinders under these conditions
was almost nil for the mixed jet, though a cylinder of oxygen
would often be purchased to save the trouble of making it,
or to take it portably to the place of exhibition, where it
would be used to fill a bag, which would then be worked in
the ordinary way.

43. Use of Cylinders alone. — With the oxy-gas jet it was
different. Only one of the gases had to be compressed and
paid for with this system, and for it cylinders came into more
or less use. For such jets very precise regulation is not
required, and a little more can be readily turned on from
time to time. Even now such jets are often used in this
simple way, direct from the cylinders and without regulation.
In working thus, the cylinder is simply secured as near the
jet as possible (in order to use a short length of elastic tube),
and connected with it as if it were a bag. Some advise
tying on the tube at both ends with strong tape as strongly
as possible, but this is not advisable, as if too much pressure
is turned on, the tube itself is apt to be ripped up with rather
a loud pop, which might startle the audience. The best plan
is to turn back outside an inch or so of each end of the rubber
tube which will tighten the ends quite as much as is desirable.

42

84 OPTICAL PROJECTION

The house-gas is connected as usual, and then the oxygen is
turned on very cautiously till there is enough supply. When-
ever the light goes down appreciably, a very little more is
turned on ; a bottle containing 6 cubic feet being sufficient,
with care, to last a single lantern nearly two hours.

Without care, however, it will not do so ; and it will be
found particularly easy to use all the oxygen in less than one
hour. Care must be taken to use as little as is sufficient, and
to turn on as little more as possible at one time. It is only
further necessary in using oxygen thus, to be careful never to
turn off the oxygen at the jet. If this be done, the full pres-
sure of the cylinder rapidly accumulates behind the stop- cock,
and blows off the rubber tube. There is not the slightest
danger in this ; but it causes a serious waste of gas, which
may unpleasantly shorten the exhibition.

Before compressed gas could come into at all general use
with all kinds of jets and lanterns, it was necessary to improve
the cylinders, to provide efficient regulators of the pressure,
and to reduce the price of the gas.

44. The Cylinders. — These are now made of a fine
quality of steel, less than one third of the weight and bulk of
the iron once formerly manufactured, much higher pressure
being employed. The common pressure for full cylinders is
1800 Ibs. to the square inch, but every cylinder is tested to
more than double its utmost real pressure before being used.
The following are fair average dimensions and weights as now
made.

Capacity Size Weight

6 ft. 12 x 31 in. 8 Ibs.

12 „ 18 x 3| „ 13 „

20 „ 32 x 4 „ 21 „

40 „ 32 x 5| „ 38 „

The worth of a cylinder depends entirely upon the
excellence of its valves, and a great many of those advertised
at unusually cheap rates are not to be depended upon, but
gradually allow the gas to escape. I have repeatedly known

COMPRESSED GASES

this to be the case ; but good cylinders will retain their full
charge for a year or more.

45. Testing Contents of Cylinders.— It is exasperating
to find no supply of gas at the critical moment, and this
makes it important to know how to ascertain the amount
really on hand. The opticians use a Bourdon pressure-gauge,
which is screwed on by a union -joint, and gives the pressure
instantly, then by calculation we get the quantity. Usually
the cylinder is ' full ' to its capacity at 1800 Ibs. : if so, the
following will be the content, at other pressures, of the sizes
commonly in use.

1800

1500

1200

900

600

300

feet

feet

feet

feet

feet

feet

6

5

4

3

2

1

10

8*

6|

5

3£

1*

12

10

8

6

4

2

20
40

16f
33£

13±
26|

10
20

*i

18J

3i

$

Constant exhibitors will find a gauge, costing about 21. 10s.,
well worth the expense, and it is the only method of knowing
what quantity of hydrogen or house-gas is available; but
where oxygen alone is used, it is sufficient, as was pointed out?
in a letter by Mr. C. H. Cathcart, simply koiveigh the cylinder
A cubic foot of oxygen weighs as much as l'43oz., or loz=0'7
cubic foot nearly. If therefore the cylinder be private pro-
perty, let it be weighed when empty, and the weight marked,
or a note kept. Deduct this net weight from the weight at
any given time, and,

Difference in oz. x 0'7 = cubic feet of gas.

Thus, suppose a 12-feet cylinder weighs empty 13lbs. 14^oz.,
and after an hour's use on one evening, we want to see if
there is enough to last again. We weigh it, 141bs. lO^oz. —
difference 12oz. Then 12oz. xO'7=8'4, or nearly 8^ feet
of gas. When a cylinder is not known, we may assume it is

86

OPTICAL PROJECTION

full when received, and weigh it so. Then the decrease in
weight in ounces multiplied by 0'7 will give us the amount of
gas used or escaped, and of course tell us what is left. Any
pair of scales that will weigh within half an ounce, is suffici-
ently accurate for this simple method of estimation.

46. Pressure Kegulators. — To use two bottles of com-
pressed gas in comfort, however, with only the one usual

assistant, it was absolutely
necessary to deliver it to
the jets at approximately
constant pressure auto-
matically. Many attempts
were vainly made in this
direction, both in England
and America. It is per-
fectly easy to maintain a
constant pressure at the
outlet, whatever the pres-
sure behind, so long as an
outflow is kept up unob-
structed. But all early
contrivances simply kept
up a constant difference
of pressure between the
two sides of the regulating
valve, and directly the
outlet was turned off (as
at the jet) the pressure
rapidly accumulated be-
hind till the rubber tubes

blew off. It was necessary to contrive that when pressure
thus began to accumulate, it should totally close the supply
from the cylinder, and take the strain off the frail rubber
tubes. This has now been accomplished in several different
regulators.

FiG. 50.— Beard's Large Regulator

COMPRESSED GASES 87

The first was invented by Mr. E. Beard, and is shown in
section in fig. 50. It screws on to the cylinder at D, from which
the small orifice is closed at d by the valve /. The gas lifts
this valve and enters the bellows A A, which rise with the
pressure, but are weighted above to the pressure desired. The
top of the bellows has a collar c1 screwed with a very long
pitch, in which the screwed pillar F turns easily as the bellows
rises. Below the long-pitch screw F which is merely to turn
the pillar easily, is a slow-motion screw fl, the turning of
which in its corresponding collar forces the valve down into
its seat and stops the supply : thus at a given pressure the

FIG. 51.— Clarkson's Regulator FIG. 52.— Beard's Small Regulator

cylinder is cut entirely off. In practice, while any jet is in
use the supply is balanced ; and leaves by the delivery-tube
and tap d2 and d1. E E1 is only an outer casing to protect
the internal parts from injury.

This regulator worked efficiently, but was expensive,
(costing about QL), cumbrous, and heavy. Next season a
form known as the ' Clarkson ' regulator was brought out, and
is shown in fig. 51. The bellows in this case is shaped like a
gas-bag, and is made of strong india-rubber framed on steel
rods at the edges. The sides are compressed by a strong
spring, D, adjusted to the pressure required, and work to and

OPTICAL PROJECTION

fro on a spindle, B, at the edge, through which the gas enters
at A, and which has a valve so constructed, that when the
bellows opens to a certain point, the gas is cut off entirely.
It passes to the jets by the nozzle E.
This regulator only measures about 3
inches each way, works very well, and
costs about 25s.

Mr. Beard has since brought out a
small regulator, costing 80s., shown in
fig. 52, which is about one-third the
actual size. This also works with a
central valve, but, instead of a screw,
has a curious sort of * lazy-tongs ' ar-
rangement, which need not be particu-
larly described. The material fact is
that there are now at least two regulators
in the market, measuring only a few
inches each, which simply screw on to the
cylinders, and then keep the pressure
practically uniform, and allow the jets to
be regulated just as usual, or even to
be turned off. Beard's is generally found most satisfactory.
Fig. 53 shows a cylinder of compressed gas with a pressure-
gauge, one of these regulators, and a screw-key, all in position.
47. Price of Oxygen. — By a fortunate coincidence, no
sooner had efficient regulators been provided, than the price
of compressed oxygen was cheapened by the introduction of
Brin's process for manufacturing it. In this process retorts
are charged with pure anhydrous barium oxide, which is
raised to a low red heat, when common air carefully washed
and purified is pumped into the retort. The oxide under
these circumstances takes up more oxygen from the air and
becomes dioxide, when the nitrogen is conveyed away ; after
which, on raising the dioxide (Ba02) to a still higher tempera-
ture and reversing the air-pump, the dioxide is reduced again

FIG. 53.— Cylinder with
accessories

COMPRESSED GASES 89

to BaO, and the oxygen given off. The process thus uses
up no chemicals whatever, and becomes simply mechanical.
The immediate effect was to reduce the retail price of oxygen
to 4:d. per cubic foot, and in large quantities to much less.

Erin's gas is always perfectly dry, and free from chlorine ;
and when also free from nitrogen, appears to me superior to
average chlorate gas, though not to gas properly washed and
dried (see p. 78). At first, however, the quantity of nitrogen
left in it varied materially, and on several occasions I have
been greatly disappointed with the light when employing this
gas for microscopic or physical experiments. So much was
this the case, that some London opticians have turned again
to chlorate gas, and gone to the extra expense of pumping
that at the lowered rate. I believe, however, that all the
practical difficulties have now been overcome by the Brin
Company, and the more recent cylinders I have had of their
gas have been all that could be desired.

48. Using Condensed Gases.— With a good regulator
screwed upon the cylinder, this is used precisely as a gas-bag.
The regulator may be set at any given pressure in reason : I
recommend 9 inches for ordinary lantern work, and 14 inches
for high-power work. It has already been remarked that
with blow- through jets no regulator is absolutely required,
provided no taps are ever turned off, and all the regulation be
done at the cylinder valve. But with the regulator the
cylinder may be turned on, and the jet regulated by its
taps as usual : also the jet may be turned completely off if
needed. At the same time it is not wise to subject the
mechanism to the immense pressure for longer than neces-
sary ; therefore if the light is wanted off for more than ten
minutes the cylinder-valve should be turned off, and it is well
not to turn it on till just before the jet can be attended to.
As a rule no further attention will be required, the pressure
being maintained much more uniform than from bags ; and
it will be kept up till the gas left in the cylinder is no longer

90 OPTICAL PROJECTION

sufficient to do so. It is most convenient to arrange a pair
of cylinders upright in a little packing-case, which stands
firmly on the floor behind the lantern, and is also convenient
for carriage ; but when one cylinder only is used, it is often
laid flat on the table, if one is used as a stand for the lantern.
It should, however, be tied to prevent rolling off.

For a blow-through jet the pressure is better adjusted to
only 5 or 6 inches.

49. Dangers of Compressed Gas.— The use of gas in
cylinders was said by some persons to remove all danger.
Experience has not quite borne this out, though what danger
there may be, appears to have attached to the persons engaged
in pumping it, and not to the consuming public. Years ago,
a cylinder exploded at the Eoyal Institution whilst being
filled, and examination showed that the intense heat caused
by the rapid compression had ignited some particle of
carbonaceous matter, and that this had in turn ignited the
iron itself, which readily burns in oxygen. This is now
guarded against. Another bottle burst when sent to have a
* stuck ' valve loosened, because the workman put it on the
fire ! No vessel would, of course, resist such an idiotic
proceeding, which is scarcely likely to happen again. I have
heard of no cylinder failing in England, whilst in the hands
of the public ; in testing, a certain portion of course do so,
and are accordingly rejected. A pressure-gauge has burst,
with serious, though not fatal results ; and it is safest to have
such gauges well secured against, at all events, broken glass.

The only practical danger is in mixture. A terrible and
fatal explosion at Dublin, in the early part of 1889, was
caused by oxygen being pumped into a hydrogen cylinder for
an emergency, the cylinder on being sent back being filled up
again with hydrogen. It was the vendor himself who was
killed ; and he knew, and had been actually warned of, the
state of affairs and the danger, running the risk rather than
lose a few cubic feet of gas ! And a similar accident — also

COMPRESSED GASES 91

on the works, not in public use — has since occurred in the
north of England.

No oxygen ought ever to be pumped into any but the usual
black bottles ; and no hydrogen or house-gas into any other
than red bottles. These are the two colours accepted by the
trade and general consent. I also insist very strongly, that
no pumpers have any right to fill up a bottle, without first
seeing that all pressure is let off. These rules give safety, and
no other rules do so ; as to little contrivances sold for cus-
tomers to ' test ' their gas, a large proportion of operators have
not knowledge enough to use such things. If an explosion
occurred, and fatal results followed, and it were proved that
the pumpers had broken the first of the rules just mentioned,
the culprits ought to be, and I believe would be, held guilty of
manslaughter. Since the explosion just referred to, all firms
of real standing now act upon both ; and while it is necessary to
point out where any neglect has caused accident, it may be
well to repeat, that not a single accident has yet happened to
any user of the gas in England up to this date, out of thou-
sands of cylinders. It is, indeed, difficult to understand how
any accident can happen without criminal carelessness on the
part of the firm supplying the gas ; the few accidents which
have occurred ' in the trade ' itself, having pointed out the
only sources of danger very clearly. The ' mixture ' which
might possibly occur with bags, if carelessly handled, is by the
high pressure of the cylinders rendered, with them, practically
impossible.

CHAPTER VII

THE OXY-ETHER AND HYDRO-CARBON LIGHTS

50. The Oxy-ether Light. — For many years lamps have
occasionally been employed in laboratories which burnt the
vapour of some volatile fluid in place of gas ; and recently such

92 OPTICAL PROJECTION

vapour has been employed in lieu of hydrogen or house-gas in
the oxy-hydrogen jet. The fluid first employed was sulphuric
ether. To vapourise this by heat would be terribly dangerous ;
but it is found that passing fine streams of air through an ade-
quate quantity of fluid carries over with it a sufficient quantity
of vapour to burn as a combustible without being explosive.
The same occurs with a stream of oxygen ; therefore if the
oxygen supply (from bag or cylinder) be divided into two
branches, one of these passing through the ether will convey
combustible vapour to the jet, whilst the other branch may
be adjusted to supply the most calorific proportions at the
nipple, in the usual manner already described.

The oxy-ether light is thus adapted for use in just such
halls, without gas, as found employment fortheoxy-spiritjet,
but with the advantage of a far more powerful light. This
light has been chiefly brought into notice in England by Mr.
W. Broughton, of Manchester, and the late Eev. F. Hardwich,
who have worked a great deal in concert ; and in the United
States by Mr. Frank Ives, of Philadelphia, who advocates
a different form of apparatus.

Mr. Broughton adopted a tank of ether, using first of all
an apparatus essentially like a wash-bottle for washing oxygen,

but forcing the
oxygen through a
number of very
fine apertures.
This failed to satu-
rate with vapour,
and gave a flicker-
ing flame. Next,

a tallk WaS

•fca.54.-Man of Tank .

with horizontal

divisions, the oxygen being compelled to travel backwards and
forwards gathering vapour. Finally, both Mr. Broughton and
Mr. Hardwifch adopted a tank furnished with perpendicular

OXY-ETHER AND HYDRO-CARBON LIGHTS 93

partitions reaching from the bottom to the top, and in plan
arranged as in fig. 54. This being partially filled with ether,
the space above the fluid left a long sinuous course for the
gas to traverse, which gave full saturation.

With this tank the oxy-ether light is certain and safe
under skilled management. But it appears to possess special
dangers of its own, and the few oxy-hydrogen explosions which
have happened during late years have been with this form of
light. Not one has been directly fatal ; but in the panic after
one at Chadderton, a poor child was trampled to death. In
that case I am convinced that, by some carelessness, ether had
been allowed to trickle down the connecting tube to the oxy-
gen-bag itself, thus converting the oxygen into an explosive
mixture. This accident produced two extra precautions in the
apparatus used.

51. Safety Chambers. — One of these was the pumice
safety-valve or chamber of Mr. Broughton, which is the only
real safety- chamber I am acquainted with. It is made by
filling a brass tube about half an inch in diameter, and say
three-quarters of an inch in clear length, with pumice-stone
granulated into particles of a certain size. This is best pre-
pared by passing pounded pumice through a double sieve of
wire gauze, using gauze No. 40 and No. 50. The coarser
particles are stopped by the top sieve, and those too fine go
through the lower ; the particles remaining between are the
proper size. The pumice is kept in place by a piece of wire
gauze each end of the chamber, resting on shoulders or flanges ;
and the chamber should be made so as to take to pieces easily
for refilling, and, if used with rubber tube, with nozzles over
which this can be stretched.

No ordinary explosion can pass
whilst in order. Eigid experiments^
have determined this. But if a vi|
small explosion) should take place i]
be stopped by it, the latter may be partially p'ulfei^d an(

94 OPTICAL PROJECTION

disarranged by the shock, so as not to resist a second one in
the same degree. This has been proved experimentally ; and
therefore these chambers should always be examined, and if
necessary refilled with fresh-sifted material, after any notice-
able ' snap ' of a jet, or at reasonably short intervals even
when none may have been noticed. Also it is to be observed
that, as the violence of an explosion will depend upon the
amount of gas in front of the chambers, these should in all
cases be used as close to the nipples as possible. They should
never be used at the supply nozzles of the dissolver, but on
the nozzles. of the jet; and where this light is habitually
used, the safety chamber should be arranged under the nipple
itself, with only a very small empty chamber above. Here
also it should be constantly examined. This makes the only
real * safety ' jet I know ; but the obstruction to the flow is
great. This, however, can now be overcome by the higher
pressure at command from cylinders.

The other precaution was to add gas-chambers above the
tank — one from which the oxygen passed to the surface of
the ether, another into which the vapour passed before
leaving by the exit-tube. Thus, if the tank were jarred or a
little tilted, ether would not run back, but be stopped in these
chambers. At the end of 1887, however, another explosion
occurred with a tank thus constructed, under circumstances
which for a time baffled speculation. The tank was purchased
in Feb. 1887, and worked well the rest of that season ; but
when got out in Nov. 1877, it behaved differently. Before
any pure oxygen was turned on at the jet, the latter heated
the lime red hot ; and this got worse and worse, until finally
a severe explosion ruptured the tank, in spite of two pumice
chambers attached to the dissolver. The probable reasons for
the explosion passing these have already been pointed out ;
but the explosive condition of the tank itself puzzled everyone.
It was carefully examined by several, including myself ; but
none could discover any apparent leakage which might have

OXY-ETHER AND HYDRO-CARBON LIGHTS 95

led to the ' short circuiting ' of the oxygen-stream, and thereby
caused that predominance of oxygen which had somehow, it waa
evident, filled the gas-space over the tank with explosive mix-
ture, instead of merely combustible vapour. At last the tank
was ripped entirely open, and examined by an engineer very
closely, when all was explained. In the soldering of the parti-
tion which divided the safety chamber over the tank (as last de-
scribed) into two, as an entering and exit chamber, a very small
leak was discovered, with traces of corrosion. It is probable
that this at first was absolutely imperceptible, but had
gradually enlarged during the summer months ; and the re-
sult was that a certain amount of oxygen passed directly to
the exit chamber, without passing over the ether at all.

Fro. 55.— Ether Tank, side view

The remedy against this danger, as was immediately
pointed out by Mr. Hardwich, was to entirely separate the
two chambers as in fig. 55. With this construction, another
such accident would be impossible. With regard to the
soldering of the partitions in the tank, any single leak between
two compartments would not have very much consequence ;
and it is the sides and top of the partitions which would need
special care. A little leakage at the bottom of the tank, ^hich
should be the last side to put on, would have no result, since
there would always be ether above that level, unless it were
actually exhausted, which never ought to occur.

52. Porous Generators. — Mr. Ives constructs his gene-
rators differently, filling a metal tube of two inches or more

96 OPTICAL PROJECTION

diameter with a roll of woollen material. This is closed by
gas-tight caps at each end, with proper nozzles. Ether is
poured in to saturate the woollen, and the oxygen passes
through the roll. The usual form of this generator sold in
England is shown in fig. 56, the tube being double, each
length with its own roll of saturated material, the two being
connected by a U piece of brass tube. But in his own last
construction Mr. Ives prefers a single tube about 14 inches
long, which is laid flat, and has a nozzle with stop-cock pro-
jecting up at each end. He also now prefers to use petroleum
ether, which in the United States is a more common com-
mercial product than in England, and volatilises at a lower

Pro. 56.— Porous Generator

temperature. It also contains no alcohol or water, some
small quantity of which is always found in common ether.
That is an advantage, but the lower vapour-point I regard as
so far a drawback, sulphuric ether itself giving quite sufficient
trouble in this respect to make its storage and carriage a
matter of some anxiety.1

Experiments in England snow that the best saturation
is given by the tank, tank vapour requiring decidedly more

1 Since writing the above pages fundamental changes have been made
in the construction of ether saturators; space here forbids further men-
tion, but fuller details will be found in the appendix at the end of the
book.— R. S. W,

OXY-ETHER AND HYDRO-CARBON LIGHTS 97

oxygen at the jet than vapour from the other generator, and
this test being conclusive. A tank can also be made to con-
tain ether enough for any reasonable time, while it is
difficult with woollen to saturate the vapour sufficiently for
more than an hour and three quarters.

53. Management of the Ether Light. — I consider either
generator, as now made, to be practically safe with experienced
operators, and especially with such as understand the inherent
risks of using volatile fluids. But it is clear that these risks
are a serious addition to the ordinary risks of the mixed jet ;
and for this reason it is not to be desired that the oxy-ether
light should be widely used by the ordinary class of operators.
I cannot condemn it wholesale, as Sir Henry Koscoe has done,
especially in the porous form ; for I should use it without hesi-
tation if I wanted high power where I could not get gas. But
since the oxy-spirit jet supplies light for most purposes in
country rooms, and compressed gas can be procured for others,
the ordinary exhibitor would do well not to run the extra risks
of a process, which he will not probably thoroughly under-
stand all the dangers of. As the oxy-ether light will, how-
ever, probably continue to be used to some extent by skilled
operators, in certain circumstances, a few words are necessary
regarding its practical manipulation.

In the first place, the oxygen pressure must be good. The
oxygen has to supply two branches of tubing, and in addition,
each current has to be driven through the resistance of the
pumice chamber, which should always be employed. Where
this light is habitually used, the pumice chamber must be
examined, and if necessary replenished from a store of sifted
material kept always at hand, at short intervals. If a bag is
used, at least 1^ cwt., and 2 cwt. to finish, will be needed for
a good light ; if a cylinder and regulator, from 9 to 12 inches
pressure.

In the second place, the ether used must be of a certain
purity, known by its specific gravity, which should not exceed

98 OPTICAL PROJECTION

0*917 to 0*920. Heavier ether owes its density to alcohol and
water. Only the ether being vapourised at the end of an
exhibition, that which remains is of inferior quality, and will
saturate the gas less perfectly. It must always be poured
out of the tank at the finish, and if the quantity be not large,
the safest plan is to throw it away. It may, however, be kept
(separately) and used with fresh ether once or twice. After
twice, I consider the residue should be rejected. Petroleum
ether certainly has the advantage in this respect. Either
must be stored in as cool a place as can be found.

No tap or connection having to do with ether vapour can
be lubricated with any kind of grease, as the ether rapidly
dissolves it. Connections may be screwed up gas-tight with
soap, or washers of leather and soft lead. Taps may be
lubricated with vaseline. Rubber tubes are also acted upon,
and should be, therefore, of the thickest and best, and as
short as possible, placing the generator on the table, if one
is used. Any heat from the lantern will promote satura-
tion.

Large nipples cannot be used with sulphuric ether, the jet
* snapping ' with them. The aperture should not exceed 1 mm.
bore. This of course limits the light, though it is nearly equal
to the oxy-hydrogen jet of the same bore.

The connections are easily understood. The tube from the
oxygen supply is divided into two at a T or Y joint. One
branch of this goes to the oxygen nozzle of the jet or dissolver,
the other to the ingress end of the generator. There should
be at least six inches from the fork of the Y to the generator,
and there must be a stop-cock between them. From the egress
end of the generator a nozzle, also furnished with a stop-cock,
goes to the hydrogen nozzle of jet or dissolver. Every con-
nection should first be made with all taps turned off, and the
bag duly weighted. The bag or the cylinder is then turned
on, the latter with a regulator of course ; next, the tap between
the fork and the generator. Finally we turn on the hydrogen

OXY-ETHER AND HYDRO-CARBON LIGHTS 99

tap of the jet, and after a few seconds, to let air escape, turn
it off again for inflammable vapour to leave the lantern.
Turning it on again we apply a light, which burns in place of
house-gas. All the * pure ' oxygen taps, from the fork on-
wards, are meanwhile turned off.

It is always to be remembered that full saturation with
ether vapour is the condition of silent combustion and safe-
guard against explosion ; the explosive mixture with oxygen
only being allowed at the jet itself. The test of this is the
condition of the flame. While all is safe, this is white or
yellowish-white. As oxygen increases, this changes to deep
yellow, and finally to a purplish or blue colour. This last
shows an explosive condition, and whenever it occurs with what
stands for the hydrogen supply alone, proceedings should at
once be stopped. This test refers to naked flame only. When
the lime is adjusted close to the jet, even a dangerous mixture
may appear to burn yellow, and therefore it is best always to
light the flame first, at a distance from the lime. If it is a
good light colour the lime can be adjusted and warmed, after
which the oxygen is gradually turned on and adjusted in the
usual manner, till the brightest light is attained. As the
work proceeds, however, if it be found that the pure oxygen
has to be gradually turned off, it shows that from some cause
saturation is becoming more and more defective, and work
should be stopped and the light put out. To proceed under
such conditions is dangerous.

At the end of work, the hydrogen tap of the jet may
be turned off, so that the surplus oxygen ' snaps ' the jet
and puts it out. The condition of safety is to turn off at
the jet first, and never at the bag or cylinder till the light is
out.

When two lanterns or a bi-unial are worked with this light,
it will often be found that the dissolver (see Chapter VIII.) will
not work properly without some alteration. No oxygen bye-
pass whatever must be allowed, as is often done with the

H2

ioo OPTICAL PROJECTION

gases (see same chapter). On tlio other hand, the hydrogen
bye-pass, when used for ether vapour, must be more free than
usual, and if it is a groove round the plug, as in many, this
may have to be filed deeper. I have seen it stated that a
dissolver cannot be used, but there is no difficulty if properly
adjusted.

54. Cxy-Carbon Light.— Mr. Albert Scott recommends
the use in a similar manner of benzoline, used in a porous
saturator which is heated by a lamp to promote volatilisation.
This fluid, like rhigoline or petroleum ether, belongs to the
paraffin series, whose formula is CnH2n+2- ^r- Scott claims
for this modification that a larger nipple can be used, and that
a better light can be obtained than with mixed gases ; also that
by passing even coal-gas through the fluid before burning at
the jet, so as to ' enrich ' the gas with hydro-carbon vapour,
using the mixed gases otherwise as usual, a considerable gain
in light is obtained, concentrated in a smaller spot. This
would be a very valuable gain for microscopic work, and I
felt much interest in the question ; but after careful trial
together, both Mr. Herbert Newton and myself came to the
conclusion that not the slightest advantage could be observed.
When experimenting with the oxygen passed through warmed
benzoline, we experienced sharp though small explosions,
and Mr. Scott himself writes rather flippantly of what he
calls ' pops.' I agree with him that these ' pops ' are of no
real danger with his form of generator ; but certainly no
audience would tolerate them, or be free from panic if they
occurred; and it is manifest that the need of Jieat to get
sufficient vapour to form a non-explosive compound, is an
additional item of risk about this method. I suggested to
Mr. Scott the use of gasoline, which is a more volatile mixture
than benzoline, of the same series ; and this was adopted
with benefit ; but I should prefer to use rhigoline' a more
volatile fluid still, and needing no heat, as is done by Mr. Ives.
I also suggested the use of benzol or benzene, as richer in

OXY-ETHER AND HYDRO-CARBON LIGHTS 101

carbon ; and Mr. Scott says that this fluid gives the smallest
spot of all ; but it requires the jet as well as generator to be
heated.

It is remarkable that Mr. Ives also claims a smaller spot
for the same light than when using mixed gases ; but at such
trials as I have had any opportunity of witnessing, I was not
able to trace this, nor was it the opinion of those present that
the light was any better or more silent than that obtained by
good operators with mixed gases, as Mr. Scott claims. On
the other hand, it is indubitably true th&tfull apertures can
be used with this series of oils, and the utmost power of the
mixed jet (with such apertures) thus obtained, with only a
cylinder of oxygen in the way of gas. As to the claim, how-
ever, that still larger apertures can be used with hydro-carbon
vapour than with mixed gases, I have never heard of Mr.
Scott using more than y^-inch, whilst I have found no diffi-
culty (with the jet described in Chapter IV.) in using T^
nipples with mixed gas, so that this idea is certainly a
mistake. It is possible that different operators may obtain
rather different results. While, however, there may be
occasional convenience in being able to obtain the most
powerful light from one bag or cylinder alone, I fear that any
apparatus needing heat will be regarded with little favour,
and that Mr. Ives's rhigoline or petroleum ether will be the
general choice amongst this class of liquids.

CHAPTER VIII

•LANTERNS AND THEIR MANIPULATION

55. Single Lanterns.— Lanterns for the simple exhibition
of single slides are now practically made of one or the other
of two patterns. If a petroleum-oil lamp is used, either
always, or occasionally, the lantern-body is usually mads

102

OPTICAL PROJECTION

of sheet-iron (fig. 57). In the better class this is enclosed
rather loosely in a wooden case. Such lanterns are usually

worked (with oil)
very near the
screen, a combi-
nation-lens of 4J
inches focus being
most usual. As
regards thus using
a single lantern
with an oil-lamp,
nothing need be
added to what
has been said in
Chapter III., ex-
cept that all the
lenses will, of

Fio.57.-Iron Lantern <JOUrse, ^ Bright

and clean, or, if

not so, will be fresh wiped with a wash-leather, kept for
that purpose alone in a collar-box, or something of that
kind, and religiously preserved from grease. A wash-leather
once greased anywhere, is of no further use until washed.
The lamp is fitted to its proper place, and it only has to be
trimmed and attended to, and the slides properly focussed
with the rack and pinion.

When such a lantern is used with a tray and jet instead
of the lamp, again all the needful directions for the manage-
ment of the light have been given in Chapter IV., but it will
be necessary now to carefully centre and adjust the light in
the manner described under § 62.

A single lantern made for limelight only, however, ig
generally constructed differently. The body itself is made of
mahogany, and it is the lining of sheet-iron or tin which is
attached to this : there is usually an all-brass front, with a

LANTERNS AND THEIR MANIPULATION 103

sliding nozzle, and the whole pattern will closely resemble
fig. 58. Such a lantern is often fitted with various objectives
and a lengthening tube; and except as regards dissolving
views, or dioramic effects, exhibits single slides in as perfect
a manner as the more costly apparatus next to be described.

. 58. — Single Mahogany Lantern

56. Dissolving Views. — This well-known effect of one
picture imperceptibly melting into another, when first intro-
duced, created an extraordinary sensation, but is produced in
a perfectly simple manner. The fading and the coming slide
each require a lantern, so that two lanterns are necessary :
then the light is simply cut off gradually from one lantern,
whilst it is as gradually turned on in the other. There is
literally nothing else about it.

The popularity of dissolving as an effect is largely a thing
of the past ; but the method will be always used by exhibitors
to a large extent, from its convenience in changing slides, the
coming slide being placed ready in the dark lantern, ready to
be dissolved into the other when required. The cases where
it is of real value, are dioramic dissolving views ; such as
where a ruin, first seen by day, changes gradually into a

fo4 OPTICAL PROJECTION

moonlight scene ; or a ship, to the same ship on fire ; or
summer to winter. These effects retain all their charm, and
many sets of such can be procured ; but dissolving one slide
into quite a different one has no meaning, and, indeed, needs
constant care to avoid now and then absolutely ludicrous
sequences.

57. Registration of Slides. — The perfection of dissolving
depends upon absolutely perfect registration, as it is called.
That is, every line of the picture must dissolve precisely into
its corresponding line of the other slide, and in more ordinary
changes, at least each disc should precisely correspond with
the preceding. To ensure this, in the first place the two
objectives must be of precisely the same magnifying power;
and this should not be taken for granted without actual proof,
which is best afforded by two duplicate plain diagrams, or
slides of printed matter. In the second place the two lanterns
must be carefully adjusted that their discs may coincide upon
the screen. And besides all this, the slides themselves must
be carefully ' registered.' A hired set of slides can never be
shown in perfect registration for want of this last, which is
impossible with such a ' scratch ' set.

The best lanterns are, and should be, fitted with stages
which have adjustable screw stops, by which the precise
places of the two discs can be adjusted. If this is carefully
done, most slides will register fairly as regards their discs,
and this degree of registration satisfies what we may call the
average exhibitor. Further even than this, the mere discs
may be absolutely registered by a plan sometimes adopted of
exhibiting the slides (packed or carried loose) in a frame or
carrier with an aperture slightly less than the slide itself
shows. This slight contraction of the visible surface is
sufficient to cover any little inaccuracy in the slides.

Another method is, for the operator to watch carefully
the disc of the new slide whilst the light is only just suffi-
ciently turned on to show it almost imperceptibly, and then

LANTERNS AND THEIR MANIPULATION 105

move it into the exact position. Now and then an operator
can do this without the audience perceiving it ; but nearly
always the movement in adjusting the slide can be seen after
a while, and is very unpleasant.

No such expedients give sufficient precision for the fine
dioramic effects aimed at by the few exhibitors who have a
reputation for their exhibitions, as well as for what, apart
from this, may be most excellent lectures ; I mean such
exhibitions, for instance, as those by Mr. Maiden. Very often
in these an ' effect ' must be ' flashed ' on instantly, in most
precise position — as, for instance, the flashes from the guns
in a bombardment, or the sudden change to an explosion.
There is only one way to get this. First of all, the two or
three lanterns must be precisely centred, by an equal number
of precisely similar test- slides kept for the purpose ; three
photographs of a diagram showing simply horizontal, perpen-
dicular, and diagonal lines all crossing in a common centre,
answer admirably. These must be most precisely superposed,
so that any one may go into either stage indifferently, and
when once centred on the screen, be interchangeable without
affecting the precise superposition of the images, when once
the stage adjustments are also accurately made. Then every
slide must be framed separately, and adjusted in its frame by
slips of paper or card, until all the usual ' shake ' or looseness
is taken out, and the slide fixed precisely in its frame. Then
lastly, if the slips of paper or card, when humoured as far as
possible, are not sufficient to adjust it sufficiently, the wooden
frame itself is adjusted so as to centre the slide on the screen.
Sometimes the frame may need a slip of card stuck on ; or
it may need a thin shaving taken off. Now all is certain ;
but hours and hours are patiently spent in thus adjusting a
set of new slides, before the season really commences.

The almost microscopic adjustment of the lantern-stages
is not so necessary, provided only the slides are always used
so that each slide always goes into the same stage. To ensure

106 OPTICAL PROJECTION

this, a programme should be written if this method is adopted;
but as the sudden splitting of a lime might possibly upset
the order, it is better to carry out the system thoroughly,
and register both lantern and slides ; moreover, the lantern
registration is done once for all.

For dissolving views with oil lanterns, these must be used
side by side, and both lamps must be kept full on, as it would
be impossible to turn the lights up and down. Generally a
shade, going off at one side into taper teeth like a comb,
travels across each lantern nozzle, so as to cover and uncover
each in turn ; the two shades being connected so that as one
covers, the other uncovers. Plans have been published for
using one oil lantern over the other, but none such are really
safe, or tolerable in use.

58. Dissolving Taps. — Two lanterns with Argand gas-
burners will work dissolving views very well on a small scale.
The lanterns must still be side by side ; but the dissolving can
in this case be done by a tap which turns one light down
whilst the other is turned up, both lamps being ' on ' whilst
the tap is half-way. Such a tap is additional to the tap on
the burner itself, and is known as the ' dissolving- tap.' In

this case it is what is called
a * three-way tap,' shown in
fig. 59, there being three
branches. The centre one
comes from the gas supply,
and the lever of the tap directs
this into one or the other of
the two channels, the amount
. 59.-rLe.way Tap being regulated by their own

taps.

A similar tap is used for dissolving the oxy- spirit jet
described on p. 46, but in this case the tap governs the oxygen
supply, the spirit-flame being left on. This flame is too weak
to give any perceptible image of the ' dead ' slide upon the

LANTERNS AND THEIR MANIPULATION 107

FIG. 60. — Star Dissolving Tap

screen, but the turning on of the oxygen brings out the light
in all its brilliancy.

All kinds of jets with two gas or vapour tubes are worked
with a dissolving tap, which in this case must be a 'six-
way ' tap, as two gases, or their equivalents, are supplied
to each lantern in turn. This is the form practically known
as a dissolving tap,
and was first invented
by the late Mr. Dan-
cer, of Manchester.
It is now made in two
principal patterns, one
of which, the ' star '
tap shown in fig. 60,
was simplified out of
Mr. Dancer's model
by Mr. Maiden. In
this tap the supply-
tubes are stretched over the horizontal nozzles on each side,
and each side of the plug has a channel cut in it which
just reaches to the apertures over and under on each side,
leading to the two jets. Thus when the lever is half-way
both lanterns are ' on,' but any movement to either side cuts
off one jet. If both gases, however, were cut off altogether,
the jet would be put quite out ; and to avoid this there is a
small tube with a stop-cock in it, connecting the two jet-noz-
zles on the hydrogen side. The effect of this is to allow a
small quantity of hydrogen (adjusted by the small stop -cock)
to pass from the ' on ' nozzle, to the nozzle which is cut off,
so as to keep a small gas-flame, quite invisible as regards the
screen, alight in the cut-off lantern. Such a subsidiary stop-
cock is called a ' bye-pass.' To avoid any ' snapping ' when
a lantern is turned on, there is often a bye-pass made in this
dissolver for the oxygen supply also, and the diagram shows
both. An oxygen bye-pass must be very little indeed, so ae

loS OPTICAL PROJECTION

to barely affect even the hydrogen bye-pass name ; else it
will snap the jet out.

The connections of this dissolver are sometimes found
rather confusing, but are easily understood with a little
thought. Both the channels revolving in the same direction,
but on opposite sides of the plug, it will be seen that when
hydrogen is turned on to the top left-hand nozzle, oxygen
is turned on to the bottom right-hand nozzle. It is easily
understood if we remember that the two nozzles are ' on '
which are covered by the lever in either position. This tap does
however, involve very long india-rubber tubes between the

tap and the jets, and on this
account I prefer the other form,
shown in fig. 61. In this dis-
solver each supply nozzle, with
its pair of jet nozzles, is on the
same side of the plug, but one
above the other. Therefore
both jet nozzles are on the same
side for each jet, and the pair of

PI&. 6i.-piug Dissolving Tap nozzles are « on ' towards which

the lever is turned. The hydro-
gen bye-pass is often provided by cutting a small groove,
beyond the supply channel, right round the plug ; but a con-
necting tube and stop-cock, as in the ' star ' form, is better. I
have my own tap so modified that the jet nozzles project at
an angle towards the front, which is an improvement, further
shortening the rubber tubes. This dissolver can, if preferred,
be affixed sideways to the back of the lantern, so that the
nozzles point upwards to the top jet and downwards to the
bottom one, and very short rubber tubes then suffice.

A third form of dissolver consists of two separate three-way
taps arranged side by side.

Dissolvers often want adjusting. The apertures may be
too small to supply a good jet, and both these and the channels

LANTERNS AND THEIR MANIPULATION ICQ

in the plugs may need considerable enlarging. Some years
ago no tolerable result could be got from the lantern in use at
the then Victoria Coffee Tavern. After doing what I could
with the jets, the light was still greatly insufficient for the
long-screen distance of 70 ft., with a disc of 20 ft. diameter,
dud on examining the * star ' dissolver I found the apertures
and channels ridiculously small. I enlarged them to rather
more than double their former area, after which the light
was quite satisfactory. The bye-pass of a tap may also need
alteration ; and finally, it is advisable that the hydrogen
channel should be slightly longer than the other, so as always
to turn on the hydrogen in advance, and leave it on a little
later, and this may not have been attended to. A penknife of
good steel will cut or scrape away a channel easily, and a
suitable file will do the rest.

All taps are best lubricated with a thin smear of animal
fat or oil, except those used with ether vapour. The connec-
tions between dissolver and jets should always be made with
the best rubber tube, and all should be left in situ, and not
disconnected except when cleaning or greasing becomes
necessary. Care must be taken to grease sparingly, or the
grease may clog the passages.

In all cases where a dissolving -tap is used, the tubes from
bag or bottle, or pair of such, are attached to the supply-
nozzles of the dissolver, the jet-taps turned off, and the
supply to the dissolver turned on. Then turning the dissolver
on to each jet in turn, that jet is adjusted separately as to
its position, and the gas-supply regulated by the tap or taps
on the jet itself. Any future re-adjustment is also made by
the taps of the jet concerned.

59. The Bi-unial Lantern. — This is the usual and favourite
exhibitor's lantern, more of this pattern being used than of
any other. Fig. 62 represents the usual model, fitted with
the optical fronts recommended on page 33. Generally the
two lanterns are made really in one, as here shown ; but it

110

OPTICAL PROJECTION

is not at all unusual to make them so that the top one can be
separated, and placed by the side of the other, to use with
oil lamps. In any case the two doors should be separate.
It saves a sovereign to have doors only on one side of the
lantern; but it is very much better to have them on both
sides, as it is impossible to tell in what corners or positions
the lantern may have to be worked. To make the two discs
coincide upon the screen, each of the metal plates carrying a

FIG. 62.— Bi-unial Lantern

front is hinged, so that the two optical systems may be
inclined to each other and adjusted by screws, any slight side
adjustment necessary being made in the stages. The inclined
front carries the condenser and slide as well as the objective,
so that the whole is kept in one optic axis.

The bi-unial is much the most convenient form of double
lantern, as the slides can be manipulated freely, and if neces-
sary by two persons, or by a hand at each end. It is also

LANTERNS AND THEIR MANIPULATION in

lighter and more compact than two separate bodies, and it
can now be had at all prices, from plain japanned iron body
and fronts, or wooden lined body and japanned fronts, to highly
finished brass fronts. But as a great deal of money is often
wasted on such instruments, a few words may be useful con-
cerning what is worth paying for and what is not, which will
apply to the tri-unial lantern in even greater degree. I have
known a sum nearer 150/. than 100/. paid for a triple lantern
and apparatus, nearly half of which was absolutely thrown
away as regards any useful object, whilst the brass-work
did not fit !

First of all, it is safest to avoid all apparatus distinguished
by pretentious names or high-sounding adjectives, in any
language ; choosing such as is made and sold by opticians of
real standing, simply on their own reputation and guarantee.
Of such there are no lack, and I have invariably found such
apparatus cheaper, as well as more satisfactory. I do not
say there are no exceptions to this rule, since even good
houses may be led away in this advertising age ; but upon
the whole it will be much the safest to follow it.

In the next place, lanterns should be avoided which have
a multitude of ornamental mouldings or inlaid work about
their wooden bodies. This is not only useless, but worse
than useless. It adds weight, and the heat in the lantern
will sooner or later find out weak spots in it, and make the
mouldings warp and twist ; that is, if the apparatus be at all
regularly used. A simple body, of good sound plain mahogany
or other wood that can be depended upon not to warp, should
be chosen, and really looks better, besides being cheaper, than
tawdry ornament, which is out of place. On the other hand
good fitting and finish about the doors, their sight-holes, the
lining, &c., should be looked for. But especially the brass-
work should be examined for good sound workmanship. By
this is meant not lacquer and polish, but that the sliding
parts fit nicely and yet smoothly ; the rack-work acts

IIJ

OPTICAL PROJECTION

pleasantly yet without shake; the screw- work screws and
tmscrews with accuracy and certainty. Thick and heavy
brass-work generally masks poor workmanship.

What is best worth having in the way of objectives, has
been described in Chapter II.

60. Focus, Distance, and Disc. — It will be fully understood
from our first chapter, how the same objective which gives
a 12-ft. disc at 24 ft. away, will give a 24-ft. disc at 48 ft.
All details will be best gathered from the following table,
showing the distance required to get a disc of given diameter
with lenses of given focus, with slides of the usual 3 inches
diameter. The foci given are real or virtual foci, and in
double combinations what is called the ' back focus,' or dis-
tance of the back lens from the slide, will be less.

Disc

wanted

Focus
4iin.

Focus
6 in.

Focus
Sin.

Focus
10 in.

Focus
12 in.

Focus
15 in.

Focus
18 hi.

feet

ft. in.

ft. in.

ft. in.

ft. in.

ft. in.

ft. in.

ft. in.

9

13 6

18 0

24 0

30 0

36 0

45 0

54 0

12

18 0

24 0

32 0

40 0

48 0

60 0

72 0

15

22 6

30 0

40 0

50 0

60 0

75 0

90 0

18

27 0

36 0

48 0

60 0

72 0

90 0

108 0

20

30 0

40 0

53 4

6fi 8

80 0

100 0

120 0

25

37 6

50 0

66 8

83 4

100 0

125 0

150 0

30

45 0

60 0

80 0

100 0

120 0

150 0

180 0

Or the following formula may be useful, as giving the data
for any other slide than 3 inches. Calling the clear diameter
of the slides hi inches, slide, the objective focus in inches
focus, distance from screen in feet, distance, and the diameter
of disc in feet : then,

If we want the distance the formula will be,

disc x focus

Distance =

slide

If we want the disc, the formula is,

distance x slide

Disc =

focus

LANTERNS AND THEIR MANIPULATION n.j
If we want approximate focus, it will be,

Focus = distance x slide
disc

As regards foci of objectives, the most generally useful set
of three would probably be 6 in., 10 in., and 15 in., or there-
abouts. A 4 J in. lens is rarely wanted except to show through
a transparent screen, when space is limited. The most
useful single lens will be 6 in, or 8 in. I think myself 6 in.
is the best upon the whole ; and if ever a 6-in. lens is made
as suggested on page 29, so that one component can be used
alone for longer focus, as in some of the best of the photo-
graphic lenses, a 6-in. lens would do a vast range of work, as
the single focus would be about 10 or 11 inches.

61. An Exhibitor's Work. — We will now suppose an ex-
hibitor arrived with all his apparatus in the place where he is
to exhibit slides, with a bi-unial lantern. Let us briefly see
what he has to do, and how he will set about it.

His very first step will be to get up his screen, for which
however we will refer to the following chapter. In the next
place he has to choose his position, as just explained, accord-
ing to his focus and the size of the disc he wants to show.

The position being found by a tape-measure, or stepping,
the lantern will be unpacked and placed in position, generally
on its own box (but see next chapter). Next, if it is a cold and
damp night, and there is a fire or stove anywhere handy, it is
wise to take out the condensers and put them to warm, and
also the box of slides, opening the cover to let them dry. If
gas is wanted for blow-through jets, the next step is to bring
the supply from the most convenient place up to the lantern
(see p. 47), and then the oxygen bag, or otherwise the pair of
bags, are placed in the boards, or the cylinders placed securely
where they will be used, and the regulators screwed on.
Then we see that all taps are turned off on both the jets,
and make the rubber connections. If the lantern has to be

ill OPTICAL PROJECTION

worked in the middle of a room, it is very important to re-
serve enough space for the gas-bags or cylinders comfortably,
and to have this space protected by making a kind of fence
with some of the seats, within which no one is allowed to
come. If it is a juvenile audience, it is important to get some
elder lad appointed to act as sentinel over this.

The connections are made as in the diagram, which shows
a star-tap (as the most puzzling). The hydrogen is connected

from the bag or cylinder to the
nozzle across which the bye-pass
is fitted, if only one ; otherwise
to the one known to have the
longest channel. (If the jets
both persist in snapping out,
with a tap that has two bye-
passes, it is probable the gases
are taken to the wrong nozzles
supplying the dissolver, and that,
from the unequal length of chan-
nel, the hydrogen cuts off before
the oxygen). In the diagram,
the hydrogen goes to the left,
the oxygen to the right-hand
nozzle. The tubes carrying from
the dissolver to the jets may be
connected to either top or bottom
lantern, as regards one of the

gases, provided the other gas be connected to correspond. In
the diagram the top hydrogen nozzle goes to the top jet, and
therefore the bottom oxygen nozzle must connect to the same
jet. With the other form of dissolver (fig. 61) both nozzles
on the same side go to the same jet, and the cylinders or bags
connect with the centre pair, and no mistake can be made by
a person of ordinary sense.

The lantern lenses will now be wiped over if necessary,

FlG. 63.— Connections

LANTERNS AND THEIR MANIPULATION 115

and the condensers replaced when they have been removed
for warming. If bags are used they will be weighted, and
the tap of the hydrogen bag or cylinder turned on ; where gas
from the main is used it will be already on at the dissolver.
The latter being turned on to both lanterns, limes will be
placed on the pins, first clearing their holes, and wiping off
all loose powder with a bit of paper ; and the nipples of the
jets will be adjusted to the known distance if needful.
Turning each hydrogen tap half on in succession, the jets
will be lighted, and the flame turned down moderately, when
the limes will be turned round and warmed. When well
heated throughout, one lantern will be turned off by the
dissolver, and the ' on ' lantern have its jet adjusted for bright-
ness according to the details in Chapter IV. The next step is
a very important one.

JKJ. 64.— Adjustment of the Light

62. Adjustment of the Light. — The lime has now to be
got into the position which gives a disc evenly illuminated all
over. This can only be properly done in one way. Any slide

i2

H6 OPTICAL PROJECTION

is placed in the lantern, and the objective being slid into
approximate focus with the rack about half-way, is properly
focussed with the rack itself; then the slide is removed,
leaving nothing in the stage. The jet is now to be adjusted
according to the appearance of the bright blank disc on the
screen. Starting with the lime about 3 inches from the back
of the condensers, the disc may present any of the appear-
ances in fig. 64. If it resembles A, the lime must be moved
to the left; if like B, to the right ; like c, it must be lowered ;
like D, it must be raised ; always moving it to the side oppo-
site to the dark shade, till whatever light or shadow there
may be, is central on the disc. It may now resemble E, when
the lime must be moved nearer the condenser ; should the
centre be dark and the edge bright, it must be drawn back.
When properly adjusted the disc will be even and bright all
over, as at F.

Amateurs often fail in this adjustment of the light, through
attempting to accomplish it with a slide in the stage, by which
it can never be done properly. The only way is that here
described. The lantern first adjusted had better be the
bottom one, which is first to be given its proper inclination, so
as to show its disc centrally on the screen. Should the lime
have to be moved much to or from the condenser, it may need
a little re-centring, as this movement will take it a little out
of the optic axis, which is inclined somewhat to the slide of
the tray. Before the other lantern is similarly adjusted, its
disc must be made to coincide with the other on the screen.
It is necessary to centre the limes with the pressure full on
which is to be used ; because if centred with a lighter pressure,
when more is used the luminous surface extends farther up
the lime, which is equivalent to raising the jet, and would
throw it out of centre.

Both lanterns being adjusted, the apparatus is ready ;
except that when the condensers cannot be warmed previously
at a fire, dew will form upon them in cold weather, and time

LANTERNS AND THEIR MANIPULATION 117

must be allowed for the heat of the jets to drive it off. If the
time is at hand, this will be the next proceeding ; but if there
is some time to spare, it is best to turn off the gases at the
bags or cylinders (the oxygen first), only taking care to
re-light in time to get the condensers dry. Or a still better
plan is to use a 'cut-off jet' as described in the following
section. The final proceeding is to see that the slides are in
order and placed handy, and the reading lamp placed ready if
one is used ; when all may be left till required.

It hardly need be remarked, that it is not enough to insert
the slides upside down. For diagrams, or diorarnic effects,
just as much care must be taken to have them the right way
as regards ' right and left,' and when exhibiting through a
transparent screen, this will be opposite to what it is when
an opaque screen is used. It is therefore most important to
have all the slides conspicuously marked, the same way, so
that they may be seen in the dim light of the exhibition hall.

On a damp night, if there is no time to dry the slides,
there may be much trouble from dew upon them. To avoid
this, each slide should, if possible, be warmed iu turn on the
top of the lantern, for which a flat top is so useful.

63. Cut-off Jets. — It is very common for a lantern exhibition
to have an interval in the middle, and it is a matter of some
consequence to be able to start again with all the lighting
adjustments ready just as they were left. One lantern being
turned off by the dissolving-tap still leaves the other ' on,'
and we do not want to turn off the gas at the source, or to
alter the taps at the jet, which would cause a slight ' hitch '
when the lantern was first used again. All such incon-
venience is avoided, and other obvious useful purposes served,
by having one of the jets fitted with its own 'cut-off,' as
invented by Mr. Andrew Pringle, which allows a single lantern
also to be cut off, but left ready for immediate use in the same
way. Fig. 65 shows this cut-off as added by Messrs. Newton
& Co. to one of the jets already recommended on p. 64. An

ii8 OPTICAL PROJECTION

additional tap is interposed between each of the usual ones and
the mixing chamber. These taps are geared together by cogged
wheels on the ends of the plugs, so that both are turned simul-
taneously by a bevelled cog-wheel worked by a spindle from
the back of the jet. The plugs are so fitted that when the
key at the back is turned as far as it will go in one direction,
the jet is full on so far as these taps are concerned ; and when
turned the other way the oxygen is cut entirely off, and the
hydrogen all except a bye-pass. When full on, the light is
adjusted precisely as usual by the usual jet-taps ; therefore,
when turned off and on again, this adjustment is found pre-

FlG. 65.— Jet with Pringle's Cut-off

cisely as it was left. With a lantern thus fitted at an extra
cost of about 15s. per jet, such an intermission occasions no
difficulty whatever.

Another ' cut-off ' jet has been constructed by Mr. Oakley.
Each plug of the ordinary taps has a screw piercing down
the centre from the top, into the gas-way. The gases are
adjusted by screwing these down so as to obstruct or free the
passage, while each tap can be turned entirely off at pleasure,
the hydrogen tap having a small channel all round as a bye-
pass. The two taps have, however, to be turned separately,
and some care is requisite to keep the adjusting screws gas-
tight.

64. Tri-unial Lanterns.— By adding a third lantern to
the top of a bi-unial, we get the tri-unial lantern, as in fig. 66,

LANTERNS AND THEIR MANIPULATION 119

which shows one fitted with shutter for the curtain effect,
(p. 122), and telescopic mounts. In nearly all cases the top
lantern is made removable, as here represented, so as to be
useful as a single lantern, and allow the remainder to be
packed lighter, as a bi-unial.

With the exception of the dissolving apparatus, there is

Fid. 66.— Tri-unial Lantern

little to add about the tri-unial. The object of the third
lantern is solely to allow of further dioramic effects, with sets
of slides made to exhibit such in combination. For instance,
dissolving into a snow scene may be done with two nozzles ;
but to do so combined with the effect of falling snow, can only

120 OPTICAL PROJECTION

be done with three, and is a charming effect. Except for such
extra effects, and for exhibitors who really use these, and
know how to produce them, and can give care and skill to
registering their own stock of slides for them, the triple lantern
is a sheer waste of money ; and the greater part of those pur-
chased are never really utilised.

The centre nozzle of a tri-unial is kept ' square,' and the
whole lantern must be first tilted or adjusted by it to the
centre of the screen, and this jet first got in order. Afterwards
the top and bottom lantern discs are made to coincide with
this, and then their jets similarly adjusted. It must not be
forgotten that a triple lantern necessarily uses more gas.

All that has been said about bi-unials, concerning work-
manship and simplicity of form (with however high finish)
applies in a still greater degree to triple lanterns. I have seen
one, regarded with evident pride by both maker and owner,
which was a fair load for a cart, and must be a grievous
burden anywhere but as a fixture in some hall or other. It
should be remembered that these instruments have at least to
be taken out of their cases and put back again.

65. Triple Dissolving.— The chief point to be really con-
sidered about a tri-unial lantern, is the system of connections
and dissolving that shall be adopted. After a great deal of
ingenuity, a single dissolving tap was invented, which by
turning a lever to the proper lettered places on a disc, would
bring any single lantern or pair into play. Such a tap costs
about three guineas, but is not practical, as there is not light
to read inscriptions during an exhibition. The only really
practical tap of this kind I know of, is known in the trade
as Noakes's Triple Dissolver. In this tap there are two sets
of holes and two levers : one to ' set ' the tap as required to
any pair of lanterns, and the other to do the dissolving. The
setting handle can be adjusted in the dark with perfect
certainty by touch alone, and the other action is quite as
simple ; bye -passes can also be arranged as required.

LANTERNS AND THEIR MANIPULATION 121

FlG. 67. — Four-way Tap

General experience, however, does not go in the direction
of any triple dissolver, but prefers an ordinary bi-unial dis-
solving tap for a pair of the lanterns, and a separate four-way
tap (fig. 67) for the other one, which turns on or cuts off both
gases as required.
A very usual plan
is to have pieces
of brass tubing
plugged at the top,
fixed up each
corner of the back
of the lantern, cut
where the top
lantern joins on,
but connectedwith
an inch or two of rubber so as to act as one. The supply-
tubes are stretched over the bottom ends of these pipes, and
small side nozzles level with the dissolving taps connect with
these. Rubber tubes connect the dissolving taps with the
jets as usual. This is decidedly the most popular plan ;
but some experienced exhibitors prefer for each lantern to
have its own single four-way tap, governing its own jet alone,
supplied from brass mains as in the preceding case. Each
tap has a double arm (that is, one extending on each side of
the plug), and the upper pair of arms are connected by a
brass rod on one side, and the lower pair by a rod on the other
ends. The rods pass through sockets with pinching screws
fixed on the arms, so as to work any given arm at pleasure,
or slide loose when the screw is slackened. The result is
that either the top pair or bottom pair of lanterns can be
dissolved together at pleasure, the third being left independent
for separate use. Or the pairs can be changed in a moment
from top to bottom pairs if required.

66. Bi-unial and Lantern Effects.— A bi-unial lantern is
necessary for many other effects than dissolving views, as

123 OPTICAL PROJECTION

already hinted. A common addition to the better class is what
is called a ' curtain-slide ' of brass. This is a single brass
shutter passing through both stages, and so adjusted (by
screws) that just as much is shown on one disc up to a sharp
edge, as is covered on the other. When raised or lowered,
the effect is as if one picture pushed the other up or down.
With statuary, a slide representing a curtain is used in one
lantern, while the shutter draws up to reveal the statue. Or
a curtain-slide is often used thus at the beginning and end of
an exhibition.

The other usual effects fitted to this class of lanterns are
' flash -shutters ' and * tinters.' The first is a shutter of brass
over the front of each objective, to shut off a lantern suddenly,
or open suddenly, when the light is full on. Many dioramic
effects, such as an explosion in a bombardment scene, must
be ' flashed ' on instantaneously to have any effect ; a ' bang '
being made in some way at the same time, for which a large
gong does very well. Occasionally an axle is fixed between
the two nozzles, on which revolve two arms with a shutter at
the end of each, which can be moved out of the way when
not required, but are adjusted to give the required ' flash '
before the scene comes on. Personally (my experience being,
however, very small, and quite as an amateur in these
matters), I prefer to trust to the hand alone, held in front
of the nozzle before the light is turned on, and which can be
withdrawn in an instant, and with no risk of failure.

Tinters may add very much to the pleasing effect of plain
photographs, if used with suitable subjects. Thus, a warm
tint is easily thrown over a foreground, and a blue tint over
the sky, or sea and sky, or on a moonlight scene. Tinters
are of two kinds. Fig. 68 shows the most common. It fits
on the nozzle of the objective, and any different colours can
be fitted in the top and bottom shutters, while an opaque
shutter outside can also be folded down, so as to gradually
extinguish the picture altogether.

LANTERNS AND THEIR MANIPULATION 12

This is a favourite attachment to the single lantern also,
as the screen may be darkened while the slide is instan-
taneously changed ; for it may
be taken as a universal rule
in lantern exhibitions, where
'effect' is any object, that the
white screen should never be
shoivn. To do so makes the
slides appear much less brilliant
than if no such bright light
intervenes.

The other plan is to cut slits
through the sides of the lantern
body near the front, so that large
pieces of coloured glass, or gela-
tine films between two glass

plates, can be slid in and out of the lantern close to the back
of the condensers. This plan is more costly, and weakens the
lantern, but has an advantage in not interfering with definition,
as the edge of a glass plate in front of the objective does to a
small extent. But the ' saving of light ' mentioned in some
catalogues only exists in the imagination of the compilers.
The apertures in the body are closed by narrow brass doors
when not in use. By using one or more sheets of coloured
gelatine between glasses, any depth of colour may be had for
tinting purposes, with either form of tinter.

Statuary is best exhibited with all the slides blocked out
with black, and dissolved into blue tint, a little of which may
be left on with the statue if preferred.

Slide Effects are described in Chapter X.

124 OPTICAL PROJECTION

CHAPTER IX

SCREENS AND OTHER LANTERN ACCESSORIES

WE liave in the last chapter supposed the lantern in position,
and the screen up ; but there are a few matters yet to be con
sidered before we can dismiss practical exhibition work with
such an assumption.

67. Screens. — There is no dispute at all as to what makes
the very best screen, for all kinds of lantern work. It is, a
fine smooth surface of white plaster of Paris, and next to that
a smoothly whitewashed wall, finished with whiting, and not
with lime. Such a surface is both white and opaque, reflect-
ing back nearly all the light which falls upon it. Such a
surface, as far as I have been able to ascertain from rather
rough tests made with polarised light, gives fully 50 per cent,
more light than the best white sheet, and 25 per cent more
than the very best faced screens that can be made.

In public institutions, therefore, and especially where
physical projections sacrificing much light are made, it is well
worth while to reserve a proper space of wall at the back of the
platform for such a surface, and to prepare it by a thin coat-
ing of fine plaster, carefully whitening thereafter as required.
Even for ordinary lantern lectures it would be well worth
while, as no one will question who has once seen slides pro-
jected upon such a surface 25 feet square. It is a matter of
continual surprise to me that, now demonstrations by the
lantern have become so customary in all halls and institutions,
steps are not taken to provide for their best effect, in a simple
and economical manner which nevertheless far surpasses any-
thing possible to a casual demonstrator. The white surface
can easily be draped over when not required.

In Germany it has been the practice for some time to pre-

SCREENS AND LANTERN ACCESSORIES 125

pare portable plaster of Paris screens in an iron frame, of about
5 feet diameter, for microscopic projections ; and in this way
results have been obtained, which otherwise the instruments
employed would have been incapable of producing.

The next best are portable screens of flexible material
faced with some similar opaque coating. For any very special
occasion indeed, the best plan is to have a sheet made of the
required size — it is never worth while taking the trouble
unless the screen is wanted at least 20 feet square — out of the
stoutest unbleached calico, and tack it tightly on a light
wooden frame put up in the hall, so that it is tense all over.
Then the whole should first have a coating of size ; and when
this is dry, several coats of the cleanest whiting mixed also
with size. Screens thus stretched, and distempered for the
occasion, give such a good effect, that it is worth while to take
the pains for any public affair of special importance ; and as
the sheet itself is portable enough, and will do over and over
again, the cost is not great. For a course of lectures in an
important place, where the screen can be left up, such a plan
will amply repay the trouble.

I have read in different books that a screen of this kind
can be prepared which will roll and unroll upon a roller, and
keep clean. My own efforts have failed in making any, after
many trials ; but it is possible that some workmen with
technical skill in distemper might succeed in such a screen.
I know that the old Polytechnic screen was prepared in some
such way ; but that always appeared to me a very dirty one,
and rather an example to be avoided than otherwise.

For ordinary lantern work, a simple screen of white linen
or thick calico answers all purposes so long as it is kept clean,
and it can be washed when necessary. This sort of screen,
being the most portable, is used by travelling lecturers more often
than any other. Up to nearly 10 feet square, material can be
got for a screen in one piece. Above that it must be sewn
together, and this must be done very carefully indeed, to keep

126 OPTICAL PROJECTION

from creases or buckles. The seam should be judiciously
placed. It is best to run horizontally, but in any case should
not cross the middle. For a 15-feet screen a seam 10 feet
from the top would be best ; a 20-feet sheet would be best made
of one piece 10 feet wide, and 5 feet top and bottom. The eye
naturally goes to the centre of the picture, and in this way
the seam is less conspicuous.

The brightest portable screens are those made of some
flexible material and faced with white paper, the latter being
still better faced with some opaque material. For this latter
improvement demonstrators are indebted to Mr. JohiiD. Mason.
This gentleman — himself an experienced lanternist — was kind
enough to take a great deal of pains in obtaining various
samples of white paper for me, with a view to a screen for
microscopic projections. The best of these were a perceptible
improvement over any faced screen I had been able to obtain
before, but neither of us were satisfied, and I suggested trying
distemper on the paper. I tried various preparations, and Mr.
Mason others, but with the common result, that anything
really white speedily flaked off the surface. At length it
occurred to Mr. Mason to apply to the paper-hangers for
samples of their mineral-faced papers ; and some of the best
of these I found to give a brilliance considerably beyond any
surface of paper alone, while the expense is only about 50 per
cent, more than of any ordinary paper-faced screen. It is not
equal to a wall surface, but comes the nearest to it of any
portable screen I am acquainted with. These screens have
since become well known to, and highly approved of by all
the best London opticians. The largest I am aware of was
20 feet square, and it is much to be wished that halls which
cannot or will not provide a wall surface, should provide a
screen of this kind, to be rolled up when not in use. For an
expense of a very few pounds once for all, lecturers would then
be spared the most anxious and troublesome part of their pre-
parations, and the effect of their pictures would be much

SCREENS AND LANTERN ACCESSORIES 127

enhanced. Such a screen, faced with mineral matter, per-
ceptibly improves the light for microscopic and physical
demonstrations.

Small occasional screens are often needed in experimental
work, and sometimes in a drawing-room. The cheapest and
handiest material for such, is apiece of Whatman's continuous
drawing cartridge paper. This can be obtained 60 inches
wide of any length for a few pence per yard, and may be at-
tached to a small roller.

68. Transparent Screens. — For common lantern work, the
thinnest material in one piece ten feet square may be used,
strained tightly, and wetted before the exhibition. The simple
fact that the picture ' shows through ' in this way, sufficiently
accounts for the superiority of plaster or opaque surfaces over
a sheet. A thinner muslin material is also employed, and
this will have to be joined with as fine and imperceptible
seams as possible. A transparent screen more than ten feet
square is never used.

Transparent screens were very usual in early lantern days,
when nine-feet discs were all that could be ventured upon.
They are generally now confined to folding-doors in private
houses, or semi -microscopic exhibitions.

The best transparent screen for microscopic work is one
made of tracing paper. This can be had in continuous rolls
5 feet wide, and a screen of this width, with a light portable
frame, is both very handy, and gives some charming results,
though not equal to an opaque screen in illumination. Such
a size is even large enough for pleasing drawing-room exhibi-
tions with oil lanterns, which can be made on this scale with?
out leaving the room in darkness.

69. Sizes of Screens. — The limits of transparencies have
been stated. Faced screens are always mounted on a rod and
roller, just like a map, and hence more than 12 or 14 feet wide
is awkward to carry. For a hall there is no limit to these
screens beyond the size of the wall on which they have to be

12S OPTICAL PROJECTION

stretched in the making. For physical demonstration more
than 10 or 12 feet is not required ; for microscopic work 12
to 14 feet is better. Both for the latter, and also for pictures,
it is to be noted, that some margin improves the effect ; thus,
a 12-feet disc will look much better on a 15-feet screen,
than on a 12-feet screen. Eegular lecturers who use sheets,
generally prefer to have two ; one 12 to 14 feet, and the other
18 to 20 feet square. If only one is used, 15 or 16 feet is the
most useful size.

70. Erecting the Screen.— This is the most anxious piece of
work of any to a peripatetic lecturer. When his screen is up,
he feels at ease. He never knows, in a new place, what con-
veniences he may find, or what awkward arrangements he
may have to get over. If the place is often used for such
exhibitions, his first thought is to look aloft, as he is pretty
certain to find a couple of eyes or hooks some distance apart,
left by some one or other of his predecessors ; for it is an under-
stood courtesy amongst lecturers, that any such hooks or eyes,
once placed in awkward positions, are left to smooth the path
of successors. If these are found, then, and are at a height
and distance which fairly suit the size of sheet he means to
use, he has only to find a ladder tall enough, and his task is
easy. The eyes must be high enough to raise the bottom of
his disc above the heads of the audience, and enough apart to
be some inches wider than his sheet — how much wider matters
little to him. If there are no such attachments, he must put
them in himself ; so that it is always wise to make sure first
of all, if necessary, that a ladder long enough is available.
As to the eyes and hooks, with screwed ends, no exhibitor
travels without these. If or when the top pair are all right,
the exhibitor's next step is to screw a pair of hooks into the
floor, a foot or two wider apart than the bottom corners of his
sheet, and just under where these will be.

There are various ways of fixing up the sheet itself. My
experience has been of such an amateur character that it may

SCREENS AND LANTERN ACCESSORIES 129

riot be worth much ; but I prefer a row of brass rings sewn
a foot apart along the top of the sheet, through which is run
a piece of the finest and best sash-line, turned at each end
over a grummet ring, the line being long enough for the two
rings to come an inch or two beyond the corners of the sheet.
One corner of the sheet is tied up to its ring with a bit of
twine ; the other corner to such a position that when the line
is stretched, it slightly stretches the sheet also. The sides
of the sheet have sewn to them, every 18 inches, eyeletted
brass hooks like fig. 69, which can readily be made with a
pair of pliers. I also have a pair of small
wooden pulley blocks, each with an iron hook
for fixing ; and two long pieces of the same
best sash-line already mentioned, each with
a hook at one end. FlG 69

In getting up the sheet, each piece of line
is rove half-way through one of the pulleys, so that both ends
will hang down to the floor when this is raised ; and both
pulleys are then hooked on to the eyes fixed up aloft, with the
lines hanging down. When both lines are thus arranged, the
sheet is placed underneath, and so far opened that only the long
folds across remain unfolded. (A sheet should be always folded
up first into long folds all across it, and only then folded over
short, so that when these short folds are undone, it lies on
the floor its full width). The two hooks are hooked into the
grummet rings at the ends of the top line, the lines are hauled
up till this is stretched reasonably tight, and then the lines are
made securely fast to the hooks fixed in the floor.

The final step is to take two long pieces of stout twine, and,
fastening or hooking one of these into the top hook on each
side of the sheet, * lace ' downwards on each side between the
hooks and the line (which should be if possible about a foot
outside the sheet), thus drawing the sheet out tight and flat.
Finally, we draw down the two bottom corners, and fast n
them to the hooks below in the floor. It is as well even to

E

i^o OPTICAL PROJECTION

have a hook or two along the bottom of the sheet, which, if
necessary, can be laced to another piece of line stretched be-
tween the hooks in the floor : but this last is seldom done.

It is more usual to sew rings on the sides instead of hooks ;
but in t*hat case the lacing twine has to be passed through
every ring ; whilst with open hooks it can be ' caught ' through
them in an instant, and the lacing only takes a third of the
time. Some exhibitors prefer the line at the top edge of the
sheet to be sewn or hemmed to it ; but I prefer the loose line
(1) because the stretch of the line and sheet can always be
precisely adjusted this way, and is sure to be evenly distributed
all along the line ; and (2) because if the line has to be re-
newed, there is less trouble. Some again never use pulleys,
but pass their lines direct through the eyes aloft. This, how-
ever, greatly hinders proper stretching, and increases the force
necessary to be used, besides wearing out the lines by friction.
Moreover, a pulley can be fitted with studs to fit on a pin at
the end of a long pole, so as to be hooked on to its eye aloft
even if no ladder is available.

Small pulley blocks suitable for these purposes are easiest
obtainable at the class of shops often called ' model dockyards,'
where parts of naval models are sold.

Occasionally it may happen that the eyes, or only available
places for them, are at the sides of a wide hall, many feet
away from the edge of the largest sheet that can be used, and
at such a distance that the top edge would sag down a great
deal, with the tightest stretch that can safely be put upon the
lines and pulleys. The remedy is very simple. After getting
the sheet as high as is safe, two slender wooden strips are
procured and placed as struts or props under the line, about
a foot outside the sheet on each side. In such a case the sides
will be laced to these props.

Faced screens are furnished with three rings on the rod
at the top, and if necessary can be got up in the same way,
drawing up a line run through these rings. Of course no

SCREENS AND LANTERN ACCESSORIES 131

lacing is required, but it may be advisable to tie the ends of
the bottom roller by pieces of twine to the floor. The ring
at one end of the top roller should always be tied with twine
to the line, to prevent the screen slipping about in raising or
lowering. Care is necessary in lowering, to let the two sides
down alike, a third person in the middle rolling up the screen
tightly as it descends. If the screen has to go on a wall,
three nails will be all that is required.

A water-tight wrap of oil -cloth or macintosh should
always be provided for a screen of any kind, unless a sheet
packs in the lantern-box.

Portable frames for erecting screens can be procured of all
opticians, but are most suitable for the smaller sizes (say 10
feet and under) in private rooms. A common form is shown
in fig. 70, the poles being built of three short pieces connected
by tubular sockets on the ends, so that
the stand for a twelve-feet screen will
pack into a case about 4^ feet long by
six inches square. These stands are
to be had fitted with pulley-rollers
at each corner, when they are equally
suitable for either a sheet or a paper-
faced screen ; the latter being simply
hung, while a sheet is first strained
over the pulleys, and then laced to the
poles by twine or tapes. FlQ. 70

Another simple plan for elevating

a roller screen, is to provide four pieces of pine-wood, each
7 feet or more long, and about 3x1 inches in section. Quarter-
inch holes are bored every three inches for a couple of feet from
each end, and four bolts with flange nuts are provided. Then
two of the pieces can be clamped together by two of the bolts,
so as to make one prop, and these props can either be simply
stood on end and secured by guy-ropes, or may be fitted into
base-pieces, or may be furnished with buttress pieces at the

E2

OPTICAL PROJECTION

bottom. The top roller of the screen itself keeps them the
proper width apart.

A tracing-paper screen IB always furnished with a portable
stand, so constructed that if necessary it can be placed upon
a table.

Transparent calico screens are usually stretched across
the space occupied by folding doors. This is easily done by
lacing twine lightly to large tacks, or even stout drawing-pins,
which can be removed without leaving any conspicuous sign.

71. Light for the Lecturer. — Most of the opticians supply
a sort of fold-up desk, which shades the light from all but the
copy of the lecture ; but such apparatus is now out of date,

being superseded by a simple lamp. If
this is properly shaded, somewhat as in
fig. 71, which is a pattern very widely
adopted in general, with little variations
in detail, all necessary purposes will be
equally secured, and some sort of desk
or table can always be procured. These
lamps can be had with a red glass signal
to the operator at the lantern, who
watches for this direction when to change
the slide ; and at option with a signal-
bell also. A bell, however, is a nuisance
in a lecture. Of late the excellence and
cheapness of electric bell apparatus has introduced the fashion
of carrying a wire from lecturer to operator, which operates
a very small signal bell for him alone ; but even this is audible
unless the lantern is worked from a box or gallery. A signal
by red light is the best. Any signal should be given some
seconds before the change is required. Of course any operator
acquainted with the lecture knows when to change without
'any signalling at all.

72. Stand for the Lantern. — It is a great comfort when
the lantern can be worked from a gallery or box at the other

FIG. 71.— Lamp

SCREENS AND LANTERN ACCESSORIES 133

end of the hall. Failing this, the lantern has to be some-
where in the body of the hall, and must be raised considerably
above the floor, in order that all the rays may pass clearly
above the heads of the audience.

Nearly all the opticians figure in their catalogues some
kind or other of tripod stand upon which to mount the lan-
tern, resembling closely the portable stands used for travelling
cameras. I would warn the inexperienced against such things,
as a source of serious danger. It is easy to get a stand of
this kind which appears perfectly firm ; but the spreading legs
are much in the way, are apt to be forgotten in the dark, and
more than once have caused the whole concern to be tipped
over, to the damage of many pounds' worth of property,
besides the humiliation of the catastrophe. I never knew any
experienced exhibitor who would run such a risk. Moreover,
such a stand affords no accommodation for the slides.

All habitual lecturers prefer to stand the lantern upon the
lantern-box, or cabinet, which is arranged to be entirely open
at one side when the lantern is taken out, and furnishes
a convenient receptacle for the box of slides, limes, and other
odd matters required. If any sort of plain and substantial
table can be found in the place for the box to stand on, that
is the best, and the extra room is always useful. Every room
where exhibitions are frequent, ought to have such a table,
unless there is a box or gallery as just mentioned. If still
more height is desirable, the table may be elevated on a
couple of seats or benches, so placed that a couple of the legs
stand on each. Such benches, when used, should be arranged
across the room, when they will comprise the front side of
the square or fence which has been already mentioned as so
desirable when working in the body of a room.

But sometimes there is either no table, or only some polished
library affair which the authorities will not allow to be used.
Mr. T. C. Hepworth, a well-known and experienced lecturer,
showed me several years ago a plan he had adopted to meet

134

OPTICAL PROJECTION

such difficulties, and which enabled him to laugh at them.
He carries with him four iron legs lxT% inch in section,
turned out at the bottom into a toe, through which is a hole,
by which all four feet are screwed to the floor. That is
essential, as otherwise these narrow and somewhat spreading
legs would be liable to the same danger as the tripod stands
above condemned. The upper ends slip into iron sockets
carefully fitted to them, and screwed to the four lower corners
of the lantern box. The box has only to be mounted on
these legs, and these latter screwed to the floor, and the
whole is complete. If more height is required, the legs can
be stood upon a couple of benches in the manner above men-
tioned; and there is no practical danger then of an upset, as
the seats themselves define the places which the feet occupy.

Another expedient often
employed belongs to the next
paragraph.

73. The Lantern Box or
Cabinet. — This can be made
in many different ways, and
should always be carefully
thought out by the owner
himself, hi relation to his
own apparatus and wants ; as
a great deal of his comfort
will depend upon it. Fig.
72 shows a general method
often adopted, in order to
serve as a higher stand for
the lantern. The main body
of the box is open at both

FIG. 72.-Rising Cabinet ends and at ^16 top, and the

ends E E, carrying the lid of

the box L, slide up and down in grooves A B, A B, near the
ends inside of the two sides ; or it is better if the grooves are

SCREENS AND LANTERN ACCESSORIES 135

not cut out of the wood, but formed by guide-strips fastened
on. On each edge of each end-piece is fixed a strip of brass
pierced with holes, by which the lid L can be secured at any
required height by the fastening screws F, which pass through
holes in the sides of the lower box. The lid of a box thus
constructed can be fixed at nearly double the height of the
box when closed, and this height is generally sufficient when
the box alone is elevated upon forms or chairs.

At the back end of the lid of the box is hinged another
top board T, which can be secured at any necessary angle
by screws which act upon the brass sectors s. On this board
the lantern is placed. All boxes should be fitted with a hinged
double top of this kind, which saves time and trouble on
every occasion in tilting the lantern.

With such a box as fig. 72 the lantern must be put in at
one end. In plain boxes it is usually put in and taken out
at the side. This should not open on fixed hinges, but have
' catches ' at the bottom, fastening in at the top with a snap
sunk-ring latch, and lock and key. The object of this is that
the side may be taken entirely away and leave the box open.

The internal arrangements will depend upon the apparatus,
and individual preferences. An optician in large business
usually has several people in his employ who are sent out to
give entertainments ; and each of these is often allowed to
have his own arrangements, which will differ considerably.
The main object is, of course, to pack away all the apparatus
conveniently, and every cabinet should have one drawer or
compartment for the lime -box, matches, screw eyes and hooks,
gas-nibs, gas-pliers, and any other odds and ends which
should be always at hand. Another will be needed for the
lamp, and another for the tubing, unless this is packed in
the lanterns. The slide-box, on the whole, is better carried
separately : but if a sheet is used it is much better in the box
if possible. One case I have seen was arranged as a large
square shallow box, the open side (when in position) taking

I3& OPTICAL PROJECTION

off as a square lid. The bi-unial lantern was laid down in
the box on its side, little compartments filling up all vacant
spaces between the brass fronts, &c. ; and finally the sheet,
folded to fit, was laid over all, and served as a soft packing to
keep the whole from rattling. The box was made this way
to allow of the sheet being thus laid on the top last of all.

If the lantern has lengthening tubes and extra lenses,
precise fittings should be provided for all these ; and ' packings '
should be carefully arranged, with proper cloth linings, to
keep the whole in place quite free from shake. A box thus
carefully packed is far stronger than one in which the lantern
is at all loose ; and as the weight of a bi-unial or tri-unial
is considerable, an extra sovereign spent on the cabinet is true
economy. It only remains to add, that the cabinet should
be arranged so that all the jets, dissolvers, and their rubber
connections may be left in place ; and that a measuring-tape
should by all means form part of the contents.

CHAPTER X

SLIDES, CARRIEES, AND EFFECTS

74. Slides. — Very little need be said about ordinary slides
for the exhibition lantern. They practically consist of four
kinds : there are plain photographs, tinted and painted photo-
graphs, and hand-painted slides. These latter have been in
so much less demand of late years, owing to the excellence of
the better painted photographs, that they are not easy to pro-
cure now-a-days ; but they are still to be had by paying for
them.

Really good plain photographs are charming, and some of
the very best have been produced by amateurs. The main
thing is the extent to which the half-tones and atmospheric

SLIDES, CARRIERS, AND EFFECTS 137

effect of distance are rendered ; but I have seen many lantern
slides by my friends Mr. Washington Teasdale and Mr.
Hepworth, which were really exquisite pictures, and would
have been simply spoilt by being coloured. Mr. Hepworth
has fully described the processes he uses in his ' Book of the
Lantern,' and also the practical details of tinting photographs ;
and those who wish to attempt this branch of art with special
reference to lantern purposes, cannot do better than consult
his work ; the subject does not belong to these pages.

Painted photographs comprise the chief bulk of the slides
exhibited, and may be either wretchedly bad or exquisitely
good. It is very greatly a matter of price ; though not
altogether, since a high price may be and often is charged
for inferior work. On the other hand, however, a low-priced
coloured slide cannot be good, since excellence depends upon
having a good photograph in the first place, and then upon
spending a certain amount of skilled handwork over it, by a
competent artist. Really good hand- painted photographs of
landscapes cannot be produced under from 5s. to 7s. 6d. each,
according to the detail in the subject ; and it is a marvel to
me how they can be finished for that. By this, however, I
mean really beautiful pictures, such as are shown by exhibitors
of high reputation. Figure subjects cost much more, if well
executed. Coloured slides are sold at a much cheaper rate for
children's exhibitions and such-like — an average price being
8s. 6d., but these are a different kind of thing altogether.
Hand-painted slides may cost any money.

75. Carriers for Slides.— It has been already explained,
that for the highest class of exhibitions each slide must be
separately framed, that it may be exactly ' registered.' Many
opticians who let slides for hire, which is usually done at from
1/6 to 2/6 per dozen, according to their quality, also prefer
to send them thus framed, as less liable to accident. But it
is not really needed for the ordinary class of dissolving-view
exhibitions, and it saves a great deal of space and weight if

138 OPTICAL PROJECTION

the glass slides alone are carried in a grooved box, and used
in some form of frame or ' carrier,' inserted in the slide- stage
of each lantern. Of these carriers there are many kinds.

The simplest is a solid frame, with a grooved opening into
which the slide drops from the top. This kind of carrier has
to be withdrawn from the lantern every time a slide is changed,
inverted to drop the old slide out, and then re-inserted with
the fresh one. About this there is some risk, and these
carriers are gone out of use since better ones have been
made.

Another form, more used some years ago than now, is
known as the panoramic carrier. In this the wooden frame is
the exact length of three common slides, and is clear from
end to end, but has only one mask or opening in the middle,

FIG. 73.— Beard's Self-centering Carrier

of the proper size. The carrier is left in the lantern, the more
firmly fixed the better, and the slides are slid in, so that when
a second slide is pushed in after the first, exactly even with
the end of the carrier, the first or middle one is in place. A
third is pushed in similarly ; and after that, as a fresh slide
is pushed in, care must be taken to catch the slide which is
simultaneously pushed out. The chance of forgetting this is
a risk about the panoramic form of carrier, which also has
the great drawback of requiring both hands at the instant of
changing. By having one end of the panoramic carrier cut
into stops at different distances from the end, different sizes
can be used after each other and still be centered. On rare

SLIDES, CARRIERS, AND EFFECTS 139

occasions this is an advantage, but the panoramic carrier has
no other merit in my opinion.

If different sizes have to be shown in the same lecture, the
best carrier is probably that known in the trade as Beard's
self-centering carrier, shown in fig. 73. Whatever be the
size of the slide, it has only to be placed upon the brass runner
at the bottom and pushed up till it stops, and it will be in the
centre of the stage.

The best and simplest carrier for any single lantern, or
bi-unial, is, in my own opinion, the simple one shown in fig. 74,
but it is only possible for slides of uniform size. There are
two grooved frames with openings at the top, into which slides
can be dropped, the two combined sliding freely from side to
side in an outer frame which remains immovable in the stage.

Fia. 74.— Double Carrier

There is a stop at each end, so that when the inner frames
are pushed up to either end, one frame is centered. The slide
done with is out on one side, and can be lifted out by the
fingers and the next one dropped in, at any convenient moment.
Once dropped in it is ready, and a push in changes the slide
in an instant. To prevent the dropping in of the slide making
any noise, it is well to jam in a narrow strip of india-rubber
(flat] on the bottom of the bottom groove.

This carrier is an especial comfort to the scientific lecturer,
especially if he has himself to stand near his lantern, as often
happens. The assistant has only to concern himself to have
the next diagram ready in the frame not in use ; and can then
bring it on in place of the other in the twinkling of an eye, or

140

OPTICAL PROJECTION

the lecturer himself, if standing near, can do it with a touch
of his hand. The blank screen is never shown. A single
lantern is often fitted with an arrangement for gradually
darkening down to complete obscuration in front of the
objective ; and in that case also this carrier is best, because
in one moment, with no possibility of hitch or mistake, the
slide can be changed while the screen is dark, and the new
one will then emerge out of the darkness. This simple effect
is thought by many to be quite as pleasing as the now familiar
dissolving views.

This carrier cannot unfortunately be used with two lanterns
side by side, for obvious reasons. In that case perhaps the
best form is that shown in fig. 75, where the inner frame

FIG. 75.— Carrier for Pair of Lanterns

travels to and fro in the same way, but can only be in position
when pushed in, and has to be drawn back and out for the
slide to be changed.

There are other carriers, some patented ; and any optician
with a large lantern business can show an assortment, or at
least representations of such from catalogues ; but the above
are in my opinion the best.

76. Mechanical Slides. — These are of many sorts, and can
only be mentioned here, a very large variety being merely of
a comic character. They seem still to maintain their
popularity amongst certain audiences, and are worth at least
enumeration, as the effects produced may at any moment
furnish some useful hint for more serious purposes. (See for
an example Chapter XV.)

SLIDES, CARRIERS, AND EFFECTS 141

Slipping Slides have a portion of the picture painted upon a
separate glass, which can be suddenly drawn along in the frame
so as to produce some startling change. To allow of this,
such slides have only a wooden edge frame like a slate frame,
the picture being painted on an oblong piece of glass contained
in this, and the rest blacked out. A familiar example is some
figure carrying a dish ; the ' slip ' will draw forward and cover
the head, whilst another head appears on the dish. Or figures
will change heads, and so on. More seriously, the ' effect ' of
an explosion is sometimes done as a slipping slide.

The Uncovering Slide, as it might be called, is the same
thing worked slowly, but may be single or double. Of
the single form an example may be cited in the gradual
lengthening of a comic figure's nose. A higher type of the
double form may be found in the gradual unfolding of a car-
nation from the bud, with perhaps the final appearance of a
fairy in the centre. In these cases the complete subject is
painted on the foundation glass, and carefully fitted blacked
screens are gradually drawn away on one or both sides. A
flash of lightning is shown in the same way. Or the moon may
emerge from behind a cloud, which, with a good dissolving-
view ' effect' of moonlight, exactly registered, is very fine.

Panoramas speak for themselves, the moving parts being
painted on a long slip of glass without frame, which is
gradually drawn along in grooves over the fixed portion.
(In all these movable slides it will be understood that the
painting on both pieces of glass must be on the inner surfaces,
which are nearly in contact ; else the two could not be
tolerably focussed together.) Vessels are often made to
traverse a sea-scene in this way, or trains may be introduced;
or if the slide is made vertical, a balloon ascent may be shown
in the same way.

A very charming variation of panoramic slides is to have
a series of good landscapes painted as a panoramic slip,
moving across a mask carefully fitted to coincide with the

142 OPTICAL PROJECTION

open space of a window or verandah, which is kept on the
screen from the other nozzle of a bi-unial lantern. The effect
is as if the panorama were passing the window; but of course
this manner of exhibiting uses a great deal of gas, both
lanterns being on all the time.

The drawback to panoramas is, that having to be specially
prepared they are expensive ; and the long unprotected slips
are rather liable to breakage.

Lever slides have the movable portion painted on a
circular glass mounted in a turned brass rim which rotates in
a circular socket, and which can be thus rotated through a
certain arc only, backwards and forwards, by a lever projecting
at one side. This kind of movement is used to represent an
animal stooping its head to drink and lifting it again, a child
swinging or see-sawing, a man breaking stones with a ham-
mer, and similar alternating movements.

Backivork slides are essentially of the well-known chroma-
trope kind, wherein two symmetrical geometrical designs are
rotated in opposite directions, each being mounted in a cir-
cular brass rim rotated by a toothed pinion. Straightforward
chromatropes, as they might be called, are easy to understand;
but few would imagine the startling change of effects in some
skilful designs, which may be found in the stock of any
London optician noted for this class of work.

The same movement is applied to many other slides.
Single racks represent mills, swarms of bees, sleepers swal-
lowing rats, &c. Double racks represent fish in an aquarium,
and the magnificent effect (when well executed) of a fountain
with moving water. Single or double racks are used to
produce the ascending smoke and flame of conflagrations or
volcanic eruptions.

A combination of rack and levers makes an exquisite
representation of waves in motion, with the riding of vessels
upon the heaving water.

The highest type of rackwork slide is one constructed

SLIDES, CARRIERS, AND EFFECTS 143

with a series of circular racks so as to show the motion of all
the members of the solar system, which can be admirably
done.

Holler slides have a small roller at the top and bottom,
over which some flexible material is drawn in one steady
direction by turning a handle. A black band pierced with
small holes gives a snow effect, care being taken that the
lantern showing the snow is not worked brighter than experi-
ence has proved to be best in effect. A transparent material
painted in lines and set rather diagonally, will give a rain or
hail storm.

77. Dioramic Effects. — Even this brief summary will be
sufficient key to the variety and realistic effects obtainable for
use with the lantern. Just as a suggestion and application, let
us enumerate what may be done with one single landscape,
with slides properly registered. It may be worth while to do
this ; because, with all the increase in instruments calculated to
produce it, there seems in the present day a general disregard
of that fine dioramic effect of which the lantern is capable,
and which I cannot but think would be appreciated were it
adequately represented to the public, as in the days of the old
Polytechnic.

Let us suppose, then, an English landscape, with a wind-
mill, and mill-pond in front. It may be first thrown on the
screen in the fresh green of an English spring. Presently
the mill turns round, then the scene dissolves (without
appearing to change) into the warmer hues of autumn, and
one or two swans glide gently over the glassy lake. The
scene dissolves again into a tempestuous night, the sky
covered with clouds, and little light getting through from
the partly turned-down lantern. Flash after flash of lightning
breaks across the dark sky, followed by peals of thunder (pro-
duced by shaking a square of sheet-iron), and finally a 'rain-
slide ' is thrown on the scene, the sound of the storm being
well represented by pouring some barley into an appropriate

144 OPTICAL PROJECTION

vessel, and a flash or two of lightning being still continued.
Finally these effects are changed for one showing the moon
emerging from behind a cloud. This must be done very
gradually, and while the moon-slide is thus worked the scene
is gradually dissolved into a bright moonlight scene, with the
light effect upon the water. After a few moments a ' snow-
slide ' is put in, and when this has fallen for a few seconds
the landscape is again dissolved into a winter scene, with
snow upon the landscape and ice upon the pond, on which
skaters execute their gliding movements ; or this last may be
done when the scene is again changed to a night and bonfire
effect, with lights in the mill windows.

Such is an example of what may be called high-class
dioramic lantern exhibition ; and if the different slides match
and register exactly, the effect is indescribably beautiful. All
depends upon that, and also upon having an adequate staff of
assistants, for one alone could not possibly conduct all the
various operations described. Two would often be required
at the lantern itself, irrespective of any acoustic effects ; and
in some cases the work would be better done with four
lanterns than even three. At the Polytechnic as many as six
were occasionally employed, but four would probably perform
all that was necessary even in such imposing spectacles as the
siege of Delhi, which awakened so much admiration at the
time.

78. Experimental Slides. — Besides scenic slides and effects,
there are some often shown which are of a semi-experimental
character. If two pieces of perforated zinc, or of wire gauze,
are mounted in a double-rack chromatrope frame, they pro-
duce very interesting effects. Another slide known as the
Kaleidotrope, consists of a piece of thin metal pierced with
small holes, and hung by a pivot at the centre to the end of
a coiled spiral watch-spring. When this is twitched by the
forefinger, the metal both spins round and vibrates on the
spring, and the effect is many apparently moving circles oi*

SLIDES, CARRIERS, AND EFFECTS 145

light upon the screen, interlacing with one another, and due
to the persistence of vision. Beale's Choreutoscope rapidly
substitutes different attitudes of the same figure, and thus
gives the effect of motion, a favourite subject being a dancing
skeleton ; but this effect is better rendered by an apparatus
used by Mr. Muybridge, in which the figures are rotated,
whilst the Choreutoscope draws them along in a straight
line.

A simple application of the pantograph has produced a
lantern sketcher, which in clever hands is very attractive,
sketching out a design upon the screen ; and there is also an
ingenious slide called the Cycloidotrope, sold at 80s., which
by a combination of mechanism produces upon the screen,
cutting them through smoked glass with a tracing point,
most beautiful geometrical patterns of cycloidal curves. Real
experiments belong to the latter portion of this work.

CHAPTER XI

ACCESSORY INSTRUMENTS

SEVERAL instruments are frequently used with ordinary
exhibition lanterns for public entertainment, which demand
a few words before proceeding to the scientific class of pro-
jections.

79. The Kaleidoscope. — This beautiful invention of Sir
David Brewster for showing the symmetrical effects of multiple
reflection, was adapted to the lantern by the late Mr. J. Darker.
As is well known, two mirrors have to be fixed so as to meet
at an angle which shall be an aliquot part of a circle. They
are fixed in a tube as at A (fig. 76), and the effect is that
the phenomena between them are repeated symmetrically all

146 OPTICAL PROJECTION

round the circle as at B. For the lantern, the tube encasing
the mirrors has a lens at each end, as in c, the tube with the
pair forming an objective, which takes the place of the
ordinary one, and focusses in a sliding jacket any object in
the lantern stage. A rackwork slide containing some loose
pieces of coloured glass, beads, and other small objects,
between two glasses, is usually employed; but a revolving
chromatrope also gives magnificent effects, and ears of bearded
wheat or barley or oats, a loosely webbed feather or two, a
key, bits of lace, &c., will give interesting patterns.

The kaleidoscope requires very special management. It
is placed in the lantern so that the mirrors stand with the

FIG. 76.— Kaleidoscope

edges upright like a V, and is focussed on the screen to its
proper slide, or whatever is to be used. The effect, however,
is at first a mere nothing — scarcely visible. The lime-jet
has now to be raised on the rod for a distance found by ex-
periment, but which usually lies between f inch and 1J inch.
The disc on the screen brightens up at once ; and the light
has finally to be carefully adjusted, both as to position and
distance, so that all the segments on the screen are illu-
minated as equally as possible. Generally the instrument
has also to be turned or adjusted a very little to the right or
left, to avoid patches of darkness and get the best effect.
The lantern kaleidoscope entirely depends for success upon

ACCESSORY INSTRUMENTS

147

this careful adjustment of the light. The rays, passing
through the objects, strike rather downwards upon the mirrors,
and are again reflected rather upwards ; and in many instru-
ments the effect is better if the front lens is somewhat in-
clined, so as to be perpendicular to the course of the rays.
The kaleidoscope should always be warmed before use, to
prevent dew upon the mirrors, if the night is at all cold.

I have seen a beautiful lantern kaleidoscope, specially
constructed by Mr. Darker, in which the mirrors were adjust-
able by a screw to any angle through a considerable range.
Such a form must necessarily be expensive ; but an instrument
of this kind is far removed from a mere lantern toy, and
becomes at once an exquisite piece of experimental appa-
ratus for demonstrating the laws and phenomena of multiple
reflection,

FIG. 77

Aphengescope

FIG. T8

80. Opaque Pictures. — Another simple instrument for
exhibiting cartes de visite and other opaque objects, such as
the works of a watch, is often called the Aphengescope — the
mania for giving Greek names to lantern affairs is inexplicable
to the ordinary mind. The usual arrangements are shown in
figs. 77 and 78, one being adapted for two lanterns, and the

L2

148 OPTICAL PROJECTION

other for a single one, which latter, however, even with the
lime-light, does not afford sufficient illumination to exhibit a
carte, de visite more than three or four feet in diameter.
Taking the double arrangement in fig. 78, the two lanterns
L L are turned away from the screen, the objectives removed,
and their two nozzles fixed to holes cut to fit them in the box
B. To the back of this box the two grooved doors D D are
attached by a common hinge, so that when one door is closed
the other is outside for the attachment of a new slide. One
of the lantern objectives is fitted on the front of the box at
A, and the carte or other object is shown by the light reflected
from its surface, as shown by the dotted lines. The single
instrument M shown in fig. 77 acts on precisely the same
principle, but with only half the illumination.

A new pattern has been lately introduced by several
English and Continental opticians. The body of the lantern
itself is so constructed that when the objective is removed, a
reflector can be introduced hi front of the condenser, which
throws the light back to a place above and behind the con-
denser, where there is a fitting to receive objects or slides.
In front of this stage for opaque slides is another opening to
which the objective can be attached. There are thus two
positions in which the objective may be used, the ordinary
one, and one above for opaque slides, an oblique reflector
being always necessary when these are shown.

Coins and medals show very well with the opaque lantern.

81. Microscopic Attachment. — The ordinary form of pro-
jecting lantern is itself a microscope, since it projects upon
the screen a magnified image — generally about 50 diameters —
of the slide or object. There is no difference whatever in
principle between doing this, with a lens of say 6 inches focus,
and producing an image of 2,000 diameters with a lens of ^-
inch focus. The practical difficulties are connected with that
management of the rays discussed in Chapter I ; and these
are very great as we approach the higher powers. Up to a

ACCESSORY INSTRUMENTS 149

Certain point, however, they are not formidable, and we will
mention here some simple problems and forms of apparatus.

Fig. 79 represents the best form of the simple and old-
fashioned microscopic attachment, which has been furnished
by all opticians for many years. A still commoner form is
sold, with only one slide-stage instead of the two here figured.
In either case, the ordinary objective is unscrewed from the
lantern front, and this attachment screwed in its place. It is
usually furnished with two objectives, each composed of a
couple of plano-convex lenses with their convex sides together,
stopped down in front ; but I have known two meniscus lenses
used with their convex sides to the object ;
and there is no doubt that with care in select-
ing the curves, and in proper spacing apart
of the lenses, these simple objectives will do
very fair work. I used a 2-inch focus lens of
this kind for some time for its large field, even
in the instrument hereafter described, with
good results. Generally the foci supplied range
from 2| and 1£ inches, to 2 inches and 1 inch.
These objectives slide into the rack-front, and
thus roughly adjust focus, precisely as the
lantern-front recommended on p. 83, the higher
power having of course to be pushed farther into the tube.

With the single-stage form of attachment, all that can be
done is to roughly focus the object on the screen, and then
draw back the lime-tray a little, and adjust it to and fro in the
lantern, till the object is most brilliantly illuminated, which
will be when it is just at the focus of the cone of rays from the
condensers. This will also, for reasons already pointed out,
be the position of best definition. The object may, however,
be rather too large to be covered by such a focus, and if so the
exhibitor must alter the focus a little to get the best result he
can. These results, with a good operator and a good jet, may
be very fair up to the limits of the powers supplied, which are

i$o OPTICAL PROJECTION

better as a rule for this class of work than achromatics. Such
an object, for instance, as the head and tongue of a bee, can be
shown with the higher of the two powers very fairly, as regards
the general structure.

There is great heat at the focus of the cone of rays, and
the microscopic attachment should therefore never be used for
more than a few seconds, without a large glass trough, filled
with saturated and filtered solution of alum, being first placed
in the ordinary slide-stage of the lantern.

For exhibiting ordinary microscopic slides a wooden frame
is used, somewhat similar to the ordinary frames of lantern
slides. It is made to fit the stage of the attachment outside, and
the ordinary 3x1 inch micro-slides inside. This frame has
a movable end, by which the slides can be inserted in it as
required.

With the double-stage instrument as shown in the figure,
more can be done. The two stages were meant to place slides
in position for the different powers ; but the second stage is
far better used in another way. In the centre of a wooden
slider made to fit the stage, mount a plano-convex lens as near
H-inch diameter as the slider will allow, and If to 2-inches
focus. Whenever the 1-inch or higher powers are in use,
place this lens, with convex side to the lantern, in the stage
nearest the condensers. Adjusting the lime by hand as before,
a great increase in light will now be gained, the cone of light
being brought down into a smaller space. It will be found
that decent results can now be obtained with higher powers,
even as high as § or \ ir 3h, which will exhibit fairly such an
object as the tongue of a blow-fly. Ordinary microscopic
lenses may be used, with the help of a simple adapter.

82. Improved Microscopic Attachment. — This attachment
was first constructed by Messrs. Newton & Co., to produce the
foregoing results in a more handy way with ordinary achro-
matic microscopic objectives, and with something of the power
of the instrument hereafter described. It screws in the same

ACCESSORY INSTRUMENTS 151

manner on to the lantern front in place of the ordinary
objective, and is supplied with a secondary condensing lens
behind the stage. For those not accustomed to optical adjust-
ments, it is better that this lens should be fixed, making
adjustment of the light as far as possible by hand, with the
lime-tray alone. For those capable of using it, however, it
is better to have this condenser fitted in either a sliding or
rack-and-pinion adjustment, both of which forms are supplied.
Then by pushing this lens close up to the slide, the illumina-
tion resembles that of the lantern condenser itself, and covers
a field nearly equal to its own surface — say a circle of 1 £ inch
diameter ; while by drawing it back till it focusses upon the
slide, a smaller surface is
much more brightly illu-
minated. The stage, in this
form of attachment, is en-
tirely open, and consists of
a rotating diaphragm-plate
pierced with apertures of
appropriate sizes for the
various objects, which are
held by simple clips. The FW. so

objective fitting is screwed

for ordinary microscopic objectives, any of which may be tried,
and used if found sufficiently flat in field. If none such can be
found amongst the owner's own objectives, the most suitable
powers are the 2J inch, 1J inch, and T8«j inch from the list of
the larger instrument described in Chapter XIII. Fair results
can be obtained with powers as high as -£5 inch.

Since its first production, this attachment has found such
general acceptance, that some form closely resembling it can
now be obtained of almost any respectable optician, with or
without certain modifications presumed to be improvements.
Of most of these the reader may be left to judge for himself ;
but he should be distinctly cautioned against one, viz. a

152 OPTICAL PROJECTION

mechanical stage. The average * Shopticiau ' — a very different
being from the more serious optician — is very fond of an
instrument with as many screws, and racks, and 'adjust-
ments ' as he can possibly crowd upon it. Many a purchaser
is fond of the same ; and by all means let the one sell as much
as he can, and the other buy as much bright brass as he likes.
But the serious worker is of another sort ; and him I would
warn that all useless lumber of the kind is, to an ordinary
operator in the dark, simply a source of embarrassment and
distress. I have worked too long at this subject not to be
familiar with all manner of stages ; and affirm without hesita-
tion, that a mechanical stage is not only useless, but a positive
hindrance, for all but a limited class of work which will be
mentioned in due course.

An attachment of this kind will do a great deal of really
interesting work, including a large range of that fascinating
modern pursuit, photo-micrography. The great bulk of
ordinary pond life is within its powers, the beautiful volvox can
be shown nicely, and a very respectable exhibition can be
made of the circulation of the blood in a frog's foot, &c.

Leaving details respecting serious microscopic exhibitions
for more full treatment in Chapter XIII., it is only needful to
add here, that in all attempts at such, special care must be
taken in centering the light, as a very slight deviation from
the optical axis will cause a great falling-off in the results.
It is also needful that the lime-tray should move truly to and
fro, parallel to the optic axis of the lantern ; because at every
change of power, or whenever the operator sees that the
effect is not what he knows the instrument to be capable of,
he should try, by moving the lime-tray a little, backward or
forward, whether he cannot get a better illumination. Very
often he will find that he can ; and, generally speaking, the
lime will have to be rather more drawn back as the powers
get higher. If the tray does not slide truly, therefore, the
lime cannot be truly axial in both positions.

ACCESSORY INSTRUMENTS 153

Lastly, let it be repeated, never to forget the alum trough
in the ordinary slide-stage. Without this, ordinary balsam-
mounted slides will be melted, and living objects rapidly killed
by a heat nearly boiling.

For micro-projections of a higher class, the reader is
referred to Chapter XIII.

CHAPTER XII

APPAEATUS FOB SCIENTIFIC DEMONSTBATION

83. Demonstrating Lanterns.— Any good ordinary lantern
can, by a little modification, be made effective in almost any
class of demonstration. It may have a good microscope
fitted to it, as described in the next chapter, interchangeable
with the ordinary front. With a slide-stage open at the top,
it will project chemical and other tank experiments. With
accessory apparatus such as is presently mentioned, it will
perform optical experiments. And by withdrawing the front,
and arranging small apparatus in the rays from the condenser,
it will project that apparatus. But for certain preponderating
kinds of work, certain forms of lantern are best adapted ; and
for constant work, especially in educational institutions, the
form of lantern should therefore be determined with reference
to the kind of work to be chiefly done.

The modification and use of the lantern in a cheap and simple
way to project pieces of apparatus as well as slides, was first
systematised in the schools of Germany. Fig. 81 represents
such a German school lantern.

The essential points are that the flange-nozzle is entirely
removed, only an open spring-holder stage being left in the
front of the condensers ; that the objective is mounted in a
separate support, which can slide along the double-rail stand

154

OPTICAL PROJECTION

or base; and that a small table, adjustable for height, can
also be fixed anywhere on the same base. On this table any
piece of sufficiently small apparatus can be adjusted, and
projected as if it were a slide. The objective is surrounded
by a shade to keep stray light from the screen. Ordinary

FlG. 81. — German Lantern

slides or diagrams are placed in the spring holder, and pro-
jected as usual.

This arrangement has been a great deal popularised in
England by Mr. W. Lant Carpenter in connection with the
Gilchrist Lecturing Trust; and where actual physical apparatus,
and diagrams, are the principal subjects of projection, it can
hardly be improved upon for handiness, cheapness, and sim-
plicity ; but it is not so well adapted for optical and acoustical
experiments, on account of the stray light. Fig. 82 is an
American modification of it, arranged with a special view to

APPARATUS FOR SCIENTIFIC DEMONSTRATION 155

portability. The rails and legs are here dispensed with ; the
lantern body is a mere frame hinged together, with a metal
top, and covered round when in use with thick black cloth ; and
the whole is placed on a table. The jet is also modified for
compactness, the usual long metal tubes being dispensed with.
Otherwise the lantern is essentially the same as the German
model.

Lanterns of this type, either mounted on rails, or for

FIG. 82.— American Lantern

simply placing on a table, with little unimportant variations
in detail, are now supplied by all the leading opticians.

For optical experiments chiefly, and incidentally for other
physical demonstrations, the most popular pattern in public
institutions till lately has been the well-known Duboscq lantern
(fig. 83), fitted usually with the electric light. The square
body is of sheet brass, and is mounted upon four brass pillars.
There is a flange-nozzle fixed to the body ; and the special
feature of the arrangements is, that the condensers are fitted
in the back end of a large tube which slides freely backwards
and forwards in this nozzle, the character of the beam being

156 OPTICAL PROJECTION

modified by sliding the condensers to and from the light, instead
of moving the light to and from the condensers. An adjustable
slit, and a circular revolving diaphragm of apertures, fit into
the front of this sliding tube, An ordinary front for showing
diagrams can be interchanged with the optical flange when
required.

This arrangement of movable condensers is convenient
for purely optical experiments, and for focussing with the

well-known Foucault and Duboscq
regulators, which would be difficult
to adjust in relation to a stationary
optical system, and which as a rule
are only employed intermittently.
But for anything like a continuous
arc light, the lantern itself is much
too small, and has occasioned many
very unpleasant burns : and optic-
ally the arrangement is unsuitable
to any other projections than
those with beams or pencils of
light, because to get a good disc,
in which to exhibit either dia-
grams or apparatus, is exceedingly
difficult with it. To put it in
another way, it is chiefly suitable
for experiments without an ob-
jective. For any range of demon-
stration, there is need for more
optical power than this lantern affords.

In some cases this increase of power may be best obtained
by using two or more single lanterns, one of which might
have a front on the Duboscq plan, and another carry an
ordinary lantern stage and front for diagrams, or other fittings.
For powerful arc lights the bodies should, however, be much
larger.

FiO. 83.— Duboscq Lautcru

APPARA TVS FOR SCIENTIFIC DEMONSTRA TION 1 57

Two detached lanterns have some obvious advantages,
where detached or side experiments are of frequent occurrence.
But in perhaps the majority of cases the demonstrator, equally
with the exhibitor, may find it most advantageous to employ
a bi-unial or even tri-unial arrangement, modified, however,

_A Microscopic Bi.unial

according to his special purposes. This is especially the plan
where the lime-light is the radiant employed. The simplest
case is perhaps that of a medical school, which may desire to
supplement the demonstrations of such a lantern microscope
as is described in the following chapter, by photo-micrographs

158 OPTICAL PROJECTION

and other diagrams. A suitable bi-unial, the simplest modi-
fication of the ordinary form, is that in fig. 84, and will per-
fectly suffice for such purposes, where physical experiments
are not required,

My own bi-unial lantern, being required for experiment,
as well as exhibiting slides and microscopic projections, was
designed differently. The bottom is supported on four brass
pillars, each 4 inches high, which allow gas-pipes for coloured
flames, or any other apparatus, to be readily introduced from
below, while a black curtain round these pillars, and another
round the back of the lantern, stop all stray light when re-
quired, the dark blue sight-holes being also covered by brass
shades. The top lantern is rarely used for anything but slides
and diagrams, and another similar front for the bottom one
converts the whole into an ordinary dissolving lantern when
required. This lower front is interchangeable with the
microscope front ; or both can be removed, leaving the bottom
lantern free for parallel-beam work or physical projections.
The base-board is also made to slide out, or close up shorter
as required ; and although the whole lantern is too small to
make regular work advisable with the arc-light, the jets are
so removable that the lamp presently described could be used
if required. The wooden body is of teak, darkly stained and
dull-polished. I believe the instinct which leads a scientific
demonstrator to desire something which looks different from
the ordinary ' magic-lantern ' of the itinerant lecturer, to be a
sound one ; at all events I plead guilty to the weakness, while
the dark wood and brass pillars have actual advantages in
practice.

Such a lantern is not, however, suitable for constant work
with the arc-light. A very well-known pattern of double arc
lantern is Ladd's, better known as the Browning model, shown
in fig. 85, where two nozzles, one reserved for diagrams, and
the other generally fitted on Duboscq'ssliding-condenser plan,
centre on the same arc-light at the centre of the circle. This

APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 159

is a good arrangement forgone fixed position in a theatre, if the
optician understands his business and arranges matters so that
the arc-light is at a focus which gives an even disc at the
given distance ; but if this is not attended to, a good disc of
clear diagram will rarely be obtained. It is also best suited
for old-fashioned lamps, with the carbons in perpendicular line,
and was in fact first designed for spectrum-work, the nozzles
being so arranged that,
without moving the lan-
tern, the rays deflected
by the prisms should fall
on the same screen as
diagrams projected direct.
To accomplish this, how-
ever, the same dispersive
power must always be
used : otherwise the lan-
tern must be made to re-
volve upon the circular rim
supported by the pillars.

When thus modified,
however, it is better to
have at once a tri-unial
lantern, which is the most
complete instrument of
all, when properly con-
structed. The first at-
tempts in this direction
were not successful, the

three nozzles being arranged on three adjacent sidgs.
hexagon, copying an old exhibition
Beech ey. The objection to it is,

directed towards the screen, another, or t^o oUi^rs, afeqjf qj<
in front, and interfere with manipulatio^. ,Tto f&n is a
one, and such lanterns have never coi&e

FIG. 85. — Double Electric Lantern

i6o

OPTICAL PROJECTION

one I am aware of, constructed on sound principles, was
planned and carried out by Messrs. Newton & Co. for the Santa
Clara College of California, on the plan represented in fig. 86,

FIG. 86.— Tri-unial Electric Lantern

and shown in ground plan, with its lamp, in fig. 87. Here the
three nozzles (which in this case carried a projection -microscope
capable of 5,000 diameters, a diagram front, and an optical
front with all the usual accessories) are arranged at equal

APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 161

intervals round the whole circle, so that whichever points
forwards, the others are backwards, and out of the way.
There are three doors with shuttered sight-holes of darkened
glass, between these nozzles, so that the arc can always be
examined either from the back in the optic axis, or from
either side. The foundation circle is supported on four
strong pillars, standing rather outside the body, for steadiness ;
and the whole body revolves easily and truly upon this circle,

MICRO. FRONT
Fl(J. 87. — Plan of Lantern

and may be fixed at any angle, while a spring detent ensures
that each front ordinarily finds its exact position. Furnished
with the electric lamp and focussing- table next described,
this form of lantern is the most complete, convenient, and
powerful instrument for scientific demonstration with which I
am acquainted, and has been adopted by the Eoyal Institution.
A lantern made on this plan for the arc-light must be
pretty large on account of the heat, as the lamp may be in

* M

1 62 OPTICAL PROJECTION

action all the time with one nozzle or the other. In many
optical experiments also it is required to push the radiant to
a considerable distance from the condenser, and this also
necessitates that the cylindrical body should be fairly large.

Of recent years a great many new patterns of science
lanterns have been placed on the market by various firms,
and it is now possible to obtain an instrument suitable for
nearly all optical experiments, as compact, and nearly as cheap,
as those made for slide work only.

Space here forbids further mention of these smaller science
lanterns ; but a more detailed account of the various forms
will be found in the appendix at the end of the book.

84. Arc Lamps.— The Duboscq lamp is too familiar to
need description. It is only fit for battery use, and for inter-
mittent experiments, being incapable of giving a really steady
light. An equally serious fault is the length of the are
between the carbons, the centre of the radiant being dark,
between two luminous poles. With about forty Bunsen or
Grove cells, however, this lamp answers the purpose for
ordinary optical experiments, and to get an even disc with it
is possible, though it requires more skill than most assistants
possess. Mr. Cottrell, formerly demonstrator at the Royal
Institution, stood almost alone in his power of getting a
fairly steady and well-centred light with this form of regu-
lator.

The lamp known as the Suisse-Serrin is a very much
better lamp of this class. It uses larger carbons, and I have
known it, in the hands of my friend the Rev. P. R. Sleeman,
give a nearly steady light for a continuous half-hour. For
battery use only, this is an excellent lamp, and by setting the
positive carbon somewhat behind the other, the evils of a
double radiant can be partially avoided.

For the dynamo currents of modern days, however, this
lamp is not well adapted, and many regulators now in the
market give better results. I have seen very good work done

APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 163

with the small- sized Siemens' differential lamp, and also with
the Giilcher, which is very steady if a proper resistance coil
be interposed in the circuit. Others also doubtless give good
results. But the most important point in an optical lamp is
the concentration of the radiant into one luminous point.
All the usual lamps fail in that particular ; and in perfecting
the arrangements of the electric projecting microscope,
especially, I was continually baffled by the confusion caused
by the two luminous points or poles, which was only partially
remedied by setting the positive carbon behind the negative.
For some time, pointing this out, I endeavoured in vain to
induce various makers to construct an optical lamp with
carbons inclined at an angle of 30° or 40°, after the manner
well known in the hand- regulated ' projector ' lamps often
used at sea.

Being finally advised by Professor S. P. Thompson to
adopt for other reasons the Brockie-Pell lamp for microscopic
work ; and finding upon inquiry that Mr. Brockie had already
constructed tot Mr. Phillips a simpler arc lamp for lantern
?jse, adjusting the arc by the potential of the current, in which
the negative carbon worked up by a kind of candle-spring to
a fixed stop, so as to ' keep in focus,' while the arc could be
readily adjusted to any current within reasonable limits, I
urged upon him very strongly to further perfect it for some
microscopes then constructing at Messrs. Newton's, by giving
the carbons the slant just alluded to. He at once undertook
to do this (my sole suggestion in the matter), and the final
result of the additional alteration was a lamp whose external
appearance can be seen in the representation of the electric
microscope on p. 209. This, as an optical radiant, is far the
best I am acquainted with ; and up to this date is, I believe,
still the only focus-keeping regulator-lamp whose carbons
occupy the proper inclined position. It is extremely port-
able, steady upon its base, very simple hi mechanism, and
its cost is only fifteen pounds. It is not the lamp usually

164 OPTICAL PROJECTION

called the Brockie-Pell, but of much simpler construction ;
and the same lamp can be adjusted from about 7 to 15, or
from 10 to 20 amperes of current, according to its size. I
have seen it work for two hours without a single ' blink.'

The effect of the inclined carbons is shown in fig. 88.
The lower carbon is still set in front of the upper or positive
carbon, according to the length of arc. This brings the
crater ' considerably to the front of the carbon, while the
Incandescent portion of the negative carbon is behind, and
invisible from the front. Hence the radiation to the condensers
is reduced, as shown in fig. 88, to that from the crater, while

that crater radiates much
more directly and powerfully ;
the general result being, not
only the one radiant point,
but that of so much greater
power, that a lamp which
would usually be called of
2,000 candles' power, gives to
the condensers at least 3,000
in proportion. The gain in

FIG. 88 11 • • -, , - ,

all microscopic and high-class
optical work cannot be over-rated.

The management of this lamp is very simple. The upper
carbon, first set by experience the proper distance behind the
other, is pushed down by hand till it just touches the lower
one without pushing that down ; as the current when switched
on must draw it down a small distance in the usual manner by
an electro-magnet, and so strike the arc. The position as to
height of the point of the lower carbon, and therefore of the
arc, can be adjusted within considerable limits. The current
will first have been ascertained, at least approximately. If
the current is sufficient to work the lamp to its full power,
the milled head which regulates the arc, seen behind the
upper carbon pillar on the top of the lamp case, is turned to

APPARA TUS FOR SCIENTIFIC DEMONS TRA TION 165

the right nearly as far as it will go ; if not, each full revolution
to the left will lower the potential from \\ to 2 volts. The
arc having been struck, the lamp is run a little till it has
attained the length desired; when the regulator is turned
very gently to the left until the mechanism gives a * feed,'
with the accompanying click. When this is adjusted, the
lamp will regulate itself. Only if the current is very small
for the lamp, may it be needful to push down the lower carbon
a very little, so that the arc struck may be from the first
shorter : afterwards the procedure will be the same. For
smaller currents smaller carbons will of course be used. Hard
carbons should be used for the negative, and soft-cored ones
for the positive pole. The collars against which the lower
carbon is forced by the spring are provided of various sizes
according to the carbons, so that the point protrudes the
required distance.

The only purpose for which this arrangement is not suit-
able, is vapourising substances for spectrum projections, which
require vertical carbons and the crater at the bottom. The
needful modification which allows the Brockie lamp to be
used for this also, is described in the proper place.

The Brockie lamp does, however, require a continuous
current ; and I confess that I dread the growing tendency to
alternate currents, as regards all work in public demonstra-
tion. A lamp can easily be made to work with such currents ;
but as there is no positive * crater,' the special radiation from
one cannot be had, and we can only hide one of the two
luminous poles at the expense of fully half the light. This
is a real difficulty, only at present to be overcome by storage
batteries. Moreover the manipulation occasionally necessary
in all optical work, is liable with alternate currents to produce
very unpleasant consequences. Should a continuous current
of, say, 10 amperes at 50 volts give an accidental shock, though
unpleasant to most persons at the moment, it would have no
further effect ; but the same current taken alternately would

i66 OPTICAL PROJECTION

give a shock to the system the effects of which would be felt
for hours. Demonstrators, at all events, have reason to look
with dread and dislike upon the extension of the alternate-
current system of lighting.

For high-class optical work, and especially microscopic
work, in which one millimetre of misplacement will make a
great difference in effect, even a Brockie lamp should be
mounted on a stand constructed like a slide-rest, with three
rectangular screw movements worked by milled heads. The
lantern represented in fig. 86, p. 160, is shown so mounted,
the extra cost of which, substantially made in gun-metal, is
about 10Z., but which is well repaid in the perfect facility
with which the incandescent crater can be brought back to
focus in any direction, or moved in the line of the optic axis
as required. The illumination in a lantern thus furnished
is magnificent, and perfectly steady.

A switch for the current should be provided on the base of
the lantern itself. It should never have to be felt for, or
looked for, elsewhere. Any alternate-current lamp should
only be handled with gloves, for a shock from such, as already
hinted, is very serious. That from a continuous current is
simply unpleasant. A lamp always works best with a rather
larger current and a corresponding resistance added to the
circuit. A small gas-jet is a great convenience inside an
electric lantern.

85. Vertical Projections. — It is very often necessary to
throw the projection of fluid surfaces, &c., vertically upwards,
afterwards reflecting it to the screen ; and the same arrange-
ment is very convenient for working out a diagram before a
class. This is generally effected by some form of the vertical
attachment devised by Professor Morton, and shown in fig. 89,
a similar arrangement by Stohrer of Leipzig being shown in
fig. 116, on page 227. The lantern condense; s are arranged
so as to throw a parallel beam, which by a large reflector at an
angle of 45° is deflected vertically, where it passes through a

APPARA TUS FOR SCIENTIFIC DEMONSTRA 7 ION 167

third plano-convex lens, which converges the light, and over
whose face is the field of the instrument. A pillar at the side
supports the focussing power, a ring allowing various powers
to be used : and the same ring-fitting carries a second silvered
reflector, turning on an axis so as to throw the image at any
desired height on the screen. If a sheet of fine ground or
smoked glass be simply laid over the face of the large lens, a
diagram can readily be worked out, and is often better under-

FiG. 89

Fiii. 90

stood when so worked out, than if projected complete. It will
be still more transparent if the glass is first rubbed with a
very little paraffin oil.

Some opticians substitute a right-angled reflecting prism,
but the second reflection from a silvered mirror is practically
imperceptible except in white line diagrams drawn upon
really blackened glass. For such a prism is advisable, and is
always employed in Duboscq's form of the apparatus.

An effective apparatus can be made very cheaply as in

168

OPTICAL PROJECTION

fig. 90. The reflector is here simply set in a wooden cubical
box, with the side which is turned towards the lantern con-
densers left open. A rod at the side carries a plain lens in one
of the sliding fittings presently figured, and another sliding
socket carries the reflector.

The illumination is not even and good with such attach-
ments, unless pains are
taken to accurately adjust
the first reflector as re-
gards the parallel (or
slightly diverging) rays
from the lantern con-
densers. To ensure this
the lantern itself is often
fitted up by Hawkridge,
and other American opti-
cians, as in fig. 91 . Here
the two first lenses of
a triple condenser are
mounted in the body of
the lantern, which is sup-
ported on pillars ; while
the third or converging
lens is fitted in a plate
hinged to the top of the
lantern front. This plate
has at one side a pillar
or standard, which carries

the focussing power and second reflector. For vertical work,
the lantern is arranged as in the diagram, the front plate and
lens being supported in a horizontal position by a movable
triangular box, to whose hypotenuse side is fixed the reflector ;
but when ordinary work is required, this box is removed, when
the plate drops down to a vertical position, with the focussing
power out in front as usual. The second small reflector is

FIG. 91

APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 169

then removed, when the lantern projects direct. Several of
these lanterns have reached England ; but there seems a
sort of national 'fashion' in apparatus, and so far as I am
aware, they are thought rather awkward in use by their pos-
sessors.

Messrs, Newton & Co, have constructed a bi-unial lantern

"\ !' ' \ir~n i

;^i\ \ ii^y

PIG. 92.-Newton»s Vertical Bi-unial

which produces vertical projections in another way, shown in
fig. 92. There are two separate lanterns, the top one so hinged
to the bottom one that the top lantern can be turned back in
a moment, so as to rest against stops and point vertically

170

OPTICAL PROJECTION

upwards. An adjustable mirror can then be attached by a
very simple fitting, and will reflect the image upon the screen.
The objectives of both lanterns are mounted on the detached
system already described for physical work. One lantern is
therefore always ready for ordinary work of any kind, while
the top one can either be used in the same way (the whole
as an ordinary bi-unial exhibition lantern if necessary) or can
be converted at a moment's notice to vertical purposes. A
screw is provided to keep the tray and jet from dropping out
of the proper position.

I was at first apprehensive that the heat of the jet, when
thus brought under the condenser, might prove a fatal objec-
tion to this arrangement ; but it has not proved so, provided
the jet be kept in work while the lantern remains vertical.
If the jet is turned off so that the flame of coal-gas, released
from the pressure driving it down, rises upwards towards the
condenser, a crack will probably result; but it is easily
avoided with this caution, and more brilliant projections are
thus obtained than in the other way.

86. Erecting Prism. — For the mass of physical experiments
the ordinary inversion of an image does not signify, and is

readily understood by the
audience. For thermometric
and some other experiments,
however, it becomes of im-
portance, and in such cases
is obviated by the use of a
prism, as described by Bertin,
Miiller, and others. Fig. 93
shows the action of a right-
angled prism P used in this way. The ray a b is refracted
to c and there reflected, and again refracted as de\ while
fg is refracted to h and there reflected, coming out as i k ;
thus the top and bottom rays are inverted, and the image is
erected. The best position for the prism is where the cone

FIG. 93 —Erecting Prism

APPARA TUS FOR SCIENTIFIC DEMONS TRA T1ON 171

of rays from the lens is reduced to its smallest diameter at
the apparent crossing-point.

A right-angled prism is ordinarily employed, but is not the
best form, as a large portion of the prism at the apex P is
quite outside the field of all inverted rays. By making the
prism with all this waste portion truncated, the same effective
field can be obtained with a much less massive piece of
optical glass, and the cost thereby much reduced. Another
method is to employ a greater angle than 90°, and Zentmayei
adopted an angle of 126°, which utilises all the field to the
apex. Such a large angle, however, loses appreciably more
light by reflection ; and, on the whole, I consider the best
results are obtained by a combination of the two methods,
adopting an angle of about 105° and truncating the prism
sufficiently to have no surplus glass.

An erecting prism should not be used unnecessarily, as its
adjustment takes time, and there is considerable loss of
light.

87. Optical Front. — For general use in optical and some
other experiments, a special focussing front is desirable, of
construction some-
what different from
the ordinary one for
exhibiting slides. I
have found the ar-
rangement shown

in fig. 94 very con- "!«. 94.-oPticai Front

venient. AB is a

tube or fitting which fits nicely into the lantern nozzle like the
usual one for diagrams. B o is a tube 3 in. diameter which
screws into a collar at B, and having a rectangular opening E
cut through both sides to serve as a slide-stage. The opening
should be 1^ inches by 2f inches, so as to receive easily slides
over 1 inch thick, and 2 J inches wide. These slides are pressed
to the back by a flange L, forced against the slide by a spiral

172 OPTICAL PROJECTION

spring M, and worked by the hand through two studs working
in slots at the top and bottom of B c. Into another collar at
c screws the jacket D of the focussing tube, which may either
slide nicely, or have a rack and pinion E. The lens tube F is
about 2£ inches diameter, and carries a combination power of
two lenses, the back one a plano-convex 2 niches diameter,
plane side towards the stage, and 5 or 6 inches focus (both are
convenient), and the front lens rather over 1^ diameter, about
8 inches focus. This lens H screws into a collar from which
projects a nozzle, N, If external diameter. The exact diameters
are, of course, not material. A rack and pinion has advantages
and disadvantages. The advantages are obvious. On the
other hand, a sliding movement may be made very pleasant if
cloth-lined, has more range, and allows of easy withdrawal of
the power, and substitution of a slit or series of apertures.
With a rack front this substitution has to be effected by un-
screwing the jacket from c, and substituting a plain jacket to
carry those fittings.

Such a front is very useful for a great deal of work in
optics and acoustics. Though the lenses are simple, it is free
enough from aberrations for practical purposes, and is the best
power for a polariscope. On its smaller slides a great deal
more light can be condensed ; or a slit for spectrum work, or
an aperture for diffraction, can have sufficient light condensed
upon it to work 'direct' for many experiments, without any
other lenses. Finally, when the power is removed, and a slit
or aperture placed in the front of the jacket, by inserting in
the stage E a cylindrical or spherical lens mounted like a slide
in a wooden frame 4 x 2J inches, the aperture or slit can be
easily adjusted in the focus of the lens, and the utmost amount
of light thus brought to bear upon the phenomena.

88. Parallel Pencils. — With the arc-light as a radiant, no
further arrangement is required for small parallel pencils
than to pass a parallel beam from the condensers through the
most appropriate hole in a circle of apertures. But with the

APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 173

lime-light greater brilliance is often desirable, especially with
such a projection as a Barton's button or a Lissajous' com-
bination figure. Even a polariscope, with field of, say, If
diameter, is improved in performance by sending all the light
from the condensers through such a field. This is easily
managed by a reducing system of lenses similar to that
shown in fig. 95, but the convex and concave lenses being of
greater foci and diameter, and so adjusted (with a movable
adjustment is desirable) that when the radiant and condensers
are arranged for a parallel beam, and the system inserted
in the flange-nozzle, the rays converged by the convex lens are
re-parallelised by the concave within two inches diameter. A
parallel beam of two inches
is convenient for many '"""
purposes, besides giving
greater brilliance to polar-
iscope projections.

But still more useful
is a smaller arrangement
of the same kind, in which
the condensing lens is of

about IT, inches diameter and 2 inches focus, and the concave
paralleliser about f inch diameter, with removable caps or
apertures from about 2 mm. to 16 mm. fitting in the front, as
in fig. 95. The light from the condensers is slightly con-
verged upon the back lens, and by this further converged upon
the concave, where it is parallelised as a small pencil. By such
an attachment a very brilliant pencil of light of any diameter
from 16 mm. downwards can be obtained with the 0. H. jet.
The rays will of course scatter or diverge considerably, even
when parallelised as far as possible ; but in acoustical pro-
jections this is an advantage rather than otherwise.

Such a ' pencil ' attachment has another use. By removing
the concave paralleliser (which should be removable) a strong
conical pencil can be thrown through a very small hole, in

FIG. 95.— Pencil Attachment

174 OPTICAL PROJECTION

whose diverging rays large shadow projections will be well
denned, as described in Chapter XIV.

Such an attachment can either be fitted into the jacket c D
of the optical front (fig. 94), or if the back lens be made of
longer focus, it may simply be fitted into the small end nozzle
N of that piece of apparatus.

89. Optical Accessories. — For general focussing and other
purposes there should be provided, wherever physical demon-
stration is attempted, at least two rod or pillar stands, made
by screwing a half-inch brass tube into a heavy foot, covered

FIG. 96 FIG. 97

at the bottom with baize. On these should slide tightly
sockets as shown in fig. 96. Small holes, A A, are bored trans-
versely through the tube of each socket, and saw-cuts made
from the ends of the tube down to them ; then a slight pinch
in a pair of gas-pliers will tighten the fit at any time. To
one side is soldered the smaller tube B c. All these are one
gauge, so that every piece of apparatus will fit into any
socket-tube B c, and turn round in it to any position. One or
two sockets should be provided modified as in fig. 97, a perpen-
dicular socket A B being soldered at the end of the projecting
spur- tube. The gauge of the aperture in A B is to be the same as
in B c of fig. 96. Thus any piece of apparatus may be placed
therein and be capable of turning on a perpendicular axis.
Into sockets thus arranged we can fit any apparatus, by

APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 175

mounting it like the glass prism shown in fig. 98, with a spin-
like rod or tube A B, which fits in the sockets of the pillar-
stands. Though such a prism does not give sufficient dis-
persion for many spectrum experiments, it is very convenient
for elementary ones.

One or two focussing lenses should be mounted in the same
way ; but it is better, and shields stray light from the screen,
and thereby improves the projection, for each lens to be
mounted with a rim or shield of blackened tin round it, about
two inches wide. The size and focus of the lenses depend
upon the size of room, and screen distance. For class-rooms
a lens of 3 inches diameter and 8 to 10 inches focus will give
good projections. For a larger scale, two lenses 4 inches
diameter, and say 9 and
14 inches focus, will be
better. For good diffrac- g
lion effects, and acoustical
projections, a lens 6 inches
diameter and 14 to 18 FlG 98

inches focus, will give su-
perior effects, the long focus giving more room to work.

A plane mirror formed of a piece of good plate looking-
glass about 6x4 inches, should be mounted in the same way,
the spur projecting in the plane of the mirror from the middle
of one of the longer sides. For some experiments a larger
mirror may be useful.

Occasionally a somewhat steadier mount than these sliding
sockets may be needed. In that case there is nothing better
than the usual telescopic stem with a tightening collar, screwed
into the centre of a very heavy foot, and with a socket of the
usual gauge drilled perpendicularly into the top end. This
socket should have a set-screw in one side, so that any
apparatus can be fixed fast in the socket.

90. Stands and Manipulation. — The mounting of a de-
monstration lantern requires some consideration, much more

176

OPTICAL PROJECTION

freedom of movement and of access being required than is
needed, in the mere projection of slides. Very often, unless
the lantern itself is a revolving one, it must be readily capable
of being turned about at various angles with the screen.

A simple and effective way of working is to place lantern
and accessories on a bare and smooth wooden table, whereon

each lens or other piece of ap-
paratus can be adjusted ad
libitum. Such a table should
never be covered with cloth or
baize, as this would destroy all
freedom of movement ; but if
appearance be desired, it can
easily be draped round, and its
surface dead-blacked or ebonised.
If the condensers used for
physical work are of the proper
height, the lantern needs no
more than a circular base, on
which it can turn easily, to be
clamped in any position by a
screw under the table. The
heaviest lantern maybe mounted
so as to turn easily, by letting
a revolving metal plate carrying
the lantern, with a shallow
groove in its under surface run
upon a few bicycle balls.

Otherwise it is convenient for the revolving base to be
mounted on three short legs, so as to form a short tripod,
which stands upon the table. Each leg should in this case
have a piece of india-rubber cemented on the bottom, so that
the tripod may ' bite ' and remain steady. The subsidiary
apparatus may still be placed about the table as before, if the
heights are adjusted for such an arrangement. But it is more

Flo. 99

APPARA TUS FOR SCIENTIFIC DEMONSTRA TION 177

convenient, as a rule, to prolong the revolving base as in fig. 99,
into a board about 9 inches wide and three feet or more in
length, A B, while another cross-board, c D, can be slid along as
shown in the section below the figure, so as to form a f -piece
of the same level, at any distance from the lantern within the
compass of A B. The use of this arrangement is very obvious.
Suppose we are projecting the surface of a soap -film. This
film will stand diagonally across
A B and reflect the light to the
screen approximately in the direc-
tion of c D ; and the transverse
board c D gives us a base on which
to place our focussing or projecting
lens. So of other cases in which
the lantern has to be deflected.
The end B of the board A B is of
course supported by an upright
of proper length, which may be
hinged to it. The revolving por-
tion of the stand can be quite well
constructed of seasoned hard wood,
if two layers are glued together
with the grain across, and there
will be little friction if a judicious
use be made of black-lead.

When work is done upon a
table, there is one point to keep in
mind. If it is necessary to cant up the optical beam in order
to get a good position on the screen, it must not be done by
canting the lantern itself, as in slide exhibitions. The whole
table must be canted, by placing blocks under the front legs.
Otherwise all adjustments for height will be destroyed, as
apparatus is moved nearer to or farther from the lantern.
The optical beam used for physical work must be parallel to
the surface of the table.

FIG. 100

I7S OPTICAL PROJECTION

In theatres it is seldom convenient to work upon a table,
which would occupy too much space in the area, and be
awkward to move about, as may be often needed. \7ery often
it is more convenient to have two or three detached single
lanterns than a combination ; and the lecture table is required
for other and general purposes. The lantern is then best
mounted upon a strong wooden tripod, more or less resembling
fig. 100, with a steady adjustment for raising and lowering. On
such a tripod should be mounted a revolving top similar to
fig. 99, furnished with an adjustable wooden pillar to support
the end. This is the method of work chiefly adopted in the
Royal Institution.

91. Light for Working. — If an arrangement is at hand, as
in the Institution just named, whereby all the lights in the
auditorium can be turned up or down in-
stantly and without mishap, nothing can be
better, except for the one point immediately
mentioned. But more homely appliances
must often suffice ; mid it is distinctly bette?
for experiments which really tax the illu-
FIG. ior minating power of the apparatus, if the light
turned up be only one gas-burner sufficient
for the lecture table, inasmuch as the eyes are more sensitive
and able to observe phenomena, when not allowed to perceive
any bright light between.

An Argand burner, shaded as in fig. 101, will be found very
effective in this way, shading the direct light at all times both
from the screen and the audience, while giving perfect light
when desired. When turned down to a rather small blue flame
not in the slightest danger of going out, it will give an ap-
preciable dim light over the apparatus, while leaving the room
and screen practically in darkness. It can be mounted on
either a bracket, or a fixed or movable pillar-stand.

Another most excellent lecture appliance is a burner
known as the matchless or self-lighting burner, which turns

APPARATUS FOR SCIENTIFIC DEMONSTRA TION 179

the flame down to a small blue jet enclosed in a metal vase.
This also leaves the room in practically total darkness, with
no possibility of turning the light out, and with the power
of instantly turning it on full. It is advisable, however, to
keep this also constantly shaded from audience and screen,
during delicate experiments, that the sight of .the audience
may not be dulled. The effect which continual darkness (or
semi -darkness) has upon the ability of the eye to perceive faint
effects, is simply wonderful, and is not attended to in most
lecture theatres to the extent it should be.

It is possible to arrange things in a permanent demonstra-
tion theatre or hall, so that the very act of switching off the
electric current, or turning off the gas from the lantern, shall
turn up the gas in either of the arrangements here described.

CHAPTER XIII

THE PROJECTION MICROSCOPE

ABOUT the close of the year 1881, I was strongly urged by
several fellows of the Royal Microscopical Society and others,
who knew my proclivities, to turn my attention to the improve-
ment of the Projection Microscope, being assured that any
instrument which would display objects effectively even on the
scale of 600 or 700 diameters, would be an immense advance
upon anything then obtainable. By one or the other of those
interested, all the instruments then known were placed in my
hands, and the subject never having till then engaged my
attention, I was surprised to find how little had been done
in that direction. Not one of them, with the best lime-light
possible, would exhibit the bulk of those slides which any
demonstrator with a serious purpose in view would desire to

N2

r8o OPTICAL PROJECTION

place upon the screen. No such instrument as even the
attachment described in Chapter XL, which was itself founded
on the more complete instrument here to be described, was
in fact then obtainable.

A brief examination of the instruments then accessible
proved that the meagre results proceeded from want of appre-
ciation of the conditions. The oxy-hydrogen radiant is a
luminous surface as large as the thumb-nail. The light from
this can never be condensed into a point, but only into an
image of that surface, whose size depends upon the focus
employed. With high powers, we cannot condense all the
light upon objects, say even of 1 mm. diameter. The utmost
concentration we can obtain, is a simple function of the relative
foci of the original lantern condenser, and the substage con-
denser. But if we use for the latter a large lens of very short
focus, then we get too high an angle, and most of the light
crossing in and diverging from the object, never passes
through the objective. If, on the other hand, we use a very
small lens, most of the light never gets through this lens.
Hence, each secondary, or substage condenser, must be specially
constructed for powers of a certain range only, in focus and
angle. Except for low powers, each substage condenser is
almost necessarily composed of two lenses ; one of fairly
large size, to take up and bring down all the cone of light,
the other, to still further condense that light on the object,
without employing an angle too great for the objective. There
was further to be studied perfect protection from the heat,
which is very great ; and simplicity of parts and manipulation,
without which, work in the dark cannot be effectively per-
formed.

The matter thus presented itself to me from the first
simply as one requiring practical adjustment in these points.
All the principles concerned had, in fact, been pointed out by
the Eev. W. Kingsley so far back as 1852 ; so clearly that I
was quite surprised to be unable, after some search, to find

THE PROJECTION MICROSCOPE 18 1

any traces of his actual instrument, or any record of its per-
formance, or that it had ever come into use. This being so,
however, I had to begin de novo, and lost much time by
making my first experiments with diatoms. It was my friend
Mr. T. Curties, F.R.M.S., who guided me out of that unprofit-
able track, and gave me to understand that he should die
happy if he could only see upon a screen, bright and sharp,
' the tongue of a blow-fly six feet long. That,' he said, ' is
what we want.' As I knew already that I could give him
more than double these dimensions easily, I was considerably
relieved ; and the instrument was shortly afterwards com-
pleted. It was first publicly exhibited at a meeting of the
Eoyal Microscopical Society on Nov. 12, 1884, and again at
the Quekett Club a few days later, to the entire satisfaction
of the many experienced microscopists present on both occa-
sions.1

I shall content myself here with describing the projection-
microscope as constructed by Messrs. Newton & Co. from my
own designs, notwithstanding that since its introduction I
have seen one or two others catalogued which are stated to
perform well. I do not question this, but there are many
reasons for the course here adopted. No such instruments
were produced till some time after that here described ; 2 none
have been demonstrated in the same public manner ; none
have come into nearly such general use or met with such in-

1 See Journal, R. M. S., December 1884, p. 1006, and Journal
Quekett Mic. Club, March 1885, p. 118.

2 One exception ought to be made, in fairness, respecting a German in-
strument. Particulars were published of a projection microscope designed by
Dr. Hugo Schroeder, before my instrument was exhibited, though not till after
it was completed. Dr. Schroeder's, which has been stated to perform excel-
lently, was therefore worked out in perfect independence of mine, as mine
was of his. With the exception of the concave parallelising lens presently
described, which has been generally used, and is mentioned by Kingsley,
there is, however, little in common in practical detail to the two instruments,
and the cost of Dr. Schroeder's, which is far more complicated, was stated to
be about 200Z.

1 82

OPTICAL PROJECTION

dependent approval, especially in first-class public institutions ;
and, so far as I can learn anything at all about them — which
it has b.een most difficult to do — their success has been in
precise proportion to the degree in which the same general
arrangements have been adopted, though I have not as yet
heard of equal results having been obtained. I therefore con-
fine myself to what I know and have myself openly tested in
various public demonstrations. Moreover, everything in this
chapter except the description and explanation of the instru-
ment in detail, will apply equally to others in proportion to
the efficiency of their performance.

92. The Oxy-hydrogen Microscope. — Fig. 102 gives a sec-
tion of the instrument as constructed for the oxy-hydrogen light.

AM

Pio. 102. — Oxylijdrogen Microscope

o represents the lantern condenser, which I prefer to make
5 inches in diameter and of triple form, so as to take up an
angle of 95°, and bring the rays to a focus, if let alone, at
about 6 inches in front of the front lens. If the microscope
be required to fit an ordinary 4-inch lantern front, the arrange-
ment must be slightly different, the lime being in this case
pushed up so as to give an approximately parallel beam with
the ordinary double condenser, while a third lens either slides
or racks in the back-end of the microscope, so as to bring
the parallel rays to a focus corresponding in character to the

THE PROJECTION MICROSCOPE 183

above. The loss of light does not, I think, exceed 10 per
cent, when thus constructed, but the edge of the field is not
quite so well illuminated.

In the convergent cone of rays from the lantern condenser
is placed a parallelising plano-concave lens P, giving an ap-
proximately parallel beam of about 1^ in. diameter. This
lens is of highly dispersive glass, and therefore to a large
extent corrects the chromatic effects of the lantern condenser.
In the same position, nearly, is placed the alum trough A.
I found it advisable to employ a full inch of alum solution,
and, in addition, to form the second side of the cell of a double
plate of glass, the two cemented together by Canada balsam.
This layer of balsam absorbs any special balsam-heating rays
which get through the alum.1 With these arrangements
protection from heat is perfect. Less than this is not real
protection ; for the heat in the conjugate focus of a good lime-
light is sufficient to ignite black paper. For ordinary purposes
I cement the concave lens with balsam on the alum trough,
thus making the lens itself the second of the two glass plates.
By this expedient the loss of light at two reflecting surfaces
is avoided.

From the parallelising concave lens to the stage is about
5 inches. Less than this would suffice for mere focussing
purposes, with plain work only ; but this distance is not
enough to produce much loss of light by scattering, while it
allows of a really good-sized polarising prism being introduced
when necessary, such as will show a polarised slide of J in.
diameter — none too much for rock sections. Also it appeared
to me, from such experiments as I could make, that some
achromatic condensers gave more light and worked better
when not placed in parallel light, but just after allowing the
rays to cross from the lantern condenser, without parallelising
them at all. I therefore allowed for this, which is easily

1 I should remark that I saw a layer of balsam employed to protect balsam
elides years ago, but am not now certain by whom.

1 84 OPTICAL PROJECTION

accomplished by having a spare alum-cell with a plane second
side, instead of the concave lens. At all events, it seemed
desirable to provide the widest possible range in the optical
manipulation of substage condensers.

The rays which reach the substage condenser s c are thus
approximately parallel. They really scatter or diverge very
appreciably from the parallelising lens p, owing to the size
of the radiant, every point in which necessarily sends out its
own bundle of parallel rays from P. This is the great diffi-
culty in increasing the light by a more powerful jet ; which
latter increases the luminous surface, but not the intensity in
the centre, which alone can be condensed and made available
on a small microscopic object. Some of this scattered light
could be saved by allowing the rays from condenser to focus or
cross, and then parallelising them by a convex lens ; but an
approximately parallel beam all along the barrel of the in-
strument has so many advantages as to far outweigh this in
practical work. Where however it is absolutely certain
polarised light will never be used, the distance from p to s c
may be shortened with some slight gain in light.

In the parallel beam of light is placed the substaye con-
denser s c, winch focusses the rays on the slide by the rack
R'. For low powers this condenser consists of one lens,
which will brightly and evenly illuminate a slide of fully
1£ inch diameter for a 2^-inch power, while the same lens,
racked back, answers for all powers as high as T^ inch.
Above that a double combination is preferable, and for
^-inch or -J-inch powers, one of these is made achromatic,
or rather, with sufficient over- correction to correct the other
lens as well. For immersion lenses, this last form of con-
denser is supplemented by a third very small lens of very
short focus ; the whole then closely resembling a Kellner
eye-piece surmounted by a small hemisphere of crown, which
was kindly suggested to me for such work by Dr. R. L. Maddox.

THE PROJECTION MICROSCOPE 185

I however found some modification of the combination give
better results.

All substage condensers fit easily from the front into a ring
of the standard R. M. S. U-inch substage gauge. Thus, any
substage apparatus approved by any microscopist, can be at
least tested for projection purposes, and, if successful, adopted.
A spot lens, which will do excellent dark-ground work on a
certain class of objects, is inserted in the same way. The barrel
of the instrument being now constructed open at the top, the
substage condensers can with equal facility be inserted or ex-
changed from the back of the ring, without interfering with
the slide or focussed objective; and this arrangement also
allows of a condenser with iris diaphragm being employed,
which is very useful in high-power work, as the best definition
can only be obtained with a cone of light of some particular
angle.

The Stage s cannot be too simple for ordinary work,
even with high powers. A plain plate, with two simple clips
capable of holding either a glass trough or a slide, is to be
preferred, even for immersion lenses. One very good plan,
preferred in practice by the majority, is to make the stage
consist of a large rotating diaphragm plate, pierced with
different apertures, ranging from T\y inch, to a size sufficient
to allow of the condenser-fitting racking out for manipulation.
Another plan preferred by some, is to have a set of plates
pierced with different apertures, sliding in dovetailed grooves,
so as to be a hair's breadth below the surface of the outer
stage-plate. This pattern allows the aperture to be changed
without removing the slide. In practice, however, any other
than the largest aperture is seldom needed ; the higher power
substage condensers confining the pencil of light sufficiently.

A mechanical stage is simply a nuisance for ordinary
projections. It does not cost much, but is only useful to two
classes of persons. The first consists of such as use the
instrument for photographic purposes (for which it is admirably

1 86 OPTICAL PROJECTION

adapted) and require the most precise and delicate adjustment
of the object, for which they can afford ample time. The
second class consists of biological demonstrators ; who may
not only need equally precise adjustment, but also to pass
parts of a slide in regular succession over the screen, and
who likewise have no impatient general audience to grudge
the time such manipulation involves.

The fitting or short tube — answering to the body of the
compound microscope — which carries the objective o, and
is screwed with the standard E. M. S. screw, is roughly
focussed by the rackwork E2, and finally adjusted for higher
powers by the fine-adjustment F, substantially made on the
pattern so well known in most * histological' stands. This
fitting slides into an outer tube, and it is advisable to have
several, so that different powers may be ready screwed in
place, and simply withdrawn or inserted.

93. Amplifying the Image. — It will often happen that
the direct image from the objective alone will not give the
best results. Suppose we want to exhibit an object double
the size it appears by a ^ lens. We may either double the
size of the image shown by that lens, or we may use a lens
of double power, \. But the results are different. By
amplifying the image we double the whole image, or nearly
so, as shown by the ^, and preserve what people roughly
understand by its ' depth of focus,' and the larger working
distance ; whilst the I lens will show a much smaller portion
of the slide, and the light cannot be proportionately condensed
into so much smaller a space, though it can be considerably
more so condensed. Hence, while really minute and sharp
sections or objects are usually shown better by a higher
power, more distinct objects of larger size are often best
shown with the amplifier, if more power is required.

The image is 'amplified' for ordinary objects by inter-
posing a plano-concave lens A M between the objective and
the screen. This lens fits into the end of a tube, which

THE PROJECTION MICROSCOPE 187

slides into the body-tube carrying the objective, and must be
guarded against any ' flare ' or internal reflection from its
sides. The effect of such a lens is to lengthen the focus of
any objective in any given position ; hence its use also
increases the working distance. For photographic purposes,
the amplifier should be corrected; but for ordinary projection
a simple lens is sufficient. It is convenient to have a set of
amplifiers of various foci, which give a great range of powers,
and of working distances from the screen, enabling very similar
results to be obtained under different conditions.

The use of an amplifier has another considerable advantage.
Ordinary micro -objectives are corrected to a focal length of
about 10 inches in England, and 6 or 7 inches on the Con-
tinent. If such lenses are used to project an image, say,
25 feet away, besides the mere greater magnification of any
error, it is obvious that their ' corrections ' can no longer be
accurate. It is really wonderful that some lenses should
perform, under such trying conditions, so well as they do.
But now suppose some given lens is focussed on a slide, so as
to project an image at its own proper distance of 6 inches.
Our screen however is, we will say, 25 feet away ; a distance
I have found to give on the average perhaps the best range of
general results, though shorter and longer are better for some
purposes. We can select a concave amplifier of a focus which,
placed in a certain position, will bring the image to a focus
on the screen without altering the focus on the object, thus
maintaining the 'corrections ' of the objective at nearly their
proper value. The late Colonel W. Woodward, U.S., was, I
believe, the first to point out this fact, which is of great use
in photo -micrography.

Any concave lens can be placed somewhere so as to focus
a 6-inch or 10-inch focus on the screen, at any longer distance ;
but practically we are limited to a certain range. I have
found generally useful quite a low power, which does this at
about four inches behind the objective, and amplifies con-

1 88 OPTICAL PROJECTION

siderably, being however chiefly valuable for the reason here
explained. Such a length of tube also cuts off some of the
outer and most indistinct portion of the field. The farther
the lens is placed from the objective, the more the image is
amplified, the more it is contracted or cut off at the margin,
and the less the focus of the objective is altered from what it
would be if used alone. We may therefore use the amplifier
nearer the objective than its best ' correction ' point, for the
sake of a larger field ; or farther from it, for the sake of more
amplification ; in either case at a slight sacrifice of the best
performance of the objective.

Practically I find the most useful range of adjustment
to be an objective-fitting and amplifier-tube which will allow
the amplifier to be placed four inches from the screw-threads,
or to be pushed in as close as two inches. The last will give
as large a field as the objective alone ; at four inches some of
the outer portion of the image is cut off, but the rest is more
amplified. Now and then it is useful to employ quite a long
(six-inch) amplifier-tube, and to use a high-power lens there.
The amplification then is very great; but only the central
portion of the image reaches the end of such a long tube,
which must be carefully guarded against ' flare ' by being
lined with black velvet.

Amplification by Eye-pieces. — Another way of ampli-
fying the original image, is to employ an eye-piece, at the end
of a tube-fitting of the ordinary length, instead of the short
fitting just described. Optically, this method is more perfect
than the other, and for photography far preferable, a sharper
image being obtained, and a flatter field, with a sharp circular
black margin to the disc on the screen. Practically, for
ordinary purposes, an eye-piece entails more trouble ; but
undoubtedly it gives the finest projections.

Zeiss's well-known projection eye-pieces have lately made
this method familiar, and need no recommendation for photo-
graphic purposes ; but they are not suitable for ordinary screen

THE PROJECTION MICROSCOPE 189

projections, owing to the small field-lens exhibiting so little
of the image, which makes it very tiresome even to find the
desired portion of a slide. It is, indeed, partly to their using
only such a very small central portion of the image, that
the excellence of the final image is due. The magnifying
power is also too high in these oculars for ordinary use, being
intended for a screen distance of a few feet only. For nearly
all demonstration purposes, all of the image that can reach
the end of a tube of ordinary length is quite good enough, if
the eye-piece is properly constructed.

The eye-piece for projection, therefore, is one with a pro-
portionately larger field-lens, and with an adjustable or focus-
sing eye-lens, made achromatic. Such eye-pieces, of various
powers, will give beautiful projections with the electric
light ; and if of proportionately low power, which for a screen
distance of 25 feet should not exceed a double amplification,
with the lime-light. In using them, the stop of the eye-piece
itself is focussed sharply on the screen by the adjustable
achromatic eye-lens, and then the whole arrangement is
focussed by the fine adjustment so that the image itself also
appears sharp. The focussing is much quicker, or more
sudden, or sensitive, with an eye-piece, than when using an
amplifier, or the objective alone.

The length of the body carrying the eye-piece must
depend upon the set of lenses used. If they are corrected for
the English tube, 10 inches is required. For the Continental
length, a 6-inch body will be required, and is more compact.
A draw-tube is not desirable. The body must be velvet-lined.
It is as well to fit it with two concave amplifiers also, which
will ' correct ' lenses for screen distances of about 12 feet and
25 feet.

It is not well to screw objectives into separate long bodies,
and change these with the powers as with the short bodies ;
but objectives may be readily changed either by using a triple
nose-piece ; or what is still better, by Zeiss's now sliding

190 OPTICAL PROJECTION

objective-changers, costing 10s. each lens, and each of which
is adjusted by screws once for all, so as to centre its own
objective exactly in the axis of the substage condenper.

94. Projection with the Compound Microscope. —When-
ever a substantial microscope with focussing and centering
substage is at command, very good occasional projections can
be made with it by the aid of a simple lantern attachment,
embodying the foregoing arrangements as far as the concave
lens P, which delivers from the lantern a brilliant parallel
beam to the ordinary substage. It only needs for the sub-
stage to have added to it a tube-screen to receive the beam
and prevent stray light scattering, while a flat plate to prevent
scattering is also screwed on the nose of the microscope, which
will itself furnish all else that is required. I have designed
such an arrangement, which is perfectly effective, at a small
cost. For permanent work, however, a complete and solid
projection instrument will be found more satisfactory.

95. The Demonstration Image. — It must never be forgot-
ten that the projecting microscope produces its image under
peculiar and distinct conditions, which interpose some peculiar
difficulties. The first difficulty lies in the enormously greater
amplification necessary. This does not produce a proportionate
effect upon the spectator, owing to his greater distance from
the image ; but, on the other hand, any errors in the objective,
with their consequent woolliiiess in the image, are magnified
proportionately. A power of 1,200 diameters -is considered
very high on the compound microscope, in ordinary work ;
but is a very moderate power for screen work. The apparent
depth or thickness of the slide is magnified also in the same
proportion ; and cannot be counteracted by constant changes
of focus as in private work, because the whole audience must
see the whole at once. Hence * depth of focus ' is a very real
matter in lenses for projection.

The second difficulty is a consequent deficiency of light,
which makes it specially difficult to exhibit images of trans-
parent objects, such as diatoms. In these objects there is no

THE PROJECTION MICROSCOPE 191

real black and white, but simply different thicknesses in a
substance as transparent as glass. When we look through a
compound microscope at such a diatom as Arachnoidisctis, for
instance, we are looking apparently at an object a few inches
across, with a superabundance of light. Now the lantern
would project a screen image of even a foot across, as sharply
as could be wished ; but a very few feet away from the screen
all the detail, though far larger than on the compound, would
be invisible ; and when to meet this we have amplified the
image to, say, a couple of yards across, the lights and darks
are naturally diluted into shades of grey. Again, to get a
perfect or ' critical ' image, the photographer stops off a great
deal of light, which he can very well afford to do, because he
can make up for deficient light by a longer exposure. But
the demonstrator cannot do this. Whatever cannot be seen
on his screen at the first moment, is seen no better by being
left on for an hour. He must have light, and yet his image
of any details must be large enough to be seen at a distance.

On the other hand there is one compensating considera-
tion, in that a ' critical ' image is seldom really necessary to
him, and can rarely be appreciated at his distances even
when produced, except of coarse objects. A certain breadth
or coarseness of line is a positive advantage in an image to be
viewed many feet away. Fortunately, also, the more serious
the work in hand, the more favourable are the conditions. A
class of thirty or forty people, really studying a biological
subject, would be in a smaller room, and at a smaller screen
distance. All could probably get well within 12 feet of the
screen, where both more detail can be seen, and the image
will be brighter and sharper. Detail can thus be well shown
in a class-room, which it would be foolish to attempt with the
same illumination before a thousand people. It is the human
eye which fails, long before the microscope ; so that an object
can be shown quite successfully when the audience have opera-
glasses to soe the details with, which the eye alone cannot

192 OPTICAL PROJECTION

distinguish on the screen when it is there. Opera-glasses are
thus used in German biological classes. Lastly, for important
demonstrations in colleges there is the arc-light, which not
only gives vastly increased illumination, hut from a smaller
radiant, of which a larger proportion can he condensed upon
a small object, and from which a sharper image can be pro-
duced. The difference is exactly that which the ordinary
microscopist experiences when he changes the flat for the
edge of his flame. Hence the electric light gives not only
a brilliancy, but a sharpness, which cannot be obtained with
a powerful lime-light, and which is a marvellous tribute to
the perfection with which modern lenses are worked.

96. Objectives. — After the projecting microscope here
described was constructed, there remained the difficulty of
finding or constructing satisfactory objectives for it. The
greater part of the lenses which performed well upon the com-
pound instrument, broke down utterly upon the screen, partly
for the reason that when used in this way, direct or with
amplifiers, a field three times the usual diameter is employed,
while that field is not so flat as with an eye-piece. A lens of
-^ focus will ' project ' the whole of a blow-fly's proboscis ;
but every microscopist knows how little of this can be seen with
the same power on the usual instrument. For ordinary screen
work it is often important to cover a large field, if only for
' finding ' reasons. I am afraid to say how many lenses passed
through my hands in the course of my search, kindly lent me
by friends from all quarters ; and the curious thing was that
price was found no criterion of performance. For moderate
powers, an excellent -j% in Seibert's series of lenses was at
length found, but powers of H inch and -£$ inch had to be
worked out upon the screen itself. For large objects to be
shown with a focus of about 2J inches (this microscope ex-
hibiting up to 1^ inch diameter perfectly well) we got the best
results from an old photographic ' postage-stamp ' lens lent
nie by my friend Mr. Washington Teasdale ; and with a little

THE PROJECTION MICROSCOPE 193

alteration in the curves, such a lens, on the Petzval principle,
does all that can be desired. For the common range of work
with the lime-light, where results with lenses already possessed
are not satisfactory, the following series is recommended, and
will be found perfectly so : —

Focus Diameter covered Price

£ a. d.

2| inch (Petzval) \\ inch 0 12 6

ll inch f inch 250

j'oinch1 £inch 276

£ inch T\ inch 1 10 0

The first three of this series being worked out for projection
direct to the screen, do not behave so well with an amplifier
for anything critical, though one may be used ; but for direct
projection, though moderation of price has been studied in
their construction, and especially since their formulas have
been further perfected by the use of the new Jena glass, the
1 J inch and T8¥ inch are the finest lenses yet produced, being
far superior in illumination, in definition, and extent and flat-
ness of field, to all others constructed for the same purpose
which I have been able to test, including both American and
Continental lantern objectives of a very much higher price.
The Ti5- works equally well with or without amplifier, or with
an eye-piece, and may be pushed up to 1,500 diameters, which
means a blow-fly's proboscis 14 feet long.

Working with a proper projecting eye-piece, a much larger
number of ordinary micro-objectives will do good work, only
the best third of the field being used, and that much flattened
by the field-lens. Almost any really fine lens will do fair
work in this way. I mention the following as lenses I have
actually tested. Zeiss's aa (1-inch) gives magnificent defini-
tion in the centre of the field, both direct and with an eye-
piece, but is far from flat. Powell & Lealand's J inch of 40°
is very crisp, and works well either direct, or with amplifier

1 I am endeavouring to get this lens modified into an inch, which with the
Reichert f presently mentioned, will give a more useful gradation of powers.

0

194 OPTICAL PROJECTION

or eye-piece. Leitz's new formulae give fair results. But
the best dry lenses I have found at any moderate price are
Herr Eeichert's new series (termed by Mr. E. M. Nelson semi-
apochromatics) No. 8 (f) 20s., No. 6 (J-) 80s., and No. .7 A (|)
86s., all which work beautifully by either of the three methods.
The last is a truly wonderful lens at its price, and both it
and the J are far the best projection lenses I have been able
to find of these powers. I may add here, that some really
good lenses, when used with brilliant lights such as projection
demands, give a ' mist ' over the image purely from flare, or
reflection in the lens mount, and which is removed by care-
ful blackening. I have drawn the attention of Herr Keichert
and others specially to this, and am sure the point has not
yet received the attention which it demands, now lenses are so
often used for photographic purposes with these strong lights.

With the electric arc, Zeiss's apochromatics give fine results
over the small field of his own projection eye-pieces, but only
so used. They cannot be used with satisfaction direct, or with
an amplifier.1 The same in both respects may be said of the
wide-angled dry lenses of Messrs. Beck. Few of these lenses,
however, have the ready adaptability of the Eeichert series
for projection work. Messrs. Powell and Lealand's apochro-
matics are much better in this respect, and their j inch of
0*95 NA, for its combination of exquisite definition with a very
large flat field, appears to me at present the most ideally
perfect lens in the world. Even at its high price of 111., it is
well worth purchasing for constant histological projection
work, of which (with eye-pieces) it will cover a great range.

Of immersion lenses, the best for projection at any mode-

1 I have been greatly disappointed in these lenses for projection. They
only exhibit a very small bit of the centre of their field in focus, and appear
to be constructed chiefly with a view to photo-micrography. This seems to
me a great practical mistake, and I moreover believe that certain conclusions
of Prof. Abbe relating to the nature of the image of minute structure, some of
which have been demonstrated to be erroneous by Mr. E. M. Nelson (Quekett
Club Jo., July 1890), are chiefly due to this character of his own lenses, and
the fact that different zones of the image are in such widely different focal
planes.

THE PROJECTION MICROSCOPE 195

rate price is also Keichert's oil TV (really y^) of N A 1/25, price
§1. 5s. only. In every optical quality a selected specimen has
been pronounced on the highest authority to be scarcely, if
at all, below Zeiss's apochromatics ; and for lantern use it is
far better, owing to its longer working distance and flatter
field. It is only, and not greatly, excelled on the screen by
Powell and Lealand's apochromatics. Leitz's y1^- oil lens is
also an excellent projection objective of long working distance ;
both these have much more of the latter than Zeiss's y1^- of
same N A. This quality is a great point in lantern demonstra-
tion. Of water-immersions, the best I have tried are Seibert's
(Ty and Zeiss's Gr (|-), but I believe the last is not now made
— more's the pity, for it was the best of his water series for
this class of work.1

So far as the lantern and projection are concerned, it is
a subject for regret that opticians seem now to have confined
their immersion lenses of moderate price to y1^ focus ; only the
expensive apochromatics being now made of ^ inch or 3 mm.
focus. The latter would be a much more generally useful
lantern focus, covering a larger field and giving more light.
An old Zeiss oil-lens of J focus, though far surpassed by more
modern productions in other respects, gave me a field and
brilliancy which no TV can possibly equal ; and I venture to
express the hope that increase in projection work may lead to
the longer focus being made at a moderate price. Herr
Eeichert was good enough to send me for trial a 4 mm. oil-
immersion made to a special order, with large glasses. The
light passed by it was enormous, bringing many objects other-
wise impracticable within the O.H. light. This lens could be
supplied to order, where the most must be made of the lime-
light alone. But 3 mm. would be more generally useful.

1 I have no doubt there are many excellent lenses not here named, and
take the opportunity of saying that I shall gladly at any time test any which
the maker thinks likely, in definition, working distance, and flatness of field,
to be suitable for projection purposes. I am only here describing what have
actually gone through my hands.

o2

196 OPTICAL PROJECTION

In using immersion lenses, care must be taken to employ
a small drop of fluid, else it will drain away by its own weight.
Water lenses are convenient, but oil is far superior in bright-
ness and crispness of definition. A good Reichert oil lens
will work either direct, or with the low-power amplifier, as
well as with an eye-piece, up to 10 or 12 feet distance.

97. Management of the Instrument. — Before using the
Oxy-hydrogen Microscope, the alum-cell must be filled with
a saturated solution of alum. To make the solution add
common alum to hot water in excess, and allow the solution to
cool, then filter the clear portion twice through ordinary filter-
paper or blotting-paper. This solution may be used as often
as desired, but must be refiltered if any sediment or turbidity
appears, being used only in a perfectly limpid condition.
When replacing the alum-cell in the microscope, the side on
which the concave parallelising lens is balsamed should be
farthest away from the light. It is as well to leave the brass
stopper out of the cell when in use. As air-bubbles form on
the inside surfaces of the glass, they should be removed with
a camel-hair brush, as they impede the light, and sometimes
would appear as images upon the screen. They always appear
to some extent as the solution becomes heated.

Not the slightest fear need be felt for the most delicate
slides on account of the heat ; but if preferred, the alum-cell
may be emptied and refilled with a fresh solution after an
hour's use, as it rapidly becomes hot. The cell should be
emptied after use, and washed out with clean water, or alum
slime may collect on the glass.

Great care should be taken to procure a good light, the
full capacity of the best oxy-hydrogen jets being required for
work with high powers.

The light from the lantern benig meant to be nearly parallel
after leaving the concave lens on the alum-cell, the first step
is to adjust the light to give this. This can be done after
gradually warming up the lantern and condensers, by taking
out the objective, mount, and sub-condenser, so as to leave

THE PROJECTION MICROSCOPE 197

the course clear, and focussing on the screen, through the
alum-cell (by sliding the light to and from the condensers)
the surface of the lime cylinder, shown by the granulation of
its surface. This will be approximately the distance of the
lime from the condensers, and when that has been corrected
in use, as presently mentioned, a mark can be made on the
tray of the jet, to which it can be at once set in future.
Placing in position an objective, with its appropriate substage
condenser, the lime should also be most carefully centred.

Most of the substage condensers have their special func-
tions as marked. The lowrest power is however • used for all
objectives up to the T8¥ objective ; but for the 2^ inch and 1^
inch powers it is inserted in the front end of its fitting, in
order that it may be racked up almost close to a large slide,
and illuminate with its cone a large field, whereas with the
T^ power it is placed in the back end of its fitting, being
focussed upon the slide. If the microscope is fitted with
the rotating diaphragm-stage, and the small holes are used,
always lift the spring-clips from its surface by the milled
head, before rotating the stage. The use of an aperture gives
a better edge to the disc when no eye-piece is used, but is not
otherwise necessary.

Let us now suppose the first slide placed upon the stage
(we are here considering the commencement of an exhibition).
Choose an objective for it (with its appropriate sub-stage con-
denser) of the loivest power that will show what is required ;
bearing in mind also that, where possible, it is best to choose
one that wrell covers all that is wanted, without showing more
than is necessary. This being screwed into the fitting and
pushed into the socket, can now be focussed by the rack ; the
screw fine -adjustment is only needed with high powers.

The illumination has next to be got right. It may
probably be but poor ; and this must now be remedied by
racking the substage condenser backwards or forwards as
required. The best position will be almost instantly found,

198 OPTICAL PROJECTION

and when found will be very little altered for the same
objective, unless the thickness of the slides should differ very
greatly. But the adjustment should be tested for possible
improvement from time to time, and will require slight
change with every change of power. This however is not all ;
the position of the lime itself should now be adjusted more
precisely, and too much care cannot be devoted to these points.
The position of the lime is tested by sliding the tray of the
jet slightly in and out. It will usually be found that a slight
alteration improves the brightness of the object materially.
The lime thus readjusted on an object, after the substage
condenser has been adjusted, will be in its position for average
work. But whenever the result seems not satisfactory with
any slide, it is always worth while to try afresh whether the
lime is in its best position ; and occasional adjustments of this
and of the substage condenser will be well repaid.

There is one general rule in these occasional adjustments
of the light, without which no projection microscope can do
its best with varying powers, though so long as the same
power is used all through, they are unnecessary. The lime
will have to be pushed up nearer the lantern condensers, for
lower powers, and drawn rather farther back with higher
powers. The distance is not great ; but the difference in the
light on the screen produced by these readjustments is won-
derful. Now and then, after the lime has been readjusted, a
very slight readjustment of the substage condenser also will
still further improve the effect.

When the microscope is once started and precisely adjusted,
no alteration will be needed whilst the same power is adhered
to ; but whenever the power is changed, and especially
whenever the substage condenser has to be changed, re-
adjustment of the light should be tried for as described above ;
also when, without changing the objective itself, an amplifier
is either put on or taken away, the substage condenser will
require re-focussing upon the object. The lime will require

THE PROJECTION MICROSCOPE 199

frequent turning, and the best plan is to give a fresh surface
for every slide.

This all sounds rather formidable ; but practically the
demonstrator does it all without thinking about it, in a second
or two. It is done so easily that, although my experience
has been only that of rare and exceptional occasions, I have
found no difficulty in exhibiting fifty slides representing organs
of insects, with powers varying from 300 to 1,500 diameters,
during a lecture of an hour and a half, performing all
manipulations whilst explaining the slides, and with no
assistance whatever beyond that of a friend to hand me each
slide in order, wiped clean, in exchange for the one just used.

Unless the operator's sight is unusually keen, an opera-
glass will be found of very great assistance in focussing the
image on the screen.

It has already been said that the lowest sufficient power
should be preferred ; but a certain ' scale ' must be had to be
visible on the screen, and experience must decide what is this
necessary scale. It is not wise to attempt unknown slides
without rehearsal, after which the power required for each
should be marked on the label. Then a list should be made
out of every object in turn, with the power required for it;
and if the lecturer has to be his own demonstrator, it is
generally possible, by a little alteration in the arrangement of
the lecture, without any detriment to it, to bring most of the
objects to be shown under similar power more together than
at first, and so avoid too many sudden changes of objectives
for single slides. A series of demonstrations should always be
revised in this way, as an avoidance of needless changes not
only saves trouble, and wear and tear of apparatus, but enables
more time to be given in obtaining the very best effect, which
cannot be obtained with a fresh power in a moment of time.

Often it is advisable to give a general view of the whole
slide with a low power, and afterwards to magnify much more
some portion of it. Sometimes it may suffice to add an

200 OPTICAL PROJECTION

amplifier; sometimes a higher power gives better results.
Suppose we had a section of human skin, exhibiting both the
perspiration glands and ducts, and the touch-bodies which
terminate the nerves (though it is not often both could be
found in the same section). Probably a power of 1 inch
would exhibit the first sufficiently well ; but the -j^-inch
amplified to 1,500 diameters would be needed for the touch-
bodies, or even a good ^-inch could be used with advantage.
So the -^y-inch, amplified or not, would well exhibit the sting
of a wasp or bee ; but to exhibit the serrations of the barbed
end coarsely, will require a \ or £.

98. The Screen. — A sheet is unfit for any work with high
powers, unless stretched tight and white-washed. Paper-
faced screens, and especially the whitened screens mentioned
in § 67, are to be preferred, unless a plastered smooth wall
can be utilised. For permanent demonstration work it is
well worth while to provide the latter; the difference can
hardly be conceived without trial.

For popular subjects I like a screen distance of about
25 feet, and up to 50 feet may be used easily, this distance
however being chiefly adapted for common objects to be shown
in a large hall. High -power ' class- work ' is far better done,
on the other hand, with higher powers at shorter distances — •
as short as will allow all of the class to see the screen. With
a good -J- homogeneous immersion, or a ^-inch amplified, the
cyclosis in Vallisneria can be shown to several hundred people
pretty well, with a screen distance of 12 to 20 feet, but the
oxy-hydrogen light is not sufficient to attempt such a subject
in anything like a large hall.

Some amount of practice is needed before anyone can use
properly even a table microscope ; and it is equally necessary
before anyone can produce with the projecting microscope
what it is really capable of. Skill will however be rapidly
attained ; and when the operator can exhibit a flea, or the
proboscis of a blow-fly, 12 to 15 feet long, sharply and

THE PROJECTION MICROSCOPE 20!

brightly on the screen, he has fairly grasped the powers of the
instrument with the oxy-hydrogen light, so far as common
objects are concerned.

99. Spot-lens and Lieberktihn.— The spot-lens is used
exactly as a substage condenser, and is usually adapted for the
T%- objective with amplifier, the amplifier enabling this power
to be used by bringing it farther from the slide. Polycystina
are beautifully shown by it, but almost too small to be seen at
any great distance. Many usual dark-ground objects do not
reflect sufficient light, and only experience can decide what
are available ; but some slides are best shown in this way.

The Lieberkiihn is usually fitted for the ^ or 1 inch
power, which can be amplified if necessary. It slides upon
the objective itself, and must be ordered with it when desired.
In using it, all substage condensers are removed, that the
parallel beam may fall direct upon the Lieberkiihn, and the
lime and the Lieberkiihn must both be adjusted, to get a good
effect. Portions of iridescent wings make beautiful objects,
also some Foraminifera, but they should not be mounted on
an asphalt surface, as, even after the alum-cell, there might
be sufficient heat absorbed to soften this. Such asphalt slides
are the only ones, owing to their powerful absorption, which
it is un advisable to leave in the rays for more than half a
minute. When slides can be specially prepared, this is avoided,
and the effect improved, by having a disc of black paper on
the back of the slide, and mounting the object on the surface
of the glass itself. The disc should be as small as will cover
the object, in order that as much light as possible may pass
round it to the Lieberkiihn. The smaller of those slides
prepared with iridescent butterfly scales, in imitation of
bouquets, vases of flowers, &c., make very beautiful screen
subjects for the Lieberkiihn.

100. Polarised Light. — The addition of polarising ap-
paratus largely increases the powers of the instrument. A
polarising prism capable of rotation, and large enough to cover

202 OPTICAL PROJECTION

a slide f -inch diameter, is arranged to be used in the barrel of
the instrument ; the analyser is a square-ended Prazmowski
prism. It is needless to describe the arrangements in detail. In
the most complete form of the apparatus, the analyser is fitted
with a Klein's quartz plate, and a quarter-wave plate ; and a
system of convergent lenses has also been arranged, somewhat
similar to that described on p. 843, which will exhibit the rings
in crystals, taking in both systems up to the angle of selenite.
The ordinary microscopic crystallisation slides make mag-
nificent displays in this form of the instrument. The loss of
illumination being very great with polarised light, about the
utmost limit of the power of the instrument, with the 0. H.
light, will be to exhibit the rotating cross in one or two of
the larger starches, such as Tous-le-mois. The arc light will
of course go considerably farther.

101. Slides and Objects. — Organs and parts of insects
make excellent objects, if mounted flat ; those mounted ' with-
out pressure ' fail, being both opaque, and impossible to focus
in one plane. The essentials are flatness in the preparation,
and neither too much nor too little colour. Experience will
soon teach the proper amount of the latter, and a suitable
slide can always be found amongst any decent stock.

Transparent objects of all kinds are the most difficult, and
histological slides especially so, unless stained well and to the
proper degree. Such sections must also be thin, and it is not
easy to purchase such, stained so as to give the best results.
Where good ' differential ' staining can be got in a thin section,
the object can generally be exhibited under the greatest
illumination, and the positive colour in the structure shows
distinctly ; but with a clear preparation the substage condenser
must either be racked back from the position that gives most
light, or, what is better, if an iris diaphragm is fitted to it,
this must be contracted so as to reduce the cone of light, and
thereby get more distinctness in the phantom-like detail. To
take an instance : a section of skin with the sudorific glands

THE PROJECTION MICROSCOPE 203

and other parts well stained, will probably bear the full cone
of light, and be both bright and sharp ; but if we have a slide
consisting of only a few of the glands themselves, teased out
and mounted separately, the full cone answering to the aper-
ture of the lens will confuse detail ; and it must be reduced to
a narrower cone in order to get the convolutions of the glands
distinctly. Amateur sections are generally far thinner, and
better in that respect than can be got from the trade mounters ;
but are seldom so well differentiated by staining as the work
of the latter.

It must always be seen, that whatever it is desired to
exhibit really is in the slide, satisfactorily, which is not
always the case. Let anyone, for instance, try to find a thin
section of skin which really exhibits clearly the ' touch-
bodies ' as stated on the label, and he will appreciate what I
mean ; but it is obvious he must find it, before he can exhibit
it. Beyond that, all will depend upon the thinness and the
staining ; for it is the want of density in the image that is
the chief difficulty in demonstrating such objects. Some
further remarks on this subject will be found in § 105 in con-
nection with the higher power of the electric microscope.

Botanical sections are beautiful objects, when thin and of
proper colour ; if too clear, they occasion the same difficulties,
and require us to diminish the cone of light in order to get
distinctness. To take an example from this class also : if they
could in any way be differentiated by staining, it would be
very easy to exhibit the punctated cells in a thin longitudinal
section of pine ; as it is, this is a very difficult object with the
lime-light, though not at all needing a high power. The cone
has to be contracted a great deal, when it can be managed ;
with the arc light, so much illumination is left with quite a
narrow cone, that it is quite an easy object.

Foraminifera of all kinds make excellent objects. Poly-
cystina project easily by transmitted light, and fairly well on
the dark field with spot -lens. Diatoms are unsatisfactory and

204 OPTICAL PROJECTION

only some of the coarsest can be shown even imperfectly by
the lime-light, as the stopping down of the light to get crispness
impairs the brilliancy so seriously. A good \ or \ amplified
does the best work with most of them.

With polarised light, besides the crystallisations already
mentioned, all mineral sections which polarise exhibit ex-
cellently ; also fish scales and most organic objects.

These details, by no means complete, will be sufficient to
show how wide is the range of the projection microscope
even with oxy-hydrogen illumination.

102. Living Objects.— Pond life is always a popular sub-
ject, and the vast mass of objects can be shown with ease
in the projecting microscope. The larger beetles and larvaB
are not really microscopic, but require a large trough in the
ordinary stage of the lantern. For smaller objects a variety
of glass troughs must be provided, which can be procured
for Is. each upwards. The troughs should be so thin as
only to allow the creatures to move freely in the same plane ;
for free movement they must not be tighter than this. Often,
for more minute examination, as of the internal organs of a
water-flea, it is necessary to check the power of movement
by somewhat compressing the animal. This can be done by
using a live-box, or by slightly forcing the creature into a
rather thin trough with the end of a sable pencil, or by
placing it with a drop of water on one of the glass slips made
with a concavity on one side, and covering it with a thin
glass, held on by capillary attraction. Very small animals, such
as rotifers, are often best dealt with by placing them in a drop
of water on a plain glass slide, and covering them with a thin
slip; or if the pressure would be too great thus managed,
the creature may be encircled with a bit of cotton thread.

Glass troughs are very easily made to any thickness, by
taking an ordinary 3x1 slip, and a thin piece of cover-glass,
and cementing between them, with dried Canada balsam
dissolved in benzol, the half of a vulcanised rubber ring. If

THE PROJECTION MICROSCOPE 205

still thinner troughs are required, a thin ivory card, or piece
of thick cartridge-paper, will give the required thickness.
Botterill troughs, in which a rubber ring is pinched by screws
between two glasses, are handy for greater thicknesses, and
can be cleaned at pleasure.

The movements of living diatoms are easily shown, in-
cluding Bacillaria paradoxa ; these only need a drop of the
water on a plain glass, with a slip of cover-glass held on by
capillarity. Volvox is easily shown 8 or 10 inches diameter,
requiring a trough with thin glass face, and enough fluid
between the glasses to allow it to roll freely, else there will be
no movement. Greater magnification is only hindered by the
thickness of the globe baffling any focussing of its image with
higher powers. Of infusoria, some — such as stentors — are easy
objects ; Vorticella is rather difficult. The chief difficulty is
that their thickness and their motion make it hard to bring
high power to bear upon many of them.

Hydra, water-fleas, cy clops, and the various larvae of the
gnat family, make good objects. The larva of the true gnat is
very opaque, and too thick at the thorax to focus very sharply ;
but some of the ' blood-worms ' make capital objects. The
corethra (' phantom ') larva is an excellent object, owing to its
transparency, and so is that of the May-fly. The small worm
found in clusters in the mud of a pond, called Tubifex, is a
truly imposing object on the screen, appearing both in shape,
size, and even colour, very much like a gigantic python. The
microscopist will however readily multiply the list for himself,
and will find the manipulation precisely the same that he is
accustomed to.

For displaying the circulation of the blood in the foot of a
frog, a frog-plate may be used ; but personally I prefer the
homely and old-fashioned plan of employing a sole of thin
cork, with a hole cut in it just large enough to show
the necessary portion of the web. By tying a short, looped
bit of thread round each toe, the foot can be held in position

206 OPTICAL PROJECTION

by pins, passed through the loops so as to stretch out the
threads and consequently the toes, with more facility than in
any other way, and the stage clips hold the rough cork more
securely than polished brass. One hint is however necessary,
which applies to any other object, where a moist surface is
involved ; and the frog's foot must be moist, and the frog
kept damp in his bag. The objective must be warmed, else
the moisture will condense upon it, and baffle all attempts at
an image. It may seem ridiculous to mention so simple and
obvious a precaution ; but having myself been fairly beaten
by such an oversight on one occasion, I know how easy it is
for such precautions to be overlooked or forgotten.

Enough has now been said to indicate the range of power
of the oxy-hydrogen form of the instrument. Beyond a
certain point, the light fails us, or even the definition from so
large a radiant as we must employ. With class-room dis-
tances averaging 12 feet, and fine histological slides, powers as
high as T1^ immersion may be used even with this light, with
or without an amplifier or eye-piece ; on the other hand only
a few of the coarsest diatoms will exhibit sufficient distinct-
ness, there being no real opacity in the markings. But with
general opacity all over a slide the power available decreases ;
and as a rule £ or 1 inch, more or less amplified, is the
highest power useful with this form of the instrument. Even
such a lens can be pushed on some objects, which combine
general transparency with opacity of detail, up to 2,500
diameters or more.

103. The Electric Microscope. — By employing the electric
arc, the power of the projecting microscope is enormously
increased, definition being improved, as already explained,
together with illumination.

The general arrangements of an instrument constructed
for an arc-light do not differ from those already described.
But I found one alteration in detail to be absolutely necessary,
and another advisable. As regards the first, much more

THE PROJECTION MICROSCOPE 207

protection from heat is required. I was obliged to adopt a
trough at least three inches thick, and 5 inches in diameter;
and were any instrument ordered with a specially powerful
lamp, even another inch would be advisable, though three
inches of alum solution is practically sufficient up to nearly
3,000 candles. It is also desirable that the brass front should
be kept apart from the body of the lantern, if of metal, by
washers of non-conducting material, to prevent heat passing
by direct conduction, and to ensure that the alum be only
heated by actual absorption of the rays. In spite of all
protection, however, the heat from a powerful arc is so much
greater than from a lime, that every precaution should be
taken, and no slide at all ' green ' or unsafe left on for too
long a period. Glycerine mounts are probably the most risky ;
old and set balsam mounts I have never known injured.

The other change is advisable owing to the less handy
character of many electric lamps, which cannot be adjusted
so readily in focus as a lime-jet. The triple condenser is
replaced by a double-piano, since no lens would bear an arc
so near its surface as a lime is brought to the triple form.
The double 5 -inch condenser brings the arc 6 inches from its
back surface, which is a convenient distance. The arc is then
adjusted to give a parallel beam from the first lens ; and the
second lens of the condenser is made to adjust by a rack and
pinion, so that by its motion those changes are made in the
cone of rays which in the previous instrument are made by
moving the tray of the jet. When the lens is racked back a
smaller cone of rays is brought on the parallelising lens, just
as when the lime-light is drawn back. When Brockie's lamp
and a focussing stand are used, however, a racked condenser
is unnecessary.

These alterations make it advisable to affix the microscope
securely to the lantern, instead of sliding it into a nozzle. It
can however be easily removed, and another front, adjusted
to the same template, substituted for physical demonstration.

208 OPTICAL PROJECTION

The instrument as thus modified, and fitted with polarising
apparatus, is shown in fig. 103, which also shows Mr. Brockie's
optical lamp.

In using the electric light, I very soon found that there
was some difference in the effect of the substage condensers.
Owing to the smallness of the radiant, and its greater distance
from the condensers, the light is brought by the same lenses
into a much smaller area ; and hence the same sub -condenser
used for low powers, answers perfectly well up to T% power as
well. In the higher powers also, lower angles may often be
employed. This likewise tends to increase penetration and
definition. What a microscopist understands by * high reso-
lution ' is quite unattainable with the lantern, since the dots
on P. Angulatum, even magnified up to 5,000 diameters in
nearly black and white, would be invisible a very few feet
away from the screen.

104. The Electric Lamp. — On this head I need only refer to
the preceding chapter. It was all-important to get rid of the
double radiant, and this is fully accomplished in Mr. Brockie's
lamp there described, while all necessities of centering and
focussing are met by the screw-motion table on which the
lamp stands.

105. High-power Demonstrations.— For high-power work,
especially biological, I would repeat that a permanent screen
or surface of fine plaster of Paris, smooth, and kept carefully
whitened, is of the greatest assistance. The dark parts of the
image are as dark upon it, while the bright parts are much
brighter ; hence that contrast of shades is heightened whose
dilution is one of the great difficulties we encounter. Such
screens are used in German institutions, and enable results
to be obtained which otherwise would be beyond the apparatus
employed. Such a surface, and opera-glasses for the back rows,
will do a great deal to extend the range of detail possible.

In the majority of cases, working with a good projection
eye-piece is decidedly to be preferred. A really fine -£• or |,

THE PROJECTION MICROSCOPE

209

?io OPTICAL PROJECTION

such as described in § 96, used thus, will give considerable
power, with more ' depth of focus,' and less trouble in
' finding.' The best general distance will be 12 to 15 feet,
beyond which the performance of every high-power lens
rapidly deteriorates. Even brilliance depends much upon
crispness, and therefore a Eeichert's TA (j) at 12 feet will
give both more light and better work (though the image is same
size) than his No. 6 (-}) at 17 feet. When the highest power
is needed, it is usually better to use a good oil-immersion lens
than to increase the distance. This refers to the magnification
of minute detail; on the contrary, if the object be to exhibit
ordinary objects on a colossal scale, this is generally better
done by a screen distance of not less than 25 feet, with
amplifiers or high-power eye-pieces, or often by stretching
the distance to 50 feet or more. In this way a flea may be
shown 30 or 40 feet long with ease. But for histology, which
will always be demonstrated to moderate audiences, short
distances and high powers are far the best. Zeiss's projection
oculars will give good results with most wide-angled lenses,
where their very contracted field is sufficient for the purpose ;
but it will save time to centre the slide with one having a
larger field.

Ample light is important, and I would prefer 3,000 candle-
power to less. For a material point is to stop down the
cone of light from the substage condenser, with the iris
diaphragm, to the precise degree which gives the best result.
Mr. Nelson's experiments have determined that even on the
compound microscope this is only about three-fourths of the
angle of the objective ; in projection it will rarely exceed two
thirds. A cone will always be found that gives a distinctly
best result, focussed on the object. That is the chief point-
about the microscopic manipulation.

With powers approaching 4,000 or 5,000 diameters, all
that has been said in § 101 applies with increased force.
Histological detail will depend, even more than with the

THE PROJECTION MICROSCOPE 211

oxy-hydrogen light, upon thinness and good staining ; the
more transparent the section, the less cone can be used.
Biologists will therefore find it best to prepare their own
slides, employing stains which, like carmine, gold, and silver,
give density to the marking. German professors, I believe,
are as a rule in advance of English in these matters, and in
overcoming this difficulty, which is the principal one. In
projecting blood-corpuscles, for instance, when the images are
quite sharp on the screen, there is so little colour in them,
that they are not easy to observe. Professor Strieker gets
over this by treating the preparation with a 0-6 per cent,
solution of salt in which is dissolved some fuchsine solution.
In this red ground the amreboid corpuscles stand out white,
and their movements are readily seen, while the red discs
appear light yellow. Any such expedients are a great help
to the screen image ; and doubtless as screen demonstration
becomes more common, both such methods, and others of
differentiating details of structure — such, e.g., as the striations
in voluntary muscular fibre — will be perfected or improved.
The limit of power will be found, not in mere magnification,
or mere brightness on the screen, but either in too little con-
trast of the image in its shades, owing to want of density, or
woolliness owing to thickness of the section, or to the lens.
Bearing these considerations in mind, few objects except mi-
crobes will be found out of the range of projection, and some
even of these can be shown with a good oil-immersion lens.

106. Management of the Electric Microscope. — This is
perfectly simple. The lamp is first adjusted at a distance
from the back lens of the condenser which gives a nearly
parallel beam from that lens alone : usually this is about
6 inches, but the exact distance may be obtained with each
instrument. The arc is centred by the screw slides of the
table or stand, and the front lens of the condenser is racked
in or out, or the lamp screwed backwards or forwards, until
when the objective fitting and sub stage condenser are re-

p2

212 OPTICAL PROJECTION

moved, a central parallel beam proceeds from the parallelising
lens on the alum-cell to the screen. Thenceforth the proce-
dure is precisely that already described for the oxy-hydrogen
apparatus, only racking the front lantern condenser, or
screwing the lamp -stand to and fro, instead of moving the
lime-tray. Good centering is all-important. The substage
condenser, in critical cases, must then be carefully fociissed on
the slide. And finally (the demonstrator here using a bin-
ocular glass unless his sight is very keen) the objective itself
being focussed as accurately as possible, and the required
point in the slide brought into the centre of the field, the iris
diaphragm is to be adjusted till the best effect is found.

In using the projection microscope with the electric light,
it is particularly necessary to remove any air-bubbles on the
sides of the alum-trough from time to time. The obstruction
to the light is not of so much consequence, but they interfere
with a truly parallel beam, and usually focus upon the screen
when the adjustment of the light is otherwise at its very
best.

CHAPTER XIV

DEMONSTRATIONS OF APPARATUS, AND IN MECHANICAL
AND MOLECULAR PHYSICS.

107. Projection of Apparatus. — Referring back for a
moment to fig. 81, page 154, it will be seen that a gold-leaf
electroscope, placed in the field of the lantern, upon a small
table adjustable both for height and distance from the con-
densers, takes the place of an ordinary diagram or slide, and
is then projected upon the screen in precisely the same way.
There are some limitations and also some difficulties about
such projections, which deserve a word or two.

DEMONSTRATIONS <9/< APPARATUS 213

In the way of limitation, it is plain that, except so far as
any piece of apparatus may be transparent or semi-transparent,
the projection can only appear as a black shadow, and fluids
in tubes, however transparent, will also appear more or less
opaque. For a large class of apparatus this matters little,
however ; especially as in many cases motion of some kind is
the essence of the experiments. Thus, in the electroscope, the
effect to be shown upon the screen is simply the divergence of
the leaves, which is sufficiently clear. It is further obvious,
that the size, of apparatus to be projected in this way is limited
by the field of the condensers ; and that, in fact, the possibility
of exhibiting a vast number of experiments by the projection
method, depends simply upon the possibility of reducing the
requisite apparatus to a sufficiently small size.

The chief difficulties are also two. The first is, that the
object to be projected no longer lies in the same focal plane,
and can therefore not be all truly in focus at one time. This
difficulty is best overcome by using a projecting lens of longer
focus than would be used for diagrams alone, 9 to 12 inches
focus being most suitable. The objects are coarse compared
with the small letters and lines of a diagram, and such a lens
gives an amply large scale, while the difference in focus be-
tween various parts of the object is much less evident. With
such a lens the difficulty about focus is much less observable
than might be supposed, and a vast mass of apparatus, which
need not be particularised, is perfectly well shown. Let ua
take, as a fair illustration, a radiometer. While in the electro-
scope the glass case may be neglected, and only the leaves
focussed, in the radiometer even the active portion of the
apparatus cannot all be in focus together, but the revolving
vanes will occupy planes differing by one to two inches.
Nevertheless the instrument, and the effect of revolution, will
be shown upon the screen perfectly well.

The other difficulty is greater, and is too often allowed
to spoil the projection altogether. It is especially difficult to

214 OPTICAL PROJECTION

overcome with the Duboscq lantern, partly from the small size
of its condensers, and partly from the optical construction
already pointed out ; and to the former prevalence of this
lantern in lecture theatres, I attribute the little progress made
in this class of projections, until the Germans showed a more
excellent way. It arises from the fact that nearly all apparatus
has to be arranged, owing to its absolute thickness in the
direction of the optic axis, farther from the condensers, as
regards its ' average ' or practical focal plane, than a diagram
would be. We place the latter almost touching the front
condenser lens ; while in the apparatus the plane focussed
will probably be some inches away from it. Now this quite
upsets the uniform illumination of the disc ; a fact which
seems too often forgotten. If we are to have a good and
clear projection, the disc on which it appears must be as
carefully adjusted, as to centering of the light, and distance
of the light behind the condensers, as for a diagram. The
radiant has to be brought nearer the condensers, and even
then the cone of rays will not embrace so large a field as
the largest elide which can be shown. This is one reason
why the condensers of a demonstrating lantern, for exhibiting
solid apparatus, should be 4^ to 5 inches in diameter.

In all work throughout the following chapters, and similar
experiments, the proper adjustment of the cone of rays from
the condensers, so as to embrace the apparatus, and finally
pass through the focussing lens, and give an evenly-illu-
minated disc over the plane of the object, is the chief thing
in order to obtain a good projection, and the objective itself
'focusses' better under these conditions. The adjustment
involves some trouble, and hence is too often neglected, as
re- arrangement may be needed from time to time. A fair
average for a series of apparatus can be pretty easily secured ;
but the reason will be evident why the same adjustment does
not suffice for apparatus and diagrams, and why, therefore,
some form of double lantern is so desirable where diagrams

DEMONSTRATIONS OF APPARATUS 215

are habitually interspersed. Otherwise, the radiant must be
readily adjustable in order to obtain good results. The
practical importance of this matter in demonstrations cannot
be exaggerated.

The inversion of the image is a third difficulty, but is
met by the use of an erecting prism. Each must decide for
himself how often this is necessary ; the only practical incon-
veniences about it are the space required in front of the objec-
tive, and the time needed for adjustment. The loss of light,
when properly adjusted at the smallest point of the pencil of
rays, is not probably more than 40 per cent., and this (for most
apparatus) leaves ample margin.

108. Method and Scale of Experiment. — Whilst, how-
ever, a vast number and variety of experiments, by reducing
the size of the apparatus until it comes within the field of
the condensers, can be demonstrated by projection, it is by
no means the purpose of this work to urge that this should
always be done. That is always a question for practical con-
sideration ; weighing on the one hand the saving in space
and weight of apparatus, and on the other ths comparative
effectiveness of the direct and projection methods. In many
experiments the direct method will remain the only one
available ; while in many more, where the method is optional,
it will depend upon particular circumstances which is to be
chosen. Let us take two illustrations from experiments
connected with the elasticity of bodies.

Simple elasticity is easily illustrated in the lantern, by
adjusting upon the table, concave side upwards, a spare (or
cracked) meniscus lens from a condenser, in a cell or not
and dropping upon the centre, from the top of the field, a
well-tempered £ inch steel ball from a bicycle bearing. Or
*he concave bottom of a thick glass tumbler will answer instead
of the lens. The ball will rebound nearly to the height it
fell from, and the concavity will keep its motions within the
field ; and this small and simple apparatus will suffice. That

OPTICAL PROJECTION

may be a convenience ; but few would question that for mere
effectiveness, in such an experiment as this, a larger ball of
ivory, clearly seen in its rebound from a metal plate, is to be
preferred.

Take again the propagation of a wave in an elastic medium,
as illustrated by the well-known apparatus shown in fig. 104.
Using small glass balls, with wires for the suspending frame,
this is very easily brought within the field of the lantern, the
apparatus not exceeding, say, 4 inches in total width. That

may be a great con-
venience and advan-
tage, in storage or
portability. But if
not, then it becomes
a question whether
it is worth while

to project apparatus
FIG. 104

made on such a

miniature scale ; or whether larger apparatus, or a row of
billiard balls in a trough, shall be employed in the usual
manner, in view of the auditory.

Ail questions of adapting physical apparatus for projection,
should be considered from this practical point of view.

109. Shadow Projections. — The screen method of demon-
stration is not, however, limited to small apparatus ; a great
deal constructed on quite a large scale can be projected on the
screen by a method I first found described by Professor
Dolbear * in connection with the heliostat, and which I have
found, with a very little modification, to be quite efficient with
the lantern. Professor Dolbear condenses the solar beam by
a lens of short focus, and in the rays diverging from this
focus as from a point, the apparatus is placed, casting a very
sharp shadow upon the screen. To show the immense range
of this method, let us take the very simple apparatus illu*-

1 The Art of Projecting. Boston: Lee & Sheppard.

DEMONSTRATIONS OF APPARATUS 217

trating the equilibrium and parallelogram of forces. I pur-
posely select this, as probably almost the last experiment that
it would occur to anyone to demonstrate by this method. As
is well known, we arrange two pulleys H and K, over which
hang weights p and Q, drawing two flexible cords from the
point A, from which depends another weight E by the line A L,
the lines A B and AC measured from any point D on the per-
pendicular by lines D B and D c parallel to the cords, giving
the ratio of the weights P and Q, and A D that to both of the
weight E. Simply using rather thicker flexible cords than
usual, all this will be projected on the screen with the greatest
facility, and on a large scale ; while by supporting in a clamp
a plate of glass just in front, and in the plane, of the cords,
the parallelogram may be actually
traced out with a black brush, appear-
ing on the screen as drawn, and the
law verified by measurements.

It has been said that some little
modification is required to use this
method with the lantern ; but it is
very slight. We merely want to pro- Fm

vide as small and intense a radiant

point as possible. When the radiant in the lantern is an arc-
lamp of moderate power, sufficiently sharp projections will be
obtained from the arc direct, removing all lenses from the
lantern front. An incandescent lime is too large for sharp-
ness, however, and the best method is to place on the front
the little ' pencil ' attachment shown in fig. 95, p. 173, removing
the parallelising concave lens. The chief part of the rays
will then be passed as a conical pencil through one of the
holes on the front, the hole being reduced till the desired
sharpness is obtained.

The range of the lantern is immensely extended by thia
method of projection, which has two advantages. The image
on the screen is not inverted ; and it can be made of any size

2i8 OPTICAL PROJECTION

desired, within wide limits. The heights and other phenomena
of fluids in glass tubes are well shown in this way, and large
diagrams traced rather thickly upon plates of glass or mica,
demonstrate very well. A Sprengel air-pump can be excellently
shown in action by this method ; also large siphons, a full-
sized pyrometer, and many experiments in conduction of heat
which require more length of conductor than readily comes
within the lantern field. ' Cartesian divers ' are excellently
shown in this way. Often there will be a choice of the two
methods. Thus, the equilibrium of liquids in communicating
vessels, the principle of the hydrometer as on page 221, and
other experiments in hydrostatics, may easily be shown either
by apparatus made on a small scale, to come within the field
of the condensers ; or with equal facility by larger apparatus
and this method of shadow projection.

I fear that this simple means of producing screen projec-
tions will be regarded with some scepticism, if not derision,
by those ignorant of it. I can only state that the sharpness
and precision of outline thus attainable will astonish those
who try it for the first time ; and that in all cases where a
silhouette representation of an operation or piece of apparatus
is sufficient, as it is in a vast number of experiments, it is
perfectly satisfactory, and unequalled in its convenience for
dealing with apparatus generally, of the usual size, as it
comes to hand in most physical laboratories.

110. Intermediate Projections. — There is still another
expedient available. It may be desirable to produce a really
focussed image of some experiment or piece of apparatus,
rather than use the shadow method ; and yet the lantern
condenser may be too small. If in such a case we have at
hand a lens six or seven inches diameter, and this gives
sufficient field,1 we can adopt the arrangement shown in

1 I have never tested this method with a lens of larger diameter than that.
It might possibly be carried farther, only there would be an obvious tendency
for the field to be less illuminated at the margin than in the centre. A seven-

DEMONSTRATIONS OF APPARATUS

219

fig. 106. We push in the radiant n so that the rays diverge
considerably from the condenser c (removing the large lantern-
nozzle, if there is one, as such a tube would cut off the
diverging rays), and adjust the large lens L so that it collects
them all, and converges them upon the focussing lens F, which
projects an image upon the screen beyond s. The effect of this,
of course, is to convert the double condenser into a triple one,
with a much larger front to cover the object o. For many
purposes this method of work is very convenient.

If the ordinary condenser is so deep that it will not bear
the radiant close enough for this arrangement, it may be

Fiu. 1U6.— Enlarging the Condenser

better to remove the front lens of the lantern condenser, and
especially if the large lens L be itself pretty deep in curve.
It must of course be deep enough to converge the diverging
rays from c through F when the latter is focussed, and if it
is mounted with a shade round it to stop scattered light, the
effect will be all the better. With large projections like this,
the focussing lens F should be pretty long in focus— say 12 to
18 inches — and itself of a good size and with a marginal shade.
A vast number of projections illustrating mechanical and
molecular physics, resolve themselves simply into projections

inch lens, or even six-inch, will however immensely increase the range of ap-
paratus brought within the field.

220 OPTICAL PROJECTION

of the necessary apparatus, in action, by one or the other of the
above methods. It is both impossible and needless to make any
pretence of a complete list ; but we will run through a few typi-
cal experiments which may serve as specimens or suggestions.

111. Gravity. Atwood's Machine. — Pendulums and their
phenomena are very easily projected by the shadow method ;
and so is the working of an Atwood machine, especially if
constructed with some reference to this use — e.g. with a trans-
parent scale, thickly divided. By a little modification, how-
ever, the law of acceleration may be demonstrated in a very
superior manner. Let the descending weight consist either
of a vibrating metal fork (the sound of which is of no
consequence), or of a rigid pendulum, or of a piece of clock-
work ; either being furnished with a tracing point pressed
lightly against a long perpendicular strip of smoked glass.
The tracer should mark a bold line transversely across the
strip of glass. When now this tracer- weight is made to
descend by an additional weight as usual, its descent will cut
•a series of wave-lines, nearly of the same amplitude as regards
width (quite so if clock-work be employed) ; but the lengths of
the waves will be greater and greater with the accelerated
downward motion, as will be seen upon the screen. The
whole series will project by the shadow method ; or measure-
ments may be taken, and different wave-lengths projected in
the slide-stage of the lantern.

With a small Cavendish apparatus as constructed by Mr.
Vernon Boys, with quartz fibres,1 the direct attraction of small
masses can be easily projected on the screen.

112. Elasticity. — By using a small spiral coil, traction can
be demonstrated ; or a larger coil may be used with the shadow
method. Torsion is projected by the vertical lantern or attach-
ment, using a divided glass dial for the scale. Flexure can be
projected in almost any way, and needs no description.

113. Cohesion. — This may be projected by the method of

1 For full details see Proc. R.S., xlvi. 253.

DEMONSTRATIONS IN PHYSICS

221

PIG. 107

raising a disc attached to a balance from the surface of water,

a small tank and the disc being brought into the field ; or

a pencil of wood may be

shown suspended by the

cohesion of intervening

fluid from the lower surface

of another bar.

114. Centrifugal Force.
Such an apparatus as shown
in fig. 107, or any other of
the usual forms, is very
easily projected, the rotating
portion only being brought
into the field, and the

whirling-table supported as convenient. See also Plateau's
method as on p. 226.

115. Hydrostatics. — For
these projections, one or more
square or cubical glass tanks
should be provided. Using
such a tank about 5 in. cube,
the upward pressure of water
is easily shown in the usual
way by a smooth lead plate
supported against the bottom
of a glass tube. The princi-
ple of Archimedes and any of
its applications are projected
in a similar tank. The ap-
plication of these principles
to specific gravity, hydro-
meters, &c., may be shown by FlG 108
immersing three small test-
tubes, A, B, c (fig. 108), each loaded with the same small
weight of mercury or shot, either in larger tubes, or in three

222 OPTICAL PROJECTION

divisions of a tank, filled respectively with alcohol, water, and
heavy brine. Different fluids may also be projected in layers,
in a test-tube or small tank. These are experiments which,
to avoid any chance of misconception, should be made with
the erecting prism, unless shown on a large scale by the shadow
method. The level of liquids in differently shaped vessels, is
best shown on the large scale : the balance of a small column
by a large one can be shown equally well either way, if a prism
be at hand. For pressure fountains the shadow method must
be used.

116. Waves and Ripples. — The propagation, and cross-
ing by reflection from one end of a trough of waves on the
surface of a liquid, are easily projected by the shadow method
in a trough a few feet long with glass sides. Using the
vertical method, and a horizontal tank like fig. 117 with a glass
bottom, smaller ripples can be shown by touching the surface
with the point of a rod, but the effect is not very satisfactory ;
it is better on the surface of methylated spirit than water.
More distinct ripples are produced by rapid intermittent con-
tact of a pointed wire, which may be produced either by a
multiplying wheel actuating an arm, so as to give a rapid in-
termittent dip to the bent end of a wire attached to the arm,
or by a bent wire attached to the end of a tuning-fork, electri-
cally driven. By the latter, sharp ripples are readily produced
and focussed, and by adjusting the starting point rather to
one side (still better in one of the foci of an elliptical trough),
interferences may be shown.

See also acoustic ripples in Chapter XVII., and by substi-
tuting a vessel of mercury for the funnel of the phoneidoscope
shown in fig. 142, its surface occupying the position of the
soap -film when the funnel is in place, beautiful ripples may
be projected by touching the surface with the point of a wire
vibrated as just described,

117. Capillarity. — Capillary attraction is projected by dip-
ping small tubes of different bores, fixed in a slip of wood, ag

DEMONSTRATIONS IN PHYSICS

22 »

fig. 109, or two small glass plates inclined together and also
fitted at the top in a strip of wood (fig. 110), in a small tank of
water placed on the lantern stage. The tank may be of any

FlO. 109

FIG. 110

convenient shape, and the simple form shown in fig. Ill,
where the glass plates are mounted in a wooden frame to fit
the usual slide-stage, will answer perfectly well. The reverse

FIG. Ill

Fro. 112

phenomena shown between glass and such a fluid as mercury,
are projected by a small apparatus like fig. 112, about double
the size there shown.

224

OPTICAL PROJECTION

A pretty method of exhibiting capillary attraction has been
described by Mr. G. M. Hopkins. A piece of wire-gauze with
the wires about one-tenth of an inch apart is thinly varnished,
and projected as a lantern slide, in any way that leaves the
whole front of the gauze accessible. Then a few differently
coloured fluids of almost any kind — aniline dyes answer very
well — being provided in small saucers, with a clean hair pencil

for each, a drop of fluid is intro-
duced by the point of the brush
into .each separate mesh. This
is easily done so as to exhibit a
geometrical pattern, which when
magnified has a wonderfully fine
effect upon the screen. By
* drawing ' the brush over the
meshes the effect is different,
and will resemble that of Berlin-
wool-work upon a large scale,
the colour always running out to
the edges of exact squares.

Experiments with fluids in
tubes project better if the tubes
can be somewhat flattened whilst
red-hot. The flat sides are of
course turned towards condenser
and screen, the object being to
get more surface and a nearer
approach to one focal plane.

Endosmose is readily projected from the usual apparatus
shown in fig. 113, using a nearly capillary tube, which is pro-
jected, and rather large membranous bag, and staining the
internal fluid. If the tube is sufficiently small in propor-
tion, the rise of the fluid in it will readily be seen in pro-
gress.

118, Surface Tension,— The tension of liquid surfaces isi

FIG. 113

DEMONSTRATIONS IN PHYSICS 225

best shown by projecting flat soap-films in circular rings,1 the
arrangement of the apparatus being shown in detail at p. 827,
where the bands of colour are being projected. At present,
however, we are not dealing with colour, but simply with the
contractile power of the film. Having lifted a film from the
saucer, lay across it a loose thread of fine sewing-silk dipped
in the solution ; this thread will also be projected, and it will
be seen that it may be moved about freely. Prick on either
side of the thread with the end of a small screw of blotting-
paper, and the tensile force on the other side at once draws
the thread into a curve as fig. 114. In the same way a loose
closed loop, pricked in the centre, is at once drawn out into

FIG. 114

a circle by the tension of the film. If the ring is carefully
flattened, and a straight thin wire wetted with the solution
laid across it near one edge, it will adhere ; and if the film be
pricked in the small space between the wire and the edge of
the ring, the tension of the film will pull the wire more or
less towards the other side.

That the contractile force is greater the smaller the
bubble, is projected by blowing two bubbles, one of an inch
and the other of two inches diameter, at the open ends of a
twice-bent tube, furnished with a stop-cock between them,
and with two others which allow communication with separate

i The rings should be of wire about 1 mm. diameter, and 2 to 8 inches
diameter, and should be coated with paraffin, rubbed on them whilst hot,

Q

226

OPTICAL PROJECTION

rubber blowing-tubes to be cut off. The two bubbles being
projected and outer taps closed, on now opening the tap which
places them in communication, the small bubble will expel its
contents into the larger one.

The effect of surface tension on masses of fluid may be
projected in various ways. A small circular vessel being
focussed on the screen, it can easily be shown that it may be
filled with liquid far above the level of the rim. In the
cubical tank, the spherical shape taken by a mass of olive or
castor oil, in a mixture of alcohol and water of the same
specific gravity, may be projected as in fig. 115, direct. Even
Plateau's experiments on the modified forms produced by

rotation of the mass, may be
partially projected in the same
simple manner ; but to show
the actual detachment of a
ring of oil at a given velocity,
the vertical method must be
employed. The manner will
suggest itself, but fig. 116
shows the apparatus as ar-
Flo 115 ranged by Stohrer. The rays

from the condensers fall at

45° upon the reflecting mirror s,, to be directed upwards. In
the field is placed the cubical tank G, with a glass bottom p,
held by the screw ml. By the screw m3 is adjusted the
revolving arrangement, consisting of a perpendicular rotating
wire provided as usual with a small tin disc near the bottom
to carry the sphere of oil, and a small pulley w at the top, by
which this is rotated from the larger pulley w. By the screw
m3 the whole can be adjusted for height and in the centre of
the field; and in Stohrer's apparatus revolution is effected
by a .thread e fastened to the endless cord which embraces
both pulleys, but I myself should prefer a winch-handle.
Highe-r up the pillar v, which is attached to the mirror-box

DEMONSTRATIONS IN PHYSICS

K, is fixed by the screw m2 the
ring E, which carries the focus-
sing objective, bearing at the top
another ring /, carrying the in-
clined mirror s2, which reflects
the image to the screen.1

119. Viscosity. — Very im-
pressive experiments may be pro-
jected with a 2 per cent, solution
of saponine. With care this will
blow a bubble nearly 2 inches
diameter ; the best way is to clip
a small glass funnel f inch dia-
meter mouth downwards in a
universal holder, with a bit of
rubber tube for blowing sprung
on the small end ; this is adjusted

W

1 For details of these experi-
ments and their practical manage-
ments cf. Taylor's Memoir, xiii.
and xxi. ; Phil. Mag., xiv. 1, xvi.
23, xxii. 286. In America, details
may be found in Smithsonian
Reports, 1865, p. 207. Three lec-
tures by Mr. C. V. Boys on Soap
Bubbles (S.P.C.K. 2s. 6cZ.) are
published just as these pages go
to press, and describe other attrac-
tive projection experiments in
this branch of physics.

FIG. 116.— Platen's Experiment

Q2

228 OPTICAL PROJECTION

so that the bubble will be in focus when blown, and the sauce*
of solution is lifted to the funnel when required, and then with-
drawn. If this bubble be pricked, it does not disappear as a soap-
bubble would, but ' hangs in rags ' to the funnel, as it were. And
secondly, if the air be sucked rapidly out from the bubble, it does
not contract smoothly, but hangs in folds like a wet linen bag.
That this is owing to extraordinary surface viscosity can be
shown in a tank like fig. 117 laid on the vertical attachment,
or less perfectly by direct projection in a cubical tank. Tanks
like fig. 117 are made by cementing large metal rings or
cylinders any required depth, on pieces of plate glass. The
tank being supplied with saponine solution, it can first be
shown by projection that a magnet
suspended above the surface is readily
moved about by another magnet.
Being then lowered to the surface,
hardly any movement can be produced.

And being finally lowered below the surface, movement is
free and easy again.

120. Cohesion Figures and Motions. — In such tanks as
fig. 117 all the experiments usually known as ' Tomlinson's
Cohesion Figures ' * are readily projected by the vertical
method ; also the movements due to varying tension caused
by dropping camphor, alcohol, &c., on the surface of water.
The water must be perfectly clean for each new experiment ;
but cleaning or a fresh tank may often be dispensed with by
neatly tossing, as it were, the contents of the tank perpen-
dicularly out of it into a convenient receptacle.

Another class of cohesion figures are produced in an
ordinary slide-tank projected direct, by filling with water or
alcohol, and dropping on the surface, or against one of the
glass sides, with a glass rod, solutions of aniline dyes. Still
another class are produced by providing pairs of pieces of
plate-glass, bevelled towards the inside surfaces, and between

» For detaila see Phil. Mag. 1861.

DEMONSTRATIONS IN PHYSICS 229

which is a film of vaseline or other viscid substance free from
bubbles. This being put into any sort of a slide-frame, the
glasses being kept together by a spring, on introducing a pen-
knife between the bevelled edges and separating them, curious
arborescent figures are projected on the screen.

121. Vortex Rings. — These may be partially projected in
a tank, vertically or otherwise, dropping in milk or aniline
dye. They are best shown, however, by using a vortex-box
in the ordinary way, and sending the rings along a track
illuminated by the full beam from the nozzle of the lantern.
Rings thus illuminated are very impressive.

122. Molecular Strains and Forces, — With the usual
precautions, the breakage of a Rupert's drop or Bologna
flask in a vessel of water is very easily projected ; and the
apparently sudden annihilation of the drop is very curious.

• Crystallisation is easily projected upon plates of glass.
About 3 inches square is a convenient size, clipped at one
corner in a * holder ' and focussed in the condenser field. A
saturated solution of ammonium chloride, or urea, in water,
or of magnesium sulphate in stale beer, crystallise perhaps
most readily.1

123. Gases. — A great many of the phenomena of gases
are readily projected, using the shadow method for larger
apparatus. Apparatus is easily arranged on a small scale for
exhibiting absorption, or the expulsion by heat of occluded
gas. The whole of a manometer can be projected by the
shadow method, or the scale portion only may be projected
with the objective. A barometer, Sprengel pump, apparatus
exhibiting Marriotte's law and its applications (including all
sorts of fountains), are best shown large by the shadow
method. Siphons can be projected either way. The action
of an aspirator is projected in the condenser field with exceed-
ingly good effect. After what has been said, the methods of
arranging such projections will suggest themselves.

1 See also under ' The Projection Microscope,' and ' Polarised Light.

230 OPTICAL PROJECTION

CHAPTER XV

PHYSIOLOGICAL DEMONSTBATION

124. The Projection Microscope, — As regards histological
detail, the most important aid which can be derived from
the lantern is in the form of a projection microscope (see
Chapter XIII.). By furnishing members of a class with very
inexpensive opera-glasses, a far greater amount of detail can
be seen upon the screen, than is possible to the unassisted
eye, which, as pointed out at page 191, fails before the
microscope, and may be unable to perceive detail really upon
the screen. Opera-glasses are thus used in Vienna, and by
their aid a single demonstration not only saves much time,
but enables the lecturer to use a pointer, and combine the
demonstration with a vivd voce explanation, which is found to
be more effective than when demonstration under the table
microscope is divorced from such explanation. The electric
projection microscope, especially, will leave little to be desired
as regards by far the larger portion of the histological demon-
stration to be done.

The simpler vital movements, such as circulation in
the foot of a frog, or cyclosis in the cells of Vallisneria, as
already stated, are also projected easily; even the lime-light
making the latter visible to a class of thirty or forty, and the
arc light showing it clearly in a larger room.

125. Mechanical Slides.— But the lantern itself is capable
of wider application in this class of work. Dealing first with
such simple matters as movable mechanical slides, even
these are capable of being adapted to serious purposes, as may
be shown by the ingenious construction described by Stein
for exhibiting in graphic form the main facts of the circulating
system. Two lanterns, or nozzles, are necessary, which will
be furnished by any form of demonstrator's bi-unial. In one

PHYSIOLOGICAL DEMONSTRATION

231

of these is placed a representation of the heart, as at A (fig.
118), showing the ventricles and auricles, while the moving
slide B, actuated by the winch K, occupies the other. This
latter slide consists first of the foundation design shown in B,
on which is painted in black ground a scheme of the distribu-
tion of blood in the human body, as seen enlarged in fig. 119.
The portion a shows the circulation in the upper extremities,
head, and lungs ; b that of the intestines and lower extremities.

b

FIG. 118. — Mechanical Slide for showing Circulation

The black space 1, 2, 3, 4 in B is filled by the picture of the
heart in A, so that the points shown by 1 , 2, 3,4 coincide ;
thus the two together make up the whole figure 119. The
glass on which B is painted should be very thin. The
movable portion of the slide is shown at C, and consists
of the three toothed wheels a', h, and b', filled with glass.
On a' are drawn many radial lines halfway from circumference
to centre, and on h similar radial lines from the centre half-

232

OPTICAL PROJECTION

way to circumference. These two discs are arranged one on
each side of the upper part a a of the slide B, and are caused
to revolve in opposite directions by a crown wheel working

between them ;
this wheel is
shown at s, with
the rod r1 r1 r1"
connecting it by
the winch /with
the handle K.
The wheel V
simply gears into
a', and has com-
plete waved
radial lines: all
these lines are
cut in black
ground and
coloured red.
The circular mo-
tion of these
lines over the
painted dia-
grams, as will be
seen on reference
to the enlarged
diagram in fig.
119, gives a very
graphic idea of
the circulation as
there depicted.

. 119 126. Physio-

logical Move-
ments. — The simplest optical demonstration of any vital
phenomenon is that of the pulse by a small reflecting mirror

PHYSIOLOGICAL DEMONSTRATION 233

as in fig. 120. A disc of microscopic cover-glass silvered,
about | inch diameter, should have attached to the back two
morsels of beeswax and one of cobbler's wax, and be so placed
that the latter adheres to the spot on the wrist found to give
the best pulse. A parallel beam, or preferably the focussed
parallel beam from a small aperture in the pencil attachment
(fig. 95), is reflected from the mirror A to the screen, and the
reflected ray exhibits every beat by the oscillation of the bright
spot.

This reflection is too simple to exhibit any characteristic
pulse-trace, as taken by the sphygmograph. Some approxi-
mation to it can however be obtained by receiving the reflection
from A upon a tilting or rocking mirror, capable of giving a
motion to the spot >A
of light at right \\
angles to that given
by the pulse-motion.
Then this rocking
motion should be

given, backwardsand FlG 120._Pulse Mirror

forwards, so that the

motion synchronises with the pulse-motion ; when the break
in the curve will be seen very fairly represented on the screen.

It is true that this break can only be followed for one
pulse at a time, and not for many beats together as in a
tracing. But there is a general remark to be made here re-
specting the true place and value in demonstration of direct
projections. Exhibited thus, however roughly, they give a
sense of vivid objective reality which can be imparted to a
class in no other way, to ' tracings ' prepared by any of the
well-known methods, and which should immediately after-
wards be projected upon the screen, using the tracings as
ordinary scientific diagrams. To obtain such slides, either
the revolving blackened cylinder so usual hi tracing apparatus
must be displaced for tracing on the flat, which is most

234 OPTICAL PROJECTION

easily and simply done in ways too obvious to need descrip-
tion ; or films of smoked mica can be bent round the cylinders,
and afterwards unrolled and mounted between glass plates.
To take the case before us : when the break in the curve has
once been shown in actual motion on the screen by the pulse-
mirror, and its method of production explained, it will be
found that the meaning of the curve in any tracings subse-
quently projected will be instantly realised in a degree very
desirable.

This will be still more the case if the process of tracing
be itself projected, which can generally be done, by the ap-

FIG. 121.— Double Recording Apparatus

paratus devised by Marey. The usual means of transmitting
vital movements to the recording style or point need not be
described in detail, as it is well known that the two principal
methods are the electric current, or Marey 's tambour. By
arranging an apparatus as in fig. 121, where the screw d
adjusts upon the main pillar an electric receiving apparatus m
in electrical connection at c, while the screw e adjusts another
recording style actuated at g by the registering tambour n,
whose membrane on the top is actuated by the varying pneu-
matic pressure conveyed into the receiving tambour by a tube
at r, the motion transmitted electrically and pneumatically

PHYSIOLOGICAL DEMONSTRATION 235

may be traced upon the same blackened plate by the styles a
and b ; and if the plate and styles be adjusted in front of the
condensers, and focussed on the screen, the tracings will
appear as they are made. In some cases it may be well to
attach fine wires carrying small black discs to the ends of a
and 6, and to exhibit the motion of these discs upon the
screen, before the clockwork tracing is made.

By employing the delicately-adjustable tambour devised
by Marey for the purpose, as the transmitting instrument in

FIG. 122. — Projection of Heart Movement

a pneumatic system, or two systems, the movement of the
heart may be shown from the outside of the chest upon the
screen, or simultaneously with a pulse -tracing. All that is
really necessary is to adjust the clock-work movement and
the traverse of the tracing point, to the size of the field
covered by the condensers of the lantern. As regards the
projection itself, it is only needful to secure an evenly illu-
minated disc, in the manner described on p. 115, for the focal
plane which is to be occupied by the blackened glass. This

236 OPTICAL PROJECTION

is accomplished by focussing a plate of glass with a few lines
marked upon it, and adjusting the light for this ; and then
making every smoked plate occupy the same position as nearly
as possible, throughout the series of demonstrations.

The movements of the heart have been projected by other
methods. The apparatus shown in fig. 122 is extremely simple.
A heart freshly taken from the living frog or other animal is
embedded in sufficient warmed wax to form a steady base,
and laid on the plate a. Above it extends a light lever, fd,

FIG. 123.— Czermak's Cardioscops

bearing an index-disc, /, at the end, or it may be a style ; this
has a very sensitive joint at d i, which can be adjusted length-
ways over the heart by the screw g, and this and the other
screw g also serve to adjust lever and stand on the pillar m.
For some experiments requiring great delicacy, it may be
advisable to carefully balance the arm fc, or the somewhat
similar arms in fig. 121, so that their weight may not inter-
fere with the motion. Between the heart and the lever at c
is adjusted a small pillar b of elder pith, with a small point at
each ond, which communicates the motion, the lower point

PHYSIOLOGICAL DEMONSTRATION 237

being sunk in the heart. The motions of the heart, which can
be kept beating rhythmically by well-known methods, are
thus readily projected, as shown by the disc on a point at/.

The same apparatus projects the contractile movements of
pieces of muscle. The plate a is removed, and the arm h
arranged above the lever fc d. To a loop on h is fastened one
end of the muscle, and to a thread attached to fed the other
end ; when the contraction of the muscle will affect the lever
as before.

Czermak's projecting Cardioscope employs the direct optical

Fio. 124.— Action of the Cardioscope

method of the reflecting mirror. Small slabs of cork are laid
on the pulsating body, so as to receive the motion without
loss or suppression by weight, and resting upon these are the
horizontal arms of very light levers bent at their pivots some-
what like an L, which communicate the angular motion to
light thin mirrors. Fig. 123 represents such an apparatus
with two slabs and mirrors, one slab being placed on the

238 OPTICAL PROJECTION

ventricle of a heart and the other on the auricle. When the
auricle is compressed the slab sinks, and so does the mirror,
as shown by the dotted lines ; whilst the slab on the ventricle
rises and the other mirror is elevated. The optical arrange-
ments are as in fig. 124. The focussed parallel pencils from
two small apertures on the lantern B fall on the mirrors of
the cardioscope c, and are reflected and focussed on the screen
T. The spots move in harmony, and one of course represents
the systole and the other the diastole of the heart, as the
mirrors rise and fall.

Contraction of Muscle, which it has already been remarked
may be projected by the apparatus shown in fig. 122, may be

also demonstrated on the
screen by the telegraph of
Du Bois-Reymond, the general
scheme of which will be seen
in fig. 125. On the base gg'
the pillar D is fixed, and sup-
ports a pair of forceps A, which
can slide in and out of the
tubular socket or handle B,
FIQ. 125.— Muscle Telegraph being fixed at any length by

the screw s. The other pillar

can be fixed at a movable distance on the base by the screw z.
This pillar carries a little pulley from which projects a radial
arm a1 carrying at the end the index-disc d. Over the pulley
passes a thread, which carries at the end a small capsule or
bucket b for weighting, and at the other end has a hook x.
One 'end of a frog's muscle properly prepared with its nerve
is held in the forceps A, and the hook is passed through the
tendo Achillis, the distances are properly adjusted, and the
bucket weighted with small shot, so that the slightest con-
traction pulls the thread and raises the index-disc in the
direction of the arrow. By binding screws at s' and x wires
can be connected for applying a galvanic current. By this

PHYSIOLOGICAL DEMONSTRATION

239

simple apparatus, making the scale c c' of glass, the different
effects, both in kind, and roughly in degree, of continuous,
making, breaking, or intermittent currents can be pro-
jected.

127. Blood-pressure. — Most experiments of this class can
be demonstrated upon the screen, preparatory to exhibiting
their tracings in the same manner. Fig. 126 is a general
sketch of Ludwig's Kymographion, which makes use of the
mercury or u^ube manometer. The normal level of the
mercury is da. In the blood-vessel cc is tied a y-shaped
end of the short limb of the U-tube, the space between the
blood-vessel and the mercury being filled with a solution of
sodic bicarbonate to prevent coagulation. To the surface of
the mercury in the long limb Ludwig ap-
plied floats, bearing at the top an arm /

carrying a tracing-point which makes the
trace on the blackened plate c as the pres-
sure of the blood depresses the mercury in
one limb to a variable point a' and of course
raises the column in the other tube to b.
We have only to remove the clockwork and
blackened plate, and if desirable the float
and style as well, and place behind the
manometer a black board or card with a slit
cut in it sufficiently large to display the
long tube. The tube and slit being then focussed, the rise
and fall of the mercury will distinctly appear upon the screen,
either inverted, or correctly if an erecting prism is
Fick and Bourdon's instrument, employing a sprii
of the mercury column, may be projected witl

128. Electric Currents. — The currents

as in muscle or nerve — can be demonstp^d^i the
more than one way. In many instances th<
itself into the simple projection of a galYanQ^et'er,
ing galvanometer may be employed ; p\pjecit«i^ wSo if <

j-vj vuo ,-> uj

ft

FlO. 126.— Ludwig's
Kymographion

240 OPTICAL PROJECTION

on another part of the screen, and from the other system of a
demonstrating bi-unial lantern, the preparation itself, in order
to show the arrangements. The time will not, however, be far
distant when every medical school and college will possess an
adequate projecting microscope ; and with such an instrument
these feeble currents may be shown directly by Professor
M'Kendrick's modification of Lippmann's capillary electro-
meter, which can be easily constructed by any person at all
accustomed to scientific manipulation. A piece of narrow
and thin glass tube a & is taken, the ends bent up to form
small cups, and the middle drawn into a very fine capillary
bore. Immersing one end in mercury covered with dilute
sulphuric acid, this is so drawn into the tube that a very
minute portion of dilute acid is brought into the centre at c
between the two columns of mercury, and a platinum wire is

introduced into the mercury at
each end. To secure a sensitive
instrument, perfectly clean glass,

FIQ. i27.-capiiiai7 Electrometer acid, and mercury are necessary ;

and the slightest air-bubble must

be avoided. The instrument is then carefully mounted on a
glass slip as a slide for the microscope, and it will be better
to lay over the capillary portion a piece of thin cover-glass,
and fill the space between, surrounding the tube, with Canada
balsam, which optically abolishes the glass tube, and enables
the thread of mercury with its break of acid to be sharply
focussed. Placing the slide on the stage of the microscope,
and connecting the wires with the nerve or muscle through
non-polarisable electrodes, after the manner of Du Bois-Rey-
mond or otherwise,1 the current will produce a movement,

1 Details of all these arrangements must be sought in some of the numerous
text-books devoted to such matters. I have taken several from Professor
M'Gregor-Kobertson's Elements of Physiological Physics ; but it is no part of
my purpose to describe more than bears upon the projection of them, and
may make that portion of the subject intelligible to those whose special business
ft is to deal with it.

PHYSIOLOGICAL DEMONSTRATION 241

clearly projected on the screen by a power of about 1,200
diameters.

I must content myself with these representative experiments
in a branch of demonstration witli which I can necessarily have
little practical acquaintance ; but they will suffice to indicate
to those who are familiar with the subject, the applicability
to it of the projection method. That method will be found
more and more convenient, by reason of the number who can
simultaneously receive the same instruction, the more it is
practised.

CHAPTER XVI

CHEMISTRY

OP course the only chemical experiments adapted for public
projection are of the qualitative, and most of them of the
' popular ' class. Of that scrupulously accurate quantitative
work which forms so large a part of modern chemistry, the
lantern can take no cognisance. It can only impart a popu-
lar knowledge of those general phenomena which comprise
what the majority of people understand as chemistry; but
even this has its value as a part of general education, and
as giving some intelligent understanding of a great deal that
goes on around us.

129. Tanks. — The major part of such experiments may
be classed under the general head of 'reactions.' Nearly all
of these which give any conspicuous phenomena, are better
shown in the lantern, and with the advantage of employing a
great deal less weight or bulk of material. The apparatus
most frequently required will be some sort of tank, with
parallel glass vertical sides, whose contents will exhibit on a
small scale the desired reaction, and which takes the place

B

242

OPTICAL PROJECTION

in the lantern of an ordinary slide or diagram. As these
tanks frequently require to be washed clean for other experi-
ments, one of the best methods of constructing them is that
shown in fig. 128, where a piece of smooth vulcanised tubing
bent into a semi-circle, is squeezed between two glass plates
by screws through the corners, so as to make a water-tight
vessel easily taken to pieces for cleaning, and unaffected by
most chemicals. Or the plates may be mounted between
outer plates of metal or ebonite, which enables plain or un-
perforated pieces of glass to be used. Pieces of thick solid

rubber cut to shape may also
be used between the plates, but
require more pressure and are
not so safe. If a little more
solidity is desired for the com-
pressed tubing, the latter may
be filled with fine sand after
being screwed up to the proper
tension.

Such tanks may be of any
size and thickness of content,
and a sufficiency should be pro-
vided for the proposed series of
experiments. Or the glass plates

may sometimes be more conveniently introduced into some
such wooden frame as was shown in fig. Ill, p. 223 ; and the
rubber tube then squeezed into place rather tightly, will
readily give a water-tight tank, only the glass and rubber
coming into contact with the fluid.

Occasionally it will be needful to employ tanks made
entirely of glass, cemented together, according to the nature of
the solution. For aqueous solutions, those sold as zoophyte
troughs for microscopic use are convenient, and may be had
of almost any desired size ; but for spirituous solutions marine
glue must be replaced by other cements, such as are used in

FIG. 128.— Tank

CHEMISTRY 243

optical prism bottles. Isinglass with a little acetic acid answers
very well in most cases.

130. Test-tubes. — Very often a small test-tube answers
all purposes as well as any tank, besides being cleaner and
using less material. The rather flattened form of test-tube
sometimes available is, for obvious reasons, better than circu-
lar tubes ; but even in the latter any ordinary reaction can be
exhibited perfectly well.

131. Experiments. — It is perfectly useless to attempt a
list of experiments in detail, but a few representatives may be
given, with one or two practical hints.

In precipitations it will usually be found expedient to
employ solutions more than usually diluted, as the faintest
opacity is projected on the screen with a conspicuousness that
would hardly be credited without trial. Suppose, for instance,
we want to exhibit the precipitation of silver from solution of
its nitrate by hydrochloric acid. It will be found that this is
better shown by a weak solution of one, two, or three grains
per ounce ; and anyone who has never before attempted this
method will be astonished at the apparently dense masses of
cloud which will be formed in such a solution, by a single
drop of the acid introduced at the tip of a glass rod.

In reactions involving liberation of gas the same remarks
apply. Minute bubbles are so magnified, and appear so black
upon the screen, that solutions adapted for giving what would
be called very slight reactions, or often a mere trace, give the
best results. This fact makes the lantern method peculiarly
useful for exhibiting reactions involving the more expensive
chemicals. Such an experiment as the formation of calcium
carbonate by blowing carbonic acid into clear lime-water, is
very much more impressive projected, than when shown in any
other way.

The usual test-reactions are very readily shown, in tanks
of litmus or other test solutions ; but these are better made
pretty strong, in contradiction to the preceding, in order to

244 OPTICAL PROJECTION

give sufficient colour on the screen, which is the chief thing to
be studied, and will depend upon the thickness of the tank :
hence a tank of proportionate thickness enables any strength
of solution to be used which may be suitable to the reaction.
There is also room for ingenuity, in so arranging an experi-
ment as to give the reactions in an attractive or striking manner.
Thus, a tank may be filled with the well-known infusion of
red cabbage, the top of the tank being long enough to extend
all along the diameter of the condensers. Then, if we add a
drop or two of potass solution at one end, of alum solution
in the centre, and hydrochloric acid at the other end, the
purple, green, and crimson colours will appear simultaneously.

The reactions formed by adding potassic ferrocyanide to
salts of iron, copper, bismuth, &c., are very impressive. Most
of them are better shown with dilute solutions than the fore-
going.

Electric reactions are easily shown by filling a tank with
sodium sulphate, coloured with cabbage or litmus, which pro-
jects as a blue solution. If the terminals of a small battery
are then introduced, the acid and alkaline reactions will appear
at the poles.

The action of heat upon salts of cobalt is prettily shown in
the lantern by coating a glass plate with a saturated solution
of the chloride in a solution of gelatine. The rosy tint will
gradually change to a blue in the heat of the lantern. This
may be varied by using with the plate a photographic slide,
which if judiciously chosen will give a curious apparent change
from day to moonlight ; or if a design be sketched with a
weaker solution, the gradual visibility of the so-called sympa^
thetic inks can be readily exhibited.

The main facts of popular ' domestic chemistry ' are readily
demonstrated. Bleaching, for instance, may be illustrated by
filling a tank with a solution of indigo in dilute sulphuric
acid, and projecting the tank upon the screen. By adding a
solution of calcium chloride, the colour will gradually disappear.

CHEMISTRY 245

Antiseptic operations may be illustrated by a tank filled with
sewage water, concentrated and rendered darker, if necessary,
by evaporation. To this may be added a strong solution of
potassic permanganate ; when the gradual clearing of the tank
will be conspicuously shown. The difference between pure
and ' hard ' waters can be demonstrated by showing that the
' hard ' water, on adding certain reagents, gives precipitates
which are not formed in distilled. If the water contains much
lime, a crystal of oxalic acid may be tied to a thread and dipped
into it, when distinct opaque threads of calcium oxalate will
stream from the crystal, not apparent when distilled water is
used.

In all tank projections the same pains should be taken to
get an evenly -illuminated disc, and by the same methods, as
described in Chapter XIV. And any chemical apparatus or
appliances may be readily projected, either by the focussing lens,
or by the shadow method as described in that chapter.

132. Larger and Vertical Projections. — Some experiments
and reactions are required to be shown upon a large scale,
or on a more considerable flat surface of solution as a field.
For such cases Professor Ferguson has given the arrangement
in fig. 129, which enables vertical projections to be made with
scarcely any addition besides the lantern itself, to apparatus
which every chemical demonstrator has constantly at hand.
The diagram almost explains itself. The lantern and radiant
are so arranged that a somewhat diverging beam proceeds
from the final lens, L, of the condenser. This is received
on a mirror, M, adjustable at an angle of 45°, at the end of an
arm sliding, and fixed by a screw, on the stem of a chemical
retort-stand. Above it is arranged a large retort-ring holding a
spherically -shaped glass dish ss, containing the fluid, and above
is adjusted the small vessel containing the reagent, with a drop-
ping-tube. The focussing lens — from the lantern itself, or
otherwise — fits into a ring above at i, and a second smaller
mirror in reflects the rays to the screen. The glass vessel s s

246 OPTICAL PROJECTION

must be chosen so that the under surface may form a lens of
suitable focus to act as its own condenser, and give proper con-
vergence to the cone of rays. On this the excellence of the
projection will chiefly depend, as will be understood from pre-
ceding chapters ; any further
remarks it is to be hoped will
be unnecessary, except that a
sheet of blackened card should
be interposed between the
apparatus and the screen, so
as to protect the latter from
all scattered light. This last
is the only objection to the
arrangement, but is not great
if the condenser beam, mirror,
£c., are carefully adjusted. If
however it be at hand, pro-
jections of this kind are better
shown, and more quickly
adjusted, with a vertical at-
tachment, in cells resembling
fig. 117.

Any demonstrator will be
able from these hints to
elaborate any number of ex-
periments according to his
own requirements. The chief
point is to ascertain by trial
on the screen the most effective
strength for his various solu-
tions. The phenomena of crystallisation, &c., belong to a
previous chapter, and those of electro-decomposition will be
touched upon later.

133. Photography. — The formation, development, and
fixing of a photographic image is rather an attractive lantern

. 129 — Vertical Projection

CHEMISTRY 247

demonstration, and easily carried out by anyone practically ex-
perienced in photography. A developing glass tank must be
provided of proper size, and the plate cut small enough to dip
easily into it. This plate should be a chloride plate and not
a bromide, which is too opaque, while the chloride plate is also
so much less sensitive as to be more manageable. A suitable
negative being provided, and superposed in a proper frame upon
the chloride plate, may be exposed in the parallel beam from the
condensers, time 5 to 15 seconds, according to the power of
the jet. The developing tank is then placed in the stage, with
a plate of ruby glass between it and the condensers, and a
plate of glass with some black diagram being placed in it, in
the position the photographic plate will occupy, is focussed.
This being withdrawn, the plate is taken from the frame and
placed in the tank, upside down of course. The developing
fluid, previously arranged for instant use, is then poured in,
rather a weak preparation being used. Ferrous oxalate is
best, as its own red colour will allow the ruby plate to be then
withdrawn and so allow more light to pass. The image will
then gradually appear, and when of the desired strength,
the plate can be washed and replaced in another tank of fixing
solution, which will show the film gradually getting clear. If
a tank be provided with waste-pipe and tap, the same may be
employed throughout the whole operation.

CHAPTER XVII

SOUND

A LABGE number of experiments in acoustics are of necessity
incapable of projection : those for instance which depend
entirely upon the hearing, cannot be demonstrated by another
sense. But so far as it is desired to show that the phenomena

248

OPTICAL PROJECTION

depend upon motions, and especially rhythmical vibrations,
they are for the most part capable of being thus demonstrated.
In some cases projection is upon the whole less effective
than the direct observation of large apparatus, but may
enable the phenomena to be shown with smaller and less
expensive apparatus : in others there is perhaps little choice :
in others projection is distinctly superior, or may be the only
method practically available.

134. Wave Motion.— The best method of showing the
nature of the motion of the particles of air in a sound-wave,

is to project it by
what is usually
called a Crova's
disc, constructed
as follows. A cir-
cular transparent
plate 13 to 18
inches diameter is
so mounted on an
axle that it can
be rotated with
one of its hori-
zontal radii across
the condenser-
field, and the
radial portion
there focussed as

a slide. The plate is blackened, and in the centre is struck a
circle 6 to 8 mm. diameter, which is divided into twelve divi-
sions, as fig. 181. Taking a scale divided into, say, eighths of
an inch (it may range from one-twelfth to one-eighth accord-
ing to size), take a radius of from 2 to 3 inches, and strike a
circle through the varnish, from one of the divisions on fig. 131,
cutting a pretty bold transparent line (too fine a scratch will
not be bold enough upon the screen). Extend the compasses

FIG. 130.— Orovs's disc

SOUND 249

another division of the scale— be it one-twelfth or one-eighth
— and strike another circle from the next point on the little
divided circle. Extend another division of the scale, and strike
from the next point of the circle in the same direction, and so on
round and round fig. 131, extending radius each time, till the
disc is filled, which will require going round fig. 131 two 01
three times.

In using this disc, when finished and mounted, a cap or
plate is placed on the lantern with a horizontal slit
A B, and inspection will show how the revolution of
the disc with its eccentric circles, over such a slit,
will represent by the motion of bright dots focussed
upon the screen the successive condensations and FIG.ISI
expansions in a series of aerial waves.

135. Bells and Liquids,— The fact that a sounding bell
is in a state of vibration, is best projected by adjusting a thin
goblet in the field of the lantern, with a small pith ball hung
from a wire stand so as just to touch it. On now drawing a
violin bow across the edge of the goblet, the enlarged image
of the ball will be seen to be thrown away by the vibrating
glass.

The thinnest glasses should be chosen for all acoustic ex-
periments. About 3 inches across is the best average size.1
To ensure steadiness, a good plan is to cut off the feet, and
cement the stems into solid blocks about three inches square
and one inch deep, cast in lead or type-metal. This is easily
done with plaster of Paris mixed with a little powdered char-
coal, made up with gum-water. Fixed in such heavy feet, all
the phenomena are more easily obtained.

The nodes of a bell are best shown by the ripples in con-

1 For ripple phenomena next mentioned, some pains should be taken to
get good glasses. At any shop where high-class ware is sold, a certain num-
ber should be selected of various shapes, which are thinnest and seem to ring
most clearly. Then, by experiment before a window, those which give ripples
best and most readily are easily selected. As a rule quite different glasses
will give the best results with different liquids like mercury, water, and alcohol

250

OPTICAL PROJECTION

tained liquid. These may be projected in various ways.
If a thin and pretty shallow bell-glass can be found, about 5
inches diameter, on a short leg or foot, the latter may be
cemented to the centre of a massive plate of glass, and placed
on the vertical attachment. The field lens should be removed
from the latter, as the bell when filled with liquid will form
its own converging lens. The bell being filled with the liquid
selected — water, alcohol, ether, or carbon di-sulphide— the
surface is focussed by a scrap of paper thrown on it and after-
wards removed, and the bell excited by a violin bow, or in

many cases the finger
rubbed with resin
will suffice. In this
case the fluid must
be clear.

But on the whole
reflected images are
preferable. Fig. 132
shows a very simple
arrangement, pro-
vided a ceiling, or
any kind of overhead
screen can be utilised

for the image. Slightly diverging rays are reflected downwards
from the nozzle of the lantern or optical front N, by the plane
mirror A, arranged so as not to scatter the light beyond the
surface of the liquid in the goblet or glass c, and the reflected
rays are focussed (for the surface of the liquid) upon the ceiling
or screen D. Instead of rays from the nozzle N, the parallel
or slightly convergent beam from condensers may be employed.
In this case the best effect is produced by employing water
blackened with writing ink, so that rays reflected from the
surface alone may be utilised. A violin bow is neither neces-
sary nor desirable, being far too energetic ; the resined fore-
finger dipped in the water is ample, and the nodes will follow

FIG. 132

SOUND 251

Its motions round the circle. With a glass carefully selected,
very fine effects are obtainable in this way. With a suitable
glass, circular ripples of quite another character, but of great
beauty, may often be obtained by pressing the finger very hard
upon the rim while drawn round sloiuly, so as to produce a
* groaning ' kind of note. And with somo glasses (see note
on page 241)) very similar ripples to these last, but more
evanescent, may be produced by tapping some particular point
on the goblet or its stem with a metal rod.

For a perpendicular screen, the easiest arrangement is the
apparatus shown in fig. 142. The funnel and its frame are
withdrawn, and the glass in its square metal foot substituted,
arranged for height so that the surface of the fluid occupies
the position usually taken by the phoneidoscopic film.

The vibrations in mercury, with a suitable glass, are very
fine when projected by this method. For this heavy liquid a
bow is sometimes advisable, and only certain glasses will
vibrate readily when so heavily weighted.

136. Trevelyan's Rocker,— The vibrations producing the
note of this apparatus are easily projected by cementing a
small mirror on the face — it should not however be too small.
A small parallel pencil of light is focussed on the screen,
either from such an apparatus as was shown in fig. 95, or
the rays being made parallel from the condensers, a small
aperture may be placed in the stage of the optical front, and
focussed by the lenses of that arrangement. The rocker may
then be arranged so that the mirror meets the pencil at an
angle of nearly 45°, throwing the pencil upwards, whence it is
re-reflected by the adjustable plane mirror to the screen. The
motionless spot will, when the rocker is in action, be drawn
out into a line of light ; and by turning the plane mirror side-
ways, this will further be analysed into a sinuous line, making
the individual vibrations visible.

187. Vibrating Rods.— All the leading phenomena of
these are easily projected, konig's apparatus for showing

252 OPTICAL PROJECTION

longitudinal vibrations in a metal rod fixed at its centre, by
the repulsion of an ivory ball suspended so as to touch the end,
may either be projected entire by the shadow method, or the
end of the rod and the ball only may be focussed in the field
of the condensers. Another method is to show the action of a
glass strip whilst under strain, upon polarised light, as described
in § 209, page 362.

The transverse vibrations of rods are easily projected by
their shadows alone. For all rod experiments, I have found
it most convenient to screw a small parallel vice upon a piece
of blackened board, which by a slot and thumb-screw can be
adjusted at any height upon another piece of blackened wood
furnished with a foot. In such a vice, rods can be fixed at
any height, or angle, whether to project their shadows, or
beads or mirrors on their ends. A slender knitting-needle
projecting 3 to 4 inches will give a very good fan-like shadow
when ' sprung ' in the field of the condensers and focussed ; or
the shadow method will equally answer for larger rods.

Nodes and segments are well illustrated by a long piece —
say two feet — of very thin steel, such as clock- spring, slightly
loaded at the tip. This being very flexible will show divisions
readily when tapped and ' stopped ' at proper intervals ; and
the motion being slow, is easily followed on the screen when
projected by the shadow method.

But the superposition of harmonic vibrations upon the
fundamental is best projected by Wheatstone's kaleidophonic
apparatus. A straight steel wire about 1 mm. or less in
diameter, and projecting 8 or 10 inches from the vice, in a
nearly horizontal direction, I have found best. To the top
is attached a silvered bead about £ to f-inch diameter, by
Prout's elastic glue or otherwise.1 The bead is adjusted to

1 The larger the bead, the larger the spot ; but I find this size gives the
best results. As I have had some trouble in finding suitable beads, it may bo
well to state that they can generally be obtained of theatrical costumiers, such
as are found round Covent Garden. Enough of the end of the rod to go

SOUXD 253

the height of the optic axis, and the rays from the condensers
slightly converged so as to be condensed into the space that
will be occupied by the transverse vibrations of the bead,
which must never leave the illuminated area, but otherwise
the more light is condensed upon it the better ; an area 2
inches diameter is about the thing. The beam may be
managed in several ways. The lantern may be deflected
parallel with the screen, or a little more, so that the rays fall
nearly at right angles upon the rod, which must always be
pointed to the screen ; or the light may be thrown back
nearly straight towards the end of the rod by the plane
mirror ; the object focussed upon the screen with the loose
lens being the luminous spot reflected by the spherical bead,
this will be equally bright, and the apparent motion the same,
either way. The surplus light should, however, be stopped by
a black screen if convenient. The rod may now be bowed
near its base with a violin bow well resined, or struck at
various points with a wooden stick ; and by either method all
possible varieties of beautiful harmonic scrolls will appear
rippling over the screen in lines of light.

138. Tuning-Forks.— The vibration of a common c 256 fork
is easily projected in shadow upon the screen. Or it can be
impressively shown by focussing in the field of the lantern a
small glass filled with water to the brim. On striking the
fork, and touching the surface of the water with both vibrating
prongs, water will be splashed off in both directions, pro-
ducing much more effect on the screen than would be
supposed.

Employing sympathetic vibrations excited by another fork
in unison, the vibration of a fork can be projected by the
simple arrangement shown in fig. 133, consisting of a rect-
angular wooden framework on which is reversed (i.e. the fork

through the bead, should be heated in a Bunsen burner, and bent at right
angles, and the bead fastened on this portion with the glue. This applies to
all the rods subsequently mentioned.

254

OPTICAL PROJECTION

downwards) the resonance -box of a fork mounted in the usual
manner. The top of the frame has an aperture to allow the
fork to come through. A smooth spherical ball of pith or
light cork, varnished, is hung as in the figure from an eye at
the side, and the frame adjusted so that the ball barely touches
the fork, which is focussed on the screen. On now sounding
another fork in unison, also mounted on a resonance-box, the
fork, focussed will be thrown into vibration, and the ball will
be driven away as shown on the screen. (For conditions of
success see next paragraph.)

139. Doppler's Principle. — By the same simple means

Doppler's principle
or law (that the
colour in light, or
the pitch in sound,
depending solely on
the frequency of the
periodical impulses
falling upon eye or
ear, pitch is height-
ened when a vibrat-
ing body approaches,
and falls when it
recedes) can be de-
monstrated, pro-
vided the necessary
conditions of ac-
curacy be attended
to. These have been
well described by
Professor A. M.
Mayer.1 He found

c forks of 256 vibrations the most suitable, and provided two in

exact unison, a third fork of 254 giving two inferior beats, and

1 See American Journal of Science, April 1872.

FNJ. 133

SOUND 255

a fourth of 258 giving two superior beats. The essential point
is exactness in the tuning of these forks and their cases. It was
found quite insufficient to tune two forks in apparent unison,
as one has the power to some extent of coercing the other to
its own note. The only effectual plan was found to be, tuning
both forks so as to give the same number of beats per minute
with another slightly different, which can easily be done. The
boxes can of course be tuned by measurement. With forks
thus tuned, and turning the open ends of the cases towards
each other, one fork will excite the other and throw the ball
off, when as much as GO feet apart. The ball should be 5 or
6 mm. in diameter, smoothly fashioned out of cork, and
varnished, which gives it more ' life.' It is suspended by a
fine silk thread, and pains taken to suspend it accurately, so
that however it may revolve by torsion, it may hang nicely
against the fork. The frame should also be fixed steadily.

Nothing more need be said as to the effect of unisonal
vibrations ; and the illustrations of Doppler's principle will
almost suggest themselves.

a. Taking the unisonal fork, and walking steadily up with
it from a distance (Prof. Mayer detached the fork from case,
striking it, and then placing it on case for the resonance ; but
it may be more convenient to keep it on the case, and excite
with a bow), the approach is sufficient to so destroy unison
that the fork in the lantern does not respond until the demon-
strator stops. The same in receding.

b. Taking the 254 fork and exciting at some distance, there
is no response. There is still none when slowly walking.
But on swinging the fork and case towards the lantern with
approximately the right velocity — between 8 and 9 feet per
second for 2 beats difference — the fork responds and the ball
flies off.

c. A backward swing produces the same effects with the
258 fork.

140. Lissajous' Method. — Ever since it was devised by

256

OPTICAL PROJECTION

that ingenious physicist, the optical method of M. Lissajous
for observing the vibrations of a fork has been preferred
where possible. The simplest arrangement is shown in
fig. 134, where a small aperture is placed in the stage of the
optical front, and the rays from the condensers being made
parallel, the aperture is focussed on the screen, giving a very
nearly parallel pencil of light ending in a sharp spot. This
proceeds from the nozzle N, and is reflected from a small
mirror A attached to a prong of the fork, the other prong being
similarly loaded. The reflected ray is re-reflected back to the
screen by the plane -mirror B, when the spot should be re-

focussed to allow for
the extra distance
from A to B, which
blurs the original
image (this re-fo-
cussing, or else an
allowance for extra
distance caused by
reflections, should
always be attended
to in this class of
experiments). On
now exciting the

fork, the spot is drawn out into a line of light ; and if
then the plane-mirror B be slightly turned in its socket
(which is better than holding it in the hand) the line is
drawn out into ripples CD, showing the single vibrations
separately.

Very fair ripples may be projected in this way. But they
will be much more brilliant if the pencil attachment (fig, 95)
be employed, whereby much more light is sent through a
small aperture on the front. This aperture is then focussed
by a loose lens. But the best use of this focussing lens
demands some consideration, according to the circumstances,

FIG. 134.— Lissajons' Method

SOUND 257

and still more so in projecting compound figures to be pre-
sently described.

141. Management of Acoustic Pencils of Rays.— -When
the light is brilliant and the pencil nearly parallel before
focussing, as from an electric lantern, it is generally more con-
venient to first focus the spot on the screen, and then adjust
the forks or other vibrating apparatus beyond the lens, or
between it and the screen. There will be abundance of light,
more room for adjusting apparatus, and the figure will be
projected of the greatest size, which with forks is desirable.

But in projecting reeds as presently described, the greatest
size may be undesirable ; or light may be deficient. In the
latter case, more light can be passed through a small aperture
by somewhat converging the rays upon it, which diverge
again. Suppose we use a pencil thus diverging somewhat
from a £ inch aperture (indeed there is always considerable
' scattering ' when any but the arc-light is employed) and we
are employing mirrors f inch diameter, which is a good
average size. It is possible to bring the mirror of the first
reed up near the aperture, so that it receives the entire pencil,
and even the second mirror loses little of it ; and even if the
plane-mirror has to be used in addition to ' analyse ' the
result — as in showing ' beats ' — there will be room for all this
within the conjugate focus of a suitable lens, let us suppose
one of 6 in. diameter and 14 to 18 inches focus. The focus-
sing lens is thus placed beyond the apparatus, and next the
screen. In such a case the result may be more brilliant thug
arranged.

But there is a point to be remembered. The reflected pen-
cils diverge by the vibration of the mirrors ; and the focussing
Inns acts upon these divergencies the same as upon others.
The effect is to reduce the size of the figure on the screen ;
which may be either an advantage, or disadvantage. If the
lens focussed the actual vibrating mirror, there would be no
figure at all, focussed in this way ; but inasmuch as the lens

258 OPTICAL PROJECTION

focusses the aperture, some way behind the mirrors, the effect
on the size of the figure is intermediate, and variable. The
figure is least converged or shrunk when the vibrating
mirrors are as far as possible from the aperture and nearest
the lens ; it is smallest under the contrary conditions. It is
also less reduced, the longer the focus of the lens.

It will be seen that this choice of method gives a large
amount of control over the scale of the figure. It will also
easily appear, that for the second method a large lens of long
focus is best ; but that for the first, a smaller lens of moderate
focus is better, as it converges the pencil upon the mirrors
before the rays have scattered so much.

142. Lissajous' Figures. — The compound figures produced
by two sets of vibrations have always had a strong fascination
for both demonstrators and students ; but they are seldom
seen well projected except with large and expensive apparatus.
I therefore give some space to simpler and inexpensive devices,
while a word or two may suffice for the more elaborate.

Two large electrically-mounted forks, one with adjustable
weights for varying the pitch, and both furnished with
mirrors, will of course produce all the phenomena with
facility, and need no explanation.

Plain forks are best made large, with prongs not less than
12 inches long and f to 1 inch wide. They can be made
inexpensively by bending up bars of rolled steel, any precise
note being immaterial. One should have two hollow sliding
blocks of metal with set-screws, to adjust for various intervals,
and these must both be of the same weight. As mirrors,
silvered circles of micro-glass may be used, balanced of course
on the other prong, unless similar mirrors are attached to
both. The most convenient method of mounting such plain
forks, so as to be excited by a violoncello bow (the best
method) is the simple wooden frame adopted by Prof.
Weinhold (fig. 185). The rectangular frame BE is slotted on
the top and side, to adjust and fit by the screws b b the blocks

SOUND

259

in which the forks are inserted. The course of the incident
pencil oa between the mirrors s{ and s2 will be seen from the
diagram, any adjustment being easily made before tightening
the screws, by either turning the perpendicular fork a little
on its axis, or moving the horizontal one in its slot. Another
block and slot k is provided for using both forks perpen-

FiG. 135.— Mounting for Two Forks

dicularly, the block k taking one out of the line of the other.
It will be seen that with this arrangement both forks can be
readily bowed by assistants, and will be firmly held.

A very simple apparatus is shown in fig. 186, as made by
Miller, of Innsbruck, from a design by Prof. Pfaundler. Two
flat steel springs, P F, are mounted in a manner that needs no
further explanation than the diagram, one being adjustable in
position, and having a sliding weight. Both are furnished with
mirrors. The springs may be bowefl, or are readily set in motion

82

260

OPTICAL PROJECTION

by twitching with the finger. As far as regards the projection,
this is cheap and efficient, but is not audible ; it has the

advantage, however, of
producing the figure by
motions of the same
kind as a couple of forks.
For projecting the
figure only, the ap-
paratus shown in fig.
137 has been devised
by Pfaundler, who de-
scribed it in the ' Pro-
ceedings ' of the Vienna
Academy (of Sciences)
and figures it in his
* Lehrbuch der Physik.'
Fl0-136 The vibration of two

springs s, s', set rect-
angularly to each other, one variable by a weight, L, is given
to two discs, D D', with slots which also cross at right angles.

The spot of light
where they cross,
evidently moves
in a path com-
pounded of the
two motions.
The discs are
arranged in front
of the condensers,
the spot being
focussed as an
ordinary dia-
gram, and the springs twitched. Stohrer has constructed a
more complicated apparatus for giving the same motions to
two discs by a wheel and gearing, methods of which will

FIG. 137

SOUND

261

suggest themselves ; and Miller of Innsbruck has constructed
a machine in which two discs are rotated, partially overlap-
ping each other, each with a white circle cut excentrically in
black varnish, the idea being essentially the same. The
principle of the harmonograph has also been applied to trace
the figures by two pendulums on a smoked glass in the stage
of the lantern, and such an instrument can be obtained of any
optician.

Coming back to vibrating mirrors and a pencil of light,
Professor Dolbear describes an arrangement he has adapted
to a horizontal whirling-table, in which two friction- wheels
rolling on opposite edges of the disc, by cranks fitted to
them, actuate two mirrors
mounted on rocking-pivots,
over opposite sides of the re-
volving table.

Mr. G. M. Hopkins, of a,
New York, uses two mirrors,
each of which is mounted as
at M on the middle part of two
parallel strained wires, A B,
in the manner first used for
other purposes by Professor
0. Rood. Two such wires
will impart to a mirror good

vibration, if properly strained, and the period may be varied
by attaching another wire, a b, with loads at the ends w w
(fig. 138) to the back of the mirrors, so that it can be varied
in angle (as c d), such an alteration altering the period of
vibration. One pair of wires is of course strained vertically
and the other pair horizontally, in parallel planes. Such an
apparatus is easily constructed ; but it is very difficult to get
exact ratios with it, and difficult to arrange for the pencil oi
light to ' clear ' both pairs of wires.

But by far the best, most effective, and most comprehensive

262 OPTICAL PROJECTION

apparatus is one of reeds mounted with mirrors, as shown in
fig. 139, which I was able to arrange for Messrs. Newton
& Co. with the help of Mr. G. Neilson in the speaking part
of the apparatus, it being particularly difficult to make
any range of reed-notes vibrate on a closed box of conve-

PlG. 139. — Lissajous' Apparatus of Reeds

nient size.1 I at last partially overcame the difficulty
by supplying the boxes with wind through short rubber
tubes of large diameter, from large intermediate wind-chests

1 I feel bound to state that the idea of this construction was not mine. It
was suggested to me entirely by seeing such a reed apparatus named, in a
catalogue of the apparatus to be sold of Mr. Thomas Harrison of Manchester.
Mr. Harrison having left England, I could learn nothing whatever about the
arrangement, except that it had been constructed by Dr. Mann, to whom there-
fore the first design of such is due. Long after, when my own was completed,
and much time and experiment de novo had overcome the chief difficulty by
quite different means, I succeeded in discovering a description of the ap-
paratus of Dr. James Dixon Mann in Proc. Manchester Lit. and Phil. Soc.
xvii. 91. It was not, however, capable of exhibiting beats, or other scroll
figures ; one reed-box being fixed horizontally.

SOUND 263

which form the base of the apparatus ; after which Mr.
Neilson succeeded in bringing the speaking-boxes into small
and compact form.1 The wind is supplied to these chests
(two divisions of one box) by a Y-tube with a stopcock on
each fork, the trunk of the Y being connected with the
bellows. Each short rubber tube is furnished with a pinch-
cock. The reeds are severally mounted upon identical
bevelled wooden slides, so that any note slides into dovetails,
and forms for the time the front of its box ; and each is
mounted with a mirror of silvered glass, f-inch diameter,
attached to its free end by a small pillar of cork. After being
fitted with mirrors (which load them), the reeds are fairly
tuned — excessive accuracy is not required (see g hereafter).
The reed-boxes are adjustable round vertical axes coincident
with the vertical diameters of the mirrors, whether the boxes
are in the horizontal or perpendicular position. One retains
a perpendicular position; the other can either be similarly
placed, or fixed in a horizontal position rectangularly to it,
being clamped in either by a screw.

As regards the action of the apparatus, if it be confined
solely to Lissajous' figures, the pencil of light might be
reflected direct to the screen from the second mirror, as from
a pair of forks. But being desirous of projecting open
* scrolls ' also, after Tyndall's method, and especially in the
case of ' beats,' I adopted the arrangement shown in plan in
fig. 140, Omitting all details of focussing, the pencil of light
from the lantern L is reflected from the mirror on the first
reed-box, E, to that on the second, E E, and is thence reflected

1 Dr. Mann's arrangement was quite different. He used much larger
boxes or speaking-chambers, with open apertures for the supply of wind. The
supply-tube came direct from the bellows, with a nozzle at the end contracted
to about one-third the size of the hole in the reed-box, and ending with a free
space of half an inch between this nozzle and the hole in the box. Had I
found his arrangement described earlier, I should probably have adopted both
it, and the ingenious inventor's conclusion that it was indispensable ; as it is
I prefer (perhaps naturally) the smaller boxes as more easily adjusted, and the
closed supply as more certain and using less wind.

264 OPTICAL PROJECTION

to the plane-mirror M, which can be revolved on the perpen-
dicular pivot P. Thence it is reflected in the direction of the
screen at s, and it will be manifest that a slight revolution of
M opens out the scroll.

This arrangement also enables us to combine any two sets
of vibrations in harmonic addition, as well as rectangularly.
Eectangularly an octave gives the well-known 8-figure ; but if
both reed-boxes are fixed perpendicularly and a scroll opened
out by rotating the mirror M, the harmonic combination is ex-
ceedingly instructive, as is also the optical representation of
any slight departure from unison, or any other intervals.

RR

FIG. 140.— Plan of Apparatus

Such an apparatus is far the best for the projection of
compound figures, being superior to the most expensive forks
in many respects. For its efficiency, it is also far the cheapest.
It possesses the following advantages, which are not found in
combination in any other apparatus so far as I know.

(a] It projects with ease all compound figures and scrolls.
To project beats, all that is necessary is to fix the reed-box
EE (fig. 140), perpendicularly, the same as the other, insert
a second reed in unison with E, and having tuned (see g below)
rotate the mirror M, to give the scroll. When scrolls are not
wanted the mirror M is simply left unmoved.

SOUND 265

(b) All the notes are audible ; more so than with forks.

(c) Any note within the speaking range of the boxes can
be added at any time for a few shillings.

(d) Any interval can be changed for any other (so far as
notes are provided) in half-a-dozen seconds. A complete set
would comprise a lower c (for tenths and twelfths), two c's (for
unisons and beats), and the diatonic scale from c to c.1 A
convenient * small ' set would be the four notes of the common
chord, with a duplicate of the lower c. This will give unison
and beats, third, fourth, fifth, and octave.2

(e) There is absolutely no trouble in manipulation. Any-
one blowing the bellows, and having adjusted the pencil of
rays once for all, the demonstrator has only to substitute
notes as required, and manipulate a pinch-cock. If the pencil
is awkward to manage, a black card with a circular aperture
may be interposed between the first mirror and lantern,
to confine it within bounds. The mirrors are so near
each other, that little light is lost, and the projection is
brilliant.

(/) The angular motion being great, the figure is on a
large and bold scale.

(g) Most important of all : the notes can be tuned in
operation, with the greatest nicety. There is no tiresome
tuning, or loading with wax. The pitch varies with the
wind-pressure^ within more than sufficient limits, and the
hand on one or other of the pinch-cocks on the tubes will
either keep any phase of the figure ' steady,' or cause it to
pass through the transitional forms with any rapidity
desired.

With this apparatus, Lissajous' figures become a delight-
fully easy and effective projection.

1 For audibility, however, a range of notes from G to G is better than C to
c, and this scale was therefore adopted.

2 We have some hopes of still further simplifying the apparatus by making
one of the reeds variable in pitch by a rack-work ; but the experiments on
that point are not concluded at the date of this.

266 OPTICAL PROJECTION

143. Kaleidophonic Figures. — The usual Lissajous' fig-
ares, but not beats, can be projected by Wheatstone'a
kaleidophonic method. There are three ways of preparing
rods in order to produce them. The most certain, and the
best where frequent demonstrations are in view, is to prepare
rectangular steel rods, which vibrate in the requisite propor-
tions. Square steel about 2mm. square is always procurable,
as also is rectangular steel about TV x J inch in section.
By filing down these and mounting each with a bead, all the
intervals can readily be obtained. Secondly, a piece of thin
round rod may have several inches at the top end bent
back into a loop, like a lady's hair-pin. Cementing a bead
on to the bend at the tip, by screwing in the vice so that
variable lengths of the longer stem project, various figures
will be obtained ; but they are not so true as by the previous
and following method. Thirdly, a straight piece of rathe:
thick clock-spring or similar steel is prepared, so that about
six inches at the tip may have its section at right angles
to the remainder, which should be some sixteen inches long.
This may be done by cutting the steel in two, and making a
saw-cut half an inch down the middle of the long piece, into
which notch the other piece fits and is brazed ; or the steel
may be heated red-hot in a Bunsen burner just at that point,
and twisted sharply round at right angles with a pair of
pliers. The top end should be somewhat tapered, and tipped
with either a bead, or a segment of a hollow sphere of bur-
nished aluminium, which will give bolder projections. By
nipping the longer stem in the vice at various points, which
can be marked, all the intervals can be produced ; either with
a bow, or by striking the rod, they may be shown either with
or without ha,rmonic vibrations superposed.

144. Vibrations of Strings.— For showing the various
loops described by a string at different points, there is no
method to equal Tyndall's, of employing a thin ribbon of
planished silver or other metal — aluminium would now be the

SOUND 267

best — and illuminating the whole by the lantern beam sent
along the string at a small angle. For a multitude of figures
a very thin and narrow ribbon rather closely twisted will be
best ; for fewer and more distinct figures, a broader one with
only a few twists. A large fork is almost indispensable for
such experiments, but may be quite a rough one, as before
described. An electrical fork is of course much the best and
most convenient, and may be obtained for about 51.

Very good figures may however be also displayed by
employing the covered string of a violoncello, on which
silvered beads are strung, mounted on a sonometer and
vibrated with a bow. (Covered strings give much larger
figures, not being so tightly strained.) Any single bead can
be projected ; and I have found it quite possible to project in
this way the figure described by the G string of a violin ; but
the effect is hardly worth the time taken in arranging the
experiment.

145. Harmonics, and Melde's Experiments. — These ex-
periments are certainly best shown direct, on a large scale,
with an electrical fork, and using strings either steeped in
fluorescent solution, in violet light from the lantern, or what
is still better, painted with Balmain's phosphorescent paint.
Or the shadow method may be employed, when the arcs and
nodes will be enlarged. But it may be useful to know that
they can be demonstrated by projection with quite small and
inexpensive apparatus, using a common c fork of 256 vibra-
tions, or the G above it, and a cord of fine sewing-silk only 6
inches long from point to point, exciting the fork with a
bow. I have found the best practical limits of weight to be
between 10 and 80 grains for this small scale. A much more
convenient apparatus could easily be constructed in which,
instead of using weights, the other end of the silk should be
attached to a helical spring, the tension of which could be
varied as required by a lever. Quite a small fork — say c 256 —
could even be fitted in this way to act electrically, bringing

268 OPTICAL PROJECTION

all within a few inches compass, upon one base, at a very
small expense.

146. Vibrations in Air, Membranes, and Plates. — Any
membrane which is pretty transparent, vibrated in any
manner so as to show the nodal lines by powder strewn on
its surface, can be projected in the vertical attachment or
lantern. Goldbeaters' skin is clear, but the wrinkles in it
project as lines, and on the whole, varnished vegetable parch-
ment answers best ; unless it be a very thin film of mica,
which is flat, thin, quite transparent, and best of all.

The vibration of a membrane covering a capsule into
which sound is conveyed by a tube, may be shown in several
ways. The simplest is to adjust the capsule so that the
membrane stands in a vertical plane, and to hang against it
a light pith ball. The ball will be thrown away when the
membrane vibrates, and this can easily be projected.

A more elegant method is what Professor Dolbear has
called the opeidoscope. The membrane may be stretched
over one end of a mere pasteboard tube, into the other end of
which notes are spoken or sung. To its centre is cemented a
small mirror of silvered micro-glass. This receives a pencil
of rays from the lantern, which after reflection is focussed as
a spot upon the screen as usual. Upon singing into the tube
the spot will describe a figure, which may possibly take any of
the Lissajous forms, though this is rare except as to the circle
or oval. Sometimes the mirror will not happen to be placed
' happily ' on the membrane to show figures ; in that case,
pressure near the edge of the membrane with the end of a
knife, will probably so shift the nodes and vibrating portions
as to remedy this. For my own use, I have devised an
instrument which is very handy in such experiments. A
brass tube about 4 inches long and 2 inches diameter is
tapered to an inch at one end to receive strained over it a
rubber speaking-tube, at the other end of which is a mouth-
piece ; and to its centre is attached a stud, by which it is fixed

SOUND

269

horizontally in one of the sockets of a pillar-stand, so as to be
adjusted at the height of the optic axis of the lantern. On
the large open end, fit any of several short brass tubes, over
which are strained membranes of very thin rubber, held by
elastic bands catching in grooves cut round the ends of
these short tubes. On these membranes mirrors are cemented ;
and by stretching the rubber a little one side or the other, or
by changing one of these caps for another, a good position is
sure to be readily obtained. This is a very simple, easy, and
elegant experiment.

147. Manometric Flames.— Another method is by Konig's
manometric flames, which are projected easily, and better seen
in some respects than by direct
vision. They can be projected even
by the simple apparatus usually sold,
with capsule and revolving mirror
upon one base-board ; in which case
the focussing lens must be of long
focus, and stand between the re-
volving mirror and the screen. The
lantern itself has no place in this
experiment, the flame only being
focussed ; and it is necessary that
this be enclosed in an opaque
chimney of some kind, only open
towards the mirror, in order that
the screen and room may be as
dark as possible. I have got fair
results this way ; and if more bril-
liance be required, it is easily ob-
tained by ' enriching ' the gas,

passing it through benzoline, or over naphthalene heated by
a Bunsen burner in the albo- carbon manner. The flame will
be very greatly brightened by this expedient.

But it is very much better to have the capsule and jet

FIG. 141

270 OPTICAL PROJECTION

mounted independently as in fig. 141, KK being the capsule,
a the sound tube, b the gas-tube, and c the flame. This must
be shaded in all but the one direction as before, and the
focussing lens brought next to it, whilst the revolving mirror
comes on the other side of the lens. The flame is thus
shielded from the draught of the revolving mirror, and this
latter receives the rays in a more favourable manner. The
gain in brilliance is very marked, and with ' enriched ' gas
very fine projections will be obtained.

With a revolving mirror of double the usual height —
in fact sufficiently tall to embrace the usual three flames on
an organ-pipe for showing nodes, the projection method has
an advantage over the direct. Using a focussing lens for each
flame, '«ihe lenses may be easily adjusted to somewhat converge
the images, so that they may be brought as close together upon
the screen for comparison as may be desired.

A concave spherical mirror of about three feet focus,
rotated on a vertical axis, will also give detached images,
but the revolving mirror is cheaper and more convenient.
The best and most brilliant method of all, however, is that
devised by Mr. C. V. Boys, who projects such phenomena
through a circle of lenses fixed round the edge of a revolving
disc.

Sensitive flames are easily projected in the same way with
enriched gas.

Tyndall's sensitive smoke-jets are very effective projected
by the shad6w method — much more so than viewed direct,
the smoke-shadows being very distinct upon the screen.

148. The Phoneidoscope. — Mr. Sedley Taylor's phoneido-
scopic figures may be projected in various ways. A soap-film
may be taken up even on the end of a lamp-chimney, and if
arranged so as to be projected, will show figures under the
influence of sonorous vibration. Or the ordinary phoneidoscope
may be used, reflecting a converging beam from the condensers
down upon the film by the plane mirror, and focussing upon

SOUND

271

the ceiling or an overhead screen. But a better and more
complete apparatus is one which Mr. C. Darker arranged for
me, as shown in fig. 142. The figure almost speaks for itself.
The plates, with apertures of various shapes which carry the
films, and which before use are blackened, heated, and then
coated thinly with solid paraffin, are laid upon the mouth of
a funnel, whose
narrow end has
an elbow over
which is strained
the speaking- tube,
the whole being
arranged in a sim-
ple frame which
can be inserted in,
or removed from,
a sort of box open
at one side. The
edge of the funnel
is pierced with
apertures to allow
air to escape from
under the film.
By slots in a board
at the back,two'in-
clined plane mir-
rors are adjust-
able in position,

and can also be turned on axes. The nearly parallel beam
from the condensers, falling on the first mirror, is reflected
down upon the film ; from this it is reflected again to the
second mirror, and thence reflected out in a horizontal direc-
tion, where is adjusted the focussing lens, which focusses the
film upon the screen. Thus the whole works direct towards the
ordinary screen, and can all be adjusted before a lecture, which

FI3L 142.— Lantern Phoneidoscope

272

OPTICAL PROJECTION

is a great advantage. Sometimes a black card or other screen
may need to be adjusted at the side next the screen, to prevent
any but the reflected rays which form the image from passing
the apparatus ; this will suggest itself. The mouth- piece
I have found it advisable to construct as in the section
(fig. 143), with a membrane of thin india-rubber across it.
TKis does not interfere with the true sound vibrations,
while it prevents the film from being prematurely rup-
tured by any actual blast of air which might result from
unskilful management of the voice. Recipes for the solution
will be found in Chapter XX., and a good film
will often last a quarter of an hour. Besides
the usual apertures, a hexagonal one should be
provided ; for these about two inches in diameter
is a good size. The beautiful figures, and the
gradual change in colour, make this one of the
most fascinating of all lantern experiments.

Withdrawing the funnel and laying larger
plates with apertures and films upon the box
itself, beautiful figures may also be obtained
by exciting a tuning-fork, and simply holding
the open end of the resonance-box towards
the open side of the box under the film. These
figures are very powerful in their vortex motions,
and perfectly stable. A cornet blown towards the box under
such a film will also excite powerful vibrations. A thin film
of mica stretched on a frame and dusted with lycopodium,
and placed instead of the soap-film, will also show very strong
nodal lines.

As already remarked, the same apparatus is very handy
for exhibiting surface ripples, or in fact any bright surface
projection by reflected light.

149. Chladni's Figures. — These may be projected in
several ways by the aid of the vertical attachment, using of
course plates of glass, with ground and polished edges, (a) A

Pia. 143

SOUND 273

plate held at the centre may be arranged so that one quad-
rant of it projects into the field and is projected, (b) A
smaller plate may be clamped in the centre, at the end of an
arm projecting from outside the field, (c) The figures may
be formed apart and then projected whole, (d) A plate held
at one edge may be vibrated in the field, by a bow, or by a
string attached to it and rubbed with resined fingers, or by a
resined string drawn through an aperture in the centre.
(e) Or vibration may be communicated to it from some other
vibrating apparatus in any of the recognised ways. (/) Or
on one corner of a large plate may be cemented a ring of
rubber, within which water is poured an eighth of an inch
deep — a favourite experiment of Professor Morton. When
the plate is bowed, a beautiful network of waves will be pro-
jected.

150. Columns of Air.— Kundt's dust- figures, showing the
nodes and segments of air in a glass tube, may be formed in any
way and then projected, or the tube may be placed in the field
of the vertical attachment and the dust shown in vibration.
A very convenient method is to plug one end of a tuba, and
insert a short whistle of appropriate pitch into a cork in the
other. Any of Mach's experiments may be projected in the
same way. Square tubes cemented together of flat glass plates
are worth the trouble of construction, the figures being so
much more accurately focussed upon the flat glass.

The projection of tracings upon smoked glass from tuning-
forks, membranes, &c., is so obvious and simple as to require
no more than mention.

274 OPTICAL PROJECTION

CHAPTEK XVIII

LIGHT I
EEFLECTTON, REFRACTION, DISPERSION, AND COLOUR

THE phenomena of Light not only lend themselves most
readily of all to demonstration by the lantern, but in studying
them we best acquire a mastery of the various manipulations
of beams and pencils of light required in the demonstration of
other subjects. For this reason, the demonstrator in other
departments, though he may not himself desire to perform
purely optical experiments, will do well to read and understand
what is said respecting their details and methods, for the
sake of the hints which these may give him in handling his
tools.

151. Kays of Light. — The image-forming power of rays of
light, the inversion of images, and the relation of the size of
the image to the distance from the aperture or lens by which
it is formed, cannot be better shown than by the experiments
described in Chapter I., pp. 2, 3.

That rays are really reflected from ordinary bodies may be
easily shown by removing everything from the nozzle of the
lantern, and receiving the full parallel beam, or still better a
slightly divergent beam, at an angle of about 45°, on a large
piece of card, on which is pasted red surf ace -paper with a
* dead ' surface. On holding another white card as a screen
some little way off, it will be seen that a strong red light is
thrown upon this by the reflected rays ; and other colours
will display similar effects. The colours should be as rich
and ' full-bodied ' as possible but a glossy surface should be
avoided, that it may be clear the effect is produced solely by
scattered reflection.

152. Scattered Reflection.— That we only see things, not

LIGHT: REFLECTION 275

light, and only see them by scattered reflection, may be shown
by the simple experiment represented in fig. 144. A confec-
tioner's glass jar A, about six inches diameter, is covered with
a flat glass plate B, after dropping in a bit of smoking brown
paper which has been dipped in a solution of saltpetre. The
plane-reflector c is adjusted to throw down the whole beam
from the lantern (either crossed from the objective as drawn,
or the parallel beam may be used). A cloudy light fills the
jar. Taking off the plate and blowing the smoke out, dark

Fia. 144.— Scattered Reflection

spaces appear, showing that where no solid particles reflect
light to the eye, we see nothing. Or the jar may be filled
with clear water, which is also nearly invisible ; but on
stirring in a teaspoonful of milk, the same lambent light
illuminates the whole room. The black particles of soot>
equally with the white (or supposed to be white) particles of
milk, reflect white light.

153. Law of Reflection. — This is simply demonstrated with
the plane-mirror A B, arranged as in fig. 145 near the nozzle

T2

276

OPTICAL PROJECTION

Fia 145

N of the lantern. A wire pointer c is easily affixed to the
centre of one side of the mirror by sticking it in a piece of
cork E, which has a groove cut in one side, by which it is

attached to the mir-

<v>N ror. Letting the

beam from a hori-
zontal slit in the
optical stage strike
the centre, it will be
readily seen that if
it impinges at right
angles it is reflected
back ; but that at any
other angle, the angle

(measured from the normal or perpendicular, as indicated
by the pointer) of the reflected ray is equal to that of the
incident ray. A whiff of smoke near the mirror will show
the beam brightly if required.

154. Use of Parallel Beams.— This experiment being an
example of a large class, in which we want a distinct pencil or
beam of light, it is well to consider the various methods of
manipulation in order to produce it. The figure shows the
simplest, which will answer for many experiments, but not for
all. A horizontal slit is here placed in the stage of the
optical front. This slit may be of black card, or in zinc or
brass, or in a disc of thin metal fixed by a spring wire in
one of the wooden frames used for polariscope slides, the
standard size for all slides used in the optical front being
4x2^ inches. Or an ' adjustable slit ' made to fit the stage
may be used. For this experiment a broad slit should be
used, not less than 3 mm. wide. A beam so produced is not
parallel, but sufficiently so for many purposes.

With the electric light, or a good mixed jet, it is better to
remove the objective, place a slit or aperture on the front of
the open nozzle, and use the ' parallel beam ' through this.

LIGHT: REFLECTION 277

With the arc this will be very sharp and clear. Parallelism is
of course obtained by pushing the arc-light or the lime forward
into the principal focus of the condensers ; or with a Duboscq
lantern by pushing in the condensers so that the arc is in
their focus.

The lime-light does not give such an accurately parallel
beam, owing to the greater size of the radiant ; there is more
or less divergence from the aperture, so that nothing like a
sharp circular spot (in the case of a circular pencil) would
appear on the screen. Should the nozzle in work have an
ordinary slide stage behind it, then by placing in this also
(whether it be the stage of the optical front or the ordinary
stage of the lantern) a slit, or circular aperture, as the case
may be, rather larger than that on the front of the nozzle,
the pencil will be sharpened considerably, and rendered very
nearly parallel. We may call this the sharpened parallel
beam.

But the mere parallel beam is not sufficient for all cases.
When a figure has to be produced on the screen by the motion
of a luminous spot, that spot must be sharp and denned, if
the figure is to be so. Circular pencils are always employed
in these cases, and should first be rendered approximately
parallel, by adjusting the light in the focus of the condensers.
If then the detached focussing lens be adjusted to focus the
aperture on the screen, we have all the necessary conditions.
The beam itself is as nearly parallel as can be, and the little
light that is scattered by divergence is brought back and
utilised by the lens, and all sharply focussed. The long-focus
lens, if one is at command, usually produces the best results.
We will call this arrangement the focussed parallel beam. An
arrangement for greatly increasing the brilliance of such
smaller beams and pencils has been shown in fig. 95, p. 173.

155. Angular Motion Doubled by Reflection. — It will
readily be seen on being pointed out, that any angle through
which the mirror is turned is doubled by the angular motion

278 OPTICAL PROJECTION

of the beam of light, which also acts as a very long pointer
working without weight or friction. Experiments to illus-
trate this will be found under ' Sound ' (§§ 136, 140, 146),
and ' Physiology ' (§ 126). It is also illustrated in any experi-
ment with the reflecting galvanometer.

156. Reflected Images. — The images produced by multiple
reflection between parallel surfaces may be illustrated by
focussing a small aperture or narrow slit upon the screen, and
interposing a very thick piece of plate -glass in the path of the
rays at a considerable obliquity. Eeflection between inclined
surfaces is best illustrated by the Lantern Kaleidoscope (see
fig. 76, p. 146). The nature of a virtual image of an object,
appearing to send out rays as if it were a real object, is well
shown by turning the lantern somewhat away from the
screen, placing a slide in the ordinary stage, and withdrawing
the objective front from the nozzle. Near the front of the
nozzle adjust the plane-mirror so as to reflect the rays to the
screen, and in their path interpose the loose focussing lens, a
long-focus one being most convenient. The slide will be
focussed on the screen quite distinctly, as if it were really
situated behind the mirror.

157. Refraction. — This is best exhibited by a rectangular
tank, partly made of tin, with the two sides and one end made
of glass, and covered with a piece of tin in which a few slits
are cut in different places. The tank should be two inches
between the sides, and about 12 inches square is enough for
an ordinary room, but for a large hall 18 inches is better.
The sides should either have circles blacked out as shown in
fig. 146, or tin sides may be cut out, so as to support the
glass.1 The tank is filled exactly to the centre of the circle
with water. To show the course of the rays there are two
methods, (a) In the water may be stirred a very little milk
or a very little eosin or other fluorescent dye ; and the air

1 The circle is chiefly to illustrate the usual diagram explaining Snell's
law.

LIGHT: REFRACTION

279

space above may be filled with a whiff or two of smoke ; or,
(b) a piece of white card or painted tin leaned a little slanting
against the farthest side, will answer the same purpose and
avoid scattering any light. Arrange the lantern with a
horizontal slit on the nozzle N, and parallel beam alone if the
tank is brought close for a single experiment ; but it is often
more convenient to have it farther away, when the focussed
beam should be employed. By the plane-mirror adjusted on
its stand behind the tank, the beam may be first thrown down
perpendicularly ; then through a slit at F near the end of the
tank, when it will
be seen that the ray
is bent down or re-
fracted ; and finally
by tilting the lantern
a little behind, the
rays may be sent into
the tank through
the glass end at B,
just above the water,
when it will be seen
to be far more bent
down, to c. In both
cases the reflected portion of the rays will also be seen.

158. Total Reflection. — By having a slit in the bottom of
the tank covered with a piece of glass, the rays may be easily
sent up from water into air, when it will be seen that the rays
are then refracted away from the perpendicular, the ray in
either case being exactly reversible. The arrangement will
be very similar to that shown in fig. 147, which depicts the
apparatus adjusted to demonstrate total reflection. By tilting
the bottom reflector the curiously instantaneous character of
the transition from transmission to total reflection within the
water, is readily shown.

A beautiful experiment in total reflection is known as the

FIG. 146.— Refraction

z8o

OPTICAL PROJECTION

'luminous cascade,' depending on the fact that if we can send
a nearly parallel beam of light ' end on ' into a stream of

... water as it issues

II . from a horizontal

Jytr.v.tj orifice, it is reflected
"" from side to side,
except as regards
small dust particles,
which apparently
convert the stream
into one of living
fire. In front of the
open nozzle some

sort of close vessel must be arranged, transparent at the back
and with the issuing orifice in front, while there must be

FIG. 147

FIG. 148. — Cascade

another orifice by which to keep the vessel supplied with water

LIGHT: REFRACTION

281

under pressure. A two-necked glass receiver, as shown in
fig. 148, will answer very well, blacked all over except for a
circle of four inches diameter opposite one orifice. This is
supported as at A. The orifice is a short piece of glass tube
fixed in cork, upon which the light will be condensed. Several
feet higher the supply of water must be placed (a bucket will
suffice) connected with the receiver by the tube B, and the
receiver is filled. On withdrawing a cork from the glass tube
at the orifice, the water will issue in a smooth stream, when
the radiant can be adjusted to give the best effect. Various
colours can be imparted to the stream by placing different
coloured glasses in the slide stage of the lantern.

Instead of this primitive apparatus, a vessel of tin, with
one glass side to go against the lantern flange, and an orifice
opposite, may be constructed. Such a vessel will be supported
more steadily, and the light can be more prc cisely adjusted
through a plane glass ; while the pencil attachment shown
in fig. 95 may be used to give more brilliance.

159. Prisms and Lenses. — The permanent deviation of a
ray or beam of light
by a prism is best
shown by placing a
small aperture in
black card or metal
in the stage and
focussing on the
screen (or a hole in
the plate of aper-
tures focussed by
the loose lens will
do equally well), and
interposing in the
path of the rays a thin prism, often called a wedge prism,
which will show no conspicuous colour. Such a prism may
be made by cementing a wedge-shaped glass cell, and filling

FIG. 1 4 J.— Spectrum

282 OPTICAL PROJECTION

it with water. A trough of this kind is very convenient for
absorption experiments also.

That lenses bend rays of light in the same way may be
shown, by passing the same beam of light through the edge
of a large lens, and also by the experiments described in
Chapter I.

160. Dispersion. — With a glass prism of 60°, or a pris-
matic bottle filled with carbon disulphide, colour phenomena
become conspicuous, while the deviation of the rays is far
greater. The simplest arrangement for projecting the spec-
trum is shown in fig. 149. A slit (from 1 to 3 mm. wide) is
placed in the optical front, and focussed on the screen. The
prism is placed just beyond where the rays cross, when the
image of the slit will be greatly turned aside, and converted
into a spectrum.

Note on Deflected Projections.— Fig. 149 only gives the
essential apparatus. The lantern may be turned aside on a
bare table, and the prism-stand adjusted thereon in its place.
But in this class of experiments, which include also reflected
rays (as those from a soap -film later on), it is on the whole most
convenient to employ such an arrangement as fig. 99, where
the slit is first focussed on the screen, then the prism placed
in position, and finally the whole arrangement, including
the lantern, rotated until the spectrum comes again upon
the screen. In this particular case all the apparatus would
still be in the optic axis of the lantern ; but often a focussing
lens has to be placed on one side of this, e.g. to focus the
soap-film (fig. 179, p. 327). In this case the sliding cross-piece
c D, of fig. 99, affords all the necessary accommodation.

Another way of producing the spectrum is to project parallel
rays through a vertical slit on the open front of the nozzle, the
objective being removed ; and to focus it with the loose lens.
Which plan is adopted will often depend upon the manner in
which the lantern was left arranged from any preceding ex-
periment—a consideration which will often vary precise details.

LIGHT: DISPERSION 283

With regard to the width of the slit, as the spectrum may be
regarded as an infinite number of images of it, differently de-
flected according to their colours, thenarroiver the slit the purer
the spectrum, especially at a small screen distance. But with
the simpler experiments a wide slit gives much more brilliance,
without any obvious confusion at a good screen distance (which
spreads out the colours more), and as much as 6 mm. wide
may sometimes be employed with advantage.

161. Minimum Deviation. — It will be found that there is
one position of the prism which refracts the rays least, called
the position of minimum deviation. This is the proper position
for the prism. Now and then, however, it may be desirable
to turn the prism more round in order to get the greater
length of spectrum thus produced.

162. Different Colours Differently Refracted.— Newton's
two experiments to prove this are both striking, and both
easy. For the first, we place a short slit— say a square aper-
ture 3 mm. long each side in the stage, and project its
spectrum in the ordinary way, with a prism bottle. Behind
this, we adjust at the proper height the glass prism with its
refracting edge horizontal, screening stray rays if necessary
by a black card pierced with an aperture. The rays are now
again deflected either up or down, according to the position of
the glass prism ; and as this is moved along in the rays of the
spectrum from the first prism, from the red end, it will be seen
that the rays are refracted more and more as we get towards
the blue end.

Newton's other experiment is particularly elegant, depend-
ing on the fact that if each colour has its own degree of
refrangibility, it must also have its own angle of total re-
flexion ; violet rays being (because more refrangible) totally
reflected at an angle which allows red rays to pass. To per-
form it with the lantern we arrange as in fig. 150, removing
the objective, and placing a perpendicular slit at N, on the
front of the nozzle. Through this we send a sharpened

lS4 OPTICAL PROJECTION

parallel beam (p. 277). The figure is a ground-plan of the
apparatus. As close to the nozzle as convenient are a pair of
right-angled prisms, P and p2, their hypotenuses or reflecting
sides together, and kept close by a rubber band round them
near each end ; but a film of air must be preserved between
them, which a morsel of tissue-paper will secure if needful. In
the paths of both direct and reflected rays are two focussing

PIG. 150.— Newton's Experiment

lenses F and F, adjusted so as to focus the slit N, and beyond
each of these is a prism bottle B or B2, adjusted to give a
spectrum. They are shown as throwing these on two separate
screens at right angles to each other, s s and s s, but it has
lately been suggested to me l that by turning the prism B the
other way, both spectra would be thrown upon the one screen

1 By Mr. John Cox, M.A.

LIGHT: COLOUR 285

s s. All being adjusted, the double prism, p and p2, is turned
round the perpendicular axis till all the rays pass through to the
prism B, which throws its spectrum as usual. Let them now
be very slowly turned in the direction of the hands of a watch.
Then, just as the film of air arrives at the ' critical angle,' as
it is called, the violet rays being totally reflected leave the
spectrum s s and appear at s s. On continuing to turn, all the
colours in succession do the same.

The fact that lenses disperse light in the same way,
convex lenses bringing blue rays to a nearer focus than red
rays, is very readily demonstrated by placing in the stage a
piece of black card with two apertures pretty close together
in a horizontal line, one covered with deep red, and the other
with blue gelatine, and focussing the parallel beam sent
through them by the top edge of the loose lens. The two
images will not both focus on the screen at the same time, or
at the same level.

163. Composition of White Light. Colour a Shadow, or
Suppression. — These two connected facts may be shown by
numerous experi
ments. Arrange as N A \

for the simple spec- "
trum, but not deflect-
ing the lantern from
the screen. The
prism A being duly

placed before the nozzle N, or in front of the loose lens if
that is employed, place in front of it a second prism, or:
bottle B, with its refracting angle turned the other way.
This second prism gathers up again all the coloured rays, and
recomposes a white image of the slit on s. If we now introduce
a black card c at one side (the second prism being placed as
far from the first as will gather up all the rays, and the
card kept close to the second one), we suppress, or stop off
part of the colours, and the image of the slit at once becomes

286

OPTICAL PROJECTION

a coloured one. This also illustrates the important fact, that
prism analysis will always give us the correct composition of
whatever light there is passing through the prism.

If instead of the card, a thin prism ground to a fine edge
is used, the intercepted portion of the rays will be simply de-
flected, instead of suppressed, and a second image of the slit,
complementary in colour to the other, will appear by its side
upon the screen.

A more obvious and mechanically simple method is to
provide seven small pieces of looking-glass, each about 2 inches
long by J inch wide, mounting them with wax on small

wooden feet. These
are to be arranged on
a piece of blackened
board A, supported by a
stand, and so arranged
that the row together,
at some little distance
from the prism p, about
covers the whole width
of the spectrum-band
at that spot. Each
mirror is then adjusted

to throw its strip of colour on the same spot on the screen s.
On taking away one or more mirrors, and so suppressing any
of the colours, again we get colour, whereas all the colours
gave white.

A better method of recompounding the colours is to use a
cylindrical lens, which can easily be so adjusted between the
prism and screen as to focus the slit again in an image
which appears sharp and white. The cylindrical lens should
be rather long in focus, from 10 to 15 inches, to produce a
good effect ; the usual short-focus cylindrical lenses make the
slit appear much too broad. A large cylindrical jar full of
warm water (else moisture will condense upon it), may be

FIG. 155

LIGHT: COLOUR

287

made to answer, but a lens is best. A coloured image, or, with
a wedge-prism, two complementary images, may be produced
as before.

But if a large convex lens — say 6 inches diameter and 14
or 15 inches in focus — forms part of the available apparatus,
it will answer just as well as a cylindrical lens. The top and
bottom of the lens may be screened with black card so as to
leave a horizontal stripe 2 inches deep, all across the centre ;
but even this is not necessary, as the rays from the prism will
only strike the centre. With this lens exactly the same ex-
periments may be
performed.

Another method
is adopted by Pro-
fessors Eli Blake and
Ogden Eood, the
coloured rays from
the prism being re-
ceived at a distance
of three feet or more
upon a strip of
the thinnest silvered
glass procurable,
about 18 inches long
by 3 inches wide. This can be flexed by the hands so as to re-
unite the colours in a white reflected image ; or the plate may
be bent in a simple frame by a screw ; or a cylindrical mirror
may be employed such as is used for producing caricature
images.

The next two methods depend upon persistence of vision,
any impression upon the retina remaining for nearly half-a-
second (the eighth of a second so often mentioned does not
nearly represent the fact with most people). The rays from
the prism p (fig. 153) are received on the plane-reflector B,
in its vertical socket, in which the mirror must move easily.

FIG. 153. — Rocking spectrum

288 OPTICAL PROJECTION

Then a rocking motion is given, and as this motion becomes
more rapid, the spectrum becomes white over all the middle
portion. If the hand is unable to rock the reflector .with
sufficient rapidity, a stand may be employed which will im-
part the motion mechanically by a short arm from a multi-
plying wheel.

A most instructive experiment is that so well known as
' Newton's disc.' A circular card which can be rapidly
rotated on its centre is painted in sectors with the principal
hues of the spectrum in their due proportions, either in one
sector of each colour, or several sets, fig. 154 showing the
disc painted in four sets. This is arranged facing the lantern,
and all the light from the nozzle is made to just cover the

disc and no more, the
lantern being so placed
that the audience can
see the face of the disc.
When this is rapidly
rotated, it appears
white.1 Newton's disc

no. 154 FrG.i55 can also be obtained as

a transparent slide for

the ordinary stage of the lantern, the cost of the latter arrange-
ment with the wheelwork being about 12s. Qd.

If now sectors of black paper are fastened, with drawing-
pins, on the card disc, or gum on the glass, so as to cover up
any of the colours, by so suppressing these again we get
colour. Or we may rotate just in front of the nozzle of the

1 If the colours are not properly painted, of course the white will not be
perfect, and a disc should always be tested before purchase, or at least before
use. But a beautiful white can be got by this method, and it is the brighter
the more brilliant the colours. It is true the disc often appears a poor grey
as usually shown, by the general light of the room ; since each portion of it can
only reflect at most about one-eighth of the spectrum ; hence the grey, which
is merely a deficiency of light. But by keeping the room dark, and concen-
trating a bright light on the disc, with a good one the white is all that can be
desired,

LIGHT: COLOUR 289

lantern another card, in which are cut two radial slots as in
fig. 155, so that the slots cross the nozzle and let flashes of
light through intermittently. Cover the nozzle itself with a
similar slot, taking care of course that the slots come just
where the rays cross from the objective. Then while the
Newtonian disc is rapidly rotated, let the other intercepting
disc also be rotated, at first slowly. The flashes by degrees, as
their speed increases, will not allow all the colours to mingle
their impressions during each, and so the disc will appear
coloured, till at last it will stand out almost distinctly in its
real colours

From all these experiments it will be readily shown and as
readily understood, that for a ' pure ' spectrum we
must employ a narrow slit. The narrowest slit
give*} its own spectrum. That a broad slit gives
many overlapping spectra, is demonstrated by using,
instead of a single slit, a double slit arranged as
in fig. 156. At top and bottom there will be a
comparatively pure spectrum ; but over the greater
portion of the band the two will overlap, diluting
the colours by mixture ; the two slits here may be FIG. 156
regarded as the edges of a single wide slit.

164. The Rainbow. — The way in which a rainbow is
produced by the prismatic dispersion of each rain-drop was
shown by Antonio de Dominis, in an experiment easily
adapted to the lantern. A glass bulb blown on a small tube,
and from \\ to 2 inches in diameter, is filled either with
water, or with some other fluid of higher dispersion, or a
glass ball ground and polished would do, to represent our
spherical rain-drop. This bulb B is placed in a Bunsen
holder c, in front of the lantern, which must be turned
towards the spectators. The objective is removed, and the
parallel beam sent through a circular diaphragm with an
aperture the same diameter as the bulb; and surrounding
the nozzle, or at least the beam, is a screen of white card or

u

290 OPTICAL PROJECTION

paper s, with a similar aperture cut in that. It must not be
forgotten to adjust a small black screen behind the bulb B, to
prevent any direct rays from incommoding the audience and
spoiling the effect. By the refraction, reflection, and disper-
sion of the bulb, a prismatic circle B will be formed upon the
screen,1 and it will be evident that wherever a colour — say blue

Fia. 157.— Rainbow Experiments

— reaches the screen, an eye whose pupil was at that spot, or
in its precise direction, would see blue from that particular
drop of rain represented by the bulb.

165. Variation in Dispersive Power.—This may be shown

1 This experiment cannot be performed with less than the lime-light ; but
if any difficulty be found with that, it will be easily surmounted by using the
focuased parallel beam (p. 277) and the attachment described on p. 173.

LIGHT: COLOUR

291

Fio. 158. — Trough prisms

by what is called a polyprism, of glasses ranging from crown
to double-dense flint ; but a cheaper and equally effective
apparatus can be obtained of glass plates cemented together
as in fig. 158, the outer strips, forming a V of about 60°, being
about 6 inches long by 2 inches wide. They may be ceni( nted
together with marine glue, in which case water, salt and
water, and sugar of
lead and water, will
give considerable
differences in disper-
sion ; but by using
glue or isinglass
mixed with bichro-
mate of potash or
chrome alum, and
then exposed well
to sunlight so as to become insoluble, they may be filled
with water, any medium solution, and carbon disulphide, or
monobromonaphthalene, which is equally dispersive and less
volatile.

166. Achromatism, and Compound Prisms. — That refrac-
tion and dispersion are not necessarily proportional, is very
readily shown by a prism of dense flint glass, of an angle
which exactly neutralises the colour produced by the trough
of water, when its refracting angle is placed the reverse way,
or at the top. It will be seen that there is still left a very
considerable amount of refraction. It will be evident that
the reverse may also be done, and the refraction might be
neutralised while still leaving prismatic dispersion ; and both
phenomena are excellently illustrated by a very convenient
apparatus devised by Professor Weinhold and shown in
fig. 159, of one third the real size, fig. 160 giving the details
more clearly. One of the rods on the stand carries by the
screw k a flint-glass prism of 20° angle, and the other by
another screw Jc two crown-glass prisms of about 26° and 45°

292

OPTICAL PROJECTION

(the exact angles of course depend upon the glasses employed).
When the flint glass prism is used with the thick crown
glass prism, as at A in fig. 160, the apparatus gives refraction
with achromatism ; when the thin crown is used with the flint,
as at B in fig. 160, there is dispersion with an undeflected
beam. Black screens s can be adjusted to stop off any stray
parts of the beam ; or if it be desired, by adjusting one prism

FIG. 159

Fm. 160

half of its height higher than the other, the effect of each
prism separately, and of the combination, and of the unde-
flected beam, can be shown together, this arrangement being
shown at c, fig. 160.

More simply, a pair of small prisms to show achromatism,
as made for the Science and Art Department, and sold for 5s.,
may be simply bound round with a rubber ring and stood

LIGHT: COLOUR

293

on end in the patli of the beam, the effects of each prism
singly being first shown.

Compound fluid prisms are very useful for many experi-
ments with the spectrum. Fig. 161 is the usual form, the
space B being filled with carbon disulphide, or monobromo-
naphthalene, or phenyl-thiocarbimide, and the ends G, of
light crown glass. Similar prisms of smaller size may be

PIG. 161

FIG. 162

constructed with the new highly- dispersive Schott glass instead
of fluid ; and such a prism fixed in a mount at the crossing
of the rays from the optical objective, is exceedingly con-
venient for many purposes. Another useful compound prism
may be made on the general principle of fig. 162, a single crown
prism of light glass, G, separating two cells B of dispersive
fluid : such a prism may be made of nearly double the disper-
sion of a prism bottle, with only the deviation of one, and

Fio. 163.— Wernicke's Prism

with much less loss of light ; or it may be made like Thollon's,
with one cell of fluid only, of greater angle. The most
generally useful in projection is a direct prism, which for all
simple and rough experiments saves deflecting the lantern.

Another improved form of direct prism for projection is
shown in fig. 163, as constructed by Messrs. E. & J. Beck for
Professor S. P. Thompson, according to a method suggestsd
by Wernicke. Cinnamic ether has the same mean refractive

294 OPTICAL PROJECTION

index as one of the new Jena glasses, but is widely different
for the blue and red ends of the spectrum. A prism is there-
fore constructed by placing a very wide-angled prism F of the
glass, in a cell of cinnamic ether c c, closed by rectangular
glass plates. The yellow ray is undeflected, but the red rays
B and violet rays v are dispersed by the second face of the
prism F as well as by the first, as shown in the diagram.
This prism gives very great dispersion, and much better
definition than carbon disulphide, while the rectangular ends
are an advantage. The cost of one not quite 4 inches long is
about 61. 10s.

167. Transparency Reciprocal with Reflection and Re-
fraction.— It is interesting to show by experiment that in
proportion as we abolish reflection and refraction, by approxi-
mating the refractive indices of the portions of a mass, and of
the surrounding medium, it becomes more transparent, or less
visible. A screen of blotting-paper may be arranged between
two lanterns, one on each side, and a portion of it either
wetted or oiled. It will be seen that the treated portion
appears darker, or reflects less light, to a spectator on the
same side as the lantern illuminating it ; while if this be
darkened and the screen illuminated on the other side, the
same portion appears brighter, because the light passes
through instead of being reflected. If we arrange a screen of
thin paper with a grease-spot of two inches diameter in the
centre, between the rays from the bare radiant in the lantern
and the most powerful gas-flame or oil lamp that can be pro-
cured, adjusting positions and distances so that the audience
can see when the surface appears evenly illuminated, we shall
have an experimental demonstration of Bunsen's photo-
meter.

An elegant experiment with a mass of powdered glass will
make this still more clear. A bottle must be prepared like a
prism bottle, but with parallel sides ; or a fair result may be
got with one of the scent-bottles ground and polished to two

LIGHT: COLOUR 295

parallel flat sides, such as may be bought for a few pence
almost anywhere. Either is nearly filled with coarsely
powdered glass. For the bottle with plane sides it does not
matter what this glass is, if homogeneous ; with the other
bottles, it is best to purchase one or two others of the same
make, and having heated one nearly red-hot and quenched
it in cold water, to pound it up. The powdered glass sold
ready-made, generally varies too much in density for this
experiment. This powdered glass, in the cell or bottle, is of
course opaque to the beam from the lantern. The bottle is
now to be filled with a fluid of the same refractive index as
the powdered glass. This is easily prepared by mixing in the
required proportion benzol (which is less dense than glass)
and carbon disulphide (which is more so). This is done by
trial and error, and when the required density is obtained (if
the powdered glass is reasonably uniform in character) the
bottle appears clear, or at least transparent. This, however,
is not all. It has been shown how dispersion varies in
different substances. Owing to this, the refractive index can
only be brought precisely the same, for one colour of the
spectrum. We therefore place a rather wide slit on the
lantern nozzle, and focus its image on the screen ; then
interpose the cell filled with the glass and fluid. The image
will now be seen, still sharp for the colour which happens to
be exactly corrected — let us suppose a green slit may appear
on the screen — while the rest of the spectrum will be diffused
round this in a nebulous haze, of the complementary colour
to that of the slit.

168. Anomalous Dispersion.— The remarkably anomalous
proportions dispersion sometimes assumes can be illustrated
on the screen ; but after further experience resulting in occa-
sional failure, I am bound to confess that with the lime-light
the experiment is hazardous and difficult. I formerly described
the use of a prism bottle in a trough with parallel sides, but
this is too uncertain to be trusted, and I have abandoned it. I

296 OPTICAL PROJECTION

find the best arrangement to be as shown in fig. 164, a glass
trough such as is used for aquatic animals under the micro-
scope, having two thin glass plates A c, B c, cemented with great
care, so that there may be no leakage, but that no surplus
cement may hinder the fluid filling the ' knife-edges ' at the
ends, c, of the end cells. On the outside of the cell over c a
stripe must be very carefully blacked, of the exact width that
will cover the cemented ends of the glass partitions, so as to
leave exposed the extreme * knife-edges ' of the cells, and no
more. A slit being placed on the nozzle (the optical front not
being used) and a parallel beam- sent through it, is focussed on
the screen ; and just where the rays are most concentrated, the
cell is placed with the side c to the lens, shading one of the

end cells with a card,

C*

and letting the rays
pass through the
other up to the edge.
The whole cell is

FIG. 164 filled with alcohol,

and with a pipette

are dropped into the end cells, drop by drop, a few drops of
saturated solution of fuchsine in the same alcohol. It will
be evident that the alcohol cell A c u exactly neutralises the
refraction and dispersion of the alcohol in the other cells ;
but as the fuchsine is added the colours separate from the
slit, red coming between blue and yellow. If the solution
is too strong, only red passes : if too dilute, there is no
visible dispersion. The use of the double cell is, that if the
first be made too saturated, the other may be tried ; and for
the same reason it is well so to adjust the glass partitions,
as shown in the figure, which is the actual size, that the two
may be of somewhat different angles, as 25° and 35°. The
most brilliant jet should be employed, in order to work
through as much as possible of the fuchsine, the effect being
chiefly produced at the thinnest part of the cell. Often the

LIGHT: COLOUR

297

effect can be coaxed out of an apparent failure, by simply
covering up, not only the other cell, but the thicker part of
the one employed, so as to diminish the preponderance of red
light which passes through, and may drown the much fainter
blue and yellow. Success in the experiment depends chiefly
upon a brilliant light, exact and clean finish of the cells, and
the proper amount of fuchsine.

169. Subjective nature of Colour Sensation. — The most
striking proof of this is an experiment shown in fig. 165,
which depends upon fatiguing the retina with any given
colour-sensation. The nerves when thus fatigued becoming
less responsive, and
this being equiva-
lent to some sup-
pression of that par-
ticular colour, pro-
duces the sensation
of its complemen-
tary, though the
screen may be white.
Eemoving all but
the condensers, hold
over the open nozzle
N a black card
pierced with a circular aperture which, focussed by the lens F,
gives a disc of 18 or 24 inches diameter upon the screen. In
ths centre of the bright disc insert a drawing-pin as a mark,
and let the spectators fix their eyes on this steadily while
30 is slowly counted — then suddenly remove the card,
lighting up the whole screen. The retina being fatigued over
the region of the previously bright disc, that spot will appear
darker than the rest of the screen.

This done, merely to explain the principle of the experi-
ment, it is repeated with a coloured disc, holding a piece of
red glass together with the card over the nozzle, and bringing

FIG. 165.— Subjective Colours

298 OPTICAL PROJECTION

both away together ; the result of retinal fatigue is now a
green disc when the screen is lit up— if the glass be blue,
the result is an apparently yellow spot. Instead of the open
nozzle and loose lens, the aperture and gla.ss may be inserted
in the ordinary stage of a lantern with the objective on ; in
that case the stage-spring should be held back, in order that
the card and glass may be withdrawn instantaneously and
without dragging.

A more beautiful experiment is to project the spec-
trum itself on the screen, through a disulphide prism. A
rather wide slit should be employed (brilliance more than
purity being required) and the 80 must be counted deliberately,
so as to give the full time. Also, at the word the nozzle
should be covered over, and the gas in the room turned up,
the lantern light itself being often rather too brilliant. The
complementary spectrum will then appear upon the screen.

Any light — even an Argand burner — will perform these
instructive experiments ; but it may be well to remark that in
every audience there will be a percentage who do not perceive
the phenomena, either from colour-blindness, or from unusual
endurance of their nervous system, or from not keeping the
eye steadily fixed on the index spot, since it is essential that
the image on the retina remains fixed in the same place.

Contrast deceives sensation in' the same way. Arrange a
powerful gas-burner (only one) or it may be a less brilliant
lantern (as a gas or oil-lantern, used with an oxy-hydrogen
one) rather nearer the screen than the lantern, and con-
siderably to one side. Throw on the screen a strong light
from the experimental lantern, with a coloured glass in the
stage or held over the nozzle. After a few seconds hold
rods, or a card with pattern-apertures cut in it, in the coloured
rays. Where the shadows fall, the screen is free from coloured
light, and is illuminated by fainter white light from the other
source. But the shadows will appear by contrast strongly
tinged of the complementary colour.

LIGHT: COLOUR

299

170. Absorption Colours. — The only real standard of
colour being the composition of its spectrum, many most
instructive experiments may be made to show that natural
colours are caused by the suppression of certain colours through
absorption by the molecules of the substance. The most con-
venient arrangement is that in fig. 166, the objective of the
lantern being removed and replaced by a perpendicular slit,
either adjustable in brass, or it may be cut in a cap of black card.
(We shall simply speak of the slit on the nozzle in future.)
Through this the light is sent ' parallel,' and focussed by the
loose lens F, beyond
which is the prism
p throwing the spec-
trum B c. Over half
the slit a coloured
glass may be held —
suppose a deep red.
It will be seen that
more or less of the
spectrum is cut out
or obstructed, as
from c to A, and that
the glass is only
transparent to the
red rays. The light is no brighter in the red, and therefore
no other light is turned into red ; it is simply that the other
colours are cut out from the full spectrum. Other coloured
glasses will show the same thing ; and it will be observed that
some glasses cut out bands in the spectrum. Very instructive
pieces of various colours can be selected with a small pocket
spectroscope, and these glasses will show by their composition
how mixed most colours really are, as we find them, and
how different is a really pure coloured light, of a definite
refrangibility, from the light reflected or transmitted by
ordinary pigments or other coloured substances, which is

FIG. 166. -Absorption

300 OPTICAL PROJECTION

shown by the prism to be generally of a most composite
character.

Another experiment may be made. Go up to within a
yard or two of the screen, and hold up a large piece of coloured
glass in the rays. It will be seen that in some rays it casts a
perfectly black shadow — it is opaque to light of that colour —
but in other rays it is more or less transparent.

With more colouring matter, absorption increases, and it
is instructive to prepare a few cells of wedge shape, or wider
one end than the other, and fill them with coloured solutions.
Placed as in fig. 166, it will be found that often the apparent
colour changes when a thicker cell is used ; a colour only
slightly obstructed by a thin layer, being completely obstructed
by a thicker layer. These experiments may be varied ad
libitum.

That the surf ace -colours of bodies are largely transparent
colours, or remainders left after other colours have been
absorbed by a layer of the surface molecules, is shown
similarly, by holding large flowers with good masses of colour
in the rays of the spectrum ; or the coloured prints sold with
floral magazines will answer very well. Each colour, in some
rays, will appear bright and natural ; in others dull grey, and
in others black. The monochromatic lights mentioned further
on have the same effect.

171. Transmitted and Reflected Colours. — Surface colours
are, however, not all due to transparency, and hence there is
sometimes a great difference between the colours a substance
reflects and transmits. This is specially true of metallic and
semi-metallic bodies, and may be shown by a film of gold-leaf
mounted as a slide. It appears green by transmitted light.
Deflect the lantern parallel with the screen, condense all the
light from the condensers through the open nozzle on its
surface, at an angle of 45°, l and focus the reflecting surface
with the loose lens, and it is yellow. The aniline dyes used for

1 Exactly as the soap-film, fig. 179, p. 827.

LIGHT: COLOUR 301

red ink, poured over glass, dried, and treated in the same wayt
are red by transmitted light, mostly yellow-green by reflected
light. Thin silver deposited on glass appears blue by trans-
mitted light, though it apparently reflects all the rays.

172. Complementary Colours.— The primary meaning of
complementary colours has been illustrated already by the
two coloured images of the slit, produced by deflecting part
of a spectrum by a wedge-prism, before re-uniting it by a lens
into a white image of the slit on the screen (see p. 286). Two
complementaries of this kind contain between them the
whole spectrum of white light, and may be varied ad libitum
by the wedge-prism, and also by using a quartz plate and
double-image prism as in Chapter XXII. But much less than
the whole spectrum will produce white, which may be shown
by cutting out a kind of comb in black card, the teeth and
spaces being about half an inch wide. Held in the spectral
rays proceeding from the re -uniting lens, this cuts out half of
them, but the image of the slit is still white. And it can be
shown that even pairs of colours produce white, by re-uniting
the spectrum with a lens, either spherical or cylindrical,
of not less than 14 inches focus and 6 inches diameter, so as
to have the spectrum ' spread ' considerably before re-uniting
it on the slit image. Then close up to the lens must be
fixed a slip of wood with a deep saw-cut along its top surface,
in which black cards with apertures cut in them can be
made to stand and be adjusted at pleasure. A slit in the red,
one rather wider in the green, and a broad band in the violet,
will give a white of three colours. A rather narrow slit in
the red, and one double the width in greenish-blue, will give
a white of two colours. A narrow slit in orange-yellow (just
the red side of the D line) and a rather broad band in full
blue, also give white. And the eye is here also deceived, for
it cannot distinguish (except in brightness) between these
two-colour whites, and that consuming the whole spectrum.
Nor can the eye distinguish — as may be shown in the same

302 OPTICAL PROJECTION

way — between a blue or a green containing nearly half the
spectrum, and a pure blue or green.

178. Composite Colours.— The results of compounding
colours have not been what are popularly supposed, blue and
yellow, for instance, making white and not green. A great
many striking experiments may be made in compounding
colours, and especially blue and yellow. Holding a blue glass
over the slit as in fig. 166, it will be seen that it transmits
blue and green ; a yellow glass transmits yellow and green :
green therefore remains when both media are superposed, and
it appears that the two produce green as the result of succes-
sive subtractions by the blue and yellow. But using good
blues and yellows separately, in the stages of a bi-unial, and
* allowing the discs to overlap, their addition makes white.
Glasses which do this can be found, or the blue may be a cell
of neutral or slightly acid copper sulphate ; and the yellow of
potash bichromate, or picric acid (both fluids highly poison-
ous). The copper and bichromate make, on the other hand,
a nearly pure green by absorption ; and a drawing in red and
yellow chalk illuminated by this light, appears done in black.
On the other hand, a cell filled with a solution of copper oxide
in strong ammonia, and one of the bichromate of potash,
can be so adjusted in strength, or in thickness of solution
by wedge-shaped cells, that one transmits nearly all of the
spectrum stopped by the other. These will also give a good
white when their separate discs are superposed, but when the
cells are superposed, stop all the light, and the screen is
nearly dark. The same may be done with a cell of potash
permanganate, and a green glass selected by trial ; and there
is in Chance's glasses a shade called ' signal green,' which,
when superposed on a full red, also practically stops all the
rays.

Experiment with the spectral colours shows that blue
and yellow are more really ' compound ' colours than violet
or green. Neither of the latter can be made by compounding

LIGHT: COLOUR 303

any pure spectrum colours ; but blue can be compounded (in
the manner described in § 172) out of a narrow slit in the
green between the b and E lines, and a broad band in the
violet over the G line, rather to the violet end. Yellow can
be compounded in several ways. With the spectrum colours
it is done by rather a wide slit in the red, and a wider slit in
the green extending a little to each side of the b and E lines.
Another way is to project the spectrum, and from another
lantern, or other nozzle of a bi-unial, to throw on it the
image of a slit covered with red glass ; this slit being shifted
along the spectrum, will find a position over the green which
gives a good yellow. Thirdly, Lord Rayleigh superposed on a
film of gelatine stained deep blue with litmus (which cuts out
yellow and orange), one stained yellow with aurine (which
stops blue and violet). The two stop all but green and red,
and the result is yellow ; which is remarkable as yet another
result of blue and yellow films superposed. A better combina-
tion, however, also due to Lord Rayleigh, is a cell of litmus
solution, cutting out yellow and orange only as before, and
bichromate of potash, which stops blue ; the two allow to pass
the green and red, and give a remarkably good yellow, while
the colours themselves are so pure, that if a small round
aperture be focussed on the screen, the prism will disperse
this into two nearly sharp discs, one red and one green.

174. Monochromatic Lights. — These may be obtained fairly
pure in many ways. The most convenient yellow is from
combustion of sodium. A spirit lamp with a salted wick, or
a bead of fused salt in a platinum wire loop in a Bunsen
burner, will give a fair light, as will gas or hydrogen passed
through a saturated solution of salt. For a large light, for a
hall full of people, a quantity of tow with salt well rubbed
into it, soaked in methylated spirit, and burnt in a wire
basket over a vessel of water, gives a striking effect, all faces
appearing black and ghastly. A brighter light is obtained by
combustion of the actual metal, as presently mentioned.

304 OPTICAL PROJECTION

Glass coloured with copper oxide is pretty pure red.
Ammoniated copper gives a fair blue ; but a much better plan
is that discovered by Mr. H. G. Madan, of superposing
Chance's signal green, which stops all red, on a rich cobalt-
blue glass, which transmits little but red and blue. The two
only transmit rays between p and G. Green is obtained by
superposing blue and yellow glass, or by the copper and
bichromate as just described.

In all colour experiments, it has to be remembered that
deeper tints are needed in proportion to the brilliancy of the
light used in the lantern.

CHAPTER XIX

THE SPECTRUM

175. Continuous Spectra. — The continuous spectrum is
easily projected in the manner already described in Chapter
XVIII. But when the object is to illustrate the principles of
spectrum analysis, it is advisable to pay special attention to
several points, which it has not been hitherto necessary to
enforce. Always with the arc -light, and generally with the
oxy-hydrogen, it is advisable to use two prism-bottles, or else
the cinnamic ether direct-prism of high dispersion. A
convenient plan is to have the two bottles set, either in a box
with open ends, or on a piece of black board, in shallow
cavities retaining both at the proper angle of medium devia-
tion.

In most typical experiments, it is permissible to converge
more light upon the slit, by adjusting the latter at the focus
of either an ordinary or cylindrical lens.

And thirdly, special care must be taken that the spectrum

THE SPECTRUM 305

is equally focused upon the screen, from end to end. To all
* line ' work of any kind this is particularly essential ; and it
is never the case unless the lens is specially adjusted for it.
The simplest method is to hold a fine wire across the slit ;
and then to adjust the lens for focus, and incline the lens,
until the shadow of the wire appears as a black line equally
sharp from one end to the other of the spectrum.

176. The Light. — A jet of the most powerful kind should
be used, with plenty of pressure and the very best limes.
Only thus can respectable results be obtained with the oxy-
hydrogen light.

The arc -light requires modification. The positive carbon
(crater) must be the lower one for spectrum work ; and both
carbons should be perpendicular and in line. The Brockie
lamp can be used without difficulty, by inserting a wedge-
shaped block under it so as to bring the carbons upright ; and
the current may be reversed by providing an extra special
carbon-holder for the lower pole, which must protrude con-
siderably. The lower carbon cannot then feed up to the
usual stop, and therefore the focus will have to be adjusted
from time to time, by the screw motion of the table. Spec-
trum experiments do not, however, require any very great
accuracy in focus-keeping ; and most demonstrators prefer
a simple hand rack-regulator, moving the two poles in due
proportion, and which, with a screw table as described in
Chapter XII. will answer all purposes, at a small cost. Such
hand-regulators are certainly best for the large excavated
carbons presently mentioned, and allow of the length of arc
being varied with much greater facility.

177. Simple Absorption. — For solutions, thick glass cells,
including some made of a wedge shape, are very convenient,
as showing the effect of stronger and stronger absorptions in
coloured media. With the two prisms and the spectrum
nicely focussed, the definite and local character of colour-
absorption may be contrasted with the general absorption

X

306 OPTICAL PROJECTION

produced by holding a piece of glass more or less smoked in
front of the slit. Coloured glasses may of course be used
ad libitum.

A very small piece of glass containing didymium or erbium,
or a small bottle filled with solution of didymium sulphate,
held in front of the slit, gives remarkably sharp and charac-
teristic absorption lines, though it can be shown by simple
projection that the medium appears nearly clear.

The analysing power of even absorption spectra, is well
illustrated by comparing the absorption of genuine port or
claret, with dilute alcohol artificially coloured. A cell filled
with diluted healthy blood, and another containing blood
poisoned by carbonic dioxide, is another instructive ex-
ample.

Ordinary vapour-absorption is best shown by heating an
ordinary sealed tube of iodine, or by a test-tube filled with
nitrous oxide, prepared by pouring nitric acid upon a few
bright copper turnings in the tube. There are other vapours
which, with a trifle more trouble, give good spectra ; but they
are too well known to those specially concerned to need
mention here. Most coloured vapours give good phenomena,
with more of the line or fluted charactei than is shown by
the majority of liquids or solids.

178. Line Spectra.— These are given with greatest facility
by the electric arc. For bright-line spectra, the positive
carbon need not be larger than usual, but should have a small
hollow made in it ; and the current should be of rather high
E.M.F., so that the arc maybe long. The metallic vapour will,
however, lengthen the arc considerably. The comparatively
dark arc should be brought into the focus of the condensers,
when the line spectrum of the metals shown will appear
between two continuous spectra thrown by the carbon points.
Sodium, lithium, silver, copper, zinc, and thallium are the
metals usually employed. If only sodium or lithium lines
are desired, they are fairly shown by soaking carbon poles,

THE SPECTRUM

307

cleaned by boiling in acid, with solutions of the chloride salts,
which are better than the rnetals for small currents.

Caution. — In all such experiments as these, or in vaporis-
ing metallic sodium in the lantern, even with the oxy-hydrogen
flame , care should be taken to cover the condensers with a
sheet of thin glass, otherwise particles of melted metal or
oxide splutter on to the condenser, and fuse into its surface.
Many a condenser has been ruined in this way. The sheet
should be curved as in petroleum
lamps, to prevent its cracking.

By employing salts of the
metals, excellent line spectra can
be projected with the oxy-hydrogen
flame, the most convenient ar-
rangement being the lamp shown
in fig. 167, devised by Edelmann.
It is practically an upright * safety '
jet, a stream of oxygen passing up
through the centre of a larger
stream of coal-gas, supplied by the
taps at H and o. The whole can
be raised or lowered by the plate
t on the stand or pillar s, and at
the top is a nozzle N, on which
can be placed a hollow cone, &, of
carbon. The inside of this cone is
covered with a paste composed of

the salt rubbed down smoothly in a mortar with picric acid,
ammonia, and alcohol ; as many cones as desired being pre-
pared beforehand. This is an excellent method for bright-
line spectra, such a burner being easily adjusted in the focus
of the condensers, and the coloured flame emerging from the
top of the carbon cone being very brilliant.

Mr. E. Cleminshaw, F.C.S., improving on Bunsen and
Debray, places in the lantern a similar jet, but with a larger

I?iCr. 1C7. — Edelmann's Spectrum
Burner

308

OPTICAL PROJECTION

internal bore J inch diameter for the hydrogen, surrounded
by oxygen. Hydrogen is made in a bottle with zinc scraps
and dilute hydrochloric acid, tke chloride of sodium, lithium,
or calcium being dissolved in the water to saturation; and
to assist the rush of gas and carry up more of the spray of
the liquid into the flame, either hydrogen or coal-gas is also
passed through the liquid by the ordinary wash -hot tie method.
When the oxygen is properly adjusted, the lines are pretty
bright. Bright lines may also be shown upon the continuous
spectrum, by melting the chlorides of the alkalies, upon the
surface of a hard lime cylinder, and using this in the ordinary
way.

The brightest method of projecting the sodium line with

PIG. 168. — Combustion Lantern

the oxy-hydrogen flame is that adopted by the late Mr.
Spottiswoode. A jet for burning the two gases is furnished
with a hollow chamber in the course of the hydrogen tube.
Into this chamber is introduced some metallic sodium, and
the chamber is heated in a Bunsen burner; the gas then
carries over with it a copious supply of sodium vapour, which
burns with intense brilliance at the orifice of the jet.

The lines of some alkaline earths may also be excellently
shown by a simple apparatus devised by Prof. Weinhold, and
shown in figs. 168 and 169, fig. 168 being a plan one-third
the natural size, fig. 169 a perspective view on a smaller
scale. The body is a small cubical lantern, with a door at the
back fastened by the catch g, and with a nozzle on the front

THE SPECTRUM

309

A

into which slides a metal tube, enlarging at the front end into
a long cone r, and at the back end bearing a plate with a slit
s. The cone r is made to extend nearly to the lens which
focusses the slit ; the latter being made thus, to slide out,
because it will need cleaning pretty frequently. To the door
at the back is attached, by a projecting piece, a metal ring, in
which can be supported capsules, b, for the burning sub-
stances.

In using this combustion-lantern, as I venture to term it,
it is arranged in front of
the ordinary lantern, in the
optic axis, with the door
open as in the second
figure, so that the light
from the optical lantern
can be passed through the
slit s, and enable the latter
to be focussed on the
screen, with its spectrum
all in focus as described
in § 175. The optical
lantern may then be turned
off or removed. The pow-
der used is then placed
in the capsule 6, in a little
heap, a piece of cotton wick
about an inch long is stuck
perpendicularly into it so
as to be half-buried in the
powder ; the wick is lighted, and the door shut ; Prof. Wein-
hold advises soaking the wick in lead chromate. As soon as
the flame reaches the powder it flames up, and gives excellent
line-spectra, though for a short time only.

Prof. Weinhold recommends the following mixtures as
effective. For sodium lines : 3 parts sodium nitrate, 1 part

FIG. 169

3io OPTICAL PROJECTION

potassium chlorate, 1 part shellac. For calcium : 2 parts
chalk, 10 parts potassium chlorate, 3 parts shellac. For
strontium : 3 parts strontium nitrate, 1 part potassic chlorate,
1 part shellac. For barium : 3 parts barium nitrate, 1 part
potassium chlorate, 1 part shellac. The shellac to be powdered
separately from the salts, which are also to be rubbed down
to powder, and the two mixed previous to use with a horn or
wooden spoon.

These methods are superior in effect to the use of glass
rods, or sticks soaked in solution ; besides which- the manipu-
lation of such sticks in a jet is a very ticklish operation.

179. Reversed Spectrum Lines,— The absorption by vapours
of the same rays which they emit, is usually shown by sodium.

FKJ. 170.— Reversal of Sodium Line

As a dark line depends upon the absorbing vapour being less
bright than the light of similar wave-length emitted by the
radiant, this experiment, of all others, is shown with marked
superiority by the electric arc.

Fig. 170 shows the most widely known arrangement, as
adopted by Dr. Tyndall, except that two prism-bottles would

THE SPECTRUM 311

be used. In front of the slit E through which parallel rays
are sent from the condensers, is a Bunsen burner, G, in the
flame of which is held an iron or platinum spoon I containing
a good pellet of sodium, which should burst into vivid com-
bustion with some white vapour, In front of the sodium is
arranged a screen s with an aperture, to shade off all direct
sodium light from the screen— hence this experiment is not
well adapted for a direct prism. The light from the slit has
all to pass through the sodium flame before it reaches the

FIG. 171.— Reversed Sodium Line

lens, L, and a dark line D appears in the orange-yellow of the
spectrum.

By modifying the arrangement as in fig. 171 (after Miiller),
detaching the slit from the front of the lantern and placing
the burner B on the opposite side of it from the focussing
lens, the lens L and prism p (really two prisms) being as
usual, both spectra can be shown together. For introducing
by hand another tin screen T between the Bunsen-flame and
the condensers, the radiating arc light is cut off from the
upper part of the slit, and consequently half the spectrum on

312

OPTICAL PROJECTION

the screen shows the radiation n, and the other half the
absorption A, from the same sodium flame.

But the simplest, easiest, and best method is to produce
the vapour between the carbon poles themselves. To do this
with certainty, the positive carbon must be considerably
larger than usual, and this is another reason for employing a
hand-regulating lamp for spectrum experiments ; besides
which, the distances have to be adjusted by trial, which is
more conveniently done by hand. I doubt if better and more
certain reversals have ever been demonstrated than by my
long-time friend and correspondent in these matters, the Kev.
P. B. Sleeman, of Bristol ; and for sodium the arrangement

adopted by him, as shown
at A, fig. 172, is probably
the best possible. The
figure is actual size, the
lower carbon being |-inch
diameter, with a groove
^-inch wide cut in the end,
round a centre of ^-inch
diameter. In this groove
small pieces of sodium are

placed (not too much). As soon as the upper carbon touches
the centre, the metal is volatilised. It will be seen at once
how and why, by this arrangement, as soon as the arc is
struck, the vapour is comparatively cool, and well outside
of the incandescent carbon. A very little adjustment of the
carbons brings out the reversal conspicuously. The ordinary
1 cup,' as in B of the same figure, will however answer very
well, and for more refractory metals like lithium is preferable,
the heat of the arc itself being needed to volatilise them.

The guiding principle, in reversal experiments, is to get
a short arc. If we use carbons as in B, and separate the
carbons to give an arc of good length, we get the bright lines
and comparatively dull carbon poles ; then, as we lower the

A B

FIG. 172.— Carbons for Reversals

THE SPECTRUM 313

negative carbon and shorten the arc, the bright lines fade
and we gradually get dark lines.

Weinhold's combustion-lantern, shown in figs. 168, 169,
would probably give good dark lines by keeping the door open
at the back, and sending the rays from the arc through the
flame ; but I have not been able to test it in this way.

With the oxy-hydrogen flame, direct reversal is more
difficult, and is practically confined to the sodium line. I
gave ! the arrangement in fig. 171 for this light, after fairly
successful trial ; but I have since found that my success was
due almost entirely to the powerful jets employed, and that
even with these, such an arrangement cannot be depended
upon, the Bunsen-flame being not cool enough in proportion,
and so nearly in focus with the slit as to overpower the yellow
portion of the radiation spectrum. Mr. E. Cleminshaw was
quite justified in some criticism to this effect at the Physical
Society,2 and I was very glad to find that his arrangements
gave better results than my own. They are as follows.
(a) The Bunsen burner and sodium-flame are placed at the
focus or crossing point of the lens, where the rays pass
through the smallest space, and can be passed through a
small flame where the vapour is most dense. The screen is
then easily shaded from direct rays, and the sodium-flame is
notfocussed on the screen. The latter is the great advantage
of this method. A spirit-lamp may be used instead of a
Bunsen burner, but with metallic sodium, not with salt.
(6) If the sodium-flame is used between the slit and the lens,
after vivid combustion has been produced, the Bunsen flame
is cooled by passing into it, along with common air, a portion
of carbonic dioxide, generated in a bottle as usual, and with two
vent-tubes, so that the supply can be graduated, (c) Another
plan is to arrange a spirit-lamp with four wicks, with a jet of
oxygen in the centre. Using oxygen in the flame, the bright

1 Light: a Course of Experimental Optics.
8 Proc. Physical Society, vii. 53.

3H OPTICAL PROJECTION

lines are produced ; then, on shutting it off, the dark band
will usually be seen. Such a lamp may also be used between
the lime-light and the slit ; but the flame should be as far
from the slit as convenient, in order to be out of focus. I
have found that the brightness of the radiation spectrum may
also be increased by condensing the rays from the condensers
upon the slit by means of a lens.

Absorption may be clearly projected by other methods.
Makers of physical apparatus prepare sealed glass tubes about
f -inch diameter (which must be of strong and hard combustion-
tube), in which are placed some small portions of clean sodium
in an exhausted atmosphere of hydrogen (in order to prevent
oxidation of the sodium). If this is carefully heated and held
in the rays of the spectrum, it will be found to cast a shadow
in the orange rays, but not elsewhere. Sometimes it can be
so heated as to show dark lines if held over the slit, but this
is rather a doubtful experiment. Or it will cast a shadow in
the rays from a Bunsen sodium-flame.

Mr. Cleminshaw has devised another pretty experiment of
this class. Using any apparatus for producing a brilliant
sodium-light in the lantern itself, as recently described, it is
adjusted so as to give a bright yellow disc on the screen
about five feet in diameter by projection through the con-
densers. A Bunsen burner is then adjusted in the lantern
near to the condensers (protected by a glass) and the orifice
about half an inch above the lower edge of the lenses.
On holding a carbon rod, or bunch of asbestos fibre, or other
medium impregnated with salt, in the flame, a distinct shadow
will be cast upon the screen.1

1 Mr. Sleeman informs me that by an arrangement somewhat similar to
this, but using an ordinary incandescent lime to give the spectrum, and
using a bunch of asbestos fibre squeezed into the lower end of a glass tube
filled with a saturated solution of salt, the brush thus constantly fed being
held in the flame in front of the lime, he has obtained sometimes a fair
reversal of the D line. I have had no opportunity of trying this method
«ince it was suggested to me.

THE SPECTRUM

315

Lastly, we may employ Bunsen's well-known apparatus,
in which hydrogen is generated by zinc in diluted acid,
saturated with sodium chloride, coal-gas being also passed
through the liquid to assist
effervescence and carry more
spray into the exit pipes.
One burner is of slit form,
and when the supply of air is
adjusted gives a hot and bril-
liant sheet of incandescent
sodium vapour. The other is
cylindrical, and has placed
over it a conical mantle which
checks combustion, and causes
a cooler flame, also coloured
with sodium. This flame ap-
pears nearly black against the
brighter one ; and shading
both with a metal case or
chimney only open in front,
both flames are readily pro-
jected on the screen by a
single lens.

180. The Invisible Spec-
trum,— Only one or two key
experiments need be men-
tioned here for demonstrating

the existence of invisible heat-waves beyond the red, and in-
visible actinic rays beyond the violet of the spectrum.

Caloresccncc is best shown by Tyndall's experiment of
passing the invisible heat rays through an opaque solution.
The safest of his arrangements with the highly inflammable
solution of iodine in carbon disulphide, and which need give
rise to no uneasiness, is to employ a box or lantern with an
open circular aperture in front, and a reflecting mirror

FIG. 173

316 OPTICAL PROJECTION

behind the arc-light to throw out a parallel beam. If from the
aperture a tube of the same diameter, silvered and polished on
the inside, extends for two or three feet, the heat is kept from
scattering. (Condensing lenses are unsuitable for this experi-
ment, as glass is a powerful absorbent of invisible heat rays.)
A thin globular flask with a neck, about 4 inches diameter, is
filled with the solution and held in the parallel beam ; at its
focus black paper and other substances will be ignited.

This method is not very suitable for the oxy-hydrogen
light, because the lime does not radiate much heat to the
back, where the mirror must be placed. The best method
is to hold or adjust the globular flask, of the thinnest glass,
five or six inches from the naked lime. It is all the better if
a reflecting tube occupy the space between, as it will concen-
trate the heat considerably ; and the densest and hardest
limes must be used, these being much superior in radiating
power. A powerful jet of -j^-inch bore will make the whole
lime incandescent, and give enormous heat, which will be
quite sufficient at the conjugate focus of the flask. So near
the lime, however, I strongly advise the employment of carbon
tetra-chloride as the solvent. It does not give an absolutely
opaque solution like carbon disulphide, a little violet struggling
through ; but this is hardly perceptible, and the solution is
not dangerous, as disulphide so close to the lime most
certainly is. This will be found a very inexpensive, simple,
and effective arrangement.

The energy of nearly invisible actinic rays may be shown
either by taking a photograph through several blue glasses ;
or by exploding (with the usual precautions) at a rather long
conjugate focus of the condensers, through similar nearly
dark glasses, thin glass bulbs filled with chlorine and
hydrogen gases, obtainable from the makers of physical
apparatus. Explosion will be prevented by red or yellow
glasses, but will at once take place through the blue, even
with a good lime -light ; but if there is any difficulty, burning

THE SPECTRUM 317

magnesium may be employed. White glass lenses are much
superior for these experiments, as with the following.

181. Fluorescence. — Many experiments in Fluorescence,
of great beauty, are easily made with the lantern. For those
which depend chiefly on the conversion of the invisible rays
at the violet end of the spectrum into blue light, it is advisable
to have a radiant of high actinic power, such as the arc, or
burning magnesium ribbon ; and even then the effect is
heightened if a quartz condensing lens and prisms can be
employed, as crown glass, and still more carbon disulphide,
strongly absorb these rays. Lenses and prisms of fine ' white '
flint, however, also give very good results. A temporary
magnesium light can be got with little trouble by passing
three ribbons through a brass tube, an assistant watching the
combustion through a darkened glass, and feeding the ribbons
accordingly by hand. The end of the tube should be in the
focus of a large condensing lens, and the whole be fitted up
separately, since magnesia makes a great mess in an ordinary
lantern. Sulphur burnt in oxygen gives a strong actinic
light, and even in air is sufficient for some effects.

With any such light, if a sheet of paper or card, washed
repeatedly with a saturated solution of quinine sulphate in
water acidulated with sulphuric acid, is pinned on the screen
so as to cover and extend beyond the visible spectrum thrown
through even a glass prism, a visible extension of the violet
light will be produced, especially if the brighter part of the
spectrum be shaded off to prevent the feeble light being over-
powered. A glass tank some inches long, with flat ends,
filled with the same solution, will mark the cone of light
passing through it from a large lens by a beautiful sky-blue
fluorescence. Solution of assculm, or a decoction of horse-
chestnut bark, also gives a brilliant blue.

The reciprocity of absorption and fluorescent power in the
beam is shown by first showing the extension of the visible
spectrum as before, and then interposing the tank of quinine ;

3i8 OPTICAL PROJECTION

the fluorescent rays, being now taken up in the cell, no longer
have the power to excite fluorescence on the screen. This is
further illustrated by the fact, that in all tank experiments the
fluorescent glow rapidly becomes leebler as the light passes
deeper into the fluid, and that in any of the following experi-
ments, the fluorescence may be stopped, or nearly so (de-
pending upon there being sufficient absorption) by interposing
a tank of the same substance in solution. But it is not so
with different substances. Thus, if we have a tank of quinine
solution, with a fluorescent cone of light traversing it, we
stop this fluorescence by interposing another tank of the
quinine, which, in becoming fluorescent, robs the beam of the
power to similarly affect the second tank. But if we inter-
pose a cell of uranine solution, its brilliant green fluorescence
does not do so, but the quinine cell is fluorescent as before.

Amongst substances which fluoresce well in the electric
light or magnesium, are turmeric in castor-oil (green), tinc-
ture of stramonium, fustic steeped in alum solution, camwood
steeped in castor-oil (this oil, though not fluorescent itself,
seems able to excite the property in several other substances)
and sesculin. Of the latter a grain in powder should be
shaken into a confectioner's jar filled with slightly ammoniated
water, placed in the lantern beam : beautiful blue fluorescence
will be seen by reflected light. Even a few bruised fragments
of horse-chestnut bark thrown on the water will do the same.

There are plenty of substances which fluoresce magnifi-
cently by the ordinary lime-light, however. Even quinine
will show fairly in a tank, with a good jet. A solution of
nettle or any other green leaves (chlorophyll) in ether,
methylated spirit, or benzol, which is green, fluoresces blood-
red. A cube of greenish-yellow uranium glass fluoresces a
bright and lovely green. Fluorescein, or any of its deriva-
tives, such as eosin, or uranine, fluoresces brilliantly in even
gas-light, and it is a beautiful experiment to scatter as much
as is ta.ken on the very end of a penknife, on the surface of a

THE SPECTRUM 319

large glass jar of slightly ammoniated water, in the rays of
the lantern. Beautifully brilliant arborescent green streams
descend through the water. Chrysoline does the same, and is
perhaps a purer green than the uranine. A few drops of almost
any red ink will show the phenomena fairly well.

Green, or rather greenish-yellow, is the commonest and
strongest fluorescent colour, and for designs painted in it
there is ample choice. The best way is to prepare a thick
size or thin glue from gelatine, and dissolve fresh some
uranine in the fluid. This can be laid on with a broad pen
or brush, and then dried, so as to give a little body of colour.
Such will shine brilliantly in almost invisible blue light.
Barium-platino-cyanide will be equally brilliant, but is far
more expensive. Tin's is best rubbed up with gum-water. A
substance called thallene by Professor Morton, prepared by
him from petroleum residues, has probably the brightest
fluorescence, of the same yellow-green. Professor Morton
kindly wrote to me that this is best prepared by grinding up
with rather thin varnish of gum-damar in benzol. Slight and
unknown impurities often impart splendid fluorescence to
various organic compounds, which are destitute of it when
really pure.

Other colours are more difficult to get, and I have not yet
obtained any of them brilliant enough to use in designs. In
cells, chlorophyll has been already mentioned for red.
Magdala rose fluoresces orange-red, but was expensive, and
is now almost impossible to obtain, being gone out of fashion,
which is capricious as to these aniline colours. The best red
I yet know of was brought to my notice by Mr. Sidney
Jewsbury, of Manchester, and is a solution of azo-resorufin,
(C24 H16 N2 07) in slightly ammoniated alcohol — methylated
will do. Little must be used, one grain is enough for a large
cell. In this substance the fluorescence is rather masked by
the natural colour of the solution being also red; but by
treating with bromine, a compound is obtained giving a blue

320

OPTICAL PROJECTION

solution, but also a red fluorescence, though rather more dull.
Cyanosine (methyl-tetraiodo-fluorescein) Mr. H. G. Madan
tells me gives a fine orange fluorescence. It too must be
dissolved in alcohol. And Mr. Jewsbury tells me that fifteen
parts glacial acetic acid and one part essential oil of pepper-
mint, heated to nearly boiling point, give a red fluorescence.
Saffron in alcohol fluoresces red-brown.

Of blues, the sodium salt of B-naphtholsulphonic acid
fluoresces rather more powerfully than quinine. Nearly all
the petroleum lubricating oils fluoresce blue more or less, and
samples can be found that do so rather powerfully with the
lime -light. Such give the strongest blue I know as yet, but
not sufficient for designs on paper. The aniido compound of
phthalic acid fluoresces a bluish-green, but best in electric or
magnesium light.

182. Phosphorescence. — This phenomenon is readily
shown by exposing a large sheet of card coated with
Balmain's luminous paint to the beams
of an electric lantern or burning
magnesium, interposing something to
cast a distinct profile or shadow.
After some 50 or 60 seconds' expo-
sure, if the room is darkened, the ex-
posed surface will be seen to emit
light.

A set of tubes containing powders
which phosphoresce of different colours
can be obtained for about ten shillings ;
and if exposed to the same kind of
light (the lime-light will practically
answer if about 3 minutes' exposure
be given, or the tubes may be previ-
ously excited by sunlight) will glow
with their proper colours when the room is darkened.

The connection between phosphorescence and fluorescence

e E

FIG. 174.— Tyndall's Phos-
phoroscope

THE SPECTRUM 321

maybe exhibited by Becquerel's phosphoroscope ; but Tyn-
dall's apparatus is simpler and more effective. A square box
A, shown in plan in fig. 174, is fitted either with an arc lamp
or magnesium burner, as already described, at c, and in one
side is a perpendicular slot B, all else being closed except a
sight-hole for regulation of the light. Outside the slit, on a
vertical axis, the wooden cylinder D revolves rapidly, driven
by bands E E from a multiplying wheel. The cylinder is
painted over with some strongly fluorescent substance. Pro-
fessor Tyndall used uranium glass powdered and laid on with
gum, but uranine in gelatine is much easier to use, and even
more effective. On rotating the cylinder, if there were no
persistence of effect, no radiation would be visible, the cylinder
being only directly illuminated through the slit ; but owing
to the duration of the vibrations set up by this momentary
illumination, the whole cylinder glows with the characteristic
fluorescent light.

CHAPTER XX

INTEBFERENCE OF LIGHT

183. Propagation of Waves.— This may be illustrated by
revolving the edge of a blackened glass disc, round the edge of
which is traced a sinuous wave-line, in front of a coarse
grating of perpendicular lines scratched on another piece of
blackened glass, after the method of Crova. But a slide
which I devised projects the phenomena in a manner both
simpler and better. It is shown in fig. 175. The grating A
of perpendicular lines scratched (pretty coarsely, and one-
sixteenth of an inch apart) on black-varnished glass, is
inserted flush with the surface of the slab of wood c which
holds the slide together, and which has an aperture cut behind

y

322

OPTICAL PROJECTION

the grating. In the top and bottom edges of the slide are
cut longitudinal grooves, B B, the whole length ; and in these
grooves slides a panoramic strip of glass about 18 inches long
which is also black-varnished, and has the form of the wave
traced boldly through it. On drawing this sliding-piece
along, the wave will appear in motion ; and if one or two of
the perpendicular scratches be distinguished by extra width,
or a wash over it of transparent colour, it can be pointed out
upon the screen that every single dot only moves up and
down, and that the wave-motion consists in the similar

motions of successive dots
being a little later in time.

I constructed another
single -wave slide in another
manner, bending a piece of
wire round a glass tube into
a helix, about |-inch dia-
meter and f -inch pitch. Re-
moving the core-tube, the
straight ends of the wire were
then brought in to coincide
with the axis of the helix,
and the whole mounted in
a wooden frame so as to be

revolved by a small winch-handle, in an aperture just a little
larger than allowed the whole helix to be seen in profile.
This being placed in the stage and projected on the screen,
on turning the winch an apparently plane wave in "black
appears in motion, which may be continued ad libitum. By
affixing two or three bits of very fine wire as spurs here and
there, or a morsel or two of wax, the purely transverse motion
of any given point may be shown in this slide also ; and it
may further be used to illustrate the propagation of a circular
wave.

J84. Interference of Waves. — The slide partly shown in

FIG. 176.— Wave Slide

INTERFERENCE OF LIGHT 323

fig. 175 was however chiefly designed to illustrate inter-
ference, which it does in a manner superior to any other.
Instead of the one simple wave-glass just described, when
demonstrating interference the sliding part is divided into
three strips as shown in fig. 176, kept together edge to edge
in one plane by a light binding or frame at each end, which
is cemented to the two outer strips, but allows the middle one
to be moved endways. On one outside strip of blackened
glass is cut a wave, and on the other a wave of double the
length ; and the middle strip has a wave of each length,
as shown in the figure. It is convenient to colour over the
long waves red and the short ones blue, with transparent
colour. All the
waves are first
shown in similar
phases, pointing
out that two vi-
brations of the
short ones take
place in the same

time as One Of the PIG. 176.— Interference Slide

long ones. Then

by drawing along the centre strip half the length of the longer
wave, it will be seen how this measured retardation of one
pair of waves brings the long (red) waves into opposite phases,
whilst the short ones (blue) are still in the same phase.

The superposition as well as destruction of vibration may
be shown by such a model as is used at the Koyal Institution.
A wave is cut in the top of a board standing on edge. Any
number of thin rods sliding perpendicularly in the same plane
in a light framework, with narrow spaces between, are cut
of such a length that, when standing on a flat surface, their
tops give a similar wave. When the frame of rods is placed
so that the longer rods stand on the hollows in the top of
the board, the profile at the top is horizontal, or the wave is

324 OPTICAL PROJECTION

destroyed ; but when so placed that crest is superposed on
crest, there is a wave of double the height. Every arrange-
ment is projected on the screen quite sharply by the shadow
method (§ 109). The balls at the top of a Powell's wave
apparatus are easily projected in the same way.

185. Thin Films. — The readiest methods of demonstrating
the interference of light are by reflection from the two surfaces
of thin films. When the lantern is worked upon a table, a
small black tray about 12 inches diameter may be partly filled
with water, may be laid down in front of it, and the lantern
either canted up behind so that the parallel beam from the
flange nozzle comes down at an angle upon the water and is
thence reflected rather upwards to the screen ; or the long-
focus lens may roughly focus the surface upon the screen,
being adjusted in the rising reflected rays ;
or the beam may be deflected downwards
from the nozzle by the plane mirror, and
received and re-deflected in their upward
path by another plane mirror, the surface
being focussed. Then dipping a rod in oil
FIG. i77.-Fiim of oxide of turpentine, and letting a drop fall upon
the water, it rapidly spreads into a thin film,
which produces beautiful colours upon the screen.

A wide-mouthed goblet of water may be placed in the
phoneidoscope apparatus shown in fig. 142, the surface of the
water occupying the place of the soap-film. Only very thin
and fresh oil of turpentine will show colours readily in a
confined space like the goblet, the glass preventing the
spreading of the film sufficiently ; but benzol containing a
very small quantity of Canada balsam or turpentine in solution
will act very well in such an apparatus.

Thin films of oxide show the same phenomena very readily,
placing a polished steel plate on a light tripod with a spirit-
lamp underneath, in the apparatus shown at fig. 142. If the
polished plate is first slightly warmed uniformly, and smeared

INTERFERENCE OF LIGHT 325

with a film of solid paraffin, then wiped nearly off again,
coloured rings will be exhibited more readily.

186. Soap Films. — These offer the most impressive de-
monstrations. Plateau's solution is made by dissolving 1
ounce of soda oleate in 40 ounces of distilled water, and
mixing this with 30 ounces of Price's best glycerine. This is
violently shaken up at intervals for several days, and then
filtered clear at as cool a temperature as practicable. When-
ever the solution becomes turbid, it must be filtered again,
and must always be brilliantly clear for use, and all vessels
(such as saucers) made scrupulously clean. In very cold
weather, however, the turbidity thus caused may be removed
by gently warming the solution and all the vessels used. I
think a little Marseilles soap (£ ounce to the above quantity)
somewhat toughens this for the variable English climate;
and for immediate use a little gelatine added to a small
quantity of Plateau mixture will add toughness, but it de-
composes, and therefore will not keep. A Eoyal Institution
recipe, not nearly so tough, but which is free from streakiness
and gives the black effect rapidly, consists of five volumes
glycerine and six volumes of a 3 per cent, solution of potash
oleate, a ' soft ' soap. I am however indebted to Herr Dahne,
of Dresden, for a still handier method, which he has worked
out with care, and which allows the solution to be mixed as
required, even in a test-tube, from separate ingredients kept
in stock. Keep in bottles (1) a saturated solution of soda
oleate in distilled water, carefully neutralised and filtered clear
in the cold ; (2) distilled water ; (3) Price's best glycerine,
tested free from acid. Then the liquid is mixed variously as
follows, according to the purpose in view :

(a) For greatest toughness or lasting properties (not
always desirable, as extreme toughness is apt to produce a
ropy or streaky appearance) : take one volume oleate solu-
tion, one volume glycerine, and two volumes distilled
water.

326

OPTICAL PROJECTION

(b) For beautiful coloured bands, quickly developing : one
volume oleate, J volume glycerine, and four volumes water.

(c) For rapid development of the black spot : one volume
oleate, five volumes water, and only a trace of glycerine.

To use soap solutions we require a little apparatus, in
the shape of wire rings. They are best made of iron, or
tinned iron wire, with a stem bent at right angles from the
junction, which must be soldered and then smoothed carefully
off. Some about 2 inches diameter, as A (fig. 178), are used by
nipping the stem horizontally (with the ring turned upwards)

in a Bunsen holder as
at c : some about 3
inches diameter are in-
serted in small wooden
feet as at B. Before
use the rings should
be made hot, and then
rubbed with solid paraf-
fin, which will run into
a thin coating, and
prevent the wire from
cutting the films.
Eings up to 4 or 5

inches diameter, dipped in a saucer of ' tough ' solution and
carefully lifted, will take up a film.

Bubbles are best blown by a small glass funnel with
ground edges, about an inch in diameter, to which is attached
a small rubber tube for the mouth. With a tough solution
I have blown a bubble over half a yard in diameter ; and if
a clean saucer be carefully soaped to the edge, and all froth
avoided, a bubble nearly that size may be blown upon the
saucer itself. A smaller size is however safer ; and if the
parallel beam from the lantern nozzle is deflected downwards
upon this and reflected to the screen, fine colour-fringes will
appear. If two or three stands, such as B in fig. 178, be

FlG. 178.— Rings for Soap Films

INTERFERENCE OF LIGHT

327

placed in a row, bubbles 9 or 10 inches diameter can be easily
placed upon the rings, and the parallel (or slightly divergent)
beam being sent through them all, will produce a very fine
effect.

Flat films produce the best optical phenomena. Taking
one up by the apparatus o in fig. 178, the whole arrange-
ments are as shown in fig. 179. Here L is the nozzle of an
ordinary lantern with the objective on, but the parallel (or
rather slightly convergent} beam from the open flange nozzle
is better; the rays
striking on the soap-
film A. The lantern is
deflected nearly par-
allel with the screen,
and the film A stands '
at an angle of 45°
with the beam, so as
to reflect the rays to
the screen, on the
way to which they
are focussed by the
loose lens p.1 Bands
of colour speedily ap-
pear across the film, & ]*z\
which gradually pass PIG 179 _plat SoapFilm
upwards (the image

being inverted) as it thins, until at length a yellowish-white,
and finally the grey-black appears, after which it soon breaks.

1 This experiment being typical, the arrangements should be noted. On a
table, the pieces of apparatus will be simply arranged separately, standing as
figured. With a revolving stand and board such as fig. 99, the holder bearing
the film A will stand on the longitudinal board ; and the cross-piece c D (fig. 99)
being brought to that spot, the lens p will stand on that at the required
distance. This general arrangement governs all experiments where focussing
has to be done after the deflection of the rays, by reflection or otherwise, hag
been effected.

H2$ OPTICAL PROJECTION

Sometimes the heat of the lantern will destroy the film
too rapidly. In that case an alum cell in the slide-stage will
remove the difficulty.

Horizontal bands being caused by the action of gravitation
on the film, if we can make the gradual thinning take a
circular form, the bands will become annular. This is
effected by a beautiful modification of the experiment often
employed by Lord Eayleigh, and which is very easy if any
sort of bellows, or weighted bag filled with air, be at hand,
to furnish a slight blast. Even the breath will answer, but
needs some practice and a thin rubber balloon to steady the
pressure. Connected with the air-blast (a gentle one) by
rubber tubing is a piece of glass tube drawn into a small but

A B

FIG. 180.— Rotating Films

not capillary orifice : one of the ' fillers ' sold for use with
fountain pens answers excellently. This is fixed in another
small Bunsen holder, and adjusted so as to direct a blast at a
small angle with the surface of the film. Adjusted near the
edge as at A (fig. 180) the above is converted into a single
whirling vortex, which shows rapid and magnificent gradation
of colour : adjusted as at B, a little to one side of the centre,
the stream divides into two vortices, in which the play of
colour is still more rapid. This experiment never fails to
'bring down the house.'

187. Films of air— Newton's Rings. — Two squares of plate
glass, with rounded edges, cleaned and polished, are readily

INTERFERENCE OF LIGHT 329

worked together till they show beautiful air-films. It is well
then to keep them together with spring wooden clips, one at
each corner, pinching one corner in the Bunsen holder, and
projecting exactly as the soap-film in fig. 179. The slightest
additional pinch between finger and thumb alters the colours,
thus leading up to Newton's experiment.

It is not easy to get a good pair of Newton's rings, which
under pressure give true circular figures. The usual three
screws are too few, causing distortion ; six screws, with another
at the centre of the back glass, give better figures. If the
back glass is black, or some dark colour, the effect is better ;
the colours not being diluted by the reflected light from its
bottom surface. The rings are projected exactly as the soap
film. It is most convenient to have a stud projecting from
the circular frame, which fits into the socket of one of the
pillar- stands.

If a wedge-prism (not too thin) be interposed between
the focussing lens and the screen, if will be shown that in
the deflected image the number of rings is very greatly in-
creased on one half of the image.

By rapidly interposing in turn a red and a blue glass
between the lantern and the glasses, it is shown that the red
rings are larger than the blue ones ; but it is better to have a
red and blue glass framed so as each to occupy half of the
space in an ordinary lantern slide-frame. Holding this as
close as possible to the lenses, but so as to allow the reflected
rays to escape to the focussing lens, one half the image will
exhibit the red and the other the blue rings. It must be rather
a light blue glass.

Having another pair of glasses, of which the lower one is
platinised on the surface (silvering reflects too much light,
and overpowers the rings), and adjusting the glasses so that
the light from the lantern falls on them at the polarising
angle, the reflection from the first or glass surface of the
film of air can be totally abolished by placing a Nicol prism

330 OPTICAL PROJECTION

on the nozzle of the lantern and turning it into the proper
position (see Chapter XXII.) and the disappearance of the
rings, though there is light upon the screen, will demonstrate
that the reflection from both surfaces is necessary to their
production.

Finally, the light destroyed by interference can be shown
by spectrum analysis of a slit crossing the rings. In blue
or red light such a slit would give an appearance like B or n
in fig. 181. It has been shown already that in blue light the
bands B are closer than in red light, hi which they appear
comparatively as at E. If, then, the slit across the rings is
dispersed into a spectrum, the largest and smallest rings

must be connected
by curved lines
somewhat as in
fig. 181. The ar-
rangements for
the experiment
are shown in fig.
FIG. isi 182. The slit

may be cut out

of black paper and fixed to the front of the glasses, as at L,
being illuminated by the parallel beam from the flange nozzle
of the lantern N, the slit being focussed by the lens P and
dispersed by the prism P as usual. In this case the lantern
must be turned quite away from the screen, and the light
fall on the lenses very nearly normal, as shown in the figure ;
else the incident light will not be able to get down to the film
and out again through the slit, owing to the thick glass.
An easier way is to place the slit on the nozzle of the lan-
tern, with the parallel beam sent through it, and, bringing
the glasses as close to it as possible, to focus the reflection
of the slit in the glasses. A spectrum will in either case
appear, crossed by beautiful parabolic dark bands.

A soap film is easily analysed in the same way, using the

INTERFERENCE OF LIGHT 331

second method described. In this case a fresh film should
always be taken up after all arrangements are made, as the
bands are most numerous when the film is thickest. They
will be seen to shift as the film thins, but care must be taken
to avoid a current of air on the film as far as possible.

The colours of a film of condensed vapour are readily pro-
jected according to Eeade's method. A plate of glass is
rubbed with soap, and then nearly cleaned off with a chamois
leather; the plate is then adjusted in the Bunsen holder.
Then taking a piece of rubber tube about a foot long and
| -inch in bore, this is gently breathed or blown through, with
the othor end directed against the centre of the soaped side,
which is projected
like the soap film.
Coloured rings will
appear on the plate.
One precaution is
necessary, however:
as the experiment
depends upon the

condensation of the pio 1S2._AnaIysIsotEiDg3

breath by cold, an

alum trough must be placed to intercept the heat of the
lantern, and the plate must be cold when placed in position.

Any piece of tolerably flat iridescent glass may also be
projected.

That light is destroyed by interference, even with films
too thick to exhibit colour, may be shown by analysing the
light from a slit, as in the analysis of the soap film, from the
film of air between two pieces of plate glass pressed but not
rubbed together ; or by carefully splitting a very thin film of
mica, and bending it round a blackened convex surface ; the
reflection from this will itself give a slit, or rather a line of
light, if placed in the parallel beam. The spectrum will be
crossed by a number of dark parallel lines ; more and thinner.

33? OPTICAL PROJECTION

in proportion to the thickness of the film, thicker and fewer
in proportion to its thinness.

188. Thick Plates. — Newton's experiment with a thick
concave glass mirror is pretty easily projected with a good
light, using a large mirror optically worked on both sides, of
2 to 4 feet focus. This is to be slightly dulled on the surface
by laying on a film of milk and water ; milk alone is much too
opaque. A radiant point must now be adjusted at the centre
of curvature, so that the incident rays are reflected back and
focussed in the same point. With the arc light the best plan
is to place a rather small aperture on the lantern-nozzle, and
converge through it all possible rays from the condensers ;
then upon a screen of white card adjusted over the aperture,
with a small hole in the centre for the rays to pass through,
beautiful interference-rings will be seen, which will be very
brilliant if all is adjusted properly. The same plan may be
adopted with the oxy-hydrogen light ; or the naked jet may
be brought out of the lantern, and adjusted with the incan-
descent face to the mirror, and the back to the screen. This
latter will thus be shielded from the direct light from the
lime, further stopped back by a small circular stop behind it
if necessary ; while the rings formed in the air in the plane
of the lime can be fairly focussed on the screen, if the light
is very brilliant, with a large lens.

189. Fresnel's Prism. — The interference bands from
Fresnel's inclined mirrors are practically impossible to project
upon a screen. Even with his bi-prism, they are rather
difficult with the 0. H. jet, but easier with the arc light.
The best arrangement is to use the optical front, with the
objective removed and replaced by the plain nozzle, carrying
an adjustable perpendicular slit. In the optical stage is placed
a cylindrical lens, about 5 niches focus, and 2 inches diameter,
mounted in a wooden frame. This is to condense a large
amount of light upon the slit, light and lens being so ad-
justed that the parallel beam is focussed upon the slit. From

INTERFERENCE OF LIGHT 333

the slit the light diverges and falls upon the bi-prism, which
should for this experiment be about 2 inches square, so as to
intercept the whole pencil of light at a convenient distance
from the slit. Perpendicular fringes will appear on the screen.
Sometimes they appear more plainly by interposing a red glass ;
and are made more conspicuous by interposing an achromatic
focussing lens between prism and screen, at considerably more
than its focal distance from the prism ; the lens then projects
the bands in the focal plane of its own conjugate focus. Much
depends upon the screen distance, brilliance of the light,
and angle and workmanship of the bi-prism. A small angle
gives the broadest fringes, and I was only able to obtain
visible results with the lime -light by employing a fine prism
ground for the purpose by Mr. Ahrens, of very small angle.
With this the fringes were
amply conspicuous for a good-
sized class-room, but would
require the arc-light for a
large lecture theatre. This is
one of the few cases in which
an achromatic focussing lens " FlG 188

gives far superior results, the

bands being more sharply defined. An achromatic of about 8
inches focus and 3| inches diameter is the most suitable for
this particular experiment.

A smaller bi-prism, about an inch square, may be obtained
for about 5s., and will project visible fringes with the arc
light ; but for the 0. H. jet must be used in another way. It
may be mounted in a short cell which fits into the nozzle of
the optical objective, and will thus divide the image of any
object in the stage into two, which cross or overlap each
other. That object in this case is a black card 4 x 2£ inches,
cut into equal stripes as in fig. 183, or the bright lines may
be scraped away on blackened glass. A few slides of different
gauges should be prepared, as the best effect is produced by

334 OPTICAL PROJECTION

different widths at different distances. On focussing the
slits upon the screen, conspicuous colour will be seen where
the images overlap, and that these are due to interference is
readily shown by covering up half of the bi-prism.

190. Billet's Lenses.— If difficulty be found in procuring
a good bi-prism, similar results can be obtained with the arc-
light by employing a convex lens cut in half, and the two
halves separated by a small distance, filled with a strip of
some opaque substance. The section is of course arranged
perpendicularly, parallel with the slit. Such a split lens is
easily mounted on any sort of stand, and is used in place of
the bi-prism. I have not however found it give sufficiently
distinct fringes with the lime-light. A lens of about 2
inches diameter and 6 inches focus is perhaps as effective as
any.

191. Diffraction.— There is not light sufficient, unless
with a powerful arc, to show on the screen the spectra from a
single slit ; but gratings, either original or photographed, give
fine projections. Of straight-line gratings on glass, copies of
Norbert's 3,000 lines to the inch pattern are most generally
useful, but the 6,000 also gives fine projections. It is sufficient
to place a metal or card slit about 2 mm. wide in the optical
stage, and focus on the screen ; then hold the grating in the
rays. Two gratings crossed give beautiful crossed spectra,
using a small hole instead of a slit. The effect is much
brighter if the aperture is used on the open optical front
with all the light from the condensers converged upon it as
far as possible, or with the attachment shown in fig. 95, and
is focussed on the screen by the loose lens. With this aid,
the diagonal spectra will be seen, as well as the primary
crossed ones. By interposing coloured glasses the spectra
may be reduced to coloured images of the slit, the red ones
being farthest apart.

A circular grating, used in the same way to diffract a
small round aperture, gives circular rainbows. I have had

INTERFERENCE OF LIGHT 335

magnificent gratings cut upon glass by Mr. Clarke, of nearly
3 inches diameter, which project brilliant spectral rings,
using a large focussing lens. A piece of coarse perforated
zinc held near the grating affords further beautiful pheno-
mena.

Wire gauze can sometimes be procured fine enough to
give perceptible spectra, using a large lens and a piece 6 inches
square ; and spectra can often be obtained by diffracting through
a piece of the finest cambric, carefully selected, or through
the web of the feather of some fowls and birds.

Circular halos are produced by diffracting a circular aper-
ture through a glass dusted with lycopodium powder, or any
similarly small spores ; on one condition. The ' resolution '
of a grating depends upon the number of lines or dots brought
into use, and a dusted plate 3 inches diameter will not give
distinctly coloured halos on the screen. But plates 6 inches
diameter, used with a lens not less, and holding the plate
close to the lens, so as to pass the rays through the whole
effective surface, give really fine halos. These dusted plates
are best prepared by shaking the spores through fine muslin
from some little height, upon clean plates which have been
washed with water containing a few morsels of gelatine. If
gently breathed upon, the spores will be caused slightly to
adhere ; and after the surplus has been shaken off, a circular
mask of black card should be laid on the surface to preserve
from contact, another clean glass plate laid over this as a
cover, and the two bound all round with gummed slips of
dark paper, like lantern slides. Such plates should be of
thick glass, or the pressure of the fingers may bring the
plates into contact and spoil the even distribution of the
spores. Gratings and dusted plates can be mounted in
frames and used on stands if desired, but the hand is practi-
cally sufficient.

192. Reflecting Gratings and Striated Surfaces. — An
ordinary photographed glass grating will project very fair

336

OPTICAL PROJECTION

spectra by reflection ; but much more brilliant spectra are
projected if it be silvered, or by gratings ruled on speculum
metal. Formerly the best of these were by Mr. Rutherford,
of New York, but for years past those ruled by a machine
constructed by Prof. H. Rowland have surpassed all others.
The gauge of these ranges from 10,000 to 30,000 lines to
the inch, the 17,000 gauge being about the best ; and they can
be obtained from Mr. W. Brashear, of Philadelphia, of various
sizes ; and either ruled on a plane surface, or one slightly
concave, so as to give a focus without lenses. The condition
of projection is exactly the same as
with transparent gratings ; the slit
must be focussed on the screen as
reflected by the surface, and only
diffracted by the ruling.

One of those metal buttons con-
structed by the late Sir John Barton,
and known still as ' Barton's buttons,'
whose surface is divided into hexa-
gonal or other portions, each of which
is ruled with fine lines in a different
direction from its neighbours, will
yield beautiful projections, the spectra
being all arranged in the directions

of a six-pointed star. The ' focussed parallel beam ' from a
small aperture is intercepted by the grating, which reflects the
spectra on the screen.

A fine piece of polished mother-of-pearl also gives beauti-
ful diffraction colours by reflection, the colours shifting as the
plane of the pearl is a little altered in position. The easiest
way of doing this is to provide a small tablet of thin blackened
wood, D, with a boss, B (fig. 184), on the back, into which
is fixed a tube, T, fitting into the sockets of the pillar-stands.
The pearl, or any other object, such as a small haliotis shell,
is easily held on this by a couple of elastic bands, A peacock-

Fio. 184

INTERFERENCE OF LIGHT 337

feather stitched with black thread on a blackened card, can
be projected in the same way. The colour of the peacock
eye will change as its plane is altered ; but as the colour is
uniform and not in spectra, in this case the colour must be
produced by a thin film. (I believe a portion of the pearl
colour to be of the same character.) The plane is readily
shifted by turning the pillar a little ; and the object can
be readily rotated in its own plane by turning the tablet
round in its socket.

193. Perforated Plates,— Perforated zinc will give interest-
ing diffraction phenomena if the light is brilliant. Every
gauge procurable should be purchased ; then discs should be
cut out and mounted in 4 x 2£ wooden frames, for the optical
stage, and spring wires bent circularly will keep them in
place. Blackened perforated cards will answer, but the heat is
apt to ignite them. Placing one in the stage, and focussing
it rather beyond the screen, the interference of the various
small pencils of light passing through the apertures in the
card will produce coloured effects, rather brilliant a few feet
from the nozzle, where they may be shown by holding a
sheet of cardboard. Different gauges should be tried for the
best results. But still better are produced if another frame
and plate be introduced, at a distance varying from J inch to
some inches in front of the other, and either the same or
some other gauge, which experiment must determine; for
every difference in the power of the radiant, and in the screen
distance, will determine gauges and distance between the
plates which give the best effect. Sometimes a piece of gauze
as the front plate will give a good result ; as a rule, the finer
gauges of zinc or card give the best, and the coarser of two
different plates should be the posterior one. A little adjust-
ment of plates and focussing will produce beautiful coloured
patterns on the screen, the pencils from the first set of
holes being diffracted by the second. If one of the plates
is mounted in a rotating frame like fig. 195 (p. 348), and

338 OPTICAL PROJECTION

rotated, some very peculiar effects may be produced. In
these experiments the light in the lantern should be care-
fully adjusted to its best distance from the condensers.

Another beautiful method of projecting perforated plates or
cards I adapt from Prof. Dolbear's ' Art of Projecting ' with
the heliostat. The effect is, indeed, largely due to a strong
prismatic dispersion by the edges of the lens ; but the beauty
of the result makes the experiment worth a place. The
radiant is drawn back in the lantern, so that the rays from
the condenser cross in a focus 5 or 6 inches in front of it —
about at the end of, or slightly within, the flange-nozzle,
answers very well. Adjust in the optic axis another lens
of about equal diameter and of deep curve, such as one of
the condenser-lenses themselves, at a distance which gives a
luminous disc 6 to 10 feet in diameter. Now hold a piece
of perforated card or zinc (very coarse is best) in a position
easily found, either in front of or behind the second lens ; the
result will be most gorgeous chromatic patterns. Instead of
a simple plate a revolving Kaleidotrope (p. 144) may be used,
when the effect will be magnificent if the zinc is of coarse
pattern. As already stated, however, it is only partially due to
diffraction.

CHAPTER XXI

LANTERN POLARISING APPARATUS

194. The Elbow Polariscope. — The simplest and cheapest
polariscope for lantern work consists of a bundle of thin glass
plates as a polariser, arranged at the back of an elbow, as in
fig. 185. The end N of the elbow is made to fit on the flange-
nozzle of the lantern, and the elbow is of courss so constructed
that nearly parallel light from the condensers entering at N
falls upon the glass plates G at the polarising angle of 56°.

LANTERN POLARISING APPARATUS

339

FIG. 185

The other end of the elbow at B has a screw-collar, into
which screws the B collar of the optical front (fig. 94), with
stage, objective, and nozzle. Into this nozzle fits, so as to
rotate easily, an analyser, usually a Nicol prism. The whole
apparatus is shown in fig. 186, and is commonly known as
the lantern polariscope, or elbow polariscope. It is also
useful and convenient as a
table instrument for many
purposes, if a plate of finely-
ground glass is fitted into
the end which fits into the
lantern ; and will perform in
a most efficient and satisfac-
tory manner all ordinary ex-
periments, which do not re-
quire the rotation of the polariser, at a very moderate expense.
In using this instrument, it is placed on or in the flange-
nozzle with the elbow lying horizontally, so that the lantern
has to be deflected from the screen ; because the optical
portion of the instrument must be preserved in a horizontal
direction. To keep
it from turning
round in the flange
from its own weight,

there is either a slot *£ffl/ %^

fitting over a pin, ^ /sJO,

or a simple bayonet
joint.

The Nicol prism
analyser will per-
form all ordinary experiments, but its performance should
be examined. A Nicol of proper proportions will, with the
objective described in Chapter XII., just ' cover ' (i.e. give a
polarised field over) slides of the standard London pattern,

z2

186. — Elbow Polariscopo

340 OPTICAL PROJECTION

which have a disc If -inch diameter in 4 x 2 \ wooden frames.
A shade more may be sometimes covered.

Some improvements have lately been made upon the Nicol
prism. Prazmowski's prism, as further improved by Messrs.
Steeg & Eeuter, is a rectangular parallelepiped, cut from
edge to edge instead of from corner to corner (thereby giving
a pencil of double the area) and with the section at right
angles to the optic axis. This utilises better the two refrac-
tive indices, and moreover the joint is made with linseed-oil
instead of balsam : the total result being a shorter prism, of
wider area and more angular field. It, however, requires
more than double the spar, and on account of the work, is
about three times the price of a Nicol of similar area. A
simpler and cheaper improved prism was devised by Professor

S. P. Thompson, and is made by
taking an ordinary crystal of spar
_7 a very little longer than would
/ suffice for a Nicol, and cutting the

^^ ~.^v/ ends off so as to slant the reverse

) way, at the lines A B, CD (fig.
187). Had the spar been made
into a Nicol the joint would have been along the dotted line
A D ; but the considerably shortened piece is now cut along
B c, the section being thus nearly at right angles to the optic
axis (shown by the arrow), as in the Prazmowski. This prism
is not so good as the latter, but is simple and easy to make.

A complete apparatus should be fitted with analysers
showing the various methods of polarising light. A tour-
maline is easily mounted in a small cell which rotates in the
nozzle of the objective. Some tourmalines exhibit very little
appreciable colour upon the screen, but such are unfortunately
now becoming very scarce and costly. An analyser of thin
glass is easily constructed as in fig. 188, the plates being fixed
at the polarising angle in a tube whose end N fits into the
nozzler An aperture, E, at the side of the tube enables the

LANTERN POLARISING APPARATUS 341

reflected rays to be used as well as the transmitted ; and

thus one image may be thrown upon the screen, and the

complementary image at the same time upon the ceiling, if

there happen to be one. Thin micro-glass is sold at per

ounce in \\ x J- size, and twenty-four such plates, selected

for flatness, make the best analyser, and will answer very well

for a home-made instrument, both images being surprisingly

good if the glass is flat. The

plates may either be mounted

in a round tube, by cutting a

cork which fits the tube

obliquely at the proper angle, u FIQ 188

and cutting the glasses into

ovals round a shape made to the cork section, after which

the cork is hollowed through longitudinally into a tube ;

or rectangular glasses may be fitted into a square tube, with

a round nozzle at the end.

A double-image prism should also be provided, unless two
are combined in a Huygens apparatus, when one of these
will of course suffice.

195. The Nicol Prism Polariscope. — The elbow in-
strument has two inconveniences : the polariser is fixed ; and
the lantern has to be de-
flected, which is awkward,
more especially if diagrams
are required between the ex-
periments. The most perfect Flo 189
instrument is unquestionably

that in which the polariser, as well as analyser, consists of a
Nicol prism. It was formerly the custom to have two prisms
of nearly equal size for polariser and analyser, and such an
arrangement is still employed at the Eoyal Institution. As I
long ago pointed out, however, hi all ordinary experiments we
have to focus the rays by some sort of objective ; and it is mani-
fest from fig. 189 that just as many rays get through a smaD

342 OPTICAL PROJECTION

prism as a large one (if not too small to transmit the smallest
diameter of the pencil). The conveniences in manipulation
are so great, while the absorption of light is less, that so far
from a large analyser being an advantage, where such are
used the screen effects are inferior to those constantly ob-
tained by more modern and improved instruments. I add to
this, as Professor S. P. Thompson once remarked at the
British Association, that it is ' almost a sin ' under present
circumstances to throw any large prisms away as analysers,
when they are so badly wanted at the other end !

To cover the standard slides, a Nicol is required as
polariser not less than 1J inches clear diameter or field, and
2 inches is of course better still. Fig. 190 on the opposite
page is a section of a complete apparatus, as I arranged it for
my own use. It was constructed for me in the first place by
Mr. Darker, and with some slight alterations Messrs. Newton
& Co. have since made many instruments after the pattern
here shown, which has also been very generally adopted by
other opticians, as best fulfilling the requirements of the de-
monstrator at the smallest expense compatible with a Nicol
polariserj The references will explain the construction, and
the external appearance of the portion of the apparatus
in front of the polariser is the same as that of the front part
of the apparatus shown in fig. 194. The stage aperture
should be made large enough to take quite a thick wooden
frame, with another frame containing a quarter-wave plate or
an even film : sometimes the instrument is made with a
separate aperture or stage for a quarter- wave plate, between
the ordinary working stage and the polariser. The crystal
apparatus can all be removed or added in a few seconds, and
I prefer the analyser mounted with differently -sized collars at
the two ends, one of which fits the nozzle of F for ordinary
work, and the other of which, when pushed in, can be adjusted
to the proper position for crystal-ring projections to pass
through it. A quarter-wave plate can be inserted if necessary

LANTERN POLARISING APPARATUS 343

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344 OPTICAL PROJECTION

between H and K. The whole instrument is about 24 inches
long.

For convenience at different distances, it is well to have
a polariscope fitted with several powers. I have a second
back lens of 6-inch focus, as well as the usual 5-inch back
lens ; then, by removing the front lens of the power when
needed, there will be a range of powers of about 3|, 4, 5, and
6 inches. They are easily adjusted by screwing in a lengthen-
ing ring or adapter. Another plan often adopted is to have a
concave amplifying lens mounted in front of the instrument,
which amplifies the image precisely as described in § 93,
Chapter XIII.

An alum-cell may be used in the ordinary lantern-stage
with this polariscope, but is not really necessary. The glas?
caps protecting the Nicol should be removed for a few minutes
after lighting up, in order to allow the dew to escape which
will at first form on the end surfaces. In ordinary work the
stage and power s F is pushed close up to the end of the
polariser, and not kept apart from it, as here shown for
clearness. Occasionally a tube of fluid may be inserted
between the polariser and the stage, then the stage with power
is drawn forward sufficiently to allow of this.

It is very easy to adapt a low microscopic power to this
polariscope, by fitting a tube into the nozzle of F with a slot
or slide-stage at the point where the lenses F condense the
light into the smallest area. In the other end of this tube
slides (for focussing) a power of about two inches, and the
analyser. Such a power will exhibit many fine rock sections
or ordinary micro slides, which the ordinary 3^-inch power
is unable to do. But for anything like real microscopic work,
resort must be had to the polarising microscope described in
Chapter XIII.

Great care should be taken of fine calcite prisms. The
ends should be cleansed only with a clean, round-nosed camel-
hair brush, often called a * sky-brush ' at artists' shops. If

LANTERN POLARISING APPARATUS 345

absolutely necessary to apply chamois leather, it should be a
piece of the softest, well beaten till free from dust, carefully
washed, and finally cleansed from all soap and grease in
alcohol. The polariser should always stand with the balsam-
joint perpendicular when the instrument is put away, else
the weight of the top piece of spar may produce ' thin film '
colours in the balsam layer.

Unfortunately Iceland spar has for some time ceased to
be imported, and been very scarce, so that it is doubtful if
material could at this moment be found for another large
prism, except the two specimen blocks in the British
Museum. This has confined opticians to smaller pieces. Mr.
Ahrens has constructed an ingenious prism of three pieces
as in fig. 191, the arrow showing the
optic axis. These form admirable ^,
polarisers, and only need spar of half \
the usual length for the same sec-
tional area : but every one of several I
have tested fails as an analyser, the
edge of the wedge producing a percep-
tible though faint duplicate image, FIG. isi.-
and the latter being distorted. He

has constructed also several polarisers of two inches square or
more, made in four separate prisms, which have been used
by Messrs. Newton & Co. for polariscopes and given satis-
faction. I have, I think, tested every prism so made ; and
the lines of junction being out of focus were not distinguish-
able, while the whole polariser was of less length than width.
Unfortunately spar has lately failed even for these prisms, so
far as projection instruments (other than microscopes) are
concerned, and this has caused attempts at constructing
' direct ' polariscopes in other ways. The most obvious ex-
pedient is a bundle of glass plates in a tube as in fig. 188,
only larger, using the transmitted light ; but unless so many
plates are used as to absorb much light, it is difficult to get

346

OPTICAL PROJECTION

a dark field. But all practical purposes have lately been secured
by a polariser described by Delezenne, and which gives a field
of any size at a very moderate expense.

196. Direct Reflecting Polariscopes.— The principle is
simply that of bringing the reflected beam from an elbow
polariscope, by another reflection, back into the original
direction of the rays from the lantern. It has been applied
in practice in two different ways. Mr. Ahrens has constructed
a combination shown in fig. 192, where T K is a totally re-
flecting prism of glass, so cut that its end faces are normal

FIG. 192

FIG. 193

to the horizontal incident, and the emergent rays, which are
totally reflected midway at the polarising angle ; then these
rays falling on the polariser p of black glass, or two or three
thin plates with a black glass at the bottom, also at the
polarising angle, are reflected in a horizontal direction into
the instrument. This polariser is efficient, but two objections
to it are the needless expense of so large a glass prism, and
that the absorption in so large a mass is very perceptible.

A cheaper and upon the whole better arrangement was
adopted by the Kev. P. E. Sleeman. In this the parallel beam
is first reflected by the silvered mirror B (fig. 193), and then

LANTERN POLARISING APPARATUS 34?

re-reflected into a horizontal direction and polarised by
the pile P with a black glass at the bottom. This arrange-
ment can be made cheaply and readily of any size desired.

It will be observed, that while the horizontal direction of
the beam from the lantern is preserved in the Delezenne
polarisers, it is necessarily deflected several inches to one
side of the axis of the flange-nozzle. In the first instruments
made on this principle, the beam was brought down below
the nozzle ; but this makes the apparatus rather deep, and
requires a deep case for packing. To avoid this the polariser

FIG. 194.— Direct Reflecting Polariscope

was turned sideways ; but that was found awkward, and also
necessitated a large case. To meet these objections I advised
reversing the polariser so as to deflect the beam upwards, and
Messrs. Newton & Co. now construct their instruments as
in fig. 194. It will be seen that it is then rendered quite
compact, and requires no larger a case than the Nicpl prism
polariscope. All the front portion is precisely the same as in
the instrument shown in section in fig. 190.

Such polarisers are liable to the same objection as the
elbow form, not being capable of rotating the polarised beam*
This can, however, be effected through a simple expedient
suggested by Professor S. P. Thompson. If we cause the po-
larised rays to pass through a mica quarter- wave plate (§ 211)

34S

OPTICAL PROJECTION

whose planes are at an angle of 45° with the plane of polarisa-
tion, the beam becomes circularly polarised ; and if in front
of this mica plate we place another quarter-wave plate which
can be rotated, the beam again becomes plane-polarised in
any plane we please, according to the position of the second
mica plate. Such an arrangement of two quarter- wave plates
can either be mounted in front of the polariser (the second
one in a divided circle with four spokes as usual), or a slot
may be provided before the slide stage, in which one mica is
placed mounted in a frame, and the other in a rotating frame
like fig. 195.

The 'bright field' produced in this way is as good as
that of the Nicol polariser ; but if the two quarter-wave plates
are gauged as usual, by the sodium flame, the dark field will
show a perceptibly reddish tinge — the ' tint of passage.'
This is not pleasant, and I consider it better to choose a mica
film very slightly thick, when the little residual colour will
be blue, which in the faint light will not appear perceptibly
inferior to a true black.

197. The Rotating Frame. — The greater part of sub-
sidiary apparatus will be best mentioned as required, but

one item should be
mentioned here, as in
continual use. It is a
frame of standard size,
in which films or other
preparations can be
rotated whilst in the
slide-stage of the polari-
scope. This is effected quite simply by a small pinion gearing
into a circular rack ; and the preparations, which are cemented
in balsam between two glass discs the same size as those used
for slides mounted in wood, are kept from falling out of the
brass cell by a spring. Several of these rotators will be found
very serviceable, the cost being only about 4s. 6d. each.

FlG. 195.— Rotating Frame

POLARISED LIGHT

349

CHAPTER XXH

POLARISED LIGHT

198. Double Refraction. — This can be demonstrated in a
simple rhomb of calcite, by employing the ' focussed parallel
beam ' (p. 277) from a pin-hole aperture on the flange-
nozzle. The spar ought not to be less than four inches long
if the separation is to be easily visible, though its other
dimensions may be quite small, such as half an inch in the
side. A wider separation is shown by placing an aperture in
the stage of the optical front, and a double-image prism on the
nozzle, focussing the aperture as small discs upon the screen.

199. Huygens' Experiment. — For this a pair of double-
image prisms are mounted somewhat as in fig. 196, N fitting
into the nozzle of the optical front, in front of the power.
The first prism A is mounted in this

tube, the second prism B in a cell
which can be rotated easily. The
two prisms must be chosen to
match, and a slot or stage s should
be provided between them. Focus-
sing the aperture, the alternate dis-
appearance and augmentation of

the images will be seen on rotating the front prism. The slit
8 is for the insertion of a film of selenite between the two
prisms, which will give tl»:e beautiful phenomena of comple-
mentary colours. Inserting rather a larger aperture in the
stage, so that the pair of discs somewhat overlap, it will be
shown that the two overlapping colours always produce white.
It is well to have a small slide in which are mounted two
such selenites, one giving a green and red, and the other blue
&nd yellow.

FlG. 196. — Huygens' Apparatus

HO OPTICAL PROJECTION

By taking out the front prism and substituting the Nicol
prism — all these parts being made to fit, for obvious reasons
— one half the phenomena will be suppressed, and the demon-
stration analysed and simplified.

200. Tourmalines. — The phenomena of tourmalines are
simple and convenient illustrations of the essential phenomena
of polarisation. Light brown or neutral tint tourmalines are
far the best ; green ones appear coarse and unpleasant. One
plate about an inch long and \ inch wide should be cemented
with balsam in the centre of a glass disc and mounted in one
of the standard wooden frames, and another about f inch
long and \ inch wide mounted on another loose disc, to be
used in the rotator (fig. 195). These different shapes and
sizes exhibit one crystal plate clearly over the other. The

two plates are placed
together in the stage

°f the optical front,
and the crystalg fo.

FIG. 197.— Tourmaline and Prism -, -, .

cussed as an object

on the screen : then as the front one is rotated the gradual
darkening will be seen, until total extinction is produced when
the two are crossed.

These phenomena will be simply correlated with those
presented by the calcite, by leaving in the stage the rotating
tourmaline alone, and placing on the nozzle a single double-
image prism. It is better if this be not one of the Huygena
pair, but constructed of two pieces of spar after the manner of
Wollaston or Rochon, so as to give double the separation
(such an arrangement gives too much colour to be desirable
for Huygens' experiment). A plate of thin metal should be
placed in the stage with the tourmaline, having a round
aperture which just clears the rotating crystal and no more;
and the discs from this aperture should stand clear from one
another on the screen. Let the tourmalines first stand ver-
tically, as at A (fig. 197). Then one image will be transparent

POLARISED LIGHT 351

and the other black ; but on rotating the crystal, this will
reverse as at B ; or it will be also reversed by rotating the
prisni to a position at 90° angle with the first.

201. Piles of Glass.— The effect of these is most conveni-
ently demonstrated by mounting a pile of from twelve to twenty
plates, B c (fig. 198), of the thinnest and most colourless glass
that can be procured, in a frame screwed to the edges B c of
two triangular pieces of board BCD, which are screwed to a
base-board B D, and whose angles are such that a horizontal ray
E F striking on the glass, is reflected at the polarising angle to
G, while the unreflected portion (i.e. of common light) is trans-
mitted to H. The thin glass analyser (fig. 188) would answer
the same purpose, but its arrangements cannot be seen in the
same way, and moreover it should be used with the present
pile. First placing A

the glass pile in the
position of the figure,

the base- boards Don E
atable-stand, and the

rotating tourmaline F m-Giass pile

in the stage of the

optical front, it will be shown that when in one position the
tourmaline image is black on the screen by transmitted light,
and transparent on the ceiling or overhead screen by reflected
light; and that rotating the tourmaline 90° reverses this.
Rotating the pile 90° (easily done by laying the pile on its
side BCD) the images are also reversed. Using an aperture
and the double-image prism, in the same way, the pair of
discs will be alternately light and dark according to the
positions ; and then substituting the Nicol, it will be shown
how one half the phenomena are suppressed or turned aside
by this piece of apparatus. Finally, substituting the glass
analyser (fig. 188) still with the aperture in the stage, it will
be seen how the transmitted beam is either reflected or again

35* OPTICAL PROJECTION

transmitted, according as the analyser pile or the separate
pile vary in position by 90°.

In all these demonstrations of the fundamental phenomena
of plane polarisation, the polariscope itself has not been
employed ; the optical front has sufficed, with the pieces of
apparatus described. These are equivalent to a polariscope,
and form a convenient method for also illustrating of what that
instrument essentially consists, and its various possible forms.

202. Polarised and Common Light.— The best illustra
tion of the probable nature of the transverse vibrations of
common and polarised light, will be found in the actual pro-
jection on the screen, by any of the apparatus described in
Chapter XVII., of Lissajous' unison figure in a state of transi-
tion, i.e. passing from planes though ellipses into circles, and so
back to planes at right angles to the former. Such a transi-
tional orbit will represent the probable nature of the vibrations
of common light. If, with the reed apparatus, the orbit then
be ' steadied ' at any one form, that will be a vivid illustration
of either plane, elliptic, or circular polarisation.

203. Interference Colours. — At this stage the polariscope
will be arranged for use, and the analyser turned to that
position which gives extinction, or what is called the * dark
field.' If now the rotating tourmaline be placed in the stage,
in two positions the field is still dark ; but directly the tour-
maline is placed at all obliquely to the polarised plane, the
vibrations of the polarised beam are ' resolved ' into others
obliquely inclined to them, so that the tourmaline appears a
light image on the field. No other phenomena appear, because,
of the two rectangular vibrations into which the original plane
is resolved, one is absorbed by the tourmaline.

But if a thin slice of a non-absorbent crystal like selenite
or mica be placed in the stage, mounted so that its polarising
planes are at an angle of 45° with the original polarised plane
and that of the analyser, both the re-polarised beams get
through, just as in the Huygens experiment, only not appre-

POLARISED LIGHT 353

ciably separated, owing to the thinness of the crystal. When
these beams, polarised in rectangular planes, are again re-
solved by the analyser, we have two similar sets of waves
brought into interference, since one set has been retarded
more than the other in the slice of crystal. If the retardation
does not exceed a few wave-lengths, therefore, we get colour,
and this is complementary in positions of the analyser differ-
ing by 900.1

It is usual to show this by a selenite giving red and green
images, and yellow and blue. The two may be mounted in
one slide. That no interference is produced whilst the vibra-
tions are in rectangular planes, is shown by removing the
analyser. On adding this and rotating, the complementary
colours appear. But when the analyser is in an intermediate
position, no colour appears, one ray from the selenite being
then suppressed by the analyser, and the other transmitted
without modification.

That the phenomena of interference are essentially similar
to those of a thin film (only that in a selenite the retardation
is that of the difference of two paths through the film, while
in a ' thin film ' one of the rays is retarded by twice the whole
thickness, so that the selenite or mica is much thicker than
a ' thin film ' producing the same retardation), is best demon-
strated by spectrum analysis, just as the Newton's rings and
soap film were analysed. A plate of crystal ground concave
or convex exhibits Newton's rings, which are readily analysed
by placing a slit in a thin plate of metal across the slide in
the stage, focussing, and interposing a prism (the compound
direct prism is much the handiest for such experiments). The
slit across a Newton's ring slide shows precisely the same

1 It must be understood that this work is solely concerned with practical
experimental conditions. No attempt is made or intended to explain pheno-
mena, or even to show their relations, further than the order of experiments may
do so. For some elementary explanation the reader is referred to my treatise
on Light : a Course of Experimental Optics, or other systematic treatises on
the subject may be consulted.

A A

354 • OPTICAL PROJECTION

parabolic fringes, and across a plain flat film exhibits straight
interference bands crossing the spectrum. And just as the
prism showed bands in the spectrum of a film of mica too
thick to show any colour, so a slice of selenite or mica 1 mm.
to 2 mm. thick will show numerous bands crossing its
spectrum, though the image of the slice appears colourless.

204. Coloured Designs. — Since colour depends on thickness
of the film, studied variations will of course give corresponding
variations of colour. The simplest case is a film of selenite
split from common samples ; this is nearly always irregular,
and therefore shows variations. The Newton's rings caused
by concave grinding have been already referred to. A slice
ground into a wedge, of course exhibits straight bands. Many
slides can be procured prepared in selenite, exhibiting
coloured stars, butterflies, birds, flowers,
chameleons, church windows, &c., show-
ing complementary colours in different
positions of the analyser.

I consider mica preparations, however,
more really instructive, and many of them
can be prepared by the demonstrator
himself, as this crystal splits more readily

FIG. i99.-Mica Design into even films- They can be Prepared
in two ways. Such simple patterns as
fig. 199 may have their outlines partially cut through a
rather thick sheet of mica, and layers of variable thickness
split off and lifted by a fine needle-point. The design will
then be displayed in various colours, care being taken to
find the polarising axes, and mount the mica so that these
stand in the proper position. But a better plan is to split
the mica into thin even sheets, and cut out geometrical
patterns from these, to be superposed one on another (taking
care that the polarising axes of all the superposed micas lie
in the same direction, anless purposely differentiated). The
films are then cemented on one another between two glass

POLARISED LIGHT 355

discs, with dried Canada balsam dissolved in benzol, as used
by microscopists. Such designs have clean and sharp edges,
and are far neater and more transparent than the former.1

205. Crossed and Superposed Films. — The most instructive
mica preparations of this kind are wedges built up of
successively narrower strips, first designed and constructed
by Mr. C. J. Fox, F.R.M.S., and which I, therefore, have
always described as ' Fox wedges.' They are especially
valuable as showing the effect of crossed films. If a film
gives a colour due to a retardation of say one wave-length (or
any other), and a second film of the same thickness be crossed
upon it at an angle of 90°, the polarised ray that was most
retarded in the first, is least so in the second, and it will be
the same as if no film were there. This is well shown by a
pair of similar wedges built up of eight steps each. When
they are crossed, along the diagonal set of squares each step
is crossed by the same thickness, and is therefore black. One
of the -wedges is mounted in a rotator, and then produces
very various effects according to position, for it will soon be
seen that a film mounted in a position to give colour (that is,
its planes at 45° with the plane of the polariser) has those
planes (or ' axes ' as often called) crossed 90° simply by
reversing the side placed next the other wedge. When the
wedges are superposed parallel, in one position the gradation

1 The films must be very carefully laid down in superposition in the cold,
the balsam being only of a slight creamy consistence, and no air-bubbles being
left between the films. The preparation, thus laid down and centred on one
glass disc, must then be left for some days to dry, before fresh balsam is placed
on the top and the cover-glass applied. This too must be left some days, under
a very small weight, to dry round the edges, then baked at not much over
100° F. till hard enough for cleaning. These precautions are necessary to
prevent the films from slipping out of place. The mica axis should be scratched
on every piece, but these scratches practically disappear when finished, the
balsam being so nearly of the same refractive index. For further details of
mica work, the reader may refer to my paper on ' Optical Combinations of
Crystalline Films ' in Proc. Physical Society, 1883 (reprinted in Phil. Mag, for
May 1883), or to my Light, published by Macmillan & Co.

AA'2

356 OPTTCAL PROJECTION

in thickness is doubled, while in that rotated 180° it is
counteracted, and a flat tint is produced equal to the sum of
the thickest band and the thinnest ; and if then one wedge
be turned over, in one position the whole appears black.

The most important and useful preparation is a long
wedge built up of twenty-four films each ^ wave thick.1
This gives the first three orders of Newton in £ wave grada-
tions. The twelfth band being 1| wave thick, if another
plate of mica of uniform 1^ wave thickness be super-
posed, with its axes crossed, the twelfth band will appear
black on the dark field, and on each side of it will appear,
apparently, a wedge resembling the original one, starting from
its thin end. Finally, if a slit be inserted in the stage with
the wedge, crossing all its films, spectrum analysis through
the prism will show exactly what waves are destroyed by
interference in each thickness. By having mica or selenite
plates of one or more waves, to superpose (the same way) on
the Fox wedge, these interference bands may be demonstrated
up to any desired order of colours.

Various and beautiful phenomena may further be shown
on the screen by rotating an uniformly thick mica or selenite
plate (commonly called an ' even ' film or superposition film)
over some other preparation, because in one position the
resultant will equal the sum of the two films, and in another

1 A film by which one of the rays is retarded ^ wave more than the other,
is called an ^\ or ^ wave film, and so we have quarter or half- wave, one wave,
two waves, &c. This can however only be precisely the case for any one
spectral colour at a time, and the adjustment is usually made for the yellow
ray, as being brightest and of medium wave-length. Taking then a whole-
wave film, or the eighth band in Fox's wedge, one yellow ray is retarded
exactly a wave, and the dark field would therefore in monochromatic light
give this band perfectly black. But of the red and blue, which are greater and
less in wave-length, a very little must be transmitted in white light, and these
residuals make the band (or a whole-wave plate) of a dull plum-colour, known
as the ' tint of passage,' or transition-tint, between the first and second of
Newton's orders. All other thicknesses have similar residuals, which are the
same in all interference colours ; these films, and especially the Fox wedge,
being the most perfect means of demonstrating them analytically in detail.

POLARISED LIGHT 357

position rotated 90° will equal their difference. If the foun-
dation be a star of differently coloured points, or with the
points arranged with polarising axes in different directions,
the result of rotating an even tint superposed will be very
different on each. Or such an even film may be rotated over
a film ground concave to show Newton's rings. If it equal
the thickness at about the middle of the radius, when crossed
the ring at that point will be black in the dark field ; and on
rotating the film, the rings will change colours in a beautiful
manner. The same will be the case with mica preparations
built up of concentric squares or circles successively smaller
in size (fig. 200).

For crossing, superposing, and rotating with other pre-
parations, superposition films giving a
good red and green, and a good blue
and yellow, should be provided ; and
also a half- wave plate, which gives the
complementary in any case — e.g. the dark
field becomes light and the bright field
dark, when a half- wave plate is employed.
This is ingeniously applied in two selenite
designs which are very amusing. In
one, the bust of a lady is composed of a nearly half- wave
film, with the result that when the analyser is rotated 90° a
fair woman is metamorphosed into a dark mulatto ; in the
other, the figure of a miller with a sack of flour, becomes
apparently a sweep carrying a bag of soot.

Home-made preparations should be mounted in their
mahogany frames with rather stale putty, in which is mixed
a little red powder of some sort.

206. Crystallisations.— Substances which can be crystal-
lised in thin films on glass plates, if managed so that the
films are of suitable thickness, give magnificent projections.
Watery solutions are generally flowed over the discs, and in
many cases, e.g. potassic chlorate, a little gum arabic in the

353 OPTICAL PROJECTION

solution increases the size of the crystals. When dry, those
which do not dissolve in balsam should be covered with
another glass cemented down by balsam in benzol, which
adds to the transparency. Acetates are generally fugitive, as
are some others ; but the vast majority give good results,
tartaric acid and salicine being particularly fine. Salicine
should be used in saturated solution, with a little alcohol
mixed in the water ; the fluid is dried gently over a lamp to
a clear amorphous film ; and on more strongly heating this,
especially if aided by a few pricks with a sharp needle, the
beautiful rosettes with rotating crosses crystallise out. Occa-
sional breathing on the film will produce circular ripples.
Several others can be treated similarly. Many of the most
beautiful crystals for the microscope are too small to show
effectively in the polariscope. but are gorgeous objects for the
polarising microscope described in Chapter XIII.

Another class of beautiful crystallisations are formed by
melting the substance between two glass discs, and allowing
the film to cool. Such are benzoic acid, cinchonine, santo-
nine, succinic acid, cinnamic acid, &c., chiefly more or less
organic in character. A splendid variety of beautiful slides are
easily prepared from crystallisations.

It often proves interesting to an audience to project coarse
specimens of the precious stones, compared with imitations in
glass. The difference illustrates that between crystalline and
non-crystalline bodies.

Crystallisation on the screen is a beautiful experiment,
easily shown by melting benzoic acid between two glass discs
held in a pair of forceps over a spirit lamp, and then quickly
placed hot in a frame made for the purpose, and so into the
stage. The splendid coloured needles quickly begin to shoot
out on the dark field. Another way is to flow a strong solu-
tion of urea in gum-water, heated in a test-tube, over a
warmed disc, and to place this in the stage, which is held
back as open as possible, in order to touch the wet film with

POLARISED LIGHT 359

a small crystal held in the end of a tube ; if the conditions
are hit rightly, which can only be done after practice, the
effect is very fine. In the polarising projection microscope,
the best plan is to purchase two or three slides of the fatty
acids prepared for this purpose ; they have simply to be gently
warmed till the crystals all melt away, and placed on the
stage, when they crystallise out again as the slide cools. The
same slide can be used dozens of times over with no further
treatment.

207. Mineral Sections.— Every one of these can be well
exhibited in the polarising projection microscope, but only
the coarse-grained specimens are bold enough for the polari-
scope. Among those I know to be effective, are the coarse
sandstones, granites, perthites, zeolites, cross sections of
small stalactites, and labradorite. Some minerals show
interesting and extraordinary differences in figure as well as
colour, when cut in different planes. Labradorite cut in one
direction, shows little but a coloured film ; in another,
beautiful straight coloured stripes ; and in a third, beautiful
bands of rotational colours (§ 213). One kind of granite
(often called * graphic ' granite) resembles ordinary granite
cut in one plane, whilst in another it displays marking almost
like an Arabic inscription.

208. Organic Substances. — Nearly all substances with
definite structure show double refraction when cut into plates
or films. For the lantern polariscope, the best objects are
plates of thin horn, pieces of thick bladder, and quill. If
quills are split up one side, and placed in boiling water, they
become soft and can be flattened out ; but a quill pen just as
it is, pushed through the stage, will show the phenomena
plainly. Where there is fibre in a definite direction, as in a
quill, one polarising plane always lies that way, and the other
at right angles. Therefore if the quill be placed parallel
to or across the polariser, the field remains dark, and the best
effect is produced at an angle of 45°, as with crystalline films.

36o

OPTICAL PROJECTION

Shrimp and prawn shells, and the larger fish scales, make
good objects. With the polarising microscope, all the usual
organic preparations are available, and starch can be fairly
shown even with the oxy-hydrogen light, and excellently with
the arc.

209. Strains.— The effects of strain or tension are most
easily shown in glass. The usual wooden press frames sold
for the purpose exhibit the phenomena distinctly enough, but
the pressure is insufficient to do so with the beauty they
are capable of. I therefore constructed a frame of solid

gun-metal, with a
screw turned by
a powerful T-key,
as in fig. 201, the
glass being com-
pressed between
the convex sur-
faces of c and A.
To avoid any
twisting strain on
the polariscope-
stage, it is better
to have a screw
at both ends of
the slide, and to
use two keys, when one twist balances the other, and more
pressure can be put on. The glass should be compressed
even to breaking, if possible ; for the most brilliant chromatic
fringes are produced under the greatest strain. Before
breakage, however, the stage with the press in it should be
rotated 45°, in order to exhibit the great difference in the
optical results in that position.

The same press, arranged as in fig. 202, with a narrower
piece of glass resting against two pieces of brass B B as abut-
ments, c pressing between them as before, gives the optical

FIG. 201

FIG. 202

POLARISED LIGHT 361

effect of strains of another character, resembling that pro-
duced by pressure on the centre of a beam.

Clear jelly compressed from above in a glass trough, will
produce similar effects ; but a simpler experiment is to cut a
strip of thin india-rubber about 2 \ inches wide, pass it from side
to side through the stage, roll each end round a rod of wood,
and then strain the ends apart. The best effect is of course
in the 45° position, as the polarising planes are in the direc-
tions of greatest and least strain. A simple bar of glass
about half an inch square, passed through the stage, can
easily be bent with the fingers alone, so as to show coloured
fringes upon the screen.

Heating a piece of glass from one point, by the local
expansion and strain it produces, exhibits the same effects.
A disc of glass may be heated in a pair of forceps over a
spirit-flame (keeping the flame at the same point only) and
placed in the stage ; the dark field will be illuminated by
fringes. A better method is to provide a rectangular sheet
iron shell open att both ends, and with a square aperture
through each of its sides. In this can be placed a square oi
thick glass held in a bottom bar of wood, so that the plate
stands between the two apertures ; when a nearly red-hot bar
of iron can be pushed in to rest on the top edge of the square.
Beautiful fringes will appear instantly.

In plates of glass highly heated and suddenly chilled
round the edges, these effects of strain are permanent and
very brilliant. Many shapes are procurable, but the simpler
ones of square, circle, oval, and triangle are most instructive ;
and the oval particularly so, as illustrating by analogy the
phenomena of a bi-axial crystal.

Nearly all massive pieces of glass commonly procurable,
such as ink-stands, glass stoppers, paper-weights, &c., show
these latter phenomena. A section of thick glass tube,
ground and polished, rarely fails ; and many optical lenses show
a conspicuous black cross, so that the lenses of the polariscope

362 OPTICAL PROJECTION

itself need to be very carefully tested. The projection of such
a doubly-refracting lens (only too easy to find) illustrates well
the necessity for such tests in delicate instruments.

The alternate compression and dilatation caused in glass
by sonorous vibration, can be readily demonstrated in the
polariscope, after the method of Biot. A strip of plate glass
about J inch thick, two inches wide, and four to six feet
long, is required ; and the sharp edges of this should be
rounded or smoothed by a file, or emery-cloth, moistened with
turpentine. The strip should be screwed up exactly at the
middle, between the two cork-lined jaws of a wooden vice on
the tops of a short pillar, so that the strip is horizontal, with
its faces in a perpendicular plane. The portion of the polari-
scope containing the stage must be drawn forward from the
polariser, so as to leave a clear space of about an inch, and
through this space, so as not to touch either the front or
back part of the instrument, the strip must be adjusted to
cross the field, as near the point held in the vice as possible,
this being the node of the bar. The polariser and analyser
are crossed, for the dark field, at 45°, from their usual position.
The other end of the bar is now to be swept between the
thumb and fingers holding a wet flannel or other woollen
cloth, so that a shrill musical note sounds from the glass.
At once light flashes on the screen, and if a chilled glass, or
selenite, be placed in the stage and focussed, the colour will
change at every note. If the polariser is kept in its usual
position the glass strip must cross at an angle of 45° ; but
the arrangement described is more convenient.

210. Composition of Vibrations. — The method in which two
vibrations in rectangular planes may be compounded into one
resultant vibration, can be illustrated on the screen in two
ways. One is to project Lissajous' unison figure, by any of
the apparatus described in Chapter XVII., showing in suc-
cession the plane orbit (converted to another plane orbit at
right angles when one of the reeds or forks is altered half a

POLARISED LIGHT 363

vibration in phase), the elliptical orbit, and the circle. The
other plan is to arrange in front of the condensers a small
pendulum apparatus made for the purpose, drawing the figures
in white on a slide of glass covered with a film of blue
printer's ink. This is projected in action on the screen, so
that the eye follows the tracing. Other methods with pen-
dulums are plentiful, but cannot be called projections.

Having thus shown that rectangular vibrations in the
same phase compound into one plane vibration at an angle of
45°, and that difference in phase of half a vibration results in
a plane vibration at right angles to this (which in polarisa-
tion reverses all chromatic phenomena), also that vibrations
differing a quarter- vibration in phase result in a circular orbit,
and intermediate differences of phase in elliptical orbits ; that
all this is so in the phenomena of polarised light is simply
demonstrated. The fourth band in Fox's wedge (§ 205) giving
half a wave difference of phase in the two rays, and the
second band and sixth band a quarter-wave difference, the
matter can be tested, large films of similar thickness being
examined in the stage. The half- wave film does reverse the
phenomena, changing the bright field to dark, and the dark
to light, and causing the complementary colour in any design
when it is placed in the stage. The quarter- wave plate also
restores light to the dark field, but in a different manner.
The analyser cannot now quench the light in any position.
The field is always evenly illuminated [experiment], and so far
the field might appear to be one of common light. If an aper-
ture be placed in the stage with the film, and the double-image
prism employed as an analyser [experiment] it will be seen that
the two images are always alike in brilliancy. Nevertheless,
by placing any selenite or other preparation in the stage
after the quarter-wave [experiment], it will be seen that the
light is polarised somehow, for the design shows colours, with
tints simply differing by a quarter of that one of Newton'a
' orders ' to which it belongs.

364 OPTICAL PROJECTION

211. Quarter- wave Plates. — The light is in fact circularly
polarised by the compounding of the two vibrations as they
leave the film; and therefore, although the analyser again
analyses the circular motion into two rectangular ones, it is
quite indifferent at what part of the circular orbit it does so.
A mica of this thickness, known as a ' quarter- wave plate,'
is therefore in constant use for many experiments in polari-
sation.

It has been pointed out l that the colour cannot be quite
the same all round the revolution of the analyser, except for
the homogeneous light to which the thickness is adjusted;
and when this is yellow (as usual), in white light a very slight
residuum of orange appears in one position, and of bluish-grey
in the other, owing to the longer and shorter wave-lengths not
being in precisely quarter-wave relations.

The direction of the principal axis (that which joins the
centres of the two systems of ' brushes ' in mica presently
described) should always be marked upon the plate in some
way. One way is to make diamond scratches on the edges of
the glass discs containing the film, at intervals of 45° ; or the
principal axis may be scratched across the diameter of the
mica itself before mounting. I prefer the latter, which is
quite unnoticeable unless carefully looked for. Such a film,
in unmounted discs, can then be used in any position in the
rotating frame (fig. 195). But as two definite positions are
constantly required in this class of experiments, I prefer to
mount two separate plates, each as large as can be inserted
in a standard frame, permanently, one with its axis perpen-
dicular, and the other at 45°. They can be put into thinner
frames this way, and are always ready adjusted, which is
convenient when only one stage is provided.

The effect of a quarter- wave plate obviously depends upon
its axes being at an angle of 45° with the preceding planes of
polarisation. It is readily seen that the plate, if placed with

1 See note to p. 856.

POLARISED LIGHT 365

its axes parallel to and at right angles with the plane of the
polariser, is by itself totally inoperative, the polarised beam
passing through in its own plane unobstructed, and therefore
unresolved further. But it is very different if there precede
the quarter-wave in the stage a selenite or other polarising
preparation, in the usual position, the quarter-wave coming
after this latter. The plane-polarised beam, let us suppose
polarised in a perpendicular plane, is, by the selenite or other
preparation, now already ' resolved ' into two planes at right
angles to each other, but at 45° angle with the original
plane. Therefore the quarter-wave, whose own axes are per-
pendicular and horizontal, being at 45° angle with those of
the selenite, now again resolves each of these. The demon-
strator should get these different actions of a quarter-wave
plate thoroughly understood, and must at least understand
them himself clearly.

It is easily shown that any film whose thickness is an odd
number of quarter-waves, gives substantially the same phe-
nomena, particularly in homogeneous yellow light. But with
increasing thickness the amount of residual colour from all
but the yellow waves, increases also.

Circular polarisation by reflection from silver, or in a
Fresnel's rhomb, can be easily projected, in a manner too
obvious to need description, but is not a very effective experi-
ment, unless in a complete course of lectures.

212. Eotary Polarisation. — But the effect of a quarter-
wave plate coming after some other preparation should be
worked out further. The axis of the plate is to be vertical,
as just now observed, and it resolves again (into perpendicular
and horizontal) the rays which passed through the first
crystal plate. For simplicity let this be a simple ' even '
film. Each of the first crystal's sets of vibrations, is now
resolved again, separately, into horizontal and perpendicular
with a quarter-wave difference of phase. So resolved, one
compounds into a circular orbit in one direction, and the

366 OPTICAL PROJECTION

other into a circular orbit in the, other direction. [This may
be worked out with pendulums, or a pendulum slide.] The
analyser deals with these two circular waves always meet-
ing, at whatever point it intersects their common circle ; and
at that point the two contrary tangential motions destroy
each other, and the radial vibration alone is left. This radial
vibration will be barred by the analyser, whose plane is at
right angles to it ; and thus any homogeneous colour may be
cut off, and the field made dark, by some position of the
analyser. This is best shown by a sodium-light. But the
wave-lengths differ for different colours ; and so it comes to
pass that in rotating the analyser, one colour is cut off after
the other, and the residuals give in succession, apparently,
and more or less perfectly, the different colours of the spectrum.
Thus it happens [experiment] that when rotating the analyser,
instead of only two complementary colours and two colourless
positions, as usual with a plate of selenite, we have in suc-
cession all the colours (more or less) in beautiful grada-
tion. About one half to two waves for the first crystal
gives the best approximation to a complete spectrum of
colours.

Using a double-image prism as analyser, and an aperture
with the two films in the stage of a size to give two over-
lapping discs, it will be seen that, through all the gradations,
the two discs are always complementary, and make white
where they overlap.

Still further, since the direction of the circular orbits
depends on the relation of the rectangular planes in each
plate ; reversing either the foundation -colour plate, or the
quarter-wave plate, will reverse the order of colours which
appear as the analyser is rotated in any given direction. A
plate of mica or selenite cut in half, and mounted with one
half reversed, or a quarter- wave plate treated similarly, will
demonstrate this. In two positions of the analyser the plate
will appear all the same colour ; but as the analyser turns

POLARISED LIGHT

367

from these, the colours change, in contrary orders in each
half, from one end of the spectrum to the other.

213. Rotational Colour Experiments. — These phenomena
open the way to singularly beautiful experiments. If the
quarter-wave plate is superposed upon a selenite or mica
wedge, the bands of colour will appear to move across the
screen.1 Superposed upon the concave selenite, as the analyser
rotates the rings expand or contract, with fine effect. If
the quarter- wave is used in a rotator, it will be shown that
after rotating it 90° the order (expansion or contraction) is
reversed. Or if the analyser be kept stationary, and the
quarter-wave itself is continuously rotated, the rings alter-
nately contract and expand, as the quarter-wave is gradually
added to, or subtracted from, the thickness of the rings.

The exact nature of what takes place is best shown by
spectrum analysis. Placing a slit in the stage with a selenite

FIG. 203.— Quarter-wave Preparations

or mica thus circularly polarised by the rotational method,
and interposing a prism (direct prisms are far the best for
these experiments) the colours destroyed by interference
appear as dark bands in the spectrum ; and as the analyser
rotates, the bands travel along the spectrum. On reversing
either quarter- wave film or colour-plate, the direction of this
motion reverses.

Mr. Fox demonstrated these phenomena by superposing
a quarter-wave plate with its axis as at A (fig. 203) on a

1 In all rotational colour experiments it is understood that the quarter-
wave comes between the pattern- plate and whichever part of the polarising
apparatus is rotated. It will act similarly next the polariser, provided this
latter is rotated instead of the analyser.

368 OPTICAL PROJECTION

geometrical design prepared in mica, the whole cemented
down as one slide. Inserted in the stage with the A plate
next the polariser, this plate has no effect,1 and the ordinary
complementary colours appear ; but with A next the analyser,
the rotational colours appear. Mr. Fox's slides were pieced
together as a mosaic, the portions being cut out of different
thicknesses; but I have produced much better effects by
designing patterns which could be built up by successively
smaller and smaller films, each laid down in the centre. This
construction gives a better gradation of colour in the different
parts of the design, and some of the tints thus obtained are
magnificent. For use superposed on his wedge, the same
gentleman prepared a quarter- wave in two halves as at B, the
effect of which is that in the upper and lower halves the
colours pass along the wedge in contrary directions.

I myself devised what I think are still more beautiful
effects, by reversing alternate sectors of the quarter-wave, as
at c D E. Either of these superposed on a concave selenite,
or on concentric rings or squares of mica, causes simultaneous
expansion and contraction of the bands in adjacent sectors.
By placing underneath the colour-design an even film in a
rotator, and rotating this slowly aa the analyser is rotated,
the foundation colours themselves are beautifully varied, as
already seen, and the result is a kind of optical chromatrope
of great fascination. The eight -sector plate shown at E is most
suitable for concentric squares. Or any suitable geometric
design (i.e. some kind of eight-pointed or four-pointed star)
may be laid down on such a plate as E, and have a single plate
A superposed on it ; then if the analyser is rotated it will be
as if E were not there ; but if the polariser is rotated, the
contrary sector rotations of E will come into play. If E be
superposed upon an even colour-film, simple contrary rotations
in adjacent sectors will be seen.

1 Unless, as in the preceding note, the analyser be kept stationary and tha
polariser is rotated instead.

POLARISED LIGHT 369

By superposing upon a colour-film cut into strips which
are reversed, a quarter-wave plate also cut into strips reversed,
the strips crossing the others at right angles, and blacking out
the margins of the preparation, a chess-board pattern can be
made without any great difficulty, which shows contrary
rotations in adjacent squares.

Very beautiful effects are also produced by superposing D
on E, or a quarter-wave made in twelve sectors, upon a square
of chilled glass,1 in either of its principal positions.

I have also constructed a design built up as a kind of
circular wedge, the circle being divided into twenty-four sectors,
each one in succession being one film thicker than its pre-
decessor. When a single quarter-wave is superposed upon
this, the result is of course an apparent revolution of the
colours round the centre ; but this is still better
shown by making the quarter-wave of two con-
centric parts with axes reversed as in fig. 204,
when the outer portion apparently revolves in the
contrary direction to the inner portion. If the cir-
cular wedge itself is crossed (as regards axes) upon
a preparation of circular rings, or a concave selenite, the
result is a series of beautiful black spirals starting from the
centre ; and upon these, again, fig. 204 may be superposed.
Again, if a sector-plate in its depolarising position (axes at 45°)
be placed in the stage as a foundation, and fig. 204 be super-
posed, the alternate areas resulting will be black and white in
contrast ; and in whatever position the second plate be placed,
on rotating the analyser to a corresponding position, this
effect will be seen. Hence the operation of Professor S. P.
Thompson's quarter-wave rotator for the polarising plane
(p. 347).

A very instructive method of showing the remarkable
differences of result from superposing films in different ways,

1 For details see the paper in Proceedings of the Physical Society already
referred to.

BB

370 OPTICAL PROJECTION

is to prepare two similar double wedges, the thicknesses of
mica successively increasing from each end, so that the
thickest band is in the centre. The bands should be about
iV ^° s\r inch wide, and as long as possible. Crossed, these
of course give a very beautiful floor-cloth kind of pattern,
with either the colours of the added films, or two diagonals of
black squares. On superposing a plate of A, B or c (fig. 203),
c being prepared not only with the quadrant sectors as drawn,
but another plate with the sectors on lines arranged diagonally,
most extraordinary differences in the phenomena will ba
observed.

The most beautiful of all the mica preparations I have de-
vised, however, so far as spectacular effect upon the screen is
concerned, are composed of two geometrical designs, each cir-
cularly polarised by a quarter-wave film, aud then superposed.
The two may be similar, or different ones designed in relation
to each other, such as a design composed of straight lines
with one composed of circles or curved lines. The first is of
course circularly polarised in the ordinary way; but these
rotational colours are again resolved, recompounded, and
again circularly polarised by the films in the second design ;
and the result is a transition and play of colour unrivalled
in magnificence, and apparently inscrutable till its components
are analysed. This is not all. If every now and then the
original polarising plane be rotated a little, the relations of
all the axes are altered, and so are the colour variations,
until when the polariser stands at 45°, the first of the two
patterns (being in the same plane) becomes quite obliterated,
and only the second single design is left upon the screen.

214. Quartz Rotation. — A plate of quartz cut transversely
to the optic axis exhibits the same rotational change of colour,
showing that plane- polarised light is resolved in its interior
into two circular waves. The best thickness for an apparently
complete range of colours is 1\ mm., which also gives the
'transition-tint' between first and second orders. If an

POLARISED LIGHT 371

aperture be placed with the quartz in the stage, and the
double image used, the two overlapping discs will always be
complementary.

215. Effects of a Revolving Analyser. — By employing an
analyser which is swiftly rotated, at the rate of 8 to 10
revolutions per second, as described by Mach and the late Mr.
W. Spottiswoode,1 many of the preceding phenomena which
are ordinarily exhibited in succession during the revolution
of the analyser, can be made to appear on the screen simul-
taneously. In Mach's arrangement the light from the
polariser, after passing through a stage in which any plate of
crystal can be placed, immediately traverses an analysing
Nicol, close to whose face either a square aperture or a slit
can be attached. The analyser with its aperture occupies one
end of a tube revolved by a multiplying wheel, at the other
end of which is a prism of glass, to deviate the beam some-
what from the optic axis of the instrument. The effect is
that, on revolving the tube, what would have been a central
spot, by persistence of vision now becomes a ring of light on
the screen. In Mr. Spottiswoode's apparatus, for the Nicol
and deviating prism is substituted a double-image prism,
giving one central image and one considerably deviated, thus
giving both the central spot and the ring. Beyond the
analyser, in either construction, is a lens to focus the aperture.
The radial deviation in both cases is arranged to be in the
plane of polarisation of the analyser, and rotates with the
latter.

If now we use the square aperture alone, focussed on the
screen, it is plain that the ring image will be bright at two
opposite points of the circumference, and dark at the two

1 Mach's instrument is described in detail in Pogg. Ann. cxlvi. (1875)
p. 169 ; and in Miiller-Pouillet's Lehrbuch der PJiysik. Mr. Spottiswoode's
is described in Phil. Mag. xlix. (1875) p. 472. There can be little doubt that
the latter was constructed, except as to the substitution of the double-image
prism, upon a prior brief notice of Mach's which had appeared in the Proc.
Vienna Academy, Jan. 4, 1875.

B B 2

372 OPTICAL PROJECTION

intermediate points where the analyser is crossed. If we
place a plate of selenite in the stage, say red and green, we
shall have red at two opposite points, green at two opposite
intermediate points, and no colour at the four points of 45°,
all graduating smoothly. If we use in the stage a quartz cut
transversely to the axis, we shall have the rotational colours,
as a spectrum round the ring, twice repeated. We have simply
what, in the ordinary way, would be successive appearances of
the image of the aperture, simultaneously appearing round a
ring.

Mach further added to his apparatus, next to the deflecting
prism, a direct -vision dispersive prism, so placed that its
dispersion is also in the plane of polarisation of the analyser ;
whilst a slit is attached to the latter, instead of the square
aperture. Thus in any given position we have a radial spec-
trum, whose violet end should be towards the centre to
equalise the fainter light ; and on rotating the whole arrange-
ment, we now have as the foundation a circular spectrum,
with the violet towards the centre. If we place in the stage
a plate of crystal too thick to show colour in the ordinary
way — say 1 to 2 mm. thick— our ring spectrum will be con-
tinuous in the two azimuths of 45°, but will be crossed by
interference bands (appearing on the ring as concentric arcs)
along the horizontal and perpendicular diameters, while the
dark bands in the one diameter will be opposite the bright
coloured bands in the other.

Finally, Mach places in the stage a plate of quartz cut
transversely to the axis— say 8 mm. thick. We have already
seen (§ 214) the gradual passage of the interference bands, on
rotating the analyser, along the spectrum of such a plate.
With the revolving apparatus it is manifest that this must be
translated into a circular spectrum traversed by black bands
in the form of beautiful continuous bold spirals. This last
experiment is one of peculiar beauty.

216. Bi-quartz Effects.— Crystals of quartz being found

POLARISED LIGHT 373

which rotate the colours in opposite directions, if a plate be
composed half of one, and half of the other, of the proper thick-
ness, it is a very sensitive test of any optical rotation, as the
purple transition -tint seen all over in one position of the
analyser, changes towards blue in one half and red in the
other, on the least rotation. Such is therefore used to demon-
strate the rotation of fluids.

Another useful and more sensitive bi- quartz preparation
is that shown in fig. 205. In the portion B, the wedge B is of
say right-handed, and A of left-handed quartz, the effect of
which is a black band across the centre where the thicknesses
are equal (i.e. in the dark field) and coloured bands on each
side. In the half c D the wedges
are reversed. The consequence is,
that on the least additional rotation
in one direction by any substance
used with the wedges, or the least
rotation of the analyser, the bands
move in opposite directions, and
the distance the analyser has to
be rotated to bring them back, is a

measure of the rotation due to the PIQ 2o5.-Bi-quartz wedges
substance examined.

Plates of quartz may be obtained in which both right- and
left-handed crystallisation occurs, and such are very beautiful
objects. Amethyst is a quartz crystal with the contrary cry-
stals arranged in narrow parallel bands, and such are still more
beautiful, but a large one is very difficult to find : one that will
cover a standard- sized disc is a gorgeous object. Plates about
| inch diameter can be obtained without difficulty. Any clean
quartz crystal of good size which is a violet colour, is almost
certain to exhibit either amethyst or cross-crystallisation, and
should be cut up into polariscope specimens.

Other crystals can be obtained which rotate the beam,
and most of them in bi-quartz form. They are, however, all

374

OPTICAL PROJECTION

small, and really more suitable for the polarising microscope.
Cinnabar and periodate of soda are among the best.

217. Mica Registers of Rotation. — It will be obvious that
a bi-plate of mica circularly polarised, as previously described,
may be used exactly as a bi-quartz. But far more effective
as a screen demonstration is a mica preparation devised by
Prof. S. P. Thompson, consisting of twenty-four sectors, in
each of which the principal axis or polarising plane is radial
to the circle. It is obvious that when the analyser is crossed,
the horizontal and vertical sectors must show a black cross,

while the others show gradual
transition towards the full
depolarising effect of the film
at 45°. Hence the best thick-
ness is either half-wave or
\\ waves, as this gives the
greatest transition from dark
to light. With the black
cross vertical, if now any
substance exerting rotary
power be introduced, the
cross is conspicuously rotated,
showing vividly what Prof.
Thompson calls the optical 'torque' or torsion of the beam.1
Of course the rotated cross becomes more or less coloured,
owing to the differing rotation of the various colours, but the
effect is none the less simply evident.

Any large crystallisation showing a ' rose ' with rotating
black cross, may be used in a similar manner.

218. Rotation in Fluids. — Many fluids may be used to
demonstrate this, in a tube about 20 cm. long and two inches
diameter, with flat glass ends screwing on. This tube should
be arranged to rest in two supports fixed on a base-board
which can be dropped into the polariscope between the

l See Proc. Boy. Inst. xii, 474, or Nature xl. 232, 257.

PIG. 206.— Thompson's Rotation Register

POLARISED LIGHT 375

polariser and the stage s (fig. 190), the latter being drawn
forward sufficiently to allow of this. Oil of lemons, which
will require about one pound (cost 10s.) to fill such a tube,
will exhibit a nearly complete rotational spectrum, like the
quartz plate ; and saturated syrup, or oil of turpentine, a very
visible change of colours, in the fluid itself. By placing a
bi-quartz in the stage, rotation may be shown in plain cells
containing only one to two inches of fluid.

Tne operation of the saccharometer is easily shown by
introducing the bi-quartz or any other apparatus (such as
Laurent's semi-circular half-wave plate) and arranging (as
can easily be done) that an index shall rotate with the analyser
round a divided glass plate in the stage, which is focussed on
the screen. The index is first adjusted to zero, and then the
degrees of rotation necessary to bring back the bi-quartz to
equilibrium after a tube of strong sugar syrup is introduced,
can be seen. Also, by having a second tube of half the length,
it can be demonstrated that the rotation is proportional to the
length of the column of fluid ; and by filling one of two tubes
with a one -half dilution, that it is proportional to the strength.

219. Electro-magnetic Rotation. — For lecture purposes
this is generally shown with a bar of Faraday's heavy glass,
having rectangular polished ends ; other heavy glass has
somewhat less effect, and common glass, and many fluids
and gases which show the same phenomena, do not do so
with sufficient prominence for public demonstration. A bar of
Faraday glass three to six inches long, and from half an inch
square, will suffice. This should be placed in a circular tube
of annealed iron, and if the bar be square, all the vacant
space be filled with annealed iron wire, and the whole sur-
rounded by an insulated coil of wire. This beinp introduced
between the polariser and the stage, and the analyser crossed
for dark field, on passing a pretty good current through the
coil, light will appear on the screen, It will depend upon
the length of glass and the current, whether any perceptible

376 OPTICAL PROJECTION

colour is visible ; but with a bi-quartz the effect is generally
very evident with a moderate current. Thompson's mica
sectors (fig. 200) may also be used.

Dr. Kerr's experiment, showing that plane-polarised light
is rotated if reflected from the polished pole of a magnet,
is too sensitive for successful demonstration, unless with
apparatus of the highest class and the arc light.

220. Rotation of Common Light.— If the light employed
is powerful enough to exhibit interference -bands on the screen
with Fresnel's bi-prism (§ 189) this can be exhibited after the
manner of Profs. Abbe and Sohncke. It is only necessary to
cover one-half of the bi-prism with a plate of left-handed,
and the other with a plate of right-handed quartz, of 1-88
mm. in thickness. This thickness rotates yellow light 45°,
and therefore if quartz rotates common light at all, the two
interfering rays are brought into orbits differing by 90° in
azimuth, when they cannot interfere. It is demonstrated
that this is so, because the bands vanish accordingly.

221. Ring and Brushes in Crystals. — For use with the
simpler forms of lantern polariscopes, like fig. 186, plates

of crystals are cut
transversely to their
optic axes, and
mounted in wooden
sliders as repre-

FIG. 207. .... _

sented in fig. 207.

By having a frame made of the usual 4x2^ size, consisting ol
two thin metal plates with a circular aperture in the centres,
separated by a strip of wood along the top and bottom edges,
and leaving a space for the small slide as a centre strip
between, this can be placed in the usual stage, when it will be
seen that the plate of crystal (unless it be a circularly-polar-
ising one) has no double refraction at all in this direction ; the
dark or light field, in the parallel beam of plane-polarised
light, remains as it was before the crystal was inserted,

POLARISED LIGHT 377

With convergent light it is of course different. For the
simpler polariscopes a crystal-stage is provided, which consists
of a tubular fitting like fig. 208. One end A fits into the
nozzle of the objective, from which the analyser is withdrawn,
the latter now fitting into the other end B of this crystal-
stage. In the centre at s is a slot or stage with springs, large
enough to receive the crystal sliders, which are about an inch
wide ; and the stage is so placed that the crystal occupies
the spot where the cone of light is of the smallest diameter.
With the low convergence from the usual power alone,
the convergent (or divergent) light passes through the
crystal and analyser straight to the screen, and needs no
focussing lens whatever, the fringes appearing simply as
shadows. The rings and brushes are shown as perfectly in
this simple and inexpensive way,
as in any other ; but the choice of

s!

bi-axial crystals is limited, as only
very small angles will allow both
systems of rings to be brought into
the field together. Practically, only
nitre, cerussite,glauberite, and some
small-angled adularias are available, on this account ; uni-
axials of course are all available. It is customary to pre-
pare plates of other bi-axials cut across one, axis, to show one,
of the systems of rings ; but beyond one or two, such are of
little interest.

With the optical arrangements shown in fig. 190, however,
any angles up to 60° or 65° can be shown.1 The system of
converging lenses c is inserted in the nozzle, and the rays after
crossing in their focus and becoming divergent are collected
by the second system. This refraction has however been too
violent to project defined figures by mere shadow fringes ; and

1 By interposing cedar-oil, as used for homogeneous immersion objectives,
between each face of the crystal and the adjacent lenses, larger angles can
be collected if deemed necessary.

378 OPTICAL PROJECTION

they Lave to be focussed by another power, as they appear at
the back of the last lens. All sorts of focal arrangements
have been employed, but I prefer that described on p. 343, for
the sake of the easy range of foci and scale which it gives.
The whole apparatus can be adjusted, and a crystal focussed, in
less than a minute, both axes of selenite being easily shown in
one field. A great variety of crystals are prepared by Messrs.
Dr. Steeg and Keuter, of Homburg vor der Hohe, who practi-
cally supply the scientific world with these objects, and from
whom a list can be obtained. Their crystals are mounted in
plates of cork If inches square. For home-made plates or
mica preparations, I myself prefer the usual microscopic 8x1
slips, because stored so easily in the usual racked boxes.
But a stage with springs, as shown in fig. 190, will accommo-
date all alike with equal facility. In demonstrations, the
front optical apparatus is fixed so as to leave about an inch
between the two convergent systems ; a crystal is inserted,
and then brought up close to the second system by racking
out the ordinary focal power. The fringes are then focussed
by the rack and pinion on the front.1

Plates of some bi-axial crystals crossed are very fine.
Mica is easily prepared thus. There should be at least
one specimen of crystals in which the rings for red light are
at right angles to those for blue in the same crystal. Such
are Brookite — which is however expensive (for a good one)
and shows considerable red or orange colour — and the triple
tartrate of soda, potash, and ammonia, which is clear. Unfor-
tunately it, like many ' soft ' crystals, gradually oxidises when
mounted in balsam, and thus becomes cloudy. A mounting

1 If the apparatus is properly adjusted, the crystal figures will be perfectly
and evenly illuminated all over the disc. Should either the centre or the
margin appear dark, the apparatus is faulty, unless the adjustable field-lens
H (fig. 190) has been accidentally placed wrongly whilst varying its position
to alter the focal power. Ample margin for all necessary adjustments is pro-
vided in my apparatus. With only medium-angled bi-axials, the front lens of
the collecting system may often be unscrewed ond removed with advantage,

POLARISED LIGHT 379

medium which will preserve these crystals clear is much to
be desired. When clear, by placing in the stage gelatines
carefully selected to absorb all but the blue and red rays,
the two systems of rings can be shown crossed on the screen
if the light is brilliant. The axes are shown easily.

Airy's Spirals are produced by a right-handed quartz, cut
perpendicularly to the optic axis, superposed upon a left-handed
one. The two need not be of equal thickness, and plates
containing cross-crystallisation will show the spirals naturally,
wherever the two forms happen to overlap each other.

It ought to be noted, that crystals which can be shown
in both forms of polariscope, require to be much thinner for
highly convergent light than for the low convergence method.
A thick plate of nitre will fill the field, with low convergence ;
for high convergence it must be very thin, and the two axes
will appear very close together in the centre of the field.
Most mica commonly obtainable has an angle of about 45°.

Mitscherlich's Experiment, showing that a crystal of
selenite, when heated, gradually becomes uni- axial, and with
further heat becomes again bi-axial with the axes at right
angles to the former direction, is easily demonstrated with
the apparatus described. I employ a slide devised for the
purpose ; but it is sufficient to procure a strip of brass or copper
about 4x1 inches, the same thickness as the crystal, drill
and file a hole through the centre which just admits the latter
loosely, and bend up rather more than an inch of each end,
so that these ends stand well away from the crystal stage like
ears, when the strip is held by the springs. Then bend a piece
of thin card round one edge, so as to cover both sides of the
centre of the strip, and cut through both cards a hole rather
smaller than the crystal ; thus the card, as it embraces the
brass with the crystal in it, keeps the latter in place. All this
being adjusted on the stage, and the rings focussed as usual,
a lighted spirit-lamp is taken in hand and applied alternately
to the projecting ears. At first a slight mist generally appears,

380 OPTICAL PROJECTION

but this soon passes off, and the heat is applied till the
desired effect is produced. One crystal will last for many
demonstrations .

Circularly polarised crystal figures are produced by placing
a quarter- wave plate in the ordinary stage. The black
brushes then disappear, nebulous lines taking their places ;
while the rings are dislocated half a wave in each alternate
quadrant. The rings may instead be analysed circularly, by
fitting a quarter-wave plate on either side of the focussing-lens,
K (fig. 190), with the same results. If the rings be both
polarised and analysed circularly, the brushes disappear
entirely, and as the analyser is rotated, the quadrants (or
halves in bi-axial rings) slide by each other, producing in the
two principal positions unbroken rings with no brushes or
interruption whatever. If at this point the quarter-wave be
rotated with the analyser, the unbroken character of the rings
is retained throughout all the rotation ; showing the perfectly
circular character of the polarisation.

Spiral Figures were discovered by myself,1 in a search
after phenomena which should more distinctly show the rela-
tion of bi-axial to uni-axial crystals, and of the two axes of a bi-
axial to the prismatic axis— or, in short, that the axis of a uni-
axial was simply a case of the coincidence of two axes. For
obvious reasons this was most likely to be brought about by the
two circular waves concerned in rotary polarisation ; and it
seemed worth while to seek for such demonstration, since
when polarised and analysed circularly, one single axis of a
bi-axial gives as unbroken a circle as a uni-axial. I sought
for phenomena which might show that each axis of a bi-axial
was only one sex, as it were, of a combination, both of which
were found in a uni-axial. This is shown by placing in the
ordinary stage of the polariscope a quartz plate 7^ mm. thick,
and introducing between the crystal and the analyser a

1 See Proceedings of the Physical Society for November 12, 1881, or Phil,
Mag. January 1882,

POLARISED LIGHT 381

quarter- wave plate. The result is that a plate cut to exhibit
one axis only of a bi-axial, shows one spiral figure surrounding
this axis. A plate cut to show both axes, exhibits a few turns
of a spiral round each axis, after which the two spirals em-
brace each other, and proceed concentrically round the whole
field. With a low-angled bi-axial cut thin, in highly conver-
gent light, the two spirals enwrap each other nearly from the
first. And finally, in a uni-axial the two separate spirals are
visible, showing that both elements of the bi-axial remain and
are combined in what is simply a limiting case. By applying
the arrangement to Mitscherlich's experiment with a heated
sslenite, the gradual drawing in of the figure, but preservation
of the two spirals through all, can be simply demonstrated.

A quartz plate alone in highly convergent circularly
polarised light, projects a double spiral, as pointed out by
Mr. Airy. The above experiments prove that this is owing to
its peculiar properties enabling it to show its own spirals as a
uni-axial crystal.

A column of fluid of adequate rotary power, such as a
column of oil of lemons 20 cm. long, employed with a crystal
instead of the quartz plate, will exhibit exactly the same
phenomena, thus affording proof that the molecular con-
stitution of the fluid resembles that of the quartz. The
polariscope I have described projects through the column of
fluid, crystal, and convergent lenses, &c., brilliantly and with
ease.

It will be very interesting to point out, that by thus
modifying the polarisation in different ways, rays of the same
convergence can produce with the same plate of crystal, either
rings with brushes, rings dislocated, unbroken rings without
any brushes, or spirals.

222. Artificial Crystals. — It has been seen that a plate
of crystal circularly polarised, roughly represents the rotary
phenomena of quartz. Keusch, by employing a preparation
built up of many thin films of mica successively rotated on

382 OPTICAL PROJECTION

each other by an aliquot part of a circle, found that all the
phenomena of quartz, either in parallel or convergent light,
are perfectly reproduced. Such a preparation treated as a
crystal gives the crystal fringes of quartz exactly, and two
of opposite rotations superposed, give Airy's spirals.

Plates of mica of equal thickness crossed at right angles,
after Norremberg, as the plates become thinner and more
numerous, give a gradual transition in convergent light from
bi-axial rings, to the circular rings and four-armed cross of
a uni-axial. Two crossed give of course the four rings of a
crossed bi-axial ; twenty-four of about J wave thickness, give
effects quite undistinguishable from those of a calcite crystal.

Plates of mica of various thicknesses crossed in different
ways1 give very beautiful and complicated figures in con-
vergent light, which may still more be varied by crossing very
thin films between much thicker ones, or interposing plates at
an angle of 45° to the others.

Mica-selenite combinations after Norremberg, built up of
elements consisting of two parallel micas, with a selenite
between them either parallel or across, these triple ' elements '
being superposed in various ways and number, crossed or
parallel, give in highly- convergent light the most beautiful
projections. With no geometrical design whatever from the
hand of man (except the crossing of films in various ways),
the most exquisite coloured patterns are produced by con-
vergent polarised light, some of them looking more like the
most brilliant designs in squares of Turkey carpet than any-
thing elso.

223. Polarisation by Small Particles. — The blue colour
and polarisation produced by all reflection of light from
sufficiently small particles (as in the sky), may be easily
demonstrated by Tyndall's method, a large glass tube with

1 For details of such crossed preparations, see my paper in Proceedings
of the Physical Society, entitled ' Optical Combinations of Crystalline Films,1
reprinted in Phil. Mag. May 1883.

POLARISED LIGHT 385

flat ends being supported horizontally in front of the polariser,
filled with suitable vapour, and the parallel polarised beam
sent through it. The particles in the tube then act as
analyser, so that the phenomena can be observed by all
spectators in a direction approximately at right angles to the
tube. In one position of the polariser, light is copiously
reflected to such spectators, while it is invisible to anyone
looking down upon the tube. When the polariser is rotated
90° the light is extinguished horizontally, but reflected above
and below. A better method, however, is to interpose between
the polariser and the tube a large plate of selenite or mica,
when the tube will glow with the usual colours.

Such a tube should not be less than 2^ or 3 inches
diameter, and 18 inches long. Professor Tyndall generally
preferred, having exhausted the tube, to introduce sufficient
nitrite of butyl vapour in air to depress the mercury-gauge -£$
inch, and to add sufficient hydrochloric acid vapour in air to
depress the gauge a further J inch. He also employed vapour
of carbon disulphide, amyl nitrite, and other compounds. But
it is very much easier, and saves the trouble of exhausting
and a great deal of rather delicate manipulation with air-
tight and expensive apparatus, to employ a whiff of tobacco
smoke, which is perfectly effectual. Instead of using the
parallel beam from the polariser, very fair results can be had
from the Nicol analyser in the nozzle of the optical front,
but the polarised beam does not then, of course, fill the tube
BO completely.

It is however easier and more convenient in every way, to
show the phenomena in liquids, which can be conveniently
done in a large rectangular glass trough or cell. Far the
best method, however, not only for economy and convenience,
but for its truly magnificent effect, is one for which I was
originally indebted to Mr. John Thomson of Dundee. For
this beautiful experiment we require a glass jar with foot,
which should be about 2^ inches diameter, and is shown at

384

OPTICAL PROJECTION

j, fig. 209. This is filled with the fluid holding small particles
in suspension, which may at a pinch be prepared by adding
a trace of soap, or even of milk, though both of these give too
great coarseness of particles for fine effect. A few drops of
a solution of five grains gum mastic, or common resin, in one
ounce of alcohol, stirred into the water, answers excellently,
as will a few drops of French polish diluted in alcohol; but I
have found the finest blue from stirring a teaspoonful of the

solution of coal-tar
in alcohol sold as
Liquor Carbonis
Detergens, into hot
water. The jar
should be clean,
and the emulsion
filtered into it, to
avoid as much as
possible ordinary
reflection from dust
particles. Lord
Bayleigh often uses
a mixture of ex-
tremely dilute so-
dium thiosulphate
with extremely di-
tto. 209 lute hydrochloric
acid, a mere trace

of each being sufficient. The advantage and disadvantage of
this method is, that the effect is progressive, and finally passes
beyond the stage of polarisation. However, the jar being filled
with the fluid, the plane reflector K is arranged over it at an
angle of 45°, so as to throw the light from the polariser N (here
shown as the Nicol analyser) down through the jar. Behind
are arranged two reflectors M M of plain looking-glass, enclos-
ing the jar within an angle of about 100°, which give by reflec-

HEAT 385

tion two images of the jar, formed by rays at right angles
with those direct from the jar to the spectators. The great
advantage of this method is, that the spectators only need
to be about the same height as the jar, to receive perfectly
polarised light all round; and the complementary effect
can always be seen at the same moment in the mirrors, so
that when the jar extinguishes direct light, the mirror
images are bright. This having been demonstrated, we
finally cover the top of the jar with a large quartz plate Q.
The jar itself will now glow with all the rotational colours
as the polariser is rotated ; differently -coloured images
being seen at the same time in the mirrors M, and the
whole forming a demonstration of indescribably delicate
beauty. It is convenient to have the mirrors hinged
together like a book, and to stand both jar and mirrors on
a circular piece of board, if no higher support be needed, in
order that the whole may be turned to face in succession all
sides of the room.

CHAPTER XXin

HEAT

A. NUMBER of experiments showing the effects, nature, and
qualitative relations of heat are easily capable of projection.
Some specimen examples may suffice.

224. Expansion. — As regards the expansion of solids,
Gravesande's ring and the various socket forms of apparatus
need no remark. Any ordinary form of pyrometer is readily
projected in action by the shadow method (§ 109) ; or a small

0 0

.386 OPTICAL PROJECTION

mirror of silvered micro-glass may be cemented to the index,
and by it a small parallel beam focussed from an aperture
reflected to the screen ; the deflection of the spot, with any
decently good instrument, will show the effect even of the
warmth of the hand. Or a series of small thermostatic com-
pound bars is easily constructed, whose action can be pro-
jected in the field of the condensers.

In liquids, any thermometer with a transparent scale can
be projected, using the erecting prism. That the expansion
is due to greater separation of the particles, and lowers the
specific gravity, is easily shown by paraffin oil
stained with aniline colour, in a U-tube, as in fig. 210.
One leg of the tube is surrounded by a much wider
tube fitted on by a cork at the bottom. The liquid
in both tubes stands at the same level ; but on
filling the large tube with boiling water, the levels
being in the field of the condensers, it will be seen
that the liquid in the heated leg stands considerably
higher than in the other.

Holding a flask of very thin glass with a very
small tube neck, which is filled with coloured liquid,
in a larger vessel, and projecting the part showing
the level in the tube, on pouring hot water into
the outer vessel, the curious effect is produced of
a momentary sinking of level, as from & to a
(fig. 211) after which the fluid rises as expected,
thus showing the expansion of the flask before the
liquid itself is heated, and the subsequent superior
expansion of the liquid.

^ne expansion of air can be projected in any
way, but is perhaps most readily shown by pro-
viding a flask with a tube of almost capillary bore, and
introducing a drop of any coloured fluid when the air is
warm, which will be drawn some way down the tube. On
projecting this, any difference of temperature will move

HEAT

387

the index fluid. Or a differential thermometer may be
projected.

FIG. 211

PIG. 212

225. Convection Currents.— The simplest apparatus for
projecting currents produced by the lowering of specific
gravity, is a mere test-tube held in a Bunsen holder and
heated as in fig. 212.
If the tube be flattened,
the effect will be better
still ; or a flattened flask
can be similarly pro-
jected. To make the
currents more visible, a
few particles of exceed-
ingly fine sawdust from
heavy wood may be
placed in the water, or
particles of blotting-
paper rubbed up in a mortar with water ; Mr. H. G. Madan
uses a crystal or two of magenta coated with gum-water and

o c 2

FIG. 213

388

OPTICAL PROJECTION

dropped into the vessel, from which coloured streams will
proceed. The little apparatus shown in fig. 213 will both show
convection currents, and illustrate their every-day applica-
tion to hot-water systems of heating. A few small bubbles
or particles of sawdust show the movement well in this
apparatus.

226. Evaporation. — Boiling in a small flask is easily pro-
jected, and by the well-known apparatus in fig. 214, boiling up

a flask of water, which is
closed with a cork and then
inverted in the field of the
lantern, the familiar experi-
ment of producing ebullition
by the application of cold
water, is also shown. The
loss of heat in evaporation
may be projected either by
wrapping the bulb of a
thermometer in a piece of-
rag moistened with the liquid,
or by applying a moistened
plate to the face of a thermo-
pile, the galvanometer in
connection with which is
projected direct, or by a re-
flected pencil. (See § 240.)
227. Conductivity.— To
show the different conducting power of various metals and
other substances, nothing more is necessary than to modify
the well-known apparatus of Ingenhouz (in which rods of
the substances project from a trough filled with boiling water,
which heats the inner ends of the rods simultaneously), so
that the rods project from the 'bottom of the trough, and their
lower ends at least stand in the field of the condensers. Glass
balls of equal weight being attached by wax softened with a

FIG. 214

HEAT

389

FIG. 215

little tallow to the ends of the rods, will be seen to drop off in
succession. They will appear to pass upwards on the screen,
unless the erecting prism is
used.

The conducting power of a
metal is projected by construct-
ing Despretz' apparatus within
the compass of the lantern, as
in fig. 215, the bulbs of the
thermometers resting at equal
distances upon a bar heated at
one end. The scales should be
of glass.

228. Mechanical and Molecular Motion. — By using a
thermo-pile with any suitable galvanometer projection, the
heat generated in any body by friction, or in a bullet by per-
cussion, or in a closed vessel of air by compression, or the
cold produced by rarefaction, can be easily shown. The
readiest way of exhibiting the heat of crystallisation is to
project the tube of an air-thermometer placed in a vessel of
the solution. Badiophony is unfortunately not a projection
experiment, though the lantern is used for it ; as the effect
is perceived only by an individual observer with the telephone.

229. Specific Heat. — The great differences in this are
best projected in the following way. Prepare balls or bullets
of equal size, about f inch to 1 inch diameter, of iron, tin, and
lead ; the two latter cast in a mould, and the iron prepared
in any way ; or zinc will answer instead of iron and can also
be cast. Each ball is furnished with a small wire hook or
handle. Prepare also a flat cake of soft wax about ^-in. to \-
in. thick and say 6x2.^ inches superficies, in a frame formed
by bending round a strip of tin or thin brass. The com-
position of the soft wax should be about 4 parts by weight
of beeswax and 3 parts of tallow, varied a little by experience
as required ; and the cake is best formed on the surface of

396 OPTICAL PROJECTION

boiling water, and left to get cold. The cake is supported
horizontally in the field of the lantern, and a cold bullet laid
on its centre is focussed on the screen and then removed.
The three bullets to be used are heated in boiling water long
enough to be certain they have all acquired the temperature
of 212° F. The leaden one is then taken out first by its
handle, and after shaking off the water, gently placed on the
centre of the wax ; then the tin one similarly laid about 1^
inches from it on one side ; and lastly the iron or zinc one
the same distance on the other side — all appearing on the
screen. Though much heaviest, the leaden bullet will scarcely
sink at all into the wax ; while if the softness and thickness
of the plate are well calculated, the tin ball will sink much
deeper, and the iron or zinc one will make its way entirely
through, and drop into some vessel placed to receive it. The
softness of the wax is to be adjusted, according to the exact
size of the balls and thickness of the cake, to secure this
result.

230. Spheroidal State. — A very shallow silver dish about
three inches diameter is supported with its convex side upper-
most over an annular Bunsen burner in the field of the lantern,
and the burner adjusted to bring the surface to a dull red
heat. Taking up in a small sponge some water as hot as the
hand will bear, and squeezing out drops to fall upon the hot
dish, the rebounding of the drops, like peas, will be clearly
projected.

Reversing the dish to its usual position, and placing the
whole lower, and more in front, the beam may be slightly
converged from the condensers and reflected down upon it,
and a focussing lens adjusted to focus the object upon the
ceiling, or a small overhead screen, at a similar angle. The
dish being heated as before, nearly boiling water is dropped
into it by degrees from a syringe or pipette, till a fluid
drachm or more has collected. The water will become a
flattened globule, in ceaseless motion, the edges especially

HEAT 391

terming beautiful curves, and the surface breaking into sym-
metrical ripples, which the lens will focus on the screen. On
turning out the burner the movement will gradually decrease
in energy, until suddenly contact is established, and the
water bursts into a cloud of steam.

Arrange in the field a smaller silver dish about two inches
diameter, which has had the bottom carefully flattened for
about one inch in the centre, convex side uppermost as in the
first experiment. By carefully adjusting a small ring on the
end of a wire about 1 mm. above its surface, and heating the
basin and in a much less degree the wire, hot water darkened
with ink or other dye can be gently delivered from the pipette
so as to be held in place by the small ring thus immersed
in it. Projection on the screen will then demonstrate that
there is a small clear space between the silver and the water,
through which the light from the lantern passes.

Heating of the water is desirable in these experiments, in
order to avoid cold water lowering the temperature of the
metal underneath to a point which will not maintain the
spheroidal state.

231. Radiant Heat in the Spectrum. — The demonstra-
tion of dark heat rays has already been mentioned under
Calorcsccnce, in § 180. The quantitative examination of the
spectrum demands expensive apparatus in the shape of a
complete train of prisms and lenses in rock-salt (which alone
will show the real heat maximum in the ultra-red region),
and a sensitive thermo-pile of the linear form, guarded by a
narrow perpendicular slit. Only the general scheme can
be given here in fig. 216, where s is the slit of the lantern
through which come parallel rays produced by a rock-salt
lens in the lantern itself, A is the salt focussing lens, B the
rock-salt prism. Then v B G B representing violet, blue, green,
and red rays, the thermo-pile will show the greatest heat at
about P, and will cease to give evidence of it at o, nearly as
far from B as the violet rays are. For these experiments a

392 OPTICAL PROJECTION

sensitive thermopile, and sensitive galvanometer, must be
employed. The needle of the latter may either travel over a
horizontal glass dial, projected in the vertical attachment, or
a pencil from an oscillating mirror may be employed.

With glass lenses and a disulphide prism, the demonstra-
tion must be comparatively rough. The maximum heat will
now be generally found in the red itself, and the galvano-
meter with perpendicular glass dial (fig. 220) can be made
sensitive enough to project all that can really be shown with
such apparatus.

It is usual to employ two lanterns or two nozzles, one
giving the spectrum in which the thermo-pile is moved, and

FIG. 216.— Examining the Spectrum

the other projecting the galvanometer movement above it ;
if two nozzles are employed on one concentric lantern, this
must be so adjusted that the common radiant is at the proper
focal distance for each of the two sets of condensers. On the
whole, a second lantern for the galvanometer is most con-
venient. It is also convenient to have at disposal two
galvanometers, one very different from the other as regards
sensitiveness. (Also see next section.)

232. Identity of Light and Radiant Heat.— By similarly
using a thermo-pile, the double refraction, polarisation, and
depolarisation of heat rays are readily shown. If luminous
heat rays are employed, the thermo-pile has simply to be

HEAT 393

placed in the rays marked out by their luminous effects. If
non-luminous rays are to be demonstrated, the positions are
first marked from the luminous rays ; after which a cell of
iodine dissolved in carbon disulphide or tetra- chloride is
interposed, and the thermo-pile introduced into the marked
positions, the galvanometer instantly responding. When
the polarised ^beam is cut off by the analyser, and the latter
is at zero, the introduction of a plate of crystal between
the crossed Nicols, at once causes a vigorous movement. In
proving electro-magnetic rotation of the dark beam, Prof.
Tyndall employed, for the sake of the greater increase of
transmission in that position, the two Nicols (or other
apparatus) with their polarising planes enclosing an angle
of 45° instead of 90°, equilibrating the thermo-pile and bring-
ing it to zero for the amount of heat thus transmitted, by
placing some other source of heat at the requisite distance
from the other face of the pile. Then upon switching on
the current, a movement of the galvanometer at once showed
the greater amount of heat transmitted by the rotated
beam.

Using a diffraction grating and slit (as in § 191) with a
red glass, and also with the iodine-cell, bands of action and
extinction as in the case of light, are readily shown by the
thermo-pile. It is just barely possible, with a fine slit on the
thermo-pile, to show heat fringes with a Fresnel prism ; but
so very difficult and uncertain is a clear result, that I
abandoned the experiment with regret. Probably others
with more skill in this sort of manipulation, and more
opportunities with the arc light than have been possible to
me, may have greater success. For such experiments, the
radio -micrometer recently perfected by Mr. C. Vernon Boys,
on the principle of d'Arsonval's thermo-galvanometer,1
gives both more delicacy in recording small variations of tem-

1 The fundamental principle is the suspension of a thermo-electric closed
circuit by a torsion-fibre in a magnetic field.

394 OPTICAL PROJECTION

perature, and more precision as regards points of action, than
any thermo-pile I can conceive of. This beautiful measuring
instrument might also be employed in the preceding experi-
ments, but is so delicate, and so rarely available for general
lecture purposes, that I must only refer concerning its con-
struction and use to the inventor's own description.1

CHAPTER XXIV

MAGNETISM AND ELECTRICITY

THE fundamental experiments in magnetism and electricity
can be projected in a manner so obvious as not to require any
detailed treatment. As a rule the needful preparations will
consist simply of reducing the apparatus to a rather small
scale, when the projection will take place under the general
conditions described in Chapter XIV. Only a few experi-
ments need be given here for the sake of illustration, or of
various remarks for which they afford occasion, and which
may be useful in other instances.

233. Polarity. — A compass needle mounted either upon a
small foot, or upon a graduated glass plate, projected with the
vertical attachment, will readily show direction, attraction,
and repulsion. Two light soft iron wires hung from the
same point by silk threads attached to one end of each, in
the field of the condensers, as a magnetic pendulum, will
also exhibit repulsion.

With the glass-bottomed trough filled with water in the
vertical attachment, and a number of magnetised steel needles
stuck perpendicularly in small cork floats, Prof. Mayer's
symmetrical figures, formed by such needles when the pole of

1 Proc. JR. S. xlii. 191, and Phil. Trans, clxxx. 159.

MAGNETISM AND ELECTRICITY 395

a magnet is held over the centre of the trough, will be pro-
jected on the screen.

Magnetic torsion may be projected in the same way, with
any form of torsion balance whose graduated dial is of glass.

234. Magnetic Induction. — Clamping an annealed iron
rod 3 inches long by the centre in a small Bunsen holder
horizontally across the field of the condensers, it will be
readily shown that it supports a key, or one or more pieces of
iron, at one end, when the other is touched by the pole of a
strong magnet. And using iron filings or small bits of wire,
it can be shown that induction takes place, though the
magnet does not actually touch.

Induction by the earth's magnetism is projected by the
usual experiment of attracting or repelling the pole of a com-
pass needle (placed on the vertical attachment) by the lower
end of an iron bar held approximately in the magnetic meridian.
Or a dipping-needle may be projected direct, and similarly
affected by the end of an iron bar.

235. Magnetic Curves. — These project in the vertical
attachment, simply laying a plain sheet of glass over the
magnet laid on the face of the condenser, sifting filings upon
the glass, and if needful tapping the plate a little. Using a
horseshoe magnet, the modifications caused by different
armatures between the poles may be shown. Elevating the
glass plate sufficiently, duplicate similar poles may be brought
up endways under it and the different character of the curves
projected : or the lines of force as modified in the neighbour-
hood of a space screened by sheets or plates of soft iron may
be shown.

236. Dia-magnetism. — Only an electro-magnet will give
sufficient power for this class of experiment. A sufficient
size for the magnet will be a cross section about J inch square,
built up of bent pieces of well-annealed hoop-iron, the total
length of each limb about 4 to 6 inches ; or it may be more,
as it is quite unnecessary for the lower part of the magnet to

396 OPTICAL PROJECTION

be in the field. There should be 1£ inches clear between
the two poles, and after all the plates are firmly screwed to-
gether and the ends filed fiat for the usual pole-pieces, the
magnet is wound until the coils come nearly into contact.
With such a magnet, two to four bichromate or Grove
cells will give ample power for most experiments, using either
small discs, or bars of metal or other substances j inch to 1
inch in length, for projection.

The same apparatus may be used to show that a cube or
disc of copper suspended between the pole-pieces, and rotated
by twisting the thread, is almost immediately stopped as
soon as the current is switched on.

For experiments on flame and vapours, however, a
magnet must be constructed differently, with the limbs much
farther apart, in order that the flame or vapour may be
introduced beneath the pole-pieces. The vapour of iodine
dropped upon a piece of heated metal is easily projected, and
the repulsion will be plainly distinguished. It is unnecessary
in dia-magnetic projections, that more than the magnetic
pole terminals should be seen in the field.

The various heaping of liquids, according to their mag-
netic character, is fairly visible in profile ; but by turning the
magnet so as to lie horizontally on the vertical attachment,
the heaping of liquids in a thin watch-glass will be con-
spicuously shown by the strong refraction produced at the
inclined surfaces.

237. Static Charges. — Most simple phenomena of this
character are demonstrated by projecting an electroscope,
especially of the gold-leaf kind, as shown in fig. 81. Static
induction is readily shown in the same way. All the usual
experiments with suspended pith-balls, chimes, feathers, hair,
&c., are too obvious to need any explanation. Any apparatus
which cannot be reduced to the size of the condensers, or the
lens in fig. 106, may generally be projected by the shadow
method (§ 109).

MAGNETISM AND ELECTRICITY

397

Quantitative measurement is more difficult, but Prof.
Mayer has lately used a pendulum electroscope in a form that
makes a most sensitive electro-
meter. He suspends a gilt
pith-ball I cm. radius (built up
of small pieces cemented) a
distance of 364 cm. from the
ceiling, by two silk fibres
fastened 52 cm. apart and
meeting at the ball, behind
which a scale is arranged. A
brass ball of the same size is
mounted on a glass rod, var-
nished while warm with
paraffin wax. A force of only
one dyne acting on the sus-
pended ball deflects it 13-3
mm. ; and charging both balls
in contact so as to give the
same charge to each, the
charge c on either was found
in absolute electrostatic units
by the formula

where D is the distance in

cm. between the centres of

the balls, and d the deflection

from the vertical in cm.

When D is over 5£ cm. the

law of inverse squares was

demonstrated within 1 per Via. siffaX^ mi/<§*' ^

cent; and by using a proof- /^^^/ ^ c^

plane, Coulomb's law of distribution on the pajp^J? a/^iin-<^

drical body was verified with close approximj

398 OPTICAL PROJECTION

apparatus is in fact too sensitive for most lecture purposes;
but diminishing this by using shorter fibres, and adjusting a
hori.-.onlal strip of glass behind the ball. di\ ided as a scale,
many useful quantitative experiments may be made ill a very
simple ai nanner.

The mere fact that, the density of a charge is greater at
the ends of a conductor than in the middle may bo shown by
Prof. Weinhold's very simple apparatus (tig. kJ17). Four

pieces Of paste-board with rounded sides and edges are couivd
with tinfoil, and fastened into a box like shape b\ strips of
ribbon also covered with foil, or by thick foil itself. To tho
middle of one angle are suspended by conducting threads a
pan of small pith balls ; and close to the middle of the oppo-
site angle are fastened two slicks of sealing' wax as insulating'
handles. The whole apparatus is shown at. A. If both
handles are so held thai the plates take the form of i:. and t lie
apparatus is gi\en a small charge, a little di\ergeitce of the
pith-balls will be observed; but if now tho handles be so

turned that the plates take the form of c, the divergence will
be increased ver\ considerably. The apparatus max be made
quite small, but is more effective if the pieces are about six
inches square, when it can bo projected by the shadow method
(§ 109).

L

Fiu. 218

288. Static Electrical Stress. — Dr. Kerr's experiments
showing by optical methods the state of stress produced by
electrical charge in di electrics, are easih projected, tig. '218
b, in-: a .-vneral sketch of the arrangements. Tho rays from
the lantern pass through a polarising apparatus, M, and then
throu.-rh a glass cell, P. In this cell are arramr> d t\\o copper
plates a very small distance apart, say from ^ to J inch, which

\GNETISM AXD F.LF.CTRTCITY 399

can bo charged -f and — by connecting with tho coats of
aLeydon jar. or the terminals of a Wiinshurst machine. The
Space between tlie platert is focussod on I lie sereen by tlie lens
D, the rays passing thro. analysing Niool, N. If

tho plates arc vertical or horizontal, the Nicola are aet at an
i >ut crossed to gi\e (lie dark field. The coll is
best filled with carbon disulphido. On charging the plates,
the Held at once becomes bright, and if the charge id increased
splendid colour-effects are produced, as with glass in a state
of mechanical stress described in § '209.

With charges HuflicienHy powerful, similar stresa can be
shown in glass itself, pierced so that Iho terminals may oomo
within a small distance.

At o may bo introduced a compensator of quartz to
measure the optical elTcct of charges, thomsolvea measured by
any sullicicnlly accurate electromotor.

ii,T.». Static Discharge. The sparks from ajar or battery,
or Winishurst machine, are beat projected by a little apparatus
arranged for the
fit-Id of the lan-
tern, and which
needs no descrip-
tion beyond tlie
diagram in fig.
'21!), from Mr. G.
M. Hopkins.1 The

halls being adjusted for striking distance, arc focussod on tbe
screen, and the light then reduced to as low a point as will
just barely make tJiem visibk. Tho place of tho image of
the discharge spark upon the screen is thus localised, which
in this case makes just nil the dillcrem-e. in a scroon demoli-
sh ;it mu, and renders it very satisfactory; the form of tho

i Tin <lu i m. ami fig*. 225, 220, 227, 281, 2B2, are from artirloi bj

Mr. llopkiiiN in tli.- .sv/Vntylc American.

400 OPTICAL PROJECTION

spark being more clearly seen than most eyes can discern it
in the brilliant spark itself.

To project the oscillating character of the discharge, as
pointed out by Dr. 0. J. Lodge in his own demonstrations,
we require a battery of considerable size or capacity, and
must introduce into the circuit a number of coils of wire. A
quarter-inch spark is a good distance. Having at hand
several large coils of wire, and introducing one after the
other into the circuit, the spark will be found to give at each
addition a lower note ; and when this reaches a certain low-
ness of pitch, it is broken up if focussed on the screen and
then reflected from a rotating mirror. A better apparatus,
however (since the breaking-up of the spark can only be seen
at one precise moment), is to project it through a number of
lenses of the same focus ranged round the edge of a rotating
disc, at slightly different radial distances, as devised by Mr.
C. V. Boys. By this means the effect cannot fail to be pro-
jected upon the screen, and there is less loss of light also
than when the mirror is employed.

The heating effect of the discharge is effectively shown by
a simple form of air thermometer employed by Prof. Weinhold.
Through a dry cork (which is sufficient insulation) in a small
bottle are passed wire terminals, connected by a rather narrow
strip of tinfoil in the shape of a U« A bent glass tube also
passing through the cork is drawn out into a small bore, and
then the bottle being slightly warmed and the tube dipped in
coloured liquid, as the air cools it draws in a small portion
which serves as an index. Projecting this, on making dis-
charge through the foil, the air in the bottle is sufficiently
heated to move the drop of fluid conspicuously upon the screen.

240. Current Electricity. — Galvanometers. — In the vast
majority of experiments, the existence or calling into action
of a current will have to be demonstrated by projecting the
movement of a galvanometer, which can be easily done in
various ways.

MAGNETISM AND ELECTRICITY

40!

Where great sensitiveness is not required, a perpendicular
glass dial and multiplier coil arranged as in fig. 220, which
represents one constructed by Stohrer of Leipzig, will be far the
most convenient and simple, needing only to be placed in front
of the condensers and focussed.
With care in balancing the
needle, this arrangement can be
made amply sensitive for all
ordinary induction or direct
current experiments, and also
serves to illustrate the action of
the needle telegraph.

Another good galvanometer
of this kind, shown in fig. 221,
was constructed, and has been
used with success in many of his
lectures to large audiences, by

Prof. S. P. Thompson. Here the coil A B c D is wound hori-
zontally, and to the centre of the needle N s is attached, at right
angles, a very light perpendicular index or pointer, the needle
and its pointer being so mounted that the centre of gravity

FIG. 220.— Lantern Galvanometer

Fia. 221.— Lecture Galvanometer

is very little below the pivot E. Long or short coils may be
used, and this pattern has the great advantage that it may be
used as an ordinary lantern slide.

In using any kind of ' face ' galvanometer, should the face

D D

40.7 OPTICAL PROJECTION

happen to lie in or near the plane of the magnetic meridian,
it will be necessary to counteract the * dipping-needle ' effect
by adjusting a bar-magnet somewhere near, preferably under-
neath the table.

The most sensitive and precisely calibrated astatic galvano-
meters are chiefly needed for very feeble thermo-pile currents,
as in investigating the heat of the spectrum in its colder
regions, or calorific interference bands (see Chapter XXIII.).
Such a galvanometer is easily projected in the vertical manner,
provided the base and dial-plate are of glass.

The lecture galvanometers most generally used, however,
where comparatively but not excessively small currents are
to be demonstrated, are of the reflecting mirror class. For
ordinary experiments, one of the dead-beat types is most
convenient ; but I am only here concerned with the projection,
which is capable of a much better result than is generally
seen. A dim and very nebulous spot on the scale seems to
satisfy most demonstrators, and is perhaps sufficient for a
small audience. But it is very much better to use a small
lime-light lantern for the index-pencil, and, omitting any
lens in the galvanometer itself, to employ the 'focussed
parallel beam ' (p. 277) from the pencil attachment shown
in fig. 95. An aperture and lens should be chosen which,
focussing on the scale, gives a sharp and brilliant disc over
its whole width (about a quarter of an inch is a good average),
and this should be crossed by two wires arranged X fashion.
The great superiority of this method will not fail to be appre-
ciated when once seen.

It is quite needless to describe in detail how every form of
current induction, either by other currents or by magnets, is
demonstrated by the galvanometer.

241. Mutual Action of Currents and Magnets. — The
general rule to be observed in devising apparatus to project
the wide range of phenomena discovered by Arago, Ampere,
apd Faraday, is always to select the most simple arrangement.

MAGNETISM AND ELECTRICITY

403

Take for illustration Oersted's fundamental experiment of
tbe deflection of a magnetic needle by the current ; it is per-

FIG. 222.— Oersted's Experiment

fectly easy to arrange an entire fixed apparatus, and project
it by the vertical method. But the truly scientific demon-
strator will rather prefer
to place the bare needle
first in the focal plane, and
bring over his wire inde-
pendently, as in fig. 222,
simply because his ex-
planation will thus be
better understood, which is
his object.

Ampere's experiments
showing the attraction or
repulsion of parallel cur-
rents may be shown by
any of the usual apparatus
made on a small scale ; but
here again the same rule
applies, and it is better to
confine the set apparatus

FIG. 223

to the movable portion,

and bring up the other current independently, as in fig. 223,
which is an arrangement by Prof. Forbes. A small arrow
may be fixed to the wire which is held; and in this way

404

OPTICAL PROJECTION

there is no room for misunderstanding. The arrangement
known as Eoget's Spiral gives another very elegant illus-
tration of the attraction of parallel currents. The wire carry-
ing the current is wound into a vertical helix, suspended
at the top from one terminal, and dipping at the bottom into
mercury, which forms the other terminal. The coils should
be not less than an inch in diameter, and wound closely
to be very easily movable : then on passing the

current the coils draw
more closely together,
which is very conspicuous
upon the screen. If more
powerful action is needed
an iron core may be intro-
duced, as in the figure, but
this will rarely be neces-
sary.

Any of the usual ap-
paratus for showing the
rotation of currents, mag-
nets, or galvanic cells, is
fc easily constructed small
% enough for projection, and
S need not be further dwelt
upon.

For many electro-mag-
netic experiments it is

convenient to arrange two parallel horizontal brass rods pro-
jecting from beneath the front of the condensers, communi-
cating with binding screws, as devised by Mr. G. M. Hopkins.
Then small pieces of apparatus, such as here shown, connect-
ing with metal spring hooks underneath, have only to be laid
on the rods and pushed home to one side, to be in contact.
Fig. 225 illustrates the simple magnetic effect of a current,
the wire being wound into a small coil at the top of each

FIG. 224.— Bogefs Spiral

MAGNETISM AND ELECTRICITY

405

FIG. 225

pillar, and then bent downwards in the middle between. The
little trough of iron filings can be lifted up first, before the
current passes, with no result ; afterwards the filings will
adhere, and show an evident tendency to range themselves at
right angles to the
current.

To explain the
construction of an
electro-magnet, two
small solenoids of
insulated wire are
arranged across a
small frame as in
fig. 226, being wound in contrary directions. They should
be wound so much apart that the turns can be clearly seen.
A stout needle may be first introduced into each, and a mag-
netised needle, sus-
pended at one end
by a thread, may be
shown to be equally
attracted by either ;
then on passing the
current, it will be
attracted by one and
repelled by the other
as shown in the
figure. Withdraw-
ing the needles, and
introducing a small
horse-shoe of an-
nealed iron wire (fig. 227), we have an electro -magnet, whoso
whole construction is clearly seen, and which will support a
nail or other piece of iron easily.

Magnetic hysteresis can be shown by any electro-magnet
small enough for the field of the lantern, or by a larger one

PIG. 226

406

OPTICAL PROJECTION

FIG. 227

in shadow. On switching off the current the armature will
be seen still attached, with even a considerable load ; but on
striking the core sharply with a small hammer so as to re-
lease the molecular
rigidity, it at once
falls.

The attractive
pull of a solenoid
upon a magnetic
core is an important
phenomenon, being
so widely employed
to regulate arc-
lamps. It is very
easily projected, fig.
228 showing one of Prof. Forbes's arrangements. The solenoid
A, here in conventional connection with a battery, is arranged
across one half of the condenser ; and the soft iron core B is

suspended by threads
so as barely to enter
one end. In order
that the iron core may
be able to enter far
enough, it is well to
prolong it by a non-
magnetic wire brazed
to one end, so that the
whole may balance
by the threads ; or the
core may be supported
in the middle of a
much longer glass

tube, whose ends are suspended by threads attached far outside
the solenoid, so as to give ample motion. Directly the current
is switched on, the core is drawn into the coil. A still simpler

MAGNETISM AND ELECTRICITY 40?

apparatus may be made by taking a piece of ' quill ' glass
tube 4 inches long, and winding one half of it with two layers
of insulated wire. Provide a soft iron wire 2J inches long,
which slides easily in the tube. Fix the tube horizontally
across the condensers, and place the iron core in the uncovered
end, so that one end rather projects. Turning on the current,
the core will be drawn into the coil, and the two ends will
be seen projecting slightly at each end. It is easy to con-
struct two small ' differential ' coils in the same way.

242. Thermo-Electricity. — Seebeck's fundamental experi-
ment is easily projected by simply placing it upon the vertical
attachment. The current from two single wires about 6
inches long, of copper and German silver, simply twisted to-
gether at one end and united by a drop of solder, can be easily
shown by a decent projection galvanometer. Any experi-
ments with the thermo-pile also afford illustrations.

243. Current and Capillarity. — Kiihne's beautiful experi-
ment is easily projected, using a very shallow watch-glass or
a plano-concave lens on the vertical

attachment. The hollow is filled with
dilute sulphuric acid containing a little
chromic acid, and enough mercury to
form an oblate spheroid half an inch
across or more, introduced into the
centre. A bright iron wire about 2 mm.
diameter and 6 inches long is then
introduced so as just to touch the edge FlG- 229

of the mercury, when the circumfer-
ence of the spheroid is thrown into rhythmic vibration by the
changes in surface-tension, set up by the intermittent contact
and consequent intermittent current.

Another simple apparatus is shown in fig. 229. The white
portion a represents mercury in the glass tube, and the
dark part b dilute sulphuric acid. On sending a current
through the whole, it will be seen that the mercury rises or

4oS OPTICAL PROJECTION

falls in the centre leg according to the direction of the
current.

Or two open ends resembling a may be connected by a
horizontal tube about 1 mm. bore, and filled with mercury,
except that in the centre of the horizontal tube is introduced
2 or 3 mm. in length of the dilute acid. Behind the tube is
a glass divided scale. This represents for easy projection
Lippmann's capillary electrometer, being simply on a larger
scale the apparatus shown in fig. 127, p. 240. Very moderate
currents will readily cause motion of the acid index-spot upon
the screen.

The beautiful experiments of Profs. A. W. Eeinold and
A. W. Eiicker, showing that an upward current retards, and
a downward current hastens the thinning of a liquid film,
are projected in exactly the same way as an ordinary soap-
film, described in Chapter XX. The usual liquid, however,
thins too slowly. It is usual to employ potash oleate, but
solution (c) on p. 826 will answer very well.1

244. Effects of the
Current. — The decomposi-
tion of acidulated water is
easily projected by a simple
cell as shown in fig. 280.
Inverting a small test-tube
filled with fluid over each
wire, the approximate
volumes of the two gases
FIG. 230 ~~ ""* will be shown. By similar

apparatus slightly modi-
fied as required, the decomposition of solutions of various salts
is projected in the same way. In this experiment, as in many

1 See for details, Proc. Phys. Society of London, vl 857. It only need be
said here that the films are easiest managed as cylinders, formed between
metal rings or short cylinders, which form terminals for the current, the whole
being enclosed in a glass case to exclude dust and draught.

MAGNETISM AND ELECTRICITY 409

others, whether or not the linage should be erected by a
prism will depend much upon both the lecturer and the
audience.

The heating effects of the current may be shown by carry-
ing a coil of platinum wire through water and boiling it ;
or by making a piece
of platinum wire in-
candescent as usual.
The last method,
however, and the
effects of heat in in-
creasing resistance,

are not as a rule pro-

. . . . FIG. 231

jection experiment?,

and therefore need not be further referred to.

Incandescence as shown in electric lamps is best projected
by Mr. Hopkins' method, of showing in the field of the
lantern the apparatus itself by as dim a light as will make it
j ust visible on the
screen, and then
sending the cur-
rent through,
when the light
itself will be also
in focus. Fig.
231 is a simple
little arrangement
for an incandes-
cent lamp, and

fig. 232 gives an FIG. 232

equally simple at-
tachment for projecting the carbon points and arc. No regu-
lator at all is needed for the latter, as anywhere within an
inch or two will project equally well by this method ; and there
being no mechanism to be worked, a smaller current will give

410 OPTICAL PROJECTION

good results. Lamps can, however, only be thus projected by
a lantern whose light can be readily and considerably lowered ;
for while a dim projection of the non-luminous parts very much
adds to the effect, any brightness in that part of the image
utterly spoils it. If the carbon-points themselves are too
brilliant, a small diaphragm may be placed over the objective,
and the light in the lantern increased accordingly. Another
advantage of this method is that the poles are projected sharply
and uncoloured, owing to the employment of the achromatic
lantern objective. Ordinarily the arc (in lantern) is projected
by the condenser alone, passing the rays through an aperture
small enough to tone down too great brilliance.

With regard to the mechanical or motor effects of the
current, their elements have been already mentioned in § 241.
If adequate range of current be at command, however, a
model made in such skeleton form as to show the essential
arrangements distinctly, may be projected in motion ; the fewer
skeleton coils producing slower motion, and so enabling the
action to be better followed. A model of a Gramme ring with
such open skeleton coils can thus be projected in action very
clearly, or can be reduced by a brake down to a visible rate if
required.

Of the optical effects of the current, the rotary power has
been already described in the chapter upon polarised light.
Whether the luminous effects in vacuo can be projected, de-
pends chiefly upon their brilliance, the size of the lens, and
the darkness of the room ; the primary apparatus itself
needing to be carefully shielded from view. De la Kive's
rotating arc can be projected fairly well, if the apparatus is
in good order ; and it depends upon its size and that of
the audience whether this method, or that of direct vision,
is to be preferred. As already pointed out at the commence-
ment of Chapter XIV., considerations of this kind must in
many cases govern the choice of method ; and they apply
to electrical experiments with especial force. The more

SCIENTIFIC DIAGRAMS 4"

massive and costly the apparatus at hand, and the more fixed,
as it often is in large public institutions, the less will projec-
tion be needed, because more can be sufficiently seen without
it. The more cost, and portability, and storage have to be
studied, the more will this method come into play. On such
considerations I would refer again to the passage jsut cited.

CHAPTER XXV

SCIENTIFIC DIAGRAMS

IT is no part of the intention of this work to enter into the
photographic production of slides for the lantern, which is of
course the best method of reproducing any printed diagram
suitable for the subject, or reducing any carefully made draw-
ing upon white paper or card. Such drawings should be
made in black Indian ink, care being taken to make any
lettering or shading as even as possible, when a reduced photo-
graph is certain to have a good effect. But ordinary diagrams
may readily be prepared by much simpler means.

245. Drawings in White Outline. —These are very effective
for suitable subjects, and easily prepared. A glass plate
should be warmed, and whilst warm rubbed with a piece of
paraffin, which should then be nearly wiped off again. Then
hold the plate over the smoke of burning camphor till suf-
ficiently black. On this black ground simple straight line or
free-hand diagrams are readily traced, but the plate should be
dropped into a sort of little socket or well slightly below the
surface on which the hand rests, that a straight-edge may be
held across it without touching.

When curves or circles have to be struck, it is better to
cover the glass with an even coat of photographer's black
varnish, or, still better, the black varnish used by slide-painters.
The drawing can be best executed before the varnish has

412 OPTICAL PROJECTION

become so dry as to be chippy. A small bit of horn should
be laid on the surface to support the leg of a pair of compasses,
and points should be provided of different fineness. Any little
injury to the black ground can easily be touched up last of all.

There is however an objection to an opaque blacli ground
for diagrams, that a pointer can scarcely be seen upon it.
This objection may be removed, and the brilliance of white-
line diagrams still secured, by using for the ground a thin
film of blue printer's ink, laid on evenly with a dabber. The
lines will appear quite as distinct upon this, while the pointer
can be seen quite well upon the blue ground.

246. Drawings in Black. — Processes for making these are
very numerous. Some employ a film of sugar solution, or
of photographer's ' mat ' varnish, upon which it is said pencil
drawings can be made, but I have never found these very
satisfactory, and they are troublesome. The following are very
simple, easy, and effectual.

For ink drawings, take a clean glass square, simply lick
it all over with the clean tongue, and let it dry in the air.
The imperceptible film of saliva thus communicated will
entirely prevent any ink from ' spreading ' on the glass from
a fine pen-point, the best of which are the crow-quill steel
pens sold for map -drawing. The plate may be laid over any
drawing, and a copy traced, or any free or ruled drawing may
be made, with common writing-ink in which a lump of sugar
is dissolved, or any of the syrupy ' copying ' inks. These
lines are not sufficiently black ; but when dry, if a little finely
powdered lampblack be rubbed lightly over them with a
morsel of cotton- wool, sufficient will adhere to the sticky
lines to make an opaque drawing.

Those who prefer, and are skilful with, a fine sable or
camel-hair pencil, will find Indian ink « take ' perfectly well
on a glass plate over which has been flowed water containing
a very small quantity of gelatine.

A photographic dry plate — chloride is better than bromide

SCIENTIFIC DIAGRAMS 413

— fixed without exposure, answers exceedingly well for ink
drawings, and is convenient for those who have photographic
material at hand.

Or the diagram may be simply scratched on a sheet of
gelatine about as thick as thin card, with steel needle-points
of appropriate fineness. This is a very convenient process,
because the gelatine may be fixed with drawing-pins over a
diagram to be copied, and can be thus traced exactly. When
finished, a little fine lamp-black, or black-lead, gently rubbed
over the lines, will make them a good black. Care need
only be taken to discard pieces already soiled or scratched,
and to keep the gelatine clean. Gelatine flowed on glass
and dried may be treated in the same way.

For more finished and permanent diagrams, the best
method is undoubtedly one largely practised by Dr. Dallinger,
the drawing being made on very finely-ground glass, such as
is used for the focussing-glasses of the best cameras. For
some work a still finer glass called ' matted ' glass, etched
with vapour of hydrofluoric acid, is better even than this,
but is sometimes difficult to procure. Dr. Dallinger made
many exquisite drawings with black-lead pencil alone, which
is capable of most delicate and yet solid shading if the glass
is fine enough. The main secret is to use the very hardest
pencils, except for shading, when as soft as H B may be used,
and to keep a very fine point. The points are cut very long
to begin with, by a sharp knife; dressed up on a piece of
emery paper or rough ground-glass; and kept constantly
dressed sharp while in use, by an occasional rub on a piece
of finer ground-glass kept at the side. Close and repeated
working over gives the solidity of shading, but black should
not be attempted. If very black outlines are wanted, they
should be put in with Indian* ink used in a fine and hard
mapping-pen, or, better still, with a fine sable brush.
The latter will put in lines as fine as a hair, and I have seen
exquisite drawings of Floscularia made by Mr. Underbill in

414 OPTICAL PROJECTION

this way, the Indian-ink lines being laid in as fine and clean
as if printed from a steel plate, To do this, however, the
plate must be perfectly free from the least greasiness, and it
helps a little to put a very little ox-gall in the last wash of
water given to it. Water-colour may be laid on also if
desired, or pencil shading be added to the ink lines. When
the drawing is finished, it is held at one corner, and a varnish
of dried Canada balsam dissolved in benzol flowed over it.
The varnish should be about as thick as cream, but the exact
consistence does not matter much. Pour it on freely so as
to cover the whole plate quickly, pour off at one corner till it
nearly ceases to drop, then cant that corner up to flow rather
back again, and then cant other sides up a little : a very few
trials will teach the operator to do this so that the varnish sets
level, and free from ridges or waves. Then protect from
dust and leave the varnish to dry ; all the grain of the glass
will disappear, and the slide be perfectly transparent. When
dry, it is mounted with a mask and cover-glass as in § 248.
Anyone with the least capacity to draw upon anything, can
produce beautiful slides in this way with the greatest ease ;
and the process is equally adapted for simple line diagrams,
or the most finished representations of microscopic detail.

247. Printed Diagrams. — It may often happen that a scien-
tific lecturer has at his command, through acquaintance with
a publisher, or perhaps because the blocks belong to his own
works, wood engravings which are exactly suitable to illustrate
his subject, not only in detail and treatment, but also in size.
It is well to know that such may be converted into excellent
lantern slides, by a simple method which occurred to myself
quite accidentally. I had collected or prepared, as I thought,
all the slides for a lecture I had promised to give, when I
came across several engravings I was very desirous to intro-
duce, only the very day before the date. There was no time
to photograph them, or to attempt imperfect copies ; but it
Suddenly occurred to me to ask the printers for proofs on

SCIENTIFIC DIAGRAMS 415

thin clear sheets of gelatine. These were taken for me, the
gelatine used being very thin, such as is used for wrapping
up Christmas crackers, but uncoloured. The printer's ink
we found should be rather stiff and fine : with such ink, and
what is called a ' hard ' impression, the result was perfectly
satisfactory, and the slides, prepared in an hour or two, were
as good as photographs.1 I have since resorted to the same
method on several occasions, and whenever there is available
material, can thoroughly recommend it as rapid, inexpensive
and perfectly satisfactory. The gelatine film appears after a
little to settle into ' crinkles ' between the protecting glass
plates ; but this is of no consequence, and in no way affects the
appearance upon the screen. The only objection is an almost
imperceptible tendency to a slight thickening of the lines.
By those at all familiar with mica work for the polariscope,
even this and the ' crinkling ' may be avoided, as I have
myself since avoided it, by splitting very thin films of mica,
cutting them into 8J-inch squares, and taking the proofs upon
these.

Even proofs upon some samples of very thin tissue-paper
(as much freedom from ' grain ' as possible is the main thing)
will make passable slides for a sudden emergency, if the proof
is afterwards fastened to a glass plate by the balsam and benzol
varnish already mentioned. This makes the tissue nearly trans-
parent, and the amount of grain visible in the paper is the
only defect, which with some papers is very slight.

248. Mounting Slides. — What are called ' masks should
always be used in mounting slides ; that is, circles, ovals, or
cushion- shapes cut out of the centre of 3|-inch squares of
thick black paper or thin card. Such can always be obtained
of opticians or lantern-dealers, and a mask should be selected
to suit the diagram. A drawing on glass (all glasses being 8|

1 Merely to indicate the class of illustrations, I may mention that the sub-
ject was ' The Eye as an Evidence and Example of Evolution,' and the
diagrams exploited related to points of histology and comparative anatomy.

4i6 OPTICAL PROJECTION

inches square) should have a mask laid on the face, and then
another covering-glass, cleaned and dried, which will be kept
from contact over the visible portion by the mask. The
double plate is then secured by an edging of thick dark paper
twice coated with gum-arabic. The way to do this is to cut
the gummed paper into strips 6 \ inches long and \ inch wide,
cutting off the corners, and corresponding notches out of each
side at the centre. A strip is first licked on the ungummed
side, and then on the gummed, and laid with gum uppermost
on the table. One corner of the double glass is then placed
on the strip at the middle notch, the glasses inclined down
each way, so that the gum may stick all along two edges,
and the strips are then bent over and worked down with
the finger and thumb. The other two sides are treated in
the same manner with another strip, and the slide is com-
pleted, any smears of gum being cleaned off when all is dry.
A proof of an engraving on a sheet of mica or gelatine is
mounted between two glass plates in a similar way, the mask
being laid on the printed side. The card masks may be used
for mica films ; but the crinkling of gelatines is less apparent
if the mask be cut out of plain dark paper, which is thinner.
This is, however, only for appearance, the most conspicuous
crinkles having no effect on the screen.

APPENDIX

TO

THE FOURTH EDITION

By RUSSELL S. WRIGHT

IT is desirable to add to this fourth edition a few particulars of
new apparatus and other details which have been designed or
improved since the last edition was published in 1895.

Oil lamps. — Petroleum-oil lamps have been very considerably
improved during the last few years by all the leading makers.
The main lines of improvement have been the same in all, and
have consisted in greatly increasing the height of the chimney
providing more ample access to the flames, and fitting a larger and
more easily adjustable reflector at the back. Fig. 233, representing
Messrs. Newton's lamp, illustrates these points, especially the first.
The bottom portion alone of the chimney is here nearly as high as
in the older forms. The lamp is to be first lit and burnt for two
or three minutes with this portion only in position, then the
upper portion of the chimney is added and the flames regulated,
the enlarged reflector being also adjusted if required. This method
of getting up the draught applies in principle to all the improved
lamps.

In Stock's pattern the chimney is raised (after lighting) by a
rack and pinion, but I am not able to trace any benefit from this
additional complication. With some of these improved four- wick
lamps 120 or 130 candle power can be obtained with a whiter light
than formerly, which is sufficient to illuminate a 12-ft. disc well.

E E

4i8 OPTICAL PROJECTION

Five wicks have been quite discarded. Three -wick lamps with
the same improvements have in some cases touched 90-candle
power.

FIG. 233.— Four-wick Oil Lamp

Incandescent Gas tamps. — This well-known form of gas-
burner is now in common use as a radiant. The light is not very
good, only amounting to some 70-candle power, and the definition
(owing to the great size of the mantle) leaves a good deal to be
desired. At the same time this form of light is so convenient,
requiring nothing but a gas-tap and a length of rubber tubing, and
is so white and clean, that in small halls and class-rooms, where a
disc of 8 or 9 feet suffices, it is, and will be, deservedly popular.

The complete fitting, including mantle and burner, with a con-
cave reflector, is now sold at prices varying from 7s. Qd. to 10s.,
and can be had of all leading opticians.

Incandescent Spirit Burner. — This is a burner essentialty the
same as the above, but using methylated spirit instead of gas,

APPENDIX 419

for use in small halls where the latter is not available (fig. 234).
The burner is cheap (about 12s. 6d.) and simple and easy to use,
but the light is not as good as even the gas-burner, and hence is
useless for discs above 7 to 8 feet in diameter.

There is a class of incandescent spirit lamps similar to this, but
in which the spirit is put under pressure, either by raising the
reservoir a few inches, or by pumping air into it, with the result
that the light is considerably increased ; but these can hardly be

FIG. 234.— Incandescent Spirit Burner

recommended for amateur lanternists, as, if the temperature of the
volatilising chamber should fall too low, the spirit is apt to overflow
into the lantern and catch fire, and though this may seldom happen
the risk is a high price to pay for the extra light, particularly as a
brilliant light can now be obtained from acetylene without any
danger.

Acetylene Gas. — This illuminant has made immense strides
during the last few years, and has now to a very large extent
superseded oil lamps, and has even made great inroads upon the
popularity of lime-light. A 1 Ib. tin of carbide of calcium can be
obtained for about Gd., and on being mixed with water will give off

420

OPTICAL PROJECTION

enough gas to supply a lantern acetylene jet for nearly two hours.
There were at first difficulties in consequence of the explosive pro-
perties of the gas, but now all modern lantern generators are so
constructed that they only produce the gas in very small quantities
as required, and under no circumstances should there ever be
enough to cause an explosion.

FIG. 235. — Acetylene Generator

The number of these generators on the market at present ia
legion, and it would be useless to attempt to describe them.

They may practically all be divided into two classes : those in
which the carbide drops a little at a time into the water, and those
in which the water is gradually admitted to the carbide, which, in
this form of generator, is usually placed in small trays or cells.

APPENDIX

421

It is claimed for the first pattern that the gas evolved is cooler
than when the water is admitted to the carbide. There does
appear to be some advantage in this respect, but the admission of
carbide, which is usually in small lumps, is difficult to control, and
on the whole the plan of gradually admitting water to the carbide
is now generally followed.

A typical lantern generator is shown above (fig. 235). As will
be seen, it acts on the gasometer principle, the gas as it is evolved
passing into the upper bell which floats and thereby lifts the
carbide out of the water ; as soon as a little gas is used, the bell
sinks and a little more water is admitted, the whole arrangement
being purely automatic and requiring no attention whatever after it
is once started — a point of
no little importance when
one has to lecture as well
as to manage the lantern.

A lantern fitted with
an acetylene generator in
this way is, of course, not
quite so portable as with
oil, as the generator is usu-
ally as large as the lantern
itself; but in the opinion
of most operators this is
more than made up for by
the increased brilliance of
the light.

A four-burner acetylene jet, as fig. 236, will well illuminate
a disc 12 to 14 feet in diameter.

Saturators. — The great extension of lantern demonstrations or
entertainments during the last ten years has greatly developed the
use of ether -saturators, which have gradually assumed more con-
venient and safer forms than either of those described in
Chapter VII. As anticipated on p. 101, the use of vapour from the
petroleum series of oils, aided by heat, has now been entirely dis-
continued. Only petroleum ether or sulphuric ether, which require
no heat, are now employed. Tanks have been discontinued, and
some form of porous generator has become universal.

Safety has been gained in two ways : (1) by entirely filling the
cavities of the generator with porous material, so that no free space

FIG. 236.— Four-burner Acetylene Jet

422 OPTICAL PROJECTION

is left for any explosive mixture ; and (2) by dispensing as much as
possible with long rubber connections.

Saturators are now either placed within the lantern itself, as in
the * Gridiron ' or ' Lawson ' forms, or hung as near to the taps of
the jet as possible. In the former case the generator itself gets
hot with the warmth of the lantern, and the evaporation of the
ether is thereby so much accelerated that the supply of oxygen
passing through the ether must gradually be turned off. After
half an hour's use in a small lantern, such a generator usually pro-
duces enough ether-gas by its own evaporation, and only free

FIG. 237.— 'Gridiron' Saturator

oxygen at the mixing chamber is required. In the * Gridiron '
form (fig. 237), which is one ofthe best of its type, there are three
taps: one to control the supply of oxygen passing through the
generator, one to control the supply of free oxygen to the mixing
chamber, and one to control the supply of ether-gas (or ether and
oxygen, as the case may be) to the chamber.

I have found in practice that, after stopping the supply of oxygen
to the ether, the evaporation still sometimes goes on so fast that
the third tap, connecting the ether chamber with the nipple, has
to be partially turned off as well. There is a disagreeable feeling
of ' sitting on the safety-valve ' in doing this, but in reality the

APPENDIX

423

pressure is never likely to become great enough to cause
danger.

When the generator is used outside the lantern, the artificial
evaporation produced by the passage of oxygen cools the vessel,
and the formation of the ether-gas is thereby retarded. This is a
serious drawback, especially as the gradual emptying of the ether
chamber leads to the same result, i.e. the lessening of the ether
supply.

A saturator to combine the advantages of both these forms was
designed in the laboratory of Christ's College, New Zealand, and is
made by Messrs. Newton & Co. (fig. 238). It consists of a brass
cylinder A, closed by a screw cap B. At c is a stopper for filling.
The space above c is filled with two rolls of
concentric packing material separated by a
brass tube. On the top are two nozzles, a.
double one E D and a single one F.

The oxygen supply is connected to D, and
divides into two branches, passing upwards out of
nozzle E to the oxygen tap of an ordinary mixed
gas-jet, the other path leading downwards into
the saturator, through the outer roll of packing
material, into the chamber below c, upwards
again through the inner roll of packing and
emerging at F, charged with ether vapour, and
on to the ordinary hydrogen tap of the jet.

Below c is a hollow space filled with liquid
ether, and from the packing chambers are hung
long cotton wicks into this tank of ether. These
wicks draw up the liquid by capillary action to
take the place of that absorbed by the oxygen,
and thereby the porous material in the upper
chamber is kept fully saturated.

The advantages of this form of generator are
threefold :

(1) The construction renders it easy to with-
draw the rolls of packing material for drying

purposes, and this should be done at least once every season, as
ether contains an appreciable quantity of moisture which does not
evaporate readily, and therefore accumulates.

(2) The saturator being kept full by means of the cotton wicks

Fio. 238

424

OPTICAL PROJECTION

aforesaid, the evaporation does not decrease nearly so much as
when no reservoir is provided, only the cooling effect having to be
reckoned with.

(3) The operator knows that so long as he has any free ether
left in the lower tank his saturator is full— a point very difficult to
be certain of with most saturators.

Mr. J. Hay Taylor, writing in the * Optical Magic Lantern
Journal,' reports that he obtained an excellent and steady light for
three hours with this saturator, using a mixed gas jet of large bore ;
with a smaller bore the supply will, of course, last even longer.

Nernst Electric Lamps. — These lamps have now come into
general use. Where a moderate supply of electric current only is
available, and the greatest perfection of light and definition is not

required, they are an ex-
tremely convenient and
simple illuminant. They
are usually mounted to fit
on to an ordinary lime-
light tray (see fig. 239),
either one or three fila-
ments being employed,
and the resistance (always
supplied with the lamp)
enclosed in a glass bulb
behind.

Each of these fila-
ments carries roughly one ampere on a 200- volt current, and by using
a lamp with three of them as much as 1,000-candle power can be
obtained, but owing to the length of these filaments the effective light,
as well as the definition, are by no means equal to the electric arc.
One peculiarity of these lamps is that no current can be passed
through them until the filaments are heated. This can be done by
means of an automatic arrangement, but for lantern work it is
usually done by hand, holding a spirit lamp (specially supplied for
the purpose) under the lamp for a few moments.

Once the current is started the spirit lamp may be removed, as
the filaments, of course, keep hot until the current is switched off.
If this is done for any reason, the spirit lamp must again be applied
before current can pass through the filaments again.

Except, however, for the trouble entailed by this initial starting,

FIG. 239. — Nernst Lamp.

APPENDIX 425

the lamps are extremely simple and easy to manage, and the
light is absolutely steady and uniform.

A filament will occasionally break, but if a few spare ones are
kept in stock this is easily replaced, a pair of forceps and a little
patience being all the tools needed.

Electric Arc Lamps. — The advance in public lighting has pro-
duced many cheap and simple pattern arc lamps for lantern use.

Those most commonly employed are of the hand-fed type, a
rack or screw adjustment moving the carbons towards or away
from each other. Fig. 240 shows a typical pattern.

FIG. 240.— Hand-fed Arc Lamp

Two screws, A and B, serve for centring the light, c is a rack
movement for bringing the carbons gradually together as they burn
away, D is a sliding movement for adjusting the position of the top
carbon with regard to the lower. It will be seen that the plan of
slanting the carbons, alluded to in Chapter XII., has been
adhered to.

These lamps are usually constructed to take carbons of varying
sizes, in accordance with the amount of current intended to be used,
and the plan is usually followed of having the upper or positive
carbon, which burns away quickest, considerably the larger of the
two, so that they shall waste at equal rates, and thereby keep the
arc approximately in the optical centre of the lantern. For a
current of 10 amperes, convenient sizes are about 13 mm. for the

426

OPTICAL PROJECTION

positive carbon, which should be cored, i.e. have a soft centre, and
8 mm. for the negative, which should be a solid or entirely hard

carbon. It must be rememberec1
that the electric arc only requires
about 45 volts, and as the street
mains usually give current at any-
thing from 100 to 250 volts' pressure,
some form of resistance must be
inserted.

This is usually made of spirals of
iron, gerinan silver platinoid, or
other metal of high electrical resis-
tance, and may be a fixed amount,
suitable for the current to be need,
or adjustable. The latter is con-
venient, as it enables the operator
to vary the light if required. A
fixed resistance is, however, con-
siderably the cheaper, and for a
great deal of work answers perfectly
well.

Fig. 241 shows an adjustable resistance constructed especially for
lantern work. The figure almost explains itself ; the frame is made
either of iron, or if required for portability, of aluminium.

The following table gives the resistance required for various
currents and voltages and also suitable size carbons to employ for
arc lamps in small lanterns.

APPROXIMATE TABLE OF BESISTANCE FOR VARIOUS CURRENTS AND SIZE
OF CARBONS.

FIG. 241.— Adjustable Resistance

Am-

Resistance in ohms for volts.

Size of carbons in
millimetres

80

100

120

150

2CO

220

240

+

-

10

4

6

8

11

16

18

20

12

7

12

2-8

4-5

6

8*

13

14*

16

13

8

14

2

4-3

5

7

10-3

12

13-5

16

10

16

1-5

2-8

4

6

9

10

11-5

16

10

18

1-3

2-5

3-9

5-5

8

9

10-3

20

13

20

1

2

3

4-5

7

8

9

20

13

APPENDIX 427

Arc Lamps with Alternating- Current. — Where the current is
alternating, it is customary to place both carbons vertical, both of
equal size and, as a rule, both cored. This is the plan recommended
in every text-book I have come across, but, after many years'
practical experience with alternating currents, I have come to the
conclusion that the arrangement is wrong. The best effect (in my
hands at any rate) is to slant the carbons as for continuous current,
to have the carbons in line instead of the upper one slightly behind,
the upper one still to be cored and the lower one solid, and the
upper one slightly the largest, though the difference in size should
not be so much as with the continuous current, as in this case the
upper one would only burn away faster because of the soft core, no
difference in the rate, because one carbon is positive and the other
negative, having to be reckoned with when the current is
alternating.

The chief disadvantages of the alternating current are :

(1) The feebler light obtained (being only above 50 per cent.) as
compared with that given by the same amount of continuous
current.

(2) The tendency of the arc to dance round the carbons, instead
of keeping on the frontal line of the two, as it should do, for a
steady light.

(3) The humming noise produced by the vibrations of the
current in the arc itself.

For this last there is no real remedy. The objection can be
reduced to a minimum by placing the whole lantern on a thick
layer of solid felt ; a pad about 1 inch thick, which can be
obtained from most saddle makers, has a marvellous effect in
reducing the noise. Much can also be done by reducing the
voltage to the right amount, and this fortunately also largely tends
to prevent the dancing about of the arc spoken of.

The alternating current, unlike the continuous, can easily and
efficiently be " transformed" to any required voltage, which point
is of so much value in commercial work that it is the real reason
why these currents are so often employed. The current, however,
must not be transformed down to the 45 volts or so necessary for
lantern work, or it will be found impossible to keep a steady light.
It is obvious that when the whole of our energy is being absorbed
in the arc itself, any variation in the length of this arc will cause a

428 OPTICAL PROJECTION

corresponding variation in the amount of current, and therefore of
the light. Under these circumstances it is necessary in practice to
keep constantly feeding the arc, and if the operator takes his hand
off the controlling mechanism for more than a second cr two, the
light will flicker or go out.

If, however, two-thirds of the current is absorbed in a fixed
resistance, and only one- third in the arc, a slight variation of this
has only one-half the effect, and consequently the arc can be left
for longer periods without feeding.

In practice a current of 80 to 100 volts is the best for lantern
work, and if the original current is supplied at 200 volts or so, it
should be transformed down to this, and then a suitable resistance
put in series. From a commercial point of view this is rank heresy,
as a resistance simply wastes the current it absorbs, but this waste
of current is quite immaterial for our purpose, and the light obtained
at this voltage is quieter, steadier, and easier to manage than at
any other. Such a transformer for a 10-ampere arc lamp costs
51. 5s. to 61. 6s., and one should always be obtained when
the alternating current is supplied at anything higher than about
120 volts.

One or two expedients have been employed in order to keep the
arc in front of the carbons. Mr. Hepworth recommended the use of
two cored carbons with the core placed eccentrically towards the
front, but I have been able to trace very little benefit from this
method, and I believe these carbons are not now manufactured. A
very similar effect can, however, be obtained by filing a flat on the
front surface of the carbons, should anyone desire to try the experi-
ment. Dr. Drysdale places a pair of electro -magnets, one each side
of the arc, in series with it, and therefore magnetised by the same
alternating current.

These magnets therefore reverse their polarity in synchronism
with the vibrations of the arc, and their inductive effect is to deflect
the arc constantly outwards towards the condenser. The same
effect can be obtained by means of a turn or two of copper wire,
which must be insulated with asbestos, placed vertically on one or
both sides of the arc, edgeways towards the condenser, and both
these methods are extremely effectual in steadying the light.

An inexpensive method of * rectifying ' or converting the current
from alternating to continuous has long been sought for, but up to
the present is not forthcoming.

APPENDIX

429

The methods adopted have been two, i.e. chemical and
mechanical.

A chemical rectifier can be made by passing the alternating
current through a bath with iron and aluminium electrodes ; the
bath being filled with a saline solution (phosphate of ammonia is
about the best) chokes back the current in one direction and only
allows a unidirectional pulsating current to pass. By using four
batlis, both phases of the alternating current can be utilised.

These rectifiers are sold under various names ; the ' Nodon
Valve ' is one of the best forms, and can be obtained from any

Fin. 2-12. — Dr. Morton's Rotary Rectifier

dealer in scientific instruments. I have used such a rectifier a
great deal for small currents in charging accumulators, &c., but for
heavy currents they are troublesome and wasteful.

Of mechanical rectifiers, the best and simplest that I have seen
was designed by Dr. E. Morton, of the London Hospital (fig. 242).
It was originally, and still is, designed chiefly for use in connection
with induction coils for #-ray work, but it struck me that the same
instrument might be utilised in connection with arc lamps. Dr.
Morton very kindly lent me one of these rectifiers for my experi-
ments ; and a great deal of good work was done with it. On one
occasion I passed a current of 30 amperes for over two hours
through this rectifier, and obtained a light that for brightness and

430 OPTICAL PROJECTION

steadiness compared well with that given even by the continuous
current.

The rectifier consists essentially of a revolving commutator or
reverser, driven in synchronism with the current by means of a
motor, and which again converts the alternating current into one
pulsating and unidirectional. The instrument, however, requires a
good deal of adjustment, and for ordinary arc-lamp demonstration
is hardly practicable.

A motor generator, or a continuous -current dynamo driven by an
alternating-current motor, is the only real way of solving the
problem up to the present, and in large halls, where first-rate results
are really required, such a plant had better be laid down. Such a
machine is, however, costly, for even for a 10-ampere arc lamp a
generator costing 407. to 501. will be required, and for general
exhibition work quite good enough results can be obtained with
the alternating current alone.

Should such a motor generator be purchased, however, it must
not be forgotten that with the continuous current, as with alternating,
80 volts or so is required, and a resistance in the circuit, in order to
avoid continual adjustment of the feeding mechanism of the arc.
This point is absolutely essential to a comfortable working of the
hand-fed arc lamp, and is too often overlooked.

Automatic Arc lamps suitable for any ordinary sized lantern
are also obtainable ; the best is probably Borland's * Scissors '
pattern, and under favourable conditions and in skilled hands this
lamp gives very satisfactory results.

As the feeding of these lamps depends on the diminution of
current consequent on the burning away of the carbons, and conse-
quent increase of resistance, one of the essential conditions of smooth
working is that a slight variation in the length of arc shall produce
as much difference in this resistance as possible. For this reason
automatic lamps, unlike the hand-fed type, usually work at their
best with a voltage as nearly as possible that required for the arc
itself, say 45 to 50 volts, and very little, if any, resistance in the
circuit. With anything over 100 volts at the mains the feeding is
apt to be ' jumpy.' As a rule, of course, there is no control over
the current supplied, and we have to put in a suitable resistance to
do the best we can, but where a dynamo is being put in for the
purpose, this distinction between hand-fed and automatic lamps
must not be lost sight of. Col. Holden has designed a semi-

APPENDIX 431

automatic lamp where the arc has to be ' struck ' or formed at
starting by touching the carbons together by a hand movement,
and then separating them, the lamp then remains burning, the
carbons as they consume away being pushed up by springs against
steel screws, somewhat after the manner of the ordinary carriage
candle lamps, and this lamp also performs well where the current
is steady and not too high in voltage ; but in practice I myself prefer
hand-fed lamps to any on account of their simplicity and freedom
from any liability to fail or become unsteady at a critical moment.

Of large automatic arc lamps little need be said ; those described
in Chapter XII. remain as the general pattern still in use. Messrs.
Oliver have brought one out on the same lines as the Brockie Pell
therein described, which is extremely steady and reliable, and is
in my opinion the best of such lamps at present on the market.

Messrs. Newton also make large hand-fed lamps for their
triple rotating and similar lanterns, and these are by some operators
preferred to even the ' Oliver ' lamp.

Projection Microscope. — Various slight improvements, both
optical and mechanical, have been made in this instrument since
1890. Of these the most important has been the introduction of a
quadruple condenser, with a view to still further reducing spherical
aberration in the lantern portion of the instrument. After many
years of experiment, the new condenser, as designed by my father,
reduces this fault to a minimum, and a considerable gain in both
light and definition is the result. With the new arrangement
apochromatic lenses especially show to more advantage than formerly .
Of lenses, several new ones have been brought out which perform
well with the projection microscope. Herr Beichert has intro-
duced an oil-immersion lens, nominally £ but really ^ focus, of
N A 1-30 to 1'32, which for moderately high power is as good an
all-round projection-lens as I know of. As this lens has a very great
working distance, it is as well to ask for a shortening of this if
required chiefly for projection, as the horizontal position tends to
drain the oil away.

The Zeiss apochromatic J of N A T40 does magnificent work,
but is very costly.

I can also speak from experience of. Messrs. Swift's ^ oil
immersion at 51. 5s. ; this also for all-round work can hardly be
beaten, and Messrs. Leitz also do a fine series at a moderate price.

Of mechanical improvements the most important has been the

432

OPTICAL PROJECTION

Fin. 243. -Vertical Pattern
Projection Microscope

introduction of a long
girder bar extending
the whole length of the
instrument, and greatly
increasing the rigidity.
A centring sub -stage
has also been added.

Sir David Salomons
has designed a modified
form of the instrument
which can be used
either horizontally or
vertically ; a large mir-
ror between the two
halves of the condenser
reflects the beam up-
wards, and a sort of
double prism above the
objective re-directs it
towards the screen.
The arrangement is
very handy for show-
ing pond life or other
objects which require
to be kept horizontal.
Both mirror and prisms
can be readily with-
drawn, and the instru-
ment then becomes an
ordinary girder pattern
projection microscope (fig. 243).

Science lanterns. — The large
growth of technical schools and poly-
technics has led to a demand for lanterns
for scientific demonstrations at moderate
prices, and several patterns of these are
now on the market.

Professor H. Jackson, of King's Col-
lege, has designed such a lantern, which
is sold complete for 101. 10s. This lantern

APPENDIX

433

(fig. 244) has an entirely open stage, even the clip for lantern-slides
being key-holed on to the front, so as to be instantly removable.
A prism fitted on the front erects the image if required, or can be
taken off if not wanted.

The front can be hinged up and a mirror slid in, which changes
the lantern instantly from a horizontal to a vertical instrument.
The above-mentioned prism, turned on a pivot into another position,
now serves to reflect the image on to the screen.

n

m §

Fio. 244.— Prof. Jackson's Science Lantern

This arrangement is extremely convenient for demonstrating
objects that require to be kept horizontal, such simple experiments,
for example, as the arrangement of lines of force shown by dusting
iron filings over a magnet are easily shown.

Such a lantern is also very useful as a substitute for black-
board illustrations. A piece of finely ground or smoked glass
being laid on the horizontal surface of the condenser, the
operator standing in front of the lantern can draw or write at will
with a pencil or needle-point, and the drawing is represented on
the screen as he proceeds.

The bar and lens of this lantern can be removed, leaving the

F F

434

OPTICAL PROJECTION

condenser entirely free, or a simple microscope, polariscope, or
other attachment can be key-holed on in place of the slide stage.

For another 63s. the lantern can be supplied with an entirely
removable front, and a long solid baseboard can then be fitted
with a large projecting microscope, or, indeed, any optical apparatus ;
or at another slight extra expense the lantern can be supplied with
the bar carrying the lens entirely sunk in the baseboard, thus carry-
ing the ' open stage ' idea to perfection. About eighteen months

FIG. 245.— Prof. Jackson's Science Lantern adapted for Opaque Work

ago I endeavoured to improve this lantern still more by adapting
it also for opaque work. This was finally achieved by tilting the
whole lantern up from the back so that it threw a powerful
beam of light down on to the baseboard. Any opaque object
up to about 5 inches square placed thereon, being thus strongly
illuminated, can be projected on the screen by means of the usual
lens and prism, and the lantern in a moment can be dropped down
again and used as just described for either horizontal or vertical
work (fig. 245).

For all-round useful work at a moderate price this lantern is
hard to beat.

APPENDIX

435

A lantern for laboratory work of very great usefulness, though
more expensive than the preceding instrument, is shown in fig. 246.
As will be seen from the illustration, it is practically a combine 1
lantern and optical bench ; the body is of ribbed aluminium, and
the various optical parts slide on an accurately planed lathe-bed
base, thus ensuring exact centring. This base makes a very rigid
support for a microscope, polariscope, or other optical instrument.
The lantern, however, from its nature is not very portable.

Another science lantern deserving of mention, though its size and

FIG. 246.— Lathe-Bed Lantern

price render it almost prohibitive, is Messrs. Zeiss's ' Epidiascope.'
This is a powerful instrument for direct, vertical, opaque, or
microscopic work. Any opaque object up to nearly a foot square can
be easily shown, the current required being 40 to 50 amperes.
Instruments of this size and price, however, are only likely to be
/required in very large institutions.

Other firms are also experimenting with somewhat similar
apparatus, so that shortly it is probable there will be choice of
patterns in these expensive instruments.

Colour Photography. — Examples of nearly all methods of
photography in natural colours are now obtainable as lantern-

436 OPTICAL PROJECTION

slides. Professor Joly's process lends itself well to lantern illustra-
tion, though the network of lines show rather prominently on the
screen. Samples of this method, however, I believe are not now
obtainable.

Messrs. Lumiere's method consists in taking three negatives
through carefully selected colour screens. A positive from one of
these is obtained on a glass plate and then dyed a corresponding
colour. The plate is then varnished and again sensitised, and the
print from the second negative imposed ; this again is dyed,
varnished, and sensitised, and the print from the third negative
superimposed again and treated likewise.

Mr. Sanger Shepheard's process is somewhat similar, but in his
method the variously dyed positives are taken on gelatine films,
and the whole then balsamed together.

In Lippman's process the photograph is taken upon a gelatine
plate containing finely divided particles of silver bromide. The
plate is ' backed ' by a thin film of mercury in absolute contact
with the sensitive surface. When the exposure is made the
mercury film reflects the image into the gelatine plate again, so to
speak, and the result is to set up a sort of wave formation in the
gelatine itself.

This, when viewed at the right angle by reflected light, gives
the image in its natural colours, the wave formation by a diffractive
effect reproducing the correct tints. Specimens of the process can,
of course, only be projected by reflected light, and then care must
be taken to ensure the beam being reflected at the correct angle ;
but the result, when enough care and pains are taken, is extremely
pleasing, and the colours really very correct.

The Photochromoscope. — This instrument, designed by
Mr. Frederick Ives, attracted great attention a few years back.
Perhaps the method can hardly be called colour photography, as
the image can only be projected or viewed by means of an instru-
ment designed for the purpose.

Three negatives are taken through suitable colour screens, as
in the Lumiere and Sanger Shepheard processes. Three separate
positives are then printed on glass from these, and the images of
these positives are then projected on the screen, each being pro-
vided with an optical system and colour screen to itself. By means
of an ingenious arrangement of mirrors, one single lime -light
jet or arc lamp suffices for the illumination of all three systems,

APPENDIX

43?

and a simple lever converges the three images into one harmonious
whole (fig. 247).

The effect, as the three crude and almost painfully glaring
coloured images are resolved into one, and an artistically coloured
picture suddenly springs into life, as it were, is an extremely
pleasing one, and the instrument as now manufactured by
Messrs. Newton & Co. forms a most useful addition to a lecturer's
outfit. As all the parts, colour screens, mirrors, &c., are removable,
the principle of the three so-called primary colours, and their

FIG. 247. — Ives' Photochromoscope

combination in the final picture, can be demonstrated step by step.
When no slide is placed in the stage the appearance on the screen
is that of three coloured discs producing white as they gradually
converge and overlap, and if then a obstacle, such as the blade of
a penknife, is placed in one of the stages, the shadow on the screen
will have the complementary colour, composed of the union of the
remaining two coloured discs.

A good series of slides for this instrument is now obtainable, or,
by means of a special dark slide which can be fitted to any good
camera, amateurs can prepare their own subjects. An instrument
for viewing these slides, instead of projecting them on the screen, is
also obtainable, but a description of it hardly belongs to these
pages. Objects for this latter instrument may be stereoscopic, as
the arrangement is binocular.

438 OPTICAL PROJECTION

Omni Focal Lenses. — Lantern lenses on the same principle aa
the Tele-photo camera lenses of Messrs. Dallmeyer are now to be
obtained, and such a lens gives very great adaptability in focal
length.

Messrs. Dallmeyer and Newton have conjointly placed on the
market an omni-focal attachment which can be adapted to almost
any good double achromatic lantern lens. A 6-inch lens fitted with
such an attachment can be used as a 6-inch, 12-inch, or any longer
focus required up to about 24-inch. The lantern simply has to be
placed at the required distance from the screen, and the lens
focussed. If the resulting picture is too large, the draw-tube
adapter must be pulled further out, and the lens can then be
focussed again in its new position, and a smaller picture will
result.

Unfortunately, up to the present, it is not possible to slightly
lengthen the focus of a lens by means of such an addition. The
omni-focal attachment, when it is screwed on, about doubles the
original focus, i.e. a 6-inch lens becomes a 12-inch lens. Beyond
that, almost any length of focus can be obtained. Of course, the
attachment can always be taken off, and the original lens used by
itself.

lantern Stereoscope. — Desire has often been expressed that
the effect of stereoscopic slides could be produced upon the screen,
and various attempts have been made without success. Two
stereoscopic pictures can, of course, be easily superimposed upon
the screen, but for stereoscopic vision some arrangement must be
made by which each image can be seen by one eye only, and this
on so large a scale introduces considerable difficulty. It had once
or twice been suggested that polarised light might solve the diffi-
culty ; but nothing, so far as I know, had ever been done in that
direction until Mr. John Anderson, of Birmingham, successfully
worked out an apparatus, which was publicly exhibited at the
Royal Society's summer conversazione in 1893.

The really new and essential point in this invention is the
screen, which is faced with thin silver leaf, varnished to prevent
oxidation. This surface, besides its greater power of reflection,
possesses the property of reflecting polarised light with its
polarised properties, not depolarising it, as most surfaces do. The
rest is easily understood. Each slide of a stereoscopic pair is shown
from one nozzle of a biunial lantern (or a pair of single ones), and
the image is transmitted through a ' pile ' of glass plates, arranged

APPENDIX 439

so that one image is polarised in one plane and the other in the
opposite (see Chap. XXII.). These images are superimposed on
the screen, but cannot be made to coincide more than approxi-
mately, the result to unaided eyes being a tangled confusion of both.
But when the screen is viewed through a pair of eye-pieces
arranged as spectacles, also containing ' piles ' of thin glass, only
one image can be seen by one eye, and the other by the other eye,
and any point of the two can be brought into perfect coincidence as
in the stereoscope. Hence we have perfect stereoscopic vision, or,
by reversing the eye-pieces, pseudoscopic effect is produced. If
the screen be viewed while the observer walks across the room,
the effect is very startling.

Plain silver-leaf screens give a very bright image viewed by an
audience directly in front, but the brilliance becomes very much
impaired as soon as the observer moves a little to the side of the
room, owing to the large amount of direct reflection, the silver
surface acting almost like a mirror, at any rate so far as illumina-
tion is concerned. To avoid this effect, my father suggested
striating the surface with fine vertical ridges, and the plan proving
perfectly successful, the screens are now made in this way.

The introduction of * anaglyphs,' or pictures where the stereo-
scopic effect is produced by printing the two images in different
colours and viewing them through spectacles with the glasses
correspondingly coloured, naturally suggested producing the same
effect on the screen, and Mr. T. E. Freshwater introduced this
method with considerable success. All that is necessary is to bind
up each slide of a pair with coloured cover-glasses, say red and
green respectively, and view the confused superimposed pictures
thus obtained through the coloured spectacles. The method has
the great advantage of cheapness and simplicity, and also of being
independent of any special screen.

Either of these methods necessitates a very strong light ; for the
colour method the electric arc, owing to its deficiency in red rays,
is not so suitable as lime-light, but by choosing colours adapted for
it, this defect could probably be overcome.

Dr. Dupre has designed a method by which a revolving shutter
closes the nozzle of each lantern alternately, and a synchronous
revolving shutter revolves before the observer's eye ; but such a
method is necessarily too complicated and expensive to ever come
into general use.

June 1906.

INDEX

ABE

ABERRATIONS of lenses, 7 ; of con-
densers, 19, 21 ; of objectives, 25
Absorption of colour, 299, 305 ;
lines in spectrum, 310

Accessory lantern apparatus, 145 ;
for scientific work, 170-8 (see
also Subjects)

Achromatism, 291

Achromatic lenses, 25

Acoustics, 247

Acoustic figures, 252, 255, 266;
pencils of light for, 257

Actinic rays, effect of, 316

Adjustment of a jet, 50 ; of the
light, 115, 214 ; kaleidoscope,
146 ; microscope, 197

Ahren's polarising prisms, 345;
reflecting polariser, 346

Air-films, colours of, 328

Air, vibrations in, 268

Airy's spirals, 379

Alternate current, danger of, 165

Alum cells, 150, 183, 207, 328,
331

American lanterns, 155, 168

Ampere's current experiments, 403

Amplifying image in the micro-
scope, 186 ; by aye-pieces, 188

Analysers for polrvriscopes, 339

Analysis, spectrum, 304

Anomalous dispersion, 295

BOD

Antiseptics, demonstration of, 245
Aphengescope, the, 147
Apochromatic objectives, 194
Apparatus, projection of, 212
Arc, projecting the, 410
Arc-lamps, 40, 162, 425-431 ; for
spectrum experiments, 159, 305
Argand lamps, 34
Atwood's machine, 220
Axes, polarising, 364

BAILEY, Mr. G. H., and zirconia, 70

Bands of colour in soap-films, 327 ;
in selenite, 354 ; quartz, 373

Barton's buttons, 336

Beads, silvered, for acoustics, 252

Beams and pencils of rays, 172,
257, 276

Beard's regulators, 86, 88

Bells, vibration of, 249

Benzoline for O.-H. light, 100

Billet's lenses, 334

Bi-prism, Fresnel's, 332, 376

Bi-quartz effects, 372

Bi-unial lanterns, 109; experi-
mental, 157 ; vertical, 169

Bleaching, 244

Blood-pressure, 239

Blow-through jets, 47

Body of a lantern, 14, 111

442

OPTICAL PROJECTION

BOB

CUR

Bores of jets, 62, 98, 101

Box or cabinet, the, 134

Boys, Mr. C. V., and Cavendish ex-
periment, 220 ; revolving lenses,
270, 400 ; radio-micrometer, 3(J3

Bright lines, mixtures for pro-
ducing, 309

Brightness of image, 8

Brin's oxygen process, 88

Brockie's optical arc-lamp, 163

Broughton's oxy-ether light, 92

Browning model lantern, 158

Brushes in crystals, 376

Bubbles, soap, 225, 326

Bunsen's photometer, 294 ; sodium
absorption apparatus, 315

Burner for line spectra, 307

Bye-pass, nature and use of, 107

CABINETS for the lantern, 133

Calcite prisms, care of, 344

Calorescence, 315

Capillarity, 222, 407

Capillary electrometer, 240, 408

Carbons, inclined, 164 ; for spec-
trum experiments, 312

Cardioscope, Czermak's, 236

Carriers for slides, 137

Cascade, luminous, 280

Cavendish experiment, 220

Centering the light in a lantern,
115, 152

Centrifugal force, 221

Charges, static electrical, 396

Chemistry, 241

Chimney of a lantern, 16

Chladni's figures, 272

Choreutoscope, the, 145

Chromatrope, 142 ; optical, 368

Cinnamic ether for prisms, 293

Circular polarisation, 363

Circulation, of blood, 205, 230 ; of
protoplasm in Vallisneria, 200

Clarkson's regulator, 87

Cleaning lenses, 102

Cleminshaw, Mr.E., on line spectra,
307, 313

Cloth-bodied lanterns, 17

Cohesion, 221 ; figures, 228

Colour, a shadow or suppression,
285,324; refraction various, 283;
a subjective sensation, 297

Colour photography, 435

Colours, composition of. 285 ; ab-
sorption, 299 ; complementary,
301 ; composite, 302 ; pure, 303 ;
interference, 324 ; of polarised
light, 353 ; rotational, 366

Coloured designs for polarised
light, 354

Combustion-lantern, 308

Comic slides, 140

Common light, rotation of, 376 ;
and polarised, 352

Complementary colours, 301

Composite colours, 302

Composition of vibrations, 362

Compound microscope, projection
with a, 190

Compressed gases, 82; manage-
ment of, 89

Concave amplifiers, 186, 344

Condensers, use of, 8 ; position of,
10 ; various, 17 ; corrections,
19, 21 ; practical points in, 22

Conductivity (heat), 388

Connections of a bi-unial, 108, 114

Contrast, effect on colour, 298

Convection currents, 387

Convergent light in crystals, 376

Correction of condensers, 19 ; of
objectives, 27

Crossed films, polarising, 355

Crossing of rays, effects of, 12

Crova's disc, 248

Crystallisation, heat of, 389 ; on
screen, 229, 358

Crystallisations, polarising, 202,
357

Crystals, rings and brushes in,
376 ; artificial, 381

Crystal-stage for polariscope, 343,
377

Current electricity, 400

Currents, action of electrical, 403 ;
in animal tissue, 239 ; continu-
ous or alternating, in demon-
stration, 165 ; and capillarity,
407

INDEX

443

CUR

Curtain for a lantern, 16
Curtain slide, 122
Curves, magnetic, 395
Cut-off jets, 117
Cycloidotrope, the, 145
Cyclosis in Vallisneria, 200
Cylinders for gas, 82, 84
Czerniak's cardioscope, 236

DAHNE, Herr, on soap-films, 325
Dallinger, Dr., on drawings for the

lantern, 413
Dancer, Mr., and dissolving taps,

107

Darkness, value of, 178
Decomposition of water, 408
Delezenne polarisers, 346
Demonstrating lanterns, 153
Demonstrations, physical, in gene-
ral, 212 ; microscopical, 190,
208 ; physiological, 230
Designs for polarised light, 354 ;
extraordinary difference in phe-
nomena, 370
Deviation, minimum, in prisms,

283

Dew, troubles from, 16, 116
Diagrams for the lantern, 411 ;

printed, 414
Diamagnetism, 395
Diameter of condensers, 23 ; ob-
jectives, 31

Didymium, spectrum of, 306
Diffraction, 334
Diorama, a lantern, 143
Dioramic effects, 103, 119, 143
Direct prisms for projection, 293
Disc, size of, 112; even illumina-
tion of, 115, 214
Discharge, static, 399
Dispersion, 282 ; anomalous, 295
Dissolving taps, 106; connections
of, 108, 114 ; adjusting, 108 ;
triple, 120

Dissolving views, 103, 106
Distances, table of, 112
Dolbear, Prof., on opeidoscope,

26* ; diffraction, 338
Doppler's principle, 254

Double combination objectives, 28
Double refraction, 349
Drawing-paper as a screen, 127
Drawings for the lantern, 411
Duboscq lantern, 155 ; lamp, 162
Du Motay and zirconia, 68

EDELMANN'S spectrum-burner, 307
Edison and Swan's lantern-lamp,

40

Effects, lantern, 122 ; slides, 140
Elasticity, 215, 220
Elbow polariscope, 338
Electricity, static, 396; current,

400
Electric currents in living tissue,

239 ; lamps, 40, 163, 425-431 ;

lanterns, 158 ; microscope, 206
Electrolysis, 408
Electro-magnetism, 395, 405
Electrometer, Prof. Mayer's, 397
Electroscopes, 212, 397
Endosmose, 224

Enlarging field of projection, 218
Enriched gas, for lime-light, 100 ;

for acoustic flames, 269
Erecting prism, 170
Erection of screen, 128, 131
Ether light, the, 91, 97, 419
Evaporation, 388
Even films for polarisation, 356
Even illumination of disc, 115, 214
Exhibition, work at an, 113
Expansion by heat, 385
Experiment, method and scale of,

215

Experimental lantern- slides, 144
Experiments, lanterns for, 153
Explosions, 51, 54, 90, 93
Eye -pieces for projection-micro-
scope, 188

FARA.DA.Y glass, magnetic rotation
in, 375

Fatty oil lamps, H4

Feathers, diffraction by, 335, 317

Fencing-off apparatus at exhibi-
tions, 114, 133

444

OPTICAL PROJECTION

FER

INV

Ferguson, Prof., and chemical
apparatus, 245

Figures, Lissajous', 255, 258

Films, thin, colours of, 324 ; soap,
225, 326 ; and electric currents,
408 ; crossed for polarising, 355,
382

Flames, manometric, 2G9 ; sensi-
tive, 270

Flashing on effects, 122

Fluorescence, 317

Focus of a lens, 5 ; and size of
image, 7, 30, 112 ; of lantern
objectives, 113

Focussing lenses, loose for project-
ing, 175

Forces, parallelogram of, 217

Forks, vibrations of, 253, 25S

Formulas for lantern distances,
discs, and foci, 112

Four-way tap, 121

Fox's mica preparations, 355

Frame, rotating, 348

Frames for portable screens, 131

Fresnel's bi-prism, 332

Fringes, interference, 332

Frog, circulation in, 205

Fronts, of lanterns, 32 ; optical,
171

Fuchsine, dispersion of, 295

GALVANOMETERS, and use, 400
Gas-bags, 42 ; cylinders, 82
Gas-burners, 40, 178, 418
Gas-supply, nozzles for, 47
Gases, phenomena of, 229
Gasoline for O.-H. light, 100
Gauze, diffraction by, 335
Gelatine for diagrams, 413
Generators for ether light, 92, 95
German model lantern, 153
Glass, compressed or heated, 360 ;

sounded, 362
Glass polarising apparatus, 340,

345

Glasses for acoustics, 249
Gratings, diffraction, 334
Gravett's condenser, 18
Gravitation, 220

Ground-glass, drawings on, 413
Giilcher arc-lamp, 103

HALOS, projection of, 335

Hardwich, Eev. F., and ether light,
93

Harmonic vibrations, 252, 267

Hawkridge's vertical lantern, 168

Heart, movements of, 235

Heat. 385 ; identity with light,
392 ; in the microscope, 183

Hepworth, Mr., on stand for the
lantern, 133

Herschel condenser, 18

Highley, Mr., on jets, 47, 67

Hopkins, G. M., electrical experi-
ments, 399

Huygen's apparatus and experi-
ment, 349

Hydrogen, preparation of, 73

Hydrostatics, 221

ICELAND spar, scarcity of, 345
Illuminating power of spirit jets,
47 ; oxy-gas jets, 50 ; mixed
jets, 62 ; arc light, 164
Image-forming power of all light-
rays, 2, 274

Image, microscopic, in demonstra-
tion, 190

Images, not formed by lenses, 1 ;
inversion of, 3, 6, 215 ; relative
size of, 3

Immersion micro-objectives, 195
Incandescent lamp, 40, 41, 418 ;
projection of, 409 ; spirit burner,
418

Inches of gas-pressure, 54
Inclination of nipple and lime in

jets, 67
Induction, magnetic, 395 ; static,

396 ; current, 402
Institutions and screens, 124, 126
Interference of light, 321 ; of

polarised light, 352 ; heat, 393
Inversion of images, 215
Invisible spectrum, the, 315

INDEX

445

IKI

Iridescent glass, 331

Iris diaphragm, use of, 202, 210,212

Ives, Mr., and the ether light, 95

JETS, improving, 60 ; oxy-spirit,
46 ; oxy-gas or safety, 47 ; mixed,
51, 59; best form of, 62, 63;
attachment and fixing, 66

Jewsbury, Mr. S., on fluorescence,
319

KALEIDOPHOXE, 252, 266
Kaleidoscope, lantern, 145
Kaleidotrope, the, 144, 338
Kerr, Dr., on magnetic rotation,

376 ; electrostatic stress, 398
Kingsley, Eev. W., and projecting

microscopes, 180
Konig's manometric flames, 269
Kiihne's surface-tension experi-
ment, 407

Kundt's dust-figures, 273
Kymographion, Ludwig's, 239

LAMPS, oil, 34, 417 ; petroleum, 36,

417 ; gas, 40, 418 ; spirit burner,

418 ; acetylene gas, 419 ; electric,
40, 163; for lecturers, 132, 178

Lantern, choice of a, 111 ;. stereo-
scope, 438

Lantern, parts of a, 13

Lantern-box, the, 134

Lantern-microscope, the, 179

Lantern-slides, see Slides

Lanterns, single, 101 ; bi-unial, 109 ;
tri-unial, 118 ; demonstrating
and scientific, 153, 432

Leakage in gas-bags, 42

Lecturer's lamp, 132, 178

Lengthening tubes, 33

Lens, action of a, 5

Lenses for projecting physical
apparatus, 175 ; ornni focal, 433

Lever slides, 142

Lieberkiihn, 201

Light (lamp) for a lecturer, 132, 178

Light, experiments in, 294-385;
rays of, 1 ; reflection, 274, re-

MEM

fraction, 278; dispersion, 282;
composition of, 285; spectrum,
304 ; fluorescence, 317 ; inter-
ference, 321 ; polarisation, 349 ;
identity with heat, 392

Light (radiant), adjusting the,
115 ; for kaleidoscope, 146 ; for
microscope, 197

Lightning, slide for, 141

Lime-light, the, 42

Limes, 47, 50, 68 ; substitutes for,
68

Lime-shield, Wood's, 66

Lime-tongs, 71

Lime-turning movements, 60, 64

Line spectra, 306 ; reversed, 310

Lippmann's capillary electro-
meter, 240, 408

Liquids, expansion in, 386

Lissajous' figures, 255, 258, 352

Living objects for microscope,
204

Lodge, Dr. 0., oscillating character
of static discharge, 400

Lubricating taps, 109

Luminous cascade, 280 ; paint,
320

Lycopodium for halos, 335

MACH'S acoustic experiments, 273 ;
revolving polariscope, 371

Magnetic curves, 395

Magnetism, 394

Management of lime-light, 71 ; of
an exhibition, 113

Manipulation of lanterns, 101

Mann, Dr. J. D., and reed project-
ing apparatus, 262

Manometric flames, 269

Marey's recording apparatus, 234

Marking slides, 117

Marriotte's law, 229

Masks for mounting slides, 415

Mason, Mr., and screens, 126

Mayer, Prof. A. M., and Doppler's
principle, 254

Mechanical slides, 140, 230

Melde's experiments, 267

Membranes vibration in, 268-272

446

OPTICAL PROJECTION

MEN

Mending gas-bags, 42

Mica designs, for polarised light,
354, 307, 374, 381

Microscope, attachment, 148 ; pro-
jecting and electric, 179 ; com-
pound, 190 ; objectives for, 192

Microscopic bi-unial, 157

Microscopic demonstration image,
the, 190

Mineral oil lamps, 36

Minerals, sections of, 359

Minimum deviation in prisms, 283

Mirror, plane, 175 ; rotating, 269,
400

Mitscherlich's experiment with
selenite, 379

Mixed gas jets, 51, 59 ; best form
of, 62, 63

Mixing gases, danger of, 51, 53, 90

Mixture for oxygen, 77

Mohture, condensed, 16, 116, 206

Molecular motion and heat, 389;
strains, 229, 360

Monochromatic lights or colours,
303

Morton, Prof., on thallene, 319 ;
his vertical apparatus, 166

Mounting experimental lanterns,
175 ; diagrams, 415 ; polarising
slides, 357

Mounts for objectives, 32

Movements, physiological, 232

Muscle-telegraph, 238

Muybridge's moving figures, 145

NERNST lamp, 41, 424

Newton's experiments in refraction
of colours, 283 ; disc, 288 ; rings,
328, 354 ; orders of colours, 356

Newton's, Messrs., triple experi-
mental lantern, 160, vertical do.,
169

Nicol prisms, 339 ; improved, 340

Nipples, 62, 65 ; inclination of, 67

Nodes, 252, 267

Norremberg's artificial crystals and
mica-selenites, 382

Npzzles for gas-supply, 4J

PHO

OAKLEY'S cut-off jet, 118
Objectives for lanterns, 24; size

and cost of, 31 ; testing ; 32, of

microscopes, 192
Objects for microscope, 202; living,

204

Oersted's experiment, 403
Oil-lamps, 34, 417
Opaque objects, 147
Opeidoscope, Dolbear's, 268
Opera-glasses in microscopic

demonstration, 191, 199, 208
Optical lantern front, 171
Organic substances, in polarised

light, 359
Oscillation in electrical discharge,

400

Osmosis in gas-bags, 43
Oxide, thin films of, 324
Oxy-carbon light, 100
Oxy-ether light, 91
Oxygen, preparation of, 74, 80 ;

price of, 88, weight of, 85
Oxy-hydrogen microscope, 182 ;

management of, 196

PACKING of jets, 60, 63, 93
Packing the lantern, 135
Painted slides, 136
Panoramic slides, 141
Pantagraph lantern-sketcher, 145
Parallel beams and pencils, 172,

257, 276

Parallelogram of forces, 217
Particles, polarisation by small,

382

Pearl, mother of, 336
Pencils of rays, management, 172,

257, 276 ; attachment to lantern

for, 173

Perforated plates, diffraction by,337
Persistence of vision, 144, 288, 371
Petroleum ether, 96
Petroleum oil lamps, 36, 417
Petzval objectives, 28, 192
Pfaundler, Prof., and apparatus for

Lissajous' figures, 259, 26Q
Phoneidoscope, the, 270
Phosphorescence, 32Q

INDEX

447

PHO

RIN

Photochromoscope, 436

Photographic slides, 136

Photography, 247 ; colour, 435

Physiological demonstration, 230

Piles of glass for polarising, 340,
345, 351

Pillar-stands, 174

Pinhole, experiments with, 3, 6

Place's lime-turning arrangement,
64

Plane mirror, 175

Plaster surface as a screen, 124,
200, 208

Plateau's experiments, 226 ; soap
solution, 325

Plates, perforated, for diffraction,
337

Plug dissolving tap, 108

Polarisation, plane, 349 ; circular,
362 ; rotary, 365 ; of heat, 393

Polarised light, 349 ; and common,
352

Polarising apparatus, 338

— miscroscope, 201

Polarity, magnetic, 394

Pond life, exhibiting, 204

Portable screens, 126, 131

Prazmowski's prism, 340

Precautions against explosions, 55,
59

Precipitations, 243

Preparation of gases, 73

Press for straining glass, 360

Pressure of gas in bags, 53, 58 ; in
cylinders, 84

Pressure-boards, 44, 55

Pressure-gauge for testing gas-
cylinders, 85

Pressure-regulators, 86

Pringle's cut-off for jets, 117

Printed diagrams, utilisation for
the lantern, 414

Prisms, action of, 4, 281 ; achro-
matic, 291 ; compound and
direct, 293 ; dispersive, 175, 282,
293 ; double-image, 349 ; Fres-
nel's, 332, 376 ; Nicol, 339 ;
Ahren's improved, 345 ; polaris-
ing, 339, 344, 349 ; wedge-form,
$81, 286

Projection : definition, 1 ; of ap-
paratus, 212 ; by shadow, 216

Projection microscope, 179, 431

Propagation of waves, 321

Proportions of gases burnt, 52

Pulse, movement of, 233

Pumice safety chambers for jets,
93

Purifying oxygen, 78

QUARTER-WAVE plates, 364, 367
Quartz in polarised light, 370, 372

HACKWORK slides, 142

Eadiant, the, 34

Radiant heat in spectrum, 391

Radio-micrometer, the, 393

Rainbow, the, 289

Rain-effect slide, 143

Rays form images, 1 ; management

of, 11

Reactions, chemical, 243
Reade's iridescope, 331
Recomposition of light, 285
Reed apparatus for projecting

Lissajous' figures, 262
Reflecting mirror, use of, 256, 277;

polariscopes, 232, 237, 261, 338,

346
Reflection of light, 274 ; angle

doubled in, 277 ; total, 279
Refraction, 4, 278 ; by prisms and

lenses, 281 ; double, 349
Refulgent petroleum lamp, 37
Registration of slides, 104
Regulators for gas-cylinders, 86
Reinold and Riicker on films and

currents, 408
Retorts for oxygen, 75
Reusch's artificial quartz, 382
Reversed spectrum lines, 310
Revolving mirrors, 270 ; lenses,

270, 400 ; polariscope, 371
Rhigoline for O.-H. light, 95, 100
Rings, Newton's, 328, 353; wire

for soap-films, 225, 326
Rings and brushes in crystals,

§76

448

OPTICAL PROJECTION

RIP

Ripples and waves in fluid, 222,
249

Bearing in jets, 61

Bods, vibrating, 251, 266, 362

Boget's spiral, 404

Boiler-slides, 143

Botary polarisation, 365

Botating frame, 348

— polarised beam, Prof. Thomp-
son's method, 347

Botation, optical, right and left-
handed, 373 ; in fluid, 381 ; mica
register for, 374 ; electro-mag-
netic, 375 ; of common light, 376

Botational colours, 366

Botator for polarising objects, 348

Bowland's, Professor, diffraction
gratings, 336

SACCHAROMETER, 375

Safety with the O.-H. light, 55,

59

Safety jets, 47 ; adjusting, 50
Saponine, properties of, 227
Saturators, 421

Scattered reflection of light, 274
Scattering of rays, 30 ; in the mi-
croscope, 184
Schroeder's projecting microscope,

181

Sciopticon, the, 36
Scott's oxy-carbon light, 100
Screens, 124, 200 ; sizes of, 127 ;

erecting, 128, 131
Screw-table for arc-lamps, 166
Scrolls, acoustic, 256, 263
Sea- waves, representation of, 142
Sector-plates, mica, 367, 374
Selenite, in polarised light, 352,

379

Sensation of colour subjective, 297
Sensitive flames and jets, 270
Serrin-Suisse arc-lamp, 162
Shadow projections, 216
Sheet, mounting of a, 128
Siemens' arc -lamp, 163
Silver screens, 425, 427
Size of condensers, 23 ; of objec-
tives, 31 ; gas-cylinders, 84

STE

Sleeman, Bev. P. B., an I reflecting
polariscopes, 346 ; spectrum
reversals, 312

Slide-stage of a lantern, 24, 104

Slides, 136 ; carriers for, 137 ;
effects, 140 ; marking, 117 ;
registration of, 104; preparation
of diagrams, 411 ; microscopic,
202

Slipping slides, 141

Slit and spectrum, 282, 289

Small particles, polarisation by.
382

Snapping in jets, 48, 52, 59, 100

Snow effect slide, 143

Soap bubbles and films, 225, 326 ;
solutions for, 325

Sodium-line, the, 308, 311, 313

Solar system, slides for, 142

Solenoid, pull of a, 406

Solutions of soap, 325

Sonorous vibration and polarised
light, 362

Sound, 247 ; and polarised light,
362

Specific heat, 389

Spectra, continuous, 304; absorp-
tion, 305 ; bright lines, 306 :
reversed lines, 310

Spectrum, the, 282, 304; the in-
visible, 315 ; heat in, 391

Spherical aberration, 19, 26

Spheroidal state, 390

Spirals, Airy's, 379 ; Wright's, 380

Spirit-jets for lime-light, 46

Spot-lens, 201

Spottiswoode's revolving polari-
scope, 371

Sprengel pump, 229

Springs, vibrations of, 266

Stands for the lantern, 132; for
apparatus, 175 ; pillar for ac-
cessories, 174

Star dissolving tap, 107

Starting an exhibition, 113

Static charges and induction, 396 ;
electrical stress, 398; discharge,
399

Statuary, exhibiting, 123

Stereoscope, lantern, 438

INDEX

449

STO

WEI

Stohrer's lantern galvanometer,
401

Strains, molecular, 229 : in po-
larised light, 360

Stress, static electrical, 398

Striated surfaces, 335

Superposition films, polarising,
356

Suppression the cause of colour,
285, 324

Surface tension of liquids, 224

Sympathetic vibrations, 253

TABLE, work upon a, 176
Talbot lines in spectrum, 331
Tanks for ether light, 92, 95
Tanks and troughs, 223, 226, 228,

241 ; for microscope, 204
Taps, dissolving, 106 ; four-way,
121 ; lubricating, 109 ; for house-
gas, 44

Telescopic draws, objection to, 32
Tension, surface, 224, 226
Testing cylinders for gas, 85 ; gas-
pressures, 54 ; objectives, 32
Test-tubes, 243, 387
Theatres, experiments in, 178
Thermo-electricity, 407
Thermopile, use of, 389, 391
Thicknesses, wave, in mica, 356
Thick-plates, colours of, 332
Thin films, colours of, 324
Thomson, Mr. J., and polarisation

by small particles, 383
Thompson, Prof. S. P., on arc-
lamps, 163 ; polarising prisms,
340; mica apparatus, 347, 374;
galvanometer, 401
Three-way tap, 106
Tinters for lanterns, 122
Tornlinson's cohesion figures, 228
Tongs for limes, 71
Top of a lantern, 16
Torque, optical, 374
Total reflection of light, 279
Tourmalines, 340, 350
Tracing-paper as a screen, 127
Transparency, nature of, 294
Transparent screens, 127, 132

Transparent objects in the micro-
scope, 202, 203, 211

Trevelyan's rocker, 251

Triple dissolving, 120

Triplet objectives, 28

Tripod stands, 176

Tri-unial lanterns, 118 ; for phy-
sical demonstration, 160

Troughs, 223, 226, 241 ; for micro-
scope, 204

Tubing for gas, 45

Tuning-forks, 253

Turpentine, film of, 324

Tyndall's phosphoroscope, 321

UKANINB, fluorescence of, 318
U -tubes, use of, 54

VAPOUR, rings of condensed, 331
Varnish for slides, 414
Vertical projections, 166, 226
Vibration of rods, 251, 266 ; forks,

253; strings, 266, membranes and

plates, 268, 273 ; columns of air,

273 ; sympathetic, 253
Vibrations, composition of, 362
Vice for acoustic experiments,

252

Viscosity, 227
Vortex-rings, 229
Vortices in soap-films, 272, 328

WASH BOTTLES, 78

Water, pure and hard, 245 ; decom-
position of, 408

Wave-motion, methods of demon-
strating, 248, 321

Waves, interference of, 322

Wave-slides, 321

Wave-thicknesses in mica, 356

Wedge-prism, 281, 286, 329

Wedges, mica, 355, 370 ; to show
Newton's orders of colours, 356

Weights, dangers from, 53, 56

Weinhold, Prof., arrangements for
tuning-forks, 258 ; achromatism,
G G

450

OPTICAL PROJECTION

WER

291 ; combustion-lantern, 308 ;

simple electroscope, 398
Wernicke's direct prism, 293
Wheatstone's kaleidophone, 252,

266

White light, composition of, 285
Wire rings for soap-films, 326
Wood's lime-jet, 65
Woodward, Col., and concave

amplifiers, 187

ZIR
Work, an exhibitor's, 113

YELLOW, compounded, 303

ZEISS'S eye-pieces, 188 ; lenses, 194

objective-changers, 189
Zirconia for O.-H. light, 68

NOTE.

The Instruments and Apparatus mentioned in this work
are made by, or can be obtained from, NEWTON & CO.,
3 FLEET STEEET, LONDON, B.C., Opticians, Scientific
and Electrical Instrument Makers by Appointment to His
Majesty the King, H.E.H. the Prince of Wales, H.M. the late
Queen, His late R.H. the Prince Consort, the Admiralty,
War Department, H.M. Training Ships, the Indian and
Foreign Governments, the Board of Education, South Ken-
sington, tfc., and by Special Appointment to the Royal Institu-
tion of Great Britain.

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