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The Art of Projecting.

IN

PHYSICS,

Chemistry, and Natural History

WITH THB

PORTE LUMIERE AND MAGIC LANTERN.
By Prof. A. E. DOLBEAR,

tuft's college.

' J 0 ' o e />

ILLUSTRATED.

BOSTON :

LEE & SHEPARD, PUBLISHERS.

NEW YORK: CHARLES T. DILlInGHAM.

1S77.

(

MAY 22 1877 ' }

Copyright :

By Lee & Shepard.

1877.

PREFACE.

The object of this treatise is to point out to teachers of physical
science, and to others who may be interested in experimentation,
the usefulness of the Magic Lantern, and especially of the Porte
Lumiere, and a few other pieces of apparatus which can mostly
be extemporized. With these a surprisingly large number of
experiments in every department of physics may be performed,
and every department of science and art may be illustrated; and
the illustrations may be upon a scale of magnitude which will
surprise one who has never witnessed them. The manipulation of
the apparatus is not at all difficult, and no one need fear he will
not succeed in doing anything described in the book, provided that
at first he masters the simple conditions of projection with a single
lens and with a condenser.

The simplest fixtures have been described, and a cut has been
inserted wherever it could make more intelligible either the forms
of the apparatus or the necessary conditions. No attempt has
been made to explain phenomena, — other books do that ; but it
is hoped that a sufficient number and variety of experiments are
plainly described to make anyone thoroughly familiar with the
art of projecting.

INDEX.

Absorption spectra 114

Acoustic curves 61

Air thermometer 144

AuJmalculc cage 33

Biaxial crystals 132

Bubbles 107

Calorescence 149

Camphor on water 47

Camera obscux'a 80

Candle power 13

<' tiame, To project . .92,100

Capillarity 49

Caustics by reflection 92

" " refraction 104

Chameleon top 143

Chemical tank 34

** reactions 157

Chladni's experiment 62

Chromatic aberration 104

Cliromatropc 142

Cloud formation 145

Cohesion 45

Cohesion figures 47

College lantern 41

Colons of thin films 107

Concave mirror, To project

with 63,91

Convection in water ...... 156

" *' air 98

Condenser: its use 26

Convex mirrors 93

Grove's apparatus 77

Crystalline substances for polar-
ized light 133

Darkened room 5

Diagrams on mica 129

DiaraagnetJsni 151

Diffraction 137

Disks for study of colors . 110, 143

Dispersion . . '. 105

Distortion 93

Divisibility of matter 44

Double refraction 126

Double salts, Prepared .... 134

Druramond light 11

Eidotropc 42

Electric light 9

" " To project .... 153

Engravings, To transfer .... 32

Etching upon glass 31

Fluorescence 119

Focal length of lenses 21

Focusing 25

Fountain, Illuminated 96

Fraunhofer's lines Ill

Galvanometer 147

Gases for lime light 11

Ghost 84

Glue, Marine 35

Gramme machine 9

Gravitation 50

Heat 144,155

Heliostat 1

Ice flowers 52

Illumination, Intensity of . . . 81

Images formed by lenses . • . 100

Interference 71

" spectra 118

Interlacing lines 70

Kaleidoscope 83

Kaleidophone 57

Lanterns 14

Lenses 19

" Magnifying power ... 33

" Mountings for 23

Light 80

" Intensity of 13

" Magnesium 10

" Lime H

" Composition of . 109, 117, 136

" Polarized 127

Lissajou's experiments .... 69

Mach's experiment 64

Magnetism 150

Magnetic phantom 150

Mauoraetric flames 62

Marine glue 35

Mer^ascope 38

Melde's experiment 58

Microscope solar 100

" attachment .... 49

Minute substances 133

Mirage 95

Monochromatic light . . . 108, 122

Newton's disk 143

rings 109

VI

IAD EX.

Objective 25

Obiocts for projection 27

Organ pipe C')

Ol)fi(l()scupe 50

Outline drawings 29

Overtones 71

Persistence of vision 139

I'epper'e gbost 84

Plateau's (txperimpnl) .... 56

Polarization of light 127

Porosity 45

Projection with single lens . . 24
•« '« condenser ... 27
" of large apparatus . 35
'* Apparatus for verti-
cal 40

Porte Lumicrc, To make ... 2

•« " its use .... 24

Pyrometer 145

Ptainbow 100

Reactions, Chemical 157

lleflections 82

Multiple 83

Refraction . . , , 97

Resultants 72

Salicine crystals 134

Screens 6

Sciopticons 18

Silver crystals 53

Singing flames 64

Sinuous lines • 69

Soap bubbles, Persistent ... 108

" " Tension of . . . 107

Solar microscope 100

♦' spectrum > Ill

Spectacle glasses. To test . . . 132

Spheroidal form C2

Spectra, Methods of project-
ing 121,153

Spectrum analysis 119

" of sodium 121

«' " " i-evcrscd . 122

Starch 134

Stroboscope 139

Sympathetic vibrations .... 75

Thermometer 144

Total reflection 94

Tuning forks 57

Vibrations of strings 59

" " forks 57

Vision, Persistence of 139

Water, Decomposed 153

" Maximum density . . . 140

" Refraction of 97

" Total reflection in . . . G4

"Waves in water 61

Whirliug-table attachment . 77

Zoetrope 140

THE ART OF PROJECTING.

A MAGNIFIED image of a picture, or of any phenom-
enon, when thrown upon a screen by means of sunlight,
and lenses, or with a magic lantern, is called a projec-
tion.

When sunlight is to be used for this purpose, it is
necessary to have some fixture to give the proper direc-
tion to the beam. The heliostat and the porte lumiere
are the devices in common use. The latter was the
earliest form, and was invented by Gravesand, a Dutch
professor of natural philosophy, in the early part of the
last century. It was afterwards reinvented by Captain
Drummond, an Englishman, who called it the heliostat.
The latter term is now only applied to an automatic
arrangement, by which a mirror is moved by clock-
work in such a way that a beam of sunlight reflected
from it may be kept in one direction all day, if it be
needed so long. Silberman and Foucault have each
devised very satisfactory instruments, but they are too
costly to be owned by any but the wealthy ; the catalogue
price of the cheapest of these being five hundred
francs. C. Gerhardt, of Bonn, however, makes a small
one, carrying a good mirror three inches in diameter,
for twenty dollars.

2 THE ART OF PROJECTING.

THE PORTE LUMIERE HOW MADE.

Thtporte lumiere is made of various patterns, and its
movements are directed by turning milled-head screws.
Ritchie makes an excellent one with three and a half
inch aperture, for about twenty-five dollars, and it is
recommended that such an one be purchased at the
outset, if it can be afforded, but as many who would
be glad to work with one cannot purchase it, directions
will be given for making one, that will enable any person
who is familiar with the use of carpenters' tools, to make
one at a trifling cost that will answer every purpose.

The room in which the porte lumiere is to be used
must, of course, be one into which the sun can shine.
A room having windows only upon the North side,
evidently cannot be used at all for such a purpose ; one
having windows only upon the East or upon the West
side could be used only in forenoon or afternoon ; while
one with windows looking to the South can be used
nearly all day. Choose then that window where the
sun is available the longest, and opposite to which can
be stretched the screen to receive the projections upon.
Next, take a well-seasoned piece of pine board a foot
or more in width, and an inch thick when dressed ;
cut it to the length of the width of the window sash,
so that it may fit into the window frame, and the sash
be brought down upon it ; this will keep it tightly in
place. With the compasses, scratch two concentric
circles in the middle of the board, one with a radius of
four inches, the other with a radius of four inches and
a half. Saw out the inner circle completely, and cut
the other but one half through the board, and then cut
away, making a square rabbet, as shown at b b. Next,
take a round piece of inch board of the same diameter

PROJECTIONS FOR THE SCHOOL-ROOM. 3

as the outer circle (namely, nine inches), cut a rabbet
upon one side of it so that it will nicely fit into the hole
of the larger board, as indicated at c c.

Make the worked edges, and touching surfaces, quite
smooth ; but the outer edge should be made a trifle
smaller than the hole, in order to allow the disk to
turn freely round in it ; then the hole may be cut in the
disk to receive the lens, four or five inches in diameter,
whichever it may chance to be.

Procure a nice piece of thin looking-glass, twelve or
fifteen inches long and five inches wide. Fasten it to
a back of wood made a little larger than itself, with
broad-headed tacks, or bits of wire driven in and the
top bent at right angles. This back will need to be
an inch thick at the bottom, but may taper like a
shingle to the top, where it need not be half an inch
thick j m is the mirror and h is the back in the figure
adjoining.

A common desk hinge h may be used to attach this
mirror-mounting to the part c in the figure below. It
must be so fastened that the mirror may swing through
ninety degrees from a horizontal plane. The accom-
panying figure will be sufficiently definite to enable
any one to make
the whole instru-
ment. When the
mirror is securely
fastened to the
part ^, the whole
can be inserted in
the board b b and
buttoned in, as is
shown at ^ and b ;
these buttons must

4 THE ART OF PROJECTING.

not bind upon the part c, as this must have an easy
rotation in its place, though they need to be tight in
the board b ; three of them will be enough. Again,
a string must be attached to the end of m, passed
through a small hole in r, and tied to a tight-fitting
thymb peg at d. As the peg is turned the mirror will
be raised or lowered. A short lever v must be made
rfast to some part of c with which to turn the whole fix-
ture around as the sun moves. The ray of light / can
rthen be always kept where it is wanted.

If the window-sill be no more than two or three feet
irom the floor, it will be better to have this fixture
-either put into a window shutter, or to remove a pane
•of glass at the proper place and fasten the board b b
into it. In this case it will be necessary to have a cap
to place over the hole when it is not in use.

The lenses will need to be purchased ; and for a
beginning I recommend a cosmorama lens five or six
inches in diameter and with a focal length of eighteen
or twenty inches ; a plano-convex lens of two and a
half or three inches diameter and eight or nine inch
focus ; also a pocket botanical glass with focus of one
or two inches. These three lenses should cost no
more than six dollars, if the two former are unmounted.
If one has got a magic lantern, or a sciopticon, the
lenses in that will answer admirably. Take one of the
glasses of the compound condenser and fasten it into
the orifice of the porte lumiere with its convex side out ;
then, taking out the front lenses, hold them with one
hand in the path ot the divergent beam of light from
the porte lumiere^ and distant but four or five inches
from it, and with the other hand hold some object
between it and the larger lens ; by moving the lens or
the object a little, a sharp outline of it will be observed

PROJECTIONS FOR THE SCHOOL-ROOM. 5

upon the opposite wall, and then will be seen what
further conveniences will be wanted, such as curtains,
screen, table, mounting for lenses, etc.

THE DARKENED ROOM.

Exhibitions with the stereopticon are almost always
given at night, and there is no trouble from exterior
light ; but the illustrations and demonstrations which
are part of the work of schools and colleges need to be
given in the daytime, and this necessitates a provision
for shutting out the light which will interfere with the
experiment.

The light may be excluded from a room by tight-
fitting shutters, or with curtains. It is very difficult to
make shutters so tight that all light is excluded by
them. It can be done much better and cheaper by
having some frames made the size of the window frames,
and covering them with what is known as enamelled
cloth, such as is used in upholstery and carriage trim-
ming. These should fit tight enough in their places to
remain when placed.

The same kind of cloth can be attached to common
curtain fixtures, and rolled up and down as wanted ;
but it will be found that a great deal of light will pass
by the edge of these curtains. This can be obviated
by tacking strips of the same material a foot wide to
the side of the casing, so that the curtain will roll down
inside of the strips. When sunlight or the lime-light
is used, it is not always necessary that the room should
be totally dark ; and, indeed, some of the best experi-
menters think it a part of their success that their work
is done in a room that is light enough for one to see to
read a newspaper. Yet there are some experiments
which require that extraneous light be shut out from

6 THE ART OF PROJECTING.

the room : for instance, the projection of the Fraunho-
fer lines in the spectrum of the sun, and the phenomena
of diffraction. For these, and the like, the darker the
room the better.

The curtain in the window that holds the porie lumi-
ere will need to have a hole cut in it large enough to
allow the beam of light to come through, and to permit
the hand to give proper motions to the mirror. A flap
should hang over this when sunlight is not wanted, and
the electric light or the lime-light is used instead.

THE SCREEN.

The white surface that receives the projected picture
is called the screen, and it may be a white finished
wall, or white cloth properly mounted. The back of a
large wall-map makes a good screen if the light is used
in front of it, and only a small disk of light is needed,
but the backs of such maps are apt to get discolored,
and to become so dark as to be useless. They maj
then be made white by painting them with whiting,
mixed in a thin solution of glue.

For a parlor exhibition, a common sheet may be
hung against the wall, or between the folding doors,
and the lantern used on either side. If the lantern is
placed back of the screen, the latter should be kept
wet, as it is made more translucent, and the pictures
will appear brighter.

When the porte lumiere^ the electric light, or the oxy-
hydrogen lantern is used, a much larger screen will be
necessary. They are sometimes made twenty-five feet
square or more, but for most purposes a screen fifteen
feet square will be large enough. Common bleached
sheeting, ten quarters wide, can be bought in most
towns. A strip of this, ten yards long, cut into two

PROJECTIONS FOR THE SCHOOL-ROOM. 7

pieces of equal length, and having the selvedges sewed
together, will make such a screen with but one seam.
That these edges may come together, but not lap, let
the sewing be done with what is called the carpet stitch.
Seme loops of tape or small rings may be sewed into
the corners, and it may be hung upon nails driven into
the wall at the proper places.

It is often convenient to have the screen so mounted
as to permit it to be rolled up when not in use, and
various devices have been invented to effect this. Per-
haps the neatest is to have a roller at the top contain-
ing a strong spring, which is wound up when the screen
is pulled down, — a large curtain fixture. A wooden
roller sixteen feet long is likely to sag in the middle,
unless it is made so large as to be cumbersome. It is
best to have one made of tin tube about three inches in
diameter.

A screen can quickly be put up in any room by pro-
curing two strips of board, two or three inches broad,
and long enough to reach from the floor to the ceiling.
Fasten the sides of the screen to these, and then wedge
them tightly between the floor and the ceiling. A
portable frame which can be adjusted to various heights
may be made by having two such strips for each side :
one of them to be provided with a collar at its end for
the other to slide through, and to be made fast together
by a thumb-screw through the collar, as in the figure.

This will permit one to adjust it to different heights to
its limit of eighteen or twenty feet, while by resting the
foot upon chairs or tables a still higher room would be
provided for.

THE ART OF PROJECTING,

CHAPTER II.

ARTIFICIAL LIGHTS.

While it is true that sun-light is much brighter than
artificial light, and is therefore very desirable as the illu-
minating agent in projections, it is also true that sun-
light is not always to be depended upon, and it will fre-
quently disappoint one, by reason of clouds, which will
entirely prevent using the porte lumiere and the ex-
periment will need to be postponed until the sky is
again clear. In some circumstances such delay would
be no serious matter, and one could very well wait ; at
other times the delay would be very inconvenient and
work some harm in our educational institutions j hence
recourse is had to artificial light and lanterns. As
nearly every kind of projection is possible in this way,
and some persons will be provided with such instru-
ments, and still others who would like to know what
can be done with lanterns, some space will be given to
descriptions of some of their more common forms and
their applications.

THE ELECTRIC LIGHT.

Chief among the artificial lights used in projecting
is the electric light, which is produced when a powerful
current of electricity is made to pass between two
carbon points which are separated a short distance from
each other. It is necessary to have a current of elec-

ARTIFICIAL LIGHTS. 9

tricity from forty or more Grove or Bunsen cells to pro-
duce this light, and there is then needed a special kind
of lamp furnished with some mechanism that will auto-
matically keep the two carbon points at a proper height
and a certain distance apart. Such lamps have been
devised by Dubosque of Paris, and Browning of Lon-
don, and others ; but the best of them are not constant,
except with a very powerful battery, and when used
with only forty or fifty cells will need personal attention
every few minutes. Browning has advertised a small
electric lamp, which he says will give a constant light
with only six or eight cells. A number of these small
lamps have been brought to this country, but, so far as
the writer knows, no one has been able to work them
with anything like so small a battery.

There are several reasons why the electric light is
not more generally used for this and other purposes.
First, its cost: the battery with lamps costing about
two hundred dollars. Second, the consumption of zinc,
acids, and mercury for amalgamation, which, with the
labor of setting it up and cleaning it after use, may
be reckoned at ten dollars a day. Third, the noxious
fumes which constantly arise from a working battery,
making it necessary to have a special battery-room, well
ventilated ; and fourth, the need of frequently over-
hauling it, re-amalgamating the zincs and filing the wire
connections. These have made every one who has ever
worked with a battery, wish that some substitute could
be found for it. The magneto-electric machines de-
vised by Wilde, Ladd, Farmer, and others, have been
more or less successful, but have been much too costly,
and require eight to ten horse-power to run them.

The machine that promises the most for us now is
the one known as the Gramme machine, a French in-

lO THE ART OF PROJECTING.

vention. All who have seen its performance speak in
high terms of praise of it. At present there is but one
of them in the United States ; that one belongs to the
University of Pennsylvania, at Philadelphia. Its cost
was about $i,ooo. It needs but one or two horse-power
to run it eight hundred revolutions a minute, when it
gives a light equal to i,6oo candles. The latest pattern,
made especially for producing the electric light, weighs
400 pounds, is run by one-horse power, and gives a light
equal to 2,000 wax candles. It would seem as though
this was the thing we have so long waited for. It is
now being used for lighting up large manufactories, only
four lamps being needed in a room three hundred feet
long, which is so well lighted as to leave no shadows.

MAGNESIUM LIGHT.

Wire made of the metal magnesium can be lighted
like a piece of pine wood, and continues to burn
with a most brilliant and dazzling light. In order to
regulate the burning of the wire or narrow ribbon,
which is generally employed, a lamp with a feed run by
clock-work is used. Sometimes two ribbons are burned
at the same time. ■ This light is not constant, and is
even more liable to go out than the electric light, and
furthermore a special arrangement of cloth tubing is
used for carrying away the product of the combustion,
magnesium oxide, a bulky white powder, which accu-
mulates very rapidly.

The cost of a lamp is about fifty dollars, and the
cost of the magnesium is about two dollars an hour.
It has the great advantage of being very compact, re-
quiring but a few minutes to prepare at any time, and
giving then a light which is amply sufficient for any ex-

ARTIFICIAL LIGHTS. II

hibition, and is especially well adapted for experiments
in fluorescence on account of the abundance of ultra
violet rays. A three-inch transparency can be magni-
fied up to thirty feet in diameter, and be well enough
lighted for a large audience to see plainly.

THE OXYHYDROGEN, OR DRUMMOND LIGHT.

A very intense light is produced by projecting a blow-
pipe flame of mixed hydrogen and oxygen gases upon a
stick of unslacked lime. The great heat raises the
lime to vivid incandescence. Sometimes magnesia is
employed instead of lime ; it is then called magnesia
light, and when zirconia is used it is called zirconia light.
The two latter are seldom used in the United States,
but the former is very common. The gases are stored
in portable tanks or india-rubber bags, which are con-
nected by flexible tubes to the jet, from which it is
driven by pressure upon the bags, and is lighted at the
outlet. There are many patterns of these jets, some
of them permitting the gases to mix before their escape,
and others not until they are ignited. The mixed jet
is the most economical one, and is to be preferred for
most purposes. Such a piece of apparatus had better
be bought of a reliable dealer.

Common illuminating gas can be used in place of
pure hydrogen, but the light is not quite so intense.
The demand for these gases has been so great during
the past three or four years that they are now manufac-
tured on a commercial scale in New York city, and are
compressed into copper tanks holdmg from ten to sixty
cubic feet. These tanks are exceedingly convenient.
They retail for twenty-two cents a foot for oxygen, and
eight cents a foot for the common gas.

12

THE ART OF PROJECTING.

An alcohol flame is sometimes used with oxygen, the
jet supplying the latter forcing the flame upon the piece
of lime. The alcohol is fed from a reservoir seen at
the back of the lantern. This gives an excellent light,

Fig, 3.

and quite sufficient for many purposes. A picture three
inches in diameter may be magnified up to fifteen feet
and be well lighted. The light produced in this way is
called the Bude light.

OTHER LIGHTS.

Common illuminating gas may be employed with
advantage where the room is small, and great intensity
is not required. The form of burner known as the
Argand, is best for this use.

Kerosene and lard oils have been, and are still

ARTIFICIAL LIGHTS. 13

largely employed. When burned under the most favor-
able conditions, kerosene will yield a light equal to
thirty or forty candles, and will illuminate a disk eight
or ten feet in diameter very well.

Illuminating gas and kerosene have this advantage,
that they are very cheap, costing but a few cents an
hour j but they can only be used to enlarge pictures
which are very transparent, outline drawings, or chem-
ical reactions in the large tank, which will be described
further on.

There is no absolute standard of the luminous inten-
sity of light. A conventional standard of its intensity
is used for convenience, and for regulating the illu-
minating power of gas. In Massachusetts, the legal
standard is the light of a sperm candle weighing two
and two-thirds ounces, when consuming one hundred
and twenty grains in an hour. In some places wax
candles are adopted, but the light does not differ much
in intensity from that given by sperm candles.

With such a standard as this, the various lights
which have been specified, have the following relative
illuminating powers under favorable circumstances :

Illuminating gas, 15

Coal oil in Argand burner, .... 20

Coal oil in Marcy's lamp, . . . '25

Magnesium, 40

The lime light with oxygen and alcohol, . . 50

*' " '* " " " common gas, 100

" *' " " '' " hydrogen, . 125

The Electric light, . . . 500 to 10,000

All of these are very variable, especially the lime
light, which depends upon the quality and pressure of
the gases, the quality of the stick of lime, and the kind
of jet used.

The electric light has a very wide range in intensity,

14 THE ART OF PROJECTING.

as will be seen. It has not been found possible yet to
make an electric light with less intensity than several
hundred candle-power, while with the new gramme
machine before mentioned, it is expected that not less
than twenty thousand candle-power will be obtained.

Sun light has about four times the intensity of the
most powerful electric light that has yet been meas-
ured. When it is reflected from a good mirror, it loses
its brilliancy somewhat, but is second to nothing but
the direct sun light. Hence, the desirability of em-
ploying the latter whenever it is possible, for both effi-
ciency and cheapness.

LANTERNS.

When sun light is used, the room must be darkened,
and the light only admitted through the opening of the
porte lumiere ; but when artificial lights are used, it be-
comes necessary, not only to have the room dark, but
to inclose the light, so that what is not used shall not
interfere with what is used. The refractive power of
lenses is made available for the purpose of securing a
larger amount of light than is possible without. This
will be understood by the diagram. Let ^ be a pomt

of light, and
^ ^ an object
three inches
long and dis-
tant six or
eight inches

from the light.
Fig. 4.

LANTERNS.

15

All the rays which will fall upon the object will be in-
cluded in the angle b a c. Interpose the lens de be-
tween the light and the object, and all the rays
included in the much greater angle d a e will now
fall upon h <r, and it will be much brighter. There is
always another lens, and sometimes two, called the
objective^ in front of the lantern, to give definition to the

picture. All the
essential parts
of a lantern are
shown i n the
accompanying
picture : — ^ is
the source of
light inclosed in
the box b : d e,
^^' ^' the lens for di-

verting a greater number of light-rays, and called the
condenser; c is the objective ; and j, the screen to receive
the light.

Fig. 6.

i6

THE ART OF PROJECTING.

The forms of lanterns differ somewhat as they are
adapted to different purposes, and they are called ster-
eopticons when the electric, the magnesium, or the
lime light is used in them.

The electric lamp is generally placed within the lan-
tern box, which is made to accommodate either it, or
the lime light, as is most convenient or desirable.

The magnesium lantern shown in Fig. 6 (p. 15) has
the lamp set into it behind, and resting upon a floor
which can be elevated or depressed by a thumb-screw.
The flexible tubing is connected with the chimney, and
the magnesia is thus prevented from distributing itself
throughout the apartment.

Fig

The above engraving (Fig. 7) represents the essen-
tial conditions for the lime light in a very convenient
form. Within the lantern may be seen the fixtures for
holding the lime and the jet, with the rubber tubing to
connect with the gas tanks. The front side and back

LANTERNS.

17

are provided with heavy black-cloth curtains which may
easily be raised to adjust the fixtures within, yet shut

Fig, 8.

in the light when allowed to hang free. The con-
denser consists of three lenses, the front one being five
inches in diameter. This is shown in the cut (Fig. 8)

ACVEBDMCfilL.

Fig. 9.

of the transverse section of it. The objective repre-
sented is also compound. This form of lantern is made

i8

THE ART OF PROJECTING.

by George Wale & Co. at the Stevens Institute, Hobo-
ken, N. J., and called the Experimenter's Lantern, be-
cause of its adaptability to many kinds of experimental
work, as well as to the exhibition of photographic trans-
parencies.

Mr. L. J. Marcy, of Philadelphia, has, in an ingen-
ious manner, made a lantern jet which can be used
with alcohol, common gas, or hydrogen, to produce the
lime light. He calls it the triple jet. The engrav-
ing (Fig. 9) shows the lantern in section. L being
the disk of lime, the gases ignited at w. This lantern
is compact, light, and has a very
convenient arrangement for hold-
ing slides, tanks, and so forth.

The oil lanterns have various
names given to them by different
makers, such as magic lanterns,
lamposcopes, sciopticons, and so

mg. 10.

on. Fig. 10 represents the new form, the Sciopticon,
which for simplicity, compactness, and brilliancy of
illumination, surpasses every other oil lantern in the

LENSES. 19

market. Neither the Sciopticon, nor any of the stere-
opticons, are difficult to manage. But little time will
be needed to learn all that is needful to know in order
to work them well, and they are not likely to become
disarranged.

The course of experiments to be given will be gen-
erally adapted to both the porte lumiere, and the lan-
tern, but the adjustments will be described for the
former. If, however, some special arrangement of the
lantern will be needed for a given experiment, it will
be pointed out.

A sufficient number and variety of experiments in
physics, chemistry, and natural history, will be de-
scribed, to make any one who is practically interested
in the work so familiar with the apparatus and its
working, that he will need no further instruction in the
art of projecting.

LENSES.

All the lanterns in the market are furnished with the
proper lenses for the ordinary kinds of projections,
such as transparencies, etc. Solar microscopes are also
generally well provided with lenses suitable for the work
to be done with them, but as this treatise is for the use of
those who have neither, yet who would like to experi-
ment, some things, necessary to be known about lenses,
and their uses are mentioned here, chiefly as to their
adaptability to the experiments with the porte lumiere.
In Fig. II the rays of light are shown to fall upon the

20

THE ART OF PROJECTING.

Fig. 11.

fall upon the screen s.
lens, used next to the

double convex
lens c d when
they are con-
verged to a
focus, and aft-
erward they
separate and
It may be noticed that this
orifice, is represented as a
double convex lens in nearly every place in the book.
Such a lens, when properly constructed, has less
spherical aberration than any other form. To be thus
properly constructed one side should have a greater
convexity than the other, the radii of curvature for
the two sides being as one to six, but such lenses are
not enough better than the more common form of hav-
ing the two sides of equal convexity, to make any dif-
ference except for special work, if the lens is used as a
condenser : so that for this purpose a common double
convex lens, with the faces of equal curvature, will be
found to answer for most purposes. But a plano-con-
vex lens with the same focal length as the convex lens
is nearly as good.

Here it may be remarked that whenever a plano-
convex lens is used it
should have its convex
side turned toward the
rays which are parallel,
or most nearly parallel.
Thus the lens (Fig. 12)
has its convex side to-
ward a^ where the rays
are parallel, and should
Tig. 12, be so whether the rays

B

^Bh

^mmn

LENSES. 21

enter that side or emerge from it, when the source
of light is at b. It must not be inferred that noth-
ing can be done except with lenses of a particular
sort. On the contrary very much can be done with
such poor lenses as are used in dark lanterns, and
are full of striae and air bubbles. The lenses that
come with ordinary magic lanterns will answer for many
purposes. Spectacle glasses, linen provers, botanical
glasses, are all very useful. A pow-
erful lens can be made out of two
watch glasses, one {a) a little larger
than the other {b). Bring the two
together under clear water. When
raised out of the water they will
adhere quite strongly, and for a time
can be used to advantage as a mag-
nifier.

FOCAL LENGTH.

Fig. 13. 'pj^g fQ^,^l length of a lens should

be known before it is brought into use, and it may be
determined experimentally in the following way :

With the lens placed as shown in Fig. 12, so that the
parallel rays from the porte lumiere fall squarely upon
it, measure the distance from the centre of the lens to
the point b, the focus. If the lens be double-convex,
add one-half its thickness to the measured line. This
number will represent the focal length of the lens. If
the lens be plano-convex, its focal length will be the
distance from its flat side (Fig. 12) to the focus. Again,
hold the lens so that the direct rays of the sun fall per-
pendicularly upon it, and measure as before the dis-
tance to the focus. Lastly, if the sun be not shining,
bring the lens close to a white wall or sheet of paper
opposite to a window, and hold it so that the light from

22 THE ART OF PROJECTING.

the window falls perpendicularly upon it ; then slowly
move the lens from the paper toward the window, until
the inverted image of some object out of doors, such as
a cloud, or house, or tree, half a mile or more away,
appears plainest. Measure the distance from the lens
to the paper, as in the other cases.

If diverging rays had been used instead of parallel
rays, the focus would have been at some greater dis-
tance ; and if converging rays, a less distance than that
indicated for parallel rays, and these differing foci may
be infinite in number ; hence, the focal length of a lens
is always specified for parallel rays.

CARE OF LENSES.

Before using lenses, or other pieces of glass appara-
tus, see to it that they are clean, and, as far as possible,
avoid touching them upon their polished surfaces with
the fingers, as the latter will always leave a mark upon
such a surface. A piece of cotton-flannel, or old fluffy
linen will do to wipe glasses with. Wet the cloth with
water, or, still better, with alcohol, if there are spots
that will not otherwise come off. One may know when
a piece of glass is clean by gently breathing upon it,
and then noticing how long the condensed moisture
remains upon its surface. If it is really clean, the
moisture will disappear in a second or two ; if it
remains for eight or ten seconds or longer, the glass is
not clean, though it may appear to be so to the eye.
Be careful to keep such pieces from touching anything
harder than the cloth they are wiped with, for even
wood will scratch a nicely polished glass surface. If
these pieces are not mounted in such a way as to pro-
tect them, they should be laid, when not in use, upon a
piece of cotton-flannel or velvet.

LENSES.

23

Fig 14.

MOUNTINGS FOR LENSES.

Lenses may be purchased already mounted in any
desirable way, but generally the mounting costs as much
or more than the lens. What is
needed is a fixture that will hold
the lens at a proper height, and
has a considerable latitude of
movement in every direction.
For many purposes an unmount-
ed lens can be held in the pinch
of a common retort-holder as in
Fig. 14.

It is often convenient, and
sometimes necessary, to have the lens so mounted as
to cut off light that would otherwise pass by its edge
and mar the effect upon the screen. To do this it is
only needful to cut a hole the size of the lens, in a
square piece of board (Fig. 15) having the proper
size and thickness, and fasten
the lens in it with small brads
or triangular pieces of tin or
zinc, such as glaziers use for fast-
ening window-glass in the sash.
The board should be thicker than
the lens, and the latter should be
Fig. 15. SQ sunk into it that its surface will

not touch the table when lying upon it. This will pre-
vent it from getting scratched. Thus mounted, the
board may be held in the retort-holder as before.

The smaller lenses, having a focal length of not more
than an inch or two, had better be bought already
mounted for carrying in the pocket, such as botanical
glasses and linen provers, or, if it can be afforded, get
one of Zentmayer's gas microscope objectives.

.24

ThE ART OF PROJECTING.

For holding pictures, and other objects, for projec-
tion, the retort-holder will answer, in many cases, just
as well as a more costly fixture. Not only may the
larger pieces, such as photographic transparencies,
leaves of trees, and the like, be held well in it, but
microscopic specimens in glass slides, also small un-
mounted objects, such as parts of flowers, insects, etc.,

may be held in

small forceps

^^' ^«- (Fig. i6), which

in turn may be held in the retort-holder. It will be

found convenient to have as many as three of the

latter.

PROJECTIONS

TO PROJECT WITH THE PORTE LUMIERE AND A
SINGLE LENS.

Fasten th^porte lumiere in its place, and so adjust it
that the beam of light / (Fig. 17) is reflected horizon-
tally, and falls upon the screen s. It will appear as

a bright spot,
five or six inch-
es in diameter.
Darken the
room, by draw-
ing the curtains
or closing the
shutters, and
the beam of
Fig. 17, light can then

PROJECTIONS. 25

be seen from the window to the screen by the light re-
flected from the dust particles, which are always in the
air. Now fasten in the retort-stand a lens 0 four or five
inches in diameter, and with a focus of a foot or more,
and place it two or three feet from the opening, in the
path of the beam, and perpendicular to it. It will at
once be noticed that the light is converged by the lens,
the rays crossing each other in front of it, at its focus,
from which they diverge, and appear upon the screen as
a large disk of light. If some object, as d, (Fig. 17),
be placed between the opening and the lens 0, a place
may be found by trial, when the image of the object
will be seen upon the screen. The outline should be
well defined ; it will be inverted and much enlarged.

Finding the right adjustment of the object and the
lens, so that the image is in its proper place, and has a
sharp outline, is called focusing. In general it is best
done by fixing the object in the path of the beam first,
and then placing the lens rather close to it, and slowly
moving the lens toward the screen, being careful to keep
it perpendicular to the beam until the image is plainest.
It will be well for a beginner to take a number of ob-
jects : some opaque, like the finger, a pencil, or a key,
and some transparent, as a grasshopper's wing, or a
piece of glass with a design drawn upon it, or a regu-
lar lantern transparency. A lens thus used to project
a picture is called an objective.

These two pieces of apparatus, the parte lumiere and
the single lens, have a much wider application than one
unfamiliar with them might suppose. Every picture
made for the magic lantern, or the stereopticon, can be
shown with these in the day-time, even better than with
the others at night. Every school in the land may have
one, for the carpenter can make the porte lumiere^ and

86 THE ART OF PROJECTING.

the lens will cost but a trifle. The pictures themselves,
though not half as costly as they were before photogra-
phy was applied in making them, can be rented of any
one who keeps them for sale, if one cannot afford to
buy them outright. Most excellent transparencies, on
all sorts of subjects, can be bought, from six to nine
dollars a dozen, of any lantern-maker or dealer in pho-
tographic materials.

If the teacher wished to give a lesson on the elements
of drawing, his copies could be prepared upon glass, by
one of the methods given a little further on. These, when
projected, would be so large that a large school could see
them as plainly as if they had been drawn upon a huge
blackboard, with chalk. The room could be light
enough for any of the required work. Geometrical fig-
ures, outline maps, botanical specimens, the kaleido-
scope, chemical reactions in a large test tube ; natural
history specimens, such as small fish, pollywogs, water
beetles, butterflies, grasshoppers ; the splendid colors
on huge soap-bubbles \ the vibrations of tuning-forks,
and of cords ; the intensity of light, reflection, refrac-
tion, magnifying power of lenses, and many more things,
may be projected, in an admirable way, with only these
two pieces.

THE CONDENSER AND ITS USE.

The rays of light reflected from the mirror a (Fig. 17)
through the aperture, are parallel, and the diameter of
the lens 0 should be as great as the thickness of the
beam, and it may have a greater diameter with advan-
tage. If it has less, some of the light will pass its
edge, and either be unused or, what would be worse,
fall upon the screen and make a bright spot in the mid-
dle of the picture. The smaller the object to be pro-

POOJECTIONS. 27

jected, the smaller must be the lens used as the object-
ive, and the shorter must be its focal length ; hence, if
a beam of parallel rays is used, it may often be so small
as to be nearly useless, for the divergence is so rapid
beyond the focus of a short focus lens that the little
light thus used would be too much scattered. It be-
comes necessary, then, to make a large beam of light
to pass through the small lens. This is accomplished
by means of a second lens, called the condenser^ because
its office is to condense a large number of light rays for
the double purpose of illuminating the object better
and making them all to pass through the smaller lens.
This condenser is usually four or five inches in diameter,
though for special purposes it is sometimes made a foot
or more in diameter. For the porte lumiere the con-
denser may be the same lens that was used as the ob-
jective, or any lens may be used that has a sufficient di-
ameter and a focus of from one to two feet. It may be
either double-convex or plano-convex.

TO PROJECT WITH A CONDENSER.

The object ^ (Fig. 18) is placed near the condenser
^^, and the objective o is brought near to it and slowly

moved toward
the screen, as
before, until
the well de-
fined image ap-
pears upon it.
It must be not-
Fig. 18. ed here that

the size and focal length of the objective must be such
that all the light passes through it when it is at its
proper distance from the object. If o be moved toward

28 THE ART OF PROJECTING.

the object, it will be seen that some of the light does
not pass through it. If the object d be moved toward
the objective, then some parts of it will not be lighted,
and consequently but a part of it would be projected.
If the object d is quite small, like a fly, or a flea, or a
small crystal, it will be necessary to bring it forward,
toward the focus of the condenser, where it will be
more strongly lighted, and allow the use of an objective
of shorter focus, and consequently higher magnifying
power. If the object be made of wood, or any kind of
tissue, be careful about bringing it very near to the
focus, as the great heat there may destroy it in a few
seconds. This danger may be somewhat lessened by
placing between the condenser and the object the chem-
ical tank, containing a strong solution of alum. The
common pocket botanical glass, having a focus of an
inch or two, will answer for very much of this work,
but Zentmayer's inch-and-a-half gas microscope object-
ive is superior to any other lens I have seen for such
projections.

This arrangement is essentially the solar microscope.
The object may be exceedingly minute if the objective
has a very short focus, say half an inch, or less.

It is possible to magnify an object a thousand diam-
eters, or a million times, and still have it so well lighted
that a large audience can see it plainly. A list of
things that are suitable for projections with this ar-
rangement is appended, mainly for the purpose of indi-
cating the breadth of its field of usefulness :

Hairs of various animals, which may be held between
two strips of glass. Down from the wings of moths and
butterflies ; these will adhere to a piece of glass with-
out any pressure. Scales of fishes. Eyes, legs, and
wings of flies, or the whole of any insect. Stings of

PROJECTIONS. 29

bees and wasps. Antennae of moths and mosquitoes.
Fibres of cotton, woolen, silk, etc. Ferns, moss, lichens,
leaves of trees. Thin sections of wood. Small flowers,
stamens and pistils, pollen. Mites in cheese. Butter-
flies, beetles, animalcules in stagnant water. Vinegar
eels. Crystallization of camphor, sulphate of copper,
and most solutions of crystal liz able substances.

Diatoms, and indeed most objects that are prepared
for the microscope, appear to good advantage upon a
screen. Any book upon the microscope will have
many valuable hints upon obtaining and preparing ob-
jects in a suitable way, and will be a very useful book
to one interested in natural history but who cannot af-
ford to buy a good microscope.

OUTLINE DRAWINGS FOR PROJECTION.

Every one who uses either a lantern or the porte
lumiere for purposes of instruction, will need to make
outline pictures to illustrate his subject, as it will be fre-
quently impracticable to get a photograph of what is
wanted. Moreover, a simple outline is often quite suf-
ficient for the illustration, as, for instance, superposi-
tion and inclination of strata in geology ; sections of
machines ; writing, or musical notes ; outlines of leaves,
roots, parts of a flower, insects, maps, etc.

The surface of transparent glass is so smooth that it
cannot be marked with either common ink or a lead
pencil. If the glass be ground, so that a pencil will
mark it, it becomes so opaque that but little light can
go through it ; hence, a surface must be prepared which
will be transparent and yet allow marking upon it.
This can be effected in many ways, and I give a num-
ber which I know to be practicable :

I. If a piece of glass be rubbed on one surface with

30 THE ART OF PROJECTING.

a piece of hard soap, enough will adhere to it to make
the glass semi-opaque. Now draw the design with a
fine-pointed stick. It will clear the soap from the glass,
and so permit the light to shine through the marks.
This has the advantage of permitting the same glass to
be used like a slate, for with a drop of water upon the
finger the old design can be rubbed out, leaving the
glass coated for another picture. The same thing can
be done with a surface of beeswax, but the glass would
need heating in order to re-spread the wax.

2. For more permanent pictures, a very good way is
to flow the glass with photographers* transparent var-
nish, and then scratch the design upon the varnish, not
cutting through to the glass. The light is so much
scattered from this scratched surface, that it appears as
a dark line, and answers a very good purpose. The
prepared plate can be laid over the design wanted if it
is to be a copy, and is of proper size ; the transparency
allows the picture to be plainly seen, and all its mark-
ings can easily be followed. The varnish is quite hard
when dry, and with a little care in handling these pic-
tures, they need not become scratched. They can be
entirely protected from that danger by covering them
with another clean glass of the same size, and binding
their edges with paper, as common lantern-pictures are
bound. Photographers have also another kind of var-
nish called ground-glass varnish, which, when spread
upon glass, gives it an appearance similar to ground
glass. This surface permits drawing with a pencil or
with ink upon it, and then a coat of the transparent
varnish will render the first coat transparent, leaving
the lines in ink or pencil ; or the design may be drawn
through the first coat of varnish, in which case, the light
will shine through the lines and appear white upon the
screen.

PROJECTIONS. 31

3. If india ink be rubbed up in water until it is quite
thick, it can be used for drawing designs upon ordinary
glass.

4. A thin sheet of gelatin may be treated like the
glass coated with the transparent varnish, and either
have the design scratched upon it, or drawn with ink.
It should be inclosed between two glasses for protection.

5. Thin sheets of transparent mica will receive lines
drawn with india ink, or the figures may be scratched
upon them with the needle or awl.

6. Designs may be nicely etched upon glass, by first
coating the glass with a thin, even coat of beeswax,
which can be well done by heating the glass over a
lamp until the wax melts and flows over its upper sur-
face. When it is cool, draw the design with a needle
point or a small awl, cutting through the wax all the
way. Take an old saucer, or some such dish which you
are willing to spoil for other use, and put into it a table
spoonfull of powdered fluor spar. Upon that pour a
table spoonful of strong sulphuric acid, and stir them
together with a stick. Fasten the glass, drawing upper-
most, to a piece of board large enough to completely
cover the dish. The fastening can be done by crowd-
ing tacks into the wood, so that the heads shall lap the
glass and keep it in its place. When thus fixed and
laid over the mixture of spar and acid, gently heat the
dish, being careful not to inhale the fumes that will
escape. When the fumes begin to appear, put the
whole either out of doors or in a good chimney draught,
and let it remain eight or ten minutes, when the wax
may be removed by heat and rubbing, and the drawing
will be found etched into the glass.

Beautiful pictures of crystals can be made in this
way, by taking various crystallizable salts, such as am-

32 THE ART OF PROJECTING,

monium chloride, cupric sulphate, etc., and making a
rather dilute solution of them, and then adding a little
dissolved gum arable. Flow the solution over the
plate, and let it remain horizontal until it is dry. The
crystals will be seen to have separated from the gum,
which will fill up all the mtermediate space. Put over
the etching dish as before. The crystals will quickly
dissolve, and their outlines will be beautifully etched
upon the glass, which may now be washed clean in
water.

7. Engravings may be transferred to glass by first
coating the glass with dammar varnish, or with Canada
balsam, and letting it dry until it is very sticky, which
will take half a day or more. The picture to be trans-
ferred should be well soaked in soft water, and care-
fully laid upon the prepared glass, and pressed upon it,
so that no air bubbles or drops of water are seen un-
derneath. This should dry a whole day before it is
touched ; then with the wetted finger, begin to rub off
the paper at the back. If this be skillfully done,
almost the whole of the paper can be removed, leaving
simply the ink upon the varnish. When the paper has
been removed, another coat of varnish will serve to
make the whole more transparent.

8. A piece of glass may be smoked in the ordinary
way, and a design marked upon it. This makes a very
good and plain picture. If the design is needed for
keeping, heat some alcohol in a cup or small porcelain
dish, and hold the smoked side of the glass in the alco-
hol vapor for a minute or two, and afterward it may be
varnished with photographers' varnish, carefully flowing
it over the plate in the same way that plates are flowed
for photographic purposes.

PROJECTIONS. 33

TO DETERMINE THE MAGNIFYING POWER OF A LENS, OR
A COMBINATION OF LENSES, IN PROJECTING.

It will be evident, upon inspection, that the farther
away the screen is from the lens, the larger will be the
picture ; but for a given projection, the simplest way of
determining the magnification is to choose some object
of known dimensions for projection, and then to meas-
ure its size upon the screen. Suppose it be a lead
pencil having a diameter of one fourth of an inch. If
its image is a foot in diameter, it is evident that it is
magnified 4 X 12 =48 diameters. If it is three feet in ^
diameter, then it has been magnified 4 X 12X3 = ^44
diameters. It will be convenient to have a scale, either
photographed or etched upon glass, for the purpose of
directly showing the magnifying power of lenses.

A vernier made upon glass by either of the described
methods, will be convenient for study, and some meas-
urements.

THE ANIMALCULE CAGE.

If one would exhibit the minute forms of life to be
seen in water, an animalcule cage will be needed.
This may be made in the following way:

Take two quite clear pieces of white glass, about four
inches long and one inch wide. Two other pieces of

the same width,
and one inch and
a half long. Put
these two short-
iig, 19. er pieces be-

tween the longer ones, so as to separate them, and
leave a space in the middle clear through. Fasten
these together with japan varnish, being careful not to
get any of the varnish into the opening. If any should

34

THE ART OF PROJECTING.

get in, wipe it carefully out. When the varnish is
dry, and the pieces are firmly fixed together, putty up
the bottom of this hole so that it will hold water.
When this is dry, it can be used to hold fluids of most
kinds, but it is especially fitted for water containing ani-
malcules, or vinegar with eels. It should be put back
of the focus of the condenser, for the great heat there
will boil the water in a little while, and the temperature
of no more than 130° Fah. will quickly kill most all
kinds of infusoria. Suitable water for examination can
be found in old rain barrels, stagnant pools, water in
which flowers have been standing for a day or two, an
infusion of hay in water, and will be found very interest-
ing. The larva of the mosquito is a lively and amus-
ing thing when magnified to five or six feet in length.

THE CHEMICAL TANK.

For chemical experiments, and a variety of others,
a tank of larger proportions will be necessary. The
accompanying diagram (Fig. 20), shows the construc-
tion. Provide two pieces of clear, white glass, of the
same size, about five inches by six, for the sides. These
may be kept apart by a strip of rubber, about one-half

of an inch thick,
bent and cut at
the corners, the
whole clamped
together by three
or four clamps,
as shown. If rub-
ber with flat sides
is not easily pro-
curable, a piece
of rubber tub-
Fiq. 20. ing will answer

PROJECTIONS. 35

nearly as well ; the tubing may be filled with sand to
keep it firm. Such a tank will hold any kind of a so-
lution, and may be quickly taken apart and cleaned.
A tank which will answer for many experiments nearly
as well can be made by cutting a semi-circular piece out
of a board, of the proper size, and fastening the glass
sides to it with cement. What is known as marine glue
will be the best for this purpose, and as it is very
convenient to have some of this glue for making
and mending apparatus, because it will adhere to any
surface, the method of preparing it is given : Dissolve,
separately, equal parts of shellac and india rubber in
naptha, and afterwards mix the solutions thoroughly,
applying heat. It may be made thinner by adding
more naptha. It may be preserved in a tin box. In
order to use it, it must be heated, as well as the sur-
faces which are to receive it. Marine glue may be dis-
solved in ether, or a solution of potash.

A METHOD FOR PROJECTING LARGE PIECES OF APPARATUS.

Many pieces of apparatus used in illustration and
demonstration are much too large to be projected in the
ordinary way, as it is obvious that the size of the lens
used as condenser will be the limit to the size of the
object that can be shown with it. Thus, if sunlight is
used, the diameter of the orifice r, ^(Fig. ii), will be
the measure of the largest picture that can be shown at
once j and if a lantern is employed, no picture larger
than the condenser can be projected.

Suppose that it is desirable to show to an audience
a piece of apparatus much too large for ordinary pro-
jection, and yet too small to be plainly seen, such, for
instance, as the electroscope ; or the movement of a
pith-ball under electrical excitement ; or the movement

36 THE ART OF PROJECTING.

of a vibrating cord, or large tuning-fork ; or the ap-
paratus for showing the linear expansion of metallic
rods, etc. The following method will be found applica-
ble to a great many such cases, where simply the outline
of the instrument is needed.

Place a short focus objective (and the shorter the
better), so near the focus of the condenser that all the
light falls upon it. After refraction the light will form
a very divergent beam and the focus in front of o will

Fig, 21,
be a sharp point, practically a luminous point, and any
object held between it and the screen s, will have a
strong shadow cast upon the latter. The magnitude of
this shadow will depend upon the distance from the
focus. There will be no penumbra — the outline will be
sharply defined.

If one has a lantern, the condensing lens above will
answer without the objective, as its focus for parallel
rays will be sufficiently short. A globular glass flask,
filled with water and placed m the path of the rays, will
also be found to be satisfactory. When a lantern is
used instead of sunlight, it will be necessary to use the
microscope attachment, which is described further on.

PROJECTIONS. 37

working in front of the lens, the same as with the
porte lumiere. •

The following is a list of apparatus and of experi-
ments which are suitable for such projection : Equil-
ibrium of the same liquid in several communicating
vessels; equilibrium of different liquids in communi-
cating vessels ; cartesian diver ; the hydrometer ; cap-
illarity j diffusion of gases ; Torricelli's experiment ;
Mariotte's law ; the manometer ; Sprengel's air pump ;
fountain in vacuo ; the siphon ; the pyrometer ; the in-
fluence of pressure upon the boiling point ; M. Des-
pretz's experiment on the conductivity of solids ; con-
vection ; the thermo-pile ; umbra and penumbra ; action
of magnets ; attraction and repulsion from electrical ex-
citation. Natural history specimens, such as birds, rats,
mice, squirrels, frogs, toads, live fishes, if in a tank with
transparent sides ; leaves of trees, ferns, etc. ; well-de-
fined crystals, such as quartz, feldspar, mica, pyrite ;
diagrams on glass of machinery, as the steam engine —
these diagrams can be drawn a foot square or more ;
silhouettes, etc., etc., are all available with this method.
There is an advantage in this plan, when it is at all
applicable, that will commend itself to every one,
namely, it is available at any point between the focus
and the screen, hence it will only be necessary to place
the object in the path of the rays to the screen at such
a point as will be convenient and will make the shadow
sufficiently large. The instructor can stand by the ob-
ject, and with a pointer like a pencil call attention to
any particular part. And again, the field is so large
that several objects can be in it at a time, if need be,
for comparison, such for instance as leaves of several
species of oaks or maples, or a range of capillary tubes
of various diameters.

38 THE ART OF PROJECTING.

THE MEGASCOPE.

Photographs that are taken especially for projection
with the magic lantern are often called transparencies
because all of the lighter parts of the pictures are made
as transparent as possible, and they are shown by light
that is transmitted through them. If one would ex-
hibit a picture like a stereoscopic view or a common
carte de visite^ it is evident that recourse must be had to
some other arrangement. The light must be reflected
from the picture, but when only the ordinary amount
which is reflected from a surface of nine square inches
is distributed over seventy-five or a hundred square feet,
it is evident that it will be but dimly visible. If a large
amount of light is concentrated upon the picture it will,
of course, reflect more, and its image will be corres-
pondingly brighter. This can be effected in two ways:
first, by using a large lens, or second, by using a large
concave mirror.

The following figures will serve to show how this may

be done. When sunlight is used, the larger the con-
denser the better. One seven or eight inches in diam-
eter, if possible, should concentrate the light upon a

PROJECTIONS. 39

second plain mirror at r, which should have such an
inclination as to reflect the converging rays upon the
object to be shown at d^ and strongly illuminate it ; the
objective at o will be used in the same way as for any
other projection. This apparatus should be in a box
made with sides a foot square and six or eight inches
deep. At the back of it a hole should be left at d, in
which the various objects for exhibition may be held.

In place of the condenser and the plain mirror, a
large concave reflector, such as is used behind lamps,
may be placed at r, and the parallel rays from the porte
lumiere allowed to fall upon it. It should be placed at
such a distance from the object d^ that it will just illu-
minate it j this will of course be determined by the
focal length of the mirror.

The room needs to be quite dark for the successful
working of this apparatus, and especial care should be
taken to prevent any of the light from the porte lumiere
from being scattered into the room ; paint the box black,
inside and out, with lampblack mixed in japan varnish.

If the lime light be used, as it generally is for such
an exhibition, it is necessary to modify the lantern very
much, — so much so as to require an entirely new in-
strument. The following is the simplest plan of one :
A square wooden box made eighteen or twenty inches
on a side, and about fifteen inches deep, may have a
little way made in it on one side for the fixtures holding
the jet / and the lime j to slide upon. A hole r cut
six inches square, may be made near the corner, and
another one on the front side for the light to come
through upon the lens <?, which is the only lens needed
for work. The size of this hole should be no greater
than that of the lens o used for the projections, but this
lens should be as large as possible. A lens six or eight

40

THE ART OF PROJECTING.

inches in diameter, with a focus of from eighteen to
twenty-four inches, will be found best for the purpose.

This may be held in the re-
tort-holder before mentioned^
and set at such a distance in
front of the hole that an ob-
ject <r, when strongly lighted,.
will be plainly projected upon
the screen s. The whole of
the back on the in side should
be covered with white paper.
Let a black cloth flap hang
over the hole at r, so that no
light will enter the room, save
what is reflected from the il-
luminated object.

With these conditions a
dark photograph of an in-
dividual, upon a white background, will show quite
well. Objects held in the hand, such as a watch with
its movements, cameo pins, small flowers, surface of
half an apple or orange. The latter, if squeezed when
being shown, presents a very amusing appearance.
Minerals, crystals, shells, bright-colored beetles, bugs,
butterflies, etc., may all be exhibited, and appear, with
the shades and shadows, like real objects. This con-
stitutes the megascope.

The accompanying cut ( Fig. 24 ) represents the
scenic effect of the human hand, as projected by the
megascope. i

Fig. 23.

THE VERTICAL ATTACHMENT.

It is often very desirable to project such phenomena
as the ripples upon the surface of water, the move-

PROJECTIONS.

41

Fig, 24.

42

THE ART OF PROJECTING.

ments of a horizontal galvanometer needle, etc., such
as cannot be exhibited with the common forms of ap-
paratus for projections. At first the awkward method
was adopted of turning the lantern up so that it rested
upon its back. This endangered the condensing lenses
of the lantern from the great heat immediately under
them. Professor
Cook and Doctor
Morton have great-
ly improved upon

Fig.

most valuable

26.

attachment to

Fig, 25.

this, and have added a
the lantern.

The cut (Fig. 25) represents this invention. It con-
sists of a plane mirror inclined at an angle of 45°, and
when so placed that the beam of light from the lantern
falls upon it, it is reflected perpendicularly upwards
upon a lens that converges the light when it passes

PROJECTIONS.

43

through the objective above it, and falls upon a
second mirror, which is so mounted as to allow re-
flection in any direction. The same device is made
a part of the standard lantern of the country, and
called the " College La?iter?ty^ manufactured by George
Wale & Co., of Hoboken, N. J. By an ingenious
arrangement the change from the horizontal to the
vertical can be made in less than half a minute. The
microscope, the polariscope, the electric-light regulator,
and several other fixtures, are fitted to this instrument,
making it a most perfect and complete lantern.

Such a vertical attachment as is shown in Fig. 25 is
applicable to the porte lumiere, but one can be extem-
porized, that will do good service, with such material as
is accessible to every one. An iron filter-stand, such

Fig. 27.

as is in common use in every chemical laboratory, may
be taken, and the condensing lens c laid upon the lower
or largest ring, and the objective, Oy upon the upper or
smaller one, as shown in Fig. 27. Below the lower ring
a plain mirror m may be placed, at such an inclination
that the beam of parallel rays falling upon it from the

44 THE ART OF PROJECTING.

porte lutniere will be reflected upward through the two
lenses upon another smaller mirror, n^ which may be
held in a retort-stand, and the beam directed to the
proper place.

PHYSICAL EXPERIMENTS.

DIVISIBILITY OF MATTER.

A good way to show the minute divisibility of matter
is to dissolve, in water, a small quantity, say a gram, of
cupric sulphate, and add enough ammonia-water to
make a clear, blue solution. Put it into the chemical
tank, having measured its capacity in cubic centimeters,
or inches, fill it with water, and project the tank by the
method described on page 183. A beautiful blue color
will appear upon the screen. With a small syphon of
bent-glass tube, draw out one-half of the solution and
fill up with pure water. The amount of coloring mat-
ter will be reduced one-half, but the solution will be
strongly colored. Remove, in the same way, another
half, and so on until the blue color is no longer visible
— comparing the color with that of pure water, pro-
jected, at the same time, in a test-tube. Keep account
of the number of dilutions, and at last, when the blue
color is on the vanishing point, calculate the weight of
cupric sulphate in each cubic centimeter of water. In
place of the copper solution, any of the analine dyes
will do as well.

The same thing can be illustrated with a soap-bubble,

PHYSICAL EXPFRIMENTS. 45

blown thin, and projected in the diverging beam (Fig.
2i). The bubble will be sharply defined upon the
screen, and its magnitude will depend upon the diverg-
ence of the beam of light, and its distance from the
screen. It may be made ten or fifteen feet in diameter,
if the lens have a short focus. The colors will begin to
appear around the pipe in bands, and computation of
the thickness may be made, and of the probable num-
ber .of molecules in its thickness. For the considera-
tion of this, see *' The New Chemistry," by Professor
Cooke, and Nature, Vol. I, p. 551; also Galloway's
"First Steps in Chemistry," article 102.

POROSITY.

The gases dissolved in common water will be ex-
pelled by gently heating some in a test-tube while the
whole is projected. The bubbles will be seen to form
and rise where nothing was before visible. The po-
rosity of water can be shown by projecting a test-tube
half filled with it, and its depth marked by a bit of
thread tied about the tube at the level of the surface.
A considerable quantity of salt or sugar can be added
to the water without noticeably increasing its bulk. A
piece of chalk dropped into a test-tube containing
warm water will at once give out quite a quantity of
included air.

The ordinary experiment of showing the porosity of
leather by forcing mercury through it by atmospheric
pressure into a partial vacuum, can be exhibited by pro-
jecting the upper part of the tube, while the exhaus-
tion is going on. The mercury will be seen to fall
upward on account of the inverting by the lens.

A mixture of equal parts of strong sulphuric acid
and water loses notably in volume when cool. Fill a

4^ THE ART OF PROJECTING.

test-tube with the fresh mixture, and tie a string about
the tube at the hight of the mixture. It will be too
hot for handling with the fingers at first, but it may be
cooled in a few minutes enough to show the shrinkage,
by stirring it in a dish of cold water. The surface will
be seen to be considerably below the string which
marked its original hight. This experiment may be
used to exhibit compressibility of liquids.

Most of the experiments which are suitable for pro-
jection of the properties of matter are chemical, and
will be found described under that head. Diagrams,
such as are given in most text-books on mechanics, can
be made upon glass by one of the processes described
on page 184, and will be found very convenient to a lec-
turer upon that subject.

COHESION.

A drop of water or other fluid exhibits this, and may
be projected with the lantern, or with the porte lumiere^
and a single lens (Fig. 28). Sprinkle a little lamp-
black or lycopodium-powder
upon one side of a strip of
glass, like a microscope slide-
and place it in the proper
place for projecting, keep-
ing it horizontal that the
dust may not slide off.
Now place a single drop of
water upon the slide ; the powder will prevent it from
spreading upon the glass, and it will gather itself up
into a round globule with some of the dust over its sur-
face, making an interesting object upon the screen.

Again, a saturated solution of zinc-sulphate is put into
a white glass square bottle, two inches square, and

Fig, 28,

PHYSICAL EXPERIMENTS. 47

three or four inches high. Let the bottle be about half
filled with this solution. Into a few drops of bisul-
phide of carbon drop a piece of iodine. It will at once
stain the bisulphide a dark-brown color, which should
then be carefully dropped upon the solution of zinc,
where it will float. If now pure water be carefully
added, so as to rest upon the solution of zinc, the bi-
sulphide will collect into an oblate spheroid, having the
appearance of brown-colored glass. A square bottle
will enable one to project it better, as a round bottle
would make a cylindrical lens, and the projection would
be indistinct, unless the vessel was quite large.

Nearly fill the large tank (Fig. 20) with alcohol, and
project the tank with the lantern, or with the single
lens and porte lumiere. Now drop upon the alcohol,
with a glass rod, or other convenient thing, any of the
aniline dyes. As soon as the dye touches the alcohol
it will go straight down for a short distance, then it will
branch, and these will shortly branch again, and so on
to the bottom of the tank, when there will be a large
number of branches. Upon the screen the appearance
will be as if a tree were growing ; if at short distances
apart in the tank drops of different colors are placed,
the branches will interlace and produce a fine effect. A
tank of coal-oil, in which is dropped a little colored
fusil oil, is said to produce an entirely different figure.

But it is with the vertical attachment that the most
novel and interesting phenomena, due to cohesion, may
be shown. For this purpose it is necessary to have a
horizontal tank, made by cementing a ring, an inch
broad and four or five inches in diameter, upon a plate
of clear glass. The ring may be made of glass, or
wood, or zinc. This is to be placed upon the hori-
zontal condenser, and half filled with pure water, the

48 THE ART OF PROJECTING.

surface of which is to be projected. Let fall, from a
height of two or three inches, a single drop of ether.
It assumes a characteristic form, will move about, but
will last only a few seconds, as it evaporates rapidly.
Rinse out the tank, and fill again with pure water, and
in like manner drop upon its surface any of the essen-
tial oils, of creosote, lavender, turpentine, sperm, and
colza oils. Each one will assume its peculiar form due
to cohesion.

Fig. 29 represents the pattern exhibited by a single
drop of oil of coriander, and Fig. 30 the appearance

Tig. 29. Tia- 30.

of oil of cinnamon. Some of these forms are very
beautiful, as, for instance, that due to oil of lavender.
This method of studying oils is used, by some experts,
to determine their kind and purity. These forms are
known as Tomlinson's Cohesion Figures.

Again, into the same tank, well cleaned and filled with
water, drop a few small pieces of camphor-gum. As
soon as they touch the water they will begin to move
rapidly, dodging each other in a wonderful way, and ap-
pearing as if they were endowed with life. Their move-
ments will be accelerated if the water is warmed to a
hundred degrees, or a little more.

PHYSICAL EXPERIMENTS. 49

A drop of a solution of camphor in sulphuric acid,
gently delivered to the surface of the water, will take a
double-convex lens shape, and will move about the
water in an eccentric manner for a long time. Several
drops may be placed upon the water at a time, but they
will avoid each other in their movements. Make a
small boat of tin-foil, and into it put a fragment of cam-
phor about the size of a pea, and place it on the tank 3
it may move round slowly, but put a piece of camphor,
about the size of a canary-seed, upon the water, and it
will spin round, dart up to the boat, and drag it about
in a lively manner, just as an insect might do.

To show the existence of the camphor-film, that forms
upon the surface as soon as it touches it, dust the sur-
face of the water with lycopodium, then gently lower a
fragment of camphor upon the middle of the tank.
The instant the camphor touches the water the dust
will be seen to open out into a circle of large di-
ameter ; then, after a moment's pause, the lycopodium
is formed into a number of wheels, arranged in pairs,
revolving in opposite directions.

A large drop of camphor dissolved in benzole,
dropped upon water, has the appearance of a double
convex lens ; it sails slowly about for a while, becoming
flatter and thinner, till at last it has sudden contrac-
tions, assuming different shapes. The contractions
multiply till at length they become so violent as to throw
off portions of the disk, or split up into smaller disks,
which, in their turn, twist and double up, and ultimately
throw out from each a tiny film of camphor, which lies
quiet upon the water.

One who is interested to pursue this subject further
will find an abundance of material by Tomlinson, in
the Philosophical Magazine for 1861. Also in " Experi-
mental Essays," Weale's Series, No. 143.

50 THE ART OF PROJECTING.

CAPILLARITY.

The chemical tank (Fig. 20), containing a little colored
water, may be projected in any convenient way. If a
small glass tube be placed vertically in the tank, the
solution will rise in it. A series of five or six tubes,
with bores of different size, may be placed in this tank
at the same time, and the whole projected. The water
will be seen to rise higher as the tube is smaller. A
plate of glass three or four inches square may be put
down into the tank, bringing one of its edges against
one side of the tank. The water will rise two or three
inches where the glasses touch, and slope away with a
beautiful curv^e, which will vary as the whole side of the
glass is nearer, or more distant from the other one.

CRYSTALLIZATION.

It is always fascinating to watch the growth of crys-
talline forms, especially when the process can be leis-
urely studied over a surface fifteen or twenty feet
square. In all cases, a high magnifying power will be
needed. Three hundred or four hundred diameters is
better than any less.

If this is to be shown by a lantern, it will be neces-
sary to have a powerful light, and the attachment known

as the microscope at-
tachment (Fig. 31),
which fits upon the
lantern (Fig. 26) when
adjusted for horizontal
projection. The lens
must run forward nine
or ten inches, and the
'''' * jet drawn back until

the maximum of light goes through the objective, which

PHYSICAL EXPERIMENTS, 51

has a short focus, and will not be more than three-
fourths of an inch in diameter. A strip of clear glass
an inch wide and three or four inches long, will answer
upon which to spread the solutions to be examined, a
few of which are given in another place. The glass
will then only need to be placed in its receptacle, and
its front focused, the same as for any microscopic
objects. Any further instructions that may be needed,
may be found under the descriptions of the method
with the solar microscope. With the porte lumiere^ and
two lenses of proper focal length, the finest effects can
be shown.

Fig. 3'4.

Let c be the condenser, with say twelve-inch focus, 0
the objective with one-inch focus ; it may be a common
pocket lens, or a linen prover, or a botanical glass.
First adjust c so as to give a disk of light upon the
screen. The rays will cross at the focus, and diverge
afterward. Place the lens 0 so that all the light may
pass through it, or as much as possible ; this will de-
pend upon the size of 0. At any rate, it will be near
the focus of c. Have ready a slip of glass three or
four inches long and an inch wide, and wet one side
with the solution to» be crystallized ; as, for instance,
ammonium chloride, sometimes called sal ammoniac.
Place it back of the objective at g, and move it until

52 THE ART OF PROJECTING.

the wetted surface appears very plain upon the screen.
Then wait until the solution begins to evaporate, as it
will, from the upper edge first, when crystallization will
begin there. See to it that the focus is right, and then
gently blow upon the plate, unless the work is going on
fast enough. The crystals will shoot out and grow
-while one looks, until they cover the entire screen with
beautiful forms.

The following are good substances for illustration
when dissolved in water : Ammonium chloride ; barium
•chloride ; copper sulphate ; camphor dissolved in water ;
common alum ; urea dissolved in alcohol.

ICE FLOWERS.

To exhibit the decrystallization of ice, which was
first shown by Tyndall, it will be necessary to saw from
a very clear piece of ice a cake three or four inches
square, and about a half or three-quarters of an inch
thick, cut parallel to the plane of freezing. When first
cut, the sawn surface will be too rough for use, but will
quickly melt smooth enough by dipping a few seconds
in water. The beam of light that falls upon it should

Fig. S3.

consist of parallel rays, and the porte lumiere is better
for projecting this experiment than any lantern.

A single lens for an objective, four or five inches

PHYSICAL EXPERIMENTS. 53

focus or longer, will answer. It is the interior of the
ice that is to be projected, and as there is a multitude
of planes within it, each one being slowly decomposed,
the light will suffer refraction, and one must not look
for such plain figures to cover the screen as is repre-
sented in Tyndall's work, and in DeschaneFs Physics.
The forms can be picked out here and there.

If a lantern be used to project these crystalline forms,
remember that the best effect will be obtained with a
beam of parallel rays, which, in most lanterns, will
necessitate the removal of the front lens of the con-
denser. .

THE LEAD TREE.

Fill the small glass tank for the solar microscope
with a rather dilute solution of the acetate of lead ; ad-
just it as for the exhibition of animalcules, using a
small lens with a short focus, not more than an inch, if
such an one is possessed. Into the solution now drop
a very narrow strip of sheet zinc, not bigger than a
common sewing-needle ; such a piece can be easily
enough cut from a sheet of zinc with a pair of shears.
This will at once have a deposit of lead upon it in a
beautiful fern structure, which, while you look upon it,
grows to be a forest. The same effect can be produced
in the larger tank, described farther on, with a smaller
magnifying power, by using a small battery of two
Grove's cells, and having fine platinum wires to dip into
the solution of lead. The lead will be deposited in the
fern-form upon one of the wires. After there is a
growth of the crystals upon a wire, attach the other end
of the wire to the other pole of the battery, and then,
completing the circuit again, the lead will be dissolved
from the first, and be deposited upon the second.

54 THE ART OF PROJECTING.

THE TIN TREE.

Take a rather dilute solution of chloride of tin, made
by dissolving the crystalline proto-chloride in water, in
the proportion of one part of the former to four or five of
the latter. This solution will precipitate its tin upon a
piece of zinc in the same manner as the lead solution
will, but the form of the crystals is very different. Use
the same tank, and a magnifying power of 400 or 500
diameters, if good sunlight can be had. The growth
will be quite rapid, and crystals six or eight feet long
ought to appear. This needs no battery. Solutions
of any degree of concentration can be used, but the
growth is so rapid in very strong solutions, that the
masses interfere with each other, and are dense and
imperfect in form. Solutions can be used that are as
dilute as twenty or more parts of water to one of the
crystalline chloride.

THE SILVER TREE.

A solution of nitrate of silver is put into the tank,
and a piece of fine copper wire put into it, the wire
being nicely focused upon the screen. Pure silver will
be immediately deposited in arborescent forms upon
the wire, but the forms will vary with the strength of
the solution. The more diluted it is, the finer will be
the threads of silver.

It is better to place
the metal w that is to
have the deposit upon
it, whether of copper or
zinc, so that it is just
below the surface {$)
of the solution, for the
Fig. 34. reason that when it is

PHYSICAL EXPERIMENTS.

55

projected it is inverted, and as the arborescent deposit
hangs upon the wire, it will appear upright upon the
screen, and so have a closer semblance to a rapid veg-
etable growth.

A neutral solution of the terchloride of gold will give
a characteristic growth upon a piece of zinc, but the
solution should be quite weak.

Salts of copper will give nodular forms upon zinc, if
very dilute, and a dense fringe of black copper, if the
solution be very strong, sometimes terminating in quite
large crystals.

GRAVITATION .

Make a frame like the picture, consisting of two up-
right posts, about one foot long and one inch square,
grooved like flooring on one
side of each. Fix these into
a board {a) about eight inches
by twelve, for a support.
Fasten a strip across the top
to hold them steady. They
should stand about five inches
apart. The flange should be
cut away from the right-hand
standard, from the top down
about five inches, so that the
weight b, which has a tongue
on each end, can be put into
its place and be free to move
up and down between the
^'S'- ^^' standards. A plate of glass

{c) of proper width must fit in the top, and be fastened
by a button {d\ or otherwise, and held firmly in place.
Procure a pocket tuning-fork, either an A or a C, and

56 THE ART OF PROJECTING.

solder to one end a piece of small copper wire, so it
will project from the side about one-eighth of an inch.
A gimlet-screw can be cut into the other end of the
fork, so that it can be tightly screwed into the wooden
weight ^, if small gimlet- holes are made before. There
should be a number of these holes bored into b, at such
a distance from the front edge that when the fork is in
one of them, the wire upon the prong will come against
the surface of the glass plate when the weight b is
raised.

In order to use this instrument, it is necessary to
coat the front side of the glass c with smoke, photo-
graphic varnish, or a very thin coat of white wax. Fix it
in its place in the frame, and then raise the weight b
(which ought to weigh two or three pounds) until the
top of the tuning-fork is above the glass. Seize the
two prongs of the fork with the thumb and forefinger,
and pinching them close together, suddenly let it drop.
The wire finger will trace a sinuous line upon the pre-
pared surface, caused by the vibration of the fork
during its descent. The undulations will be seen to
increase in length as they approach the bottom ; but as
each one was made in the same time with every other
one, it is obvious that the velocity increased as it was
falling. In order to show this, it is merely necessary
to put the apparatus near the condensing lens, and
project the face of the glass. The line traced by the
fork will be seen upon the screen. It will now be well
to measure the lengths of the undulations, which can
be very well done by having a scale in millimeters
etched, or otherwise fixed upon another glass, which
can be put just in front of the first, when the number of
divisions of the scale to each undulation can be counted,
and the result stated in mathematical terms.

PHYSICAL EXPERIMENTS. 57

plateau's experiment.

With the vertical attachment and a tank, made five or
six inches deep and with a plane glass bottom, this
beautiful experiment, which so well illustrates cohesion
and centrifugal force, may be projected. Fig. 36 shows

the proper conditions.
A wire, w^ is made to
revolve vertically i n
the tank, by means of
a little pulley driven
by a cord about a larg-
er one, at /, the whole
so made as to rest upon
the edge of the tank.
Fig, 36. and supported by ears,

as shown. The wire, w^ should have a thin disk of tin
fastened to it at s, for a surface of adhesion. Now the
solution may be one of alcohol and water, so graduated
that its specific gravity shall just equal that of the oil
used, which can only be done by trial in a test-tube ;
or it may be a solution of zinc sulphate, and the sphere
may be made of bisulphide of carbon, with a little iodine
dissolved in it, which will make it black, as in the for-
mer experiment described under the head " Cohesion."
Here, also, the solution of zinc sulphate and water will
need to be of the same density as the bisulphide of
carbon, which will be best found by trial. This fixture
must be placed upon the apparatus for vertical projec-
tion, and the focus adjusted for the sphere. If the
above fixture for producing the rotation be made of
stiff wire, it will not interfere much with the distinct-
ness of the projection. A full account of this experi-
ment, and of all the conditions to be observed, will be
found in the Smithsonian Report for 1865, p. 207.

58 THE ART OF PROJECTING.

ACOUSTICS.

THE TUNING-FORK.

The vibrations of an ordinary tuning-fork may be ex-
hibited in the following way. Having made the fork to
vibrate, hold it at a in the divergent beam (Fig. 21),
and swing it in its plane of vibration at right angles to
the beam of light. Its shadow will present a curious,
fan-like appearance. If the fork is polished it will re-
flect enough light to exhibit the same appearance when
looked at while vibrating and swmging.

Another way is to hang light pith or cork balls so
they just touch the fork, or other sounding body, and
project the ball in any convenient way. As soon as the
body begins to vibrate it will drive the ball away from
it. Two forks in unison may be used in this way, to
show sympathetic vibration. Hang a cork ball half an
inch in diameter so it will just touch the side of one of
the forks near the end, and project the ball and fork.
At some distance set the other fork to vibrating, and
put it upon its resonant case, or place the stem upon
the floor or some resonant surface. The ball will be at
once thrown off from the first fork, showing that it has
been set vibrating.

Professor Mayer has described a number of interest-
ing experiments to illustrate the change of wave-lengths
by the motion of translation of the sounding body, in
the American yournal of Science^ April, 1872.

THE KALEIDOPHONE.

To the end of a piece of steel wire, two or three feet
long, and an eighth of an inch in diameter, 0 I (Fig. 37),

ACOUSTICS.

59

fasten with marine glue, or sealing-wax, a small bit of

mirror, about
the fourth of
an inch square.
The wire must
be held tightly
at some point,
in a vice upon
a table. The
Fig. 37. light from the

porte lumiere falls upon the plane mirror vi, and is
thence reflected upon the small mirror on the end of
the wire at /, whence it is reflected to the screen. If
the wire be now carefully plucked, it will give a line of
light upon the screen, but will probably soon change
into an ellipse or a circle. If the wire be struck with
a small billet of wood, like a hammer-handle, there will
be heard two sounds, the fundamental with some over-
tone that will give a beautiful compound figure upon the
screen, some circle or ellipse made up of small undu-
lations,, which will vary as the wire is struck in different
places. If the wire be made fast at its middle, and the
other end of it be plucked, the end with the glass will
take up the vibrations at once — a case of sympathetic
vibration. If it is not fastened in the middle there will
be little or no movement when the lower end is struck.
(See Tyndall on Sound, pp. 133, 135.)

MELDE S EXPERIMENT.

To one prong of a small pocket tuning-fork tie a
piece of silk thread, six or eight inches long, and to
the other end tie a pin-hook and hang upon it a small
weight, say a shirt-button. Project this with the large

6o THE ART OF PROJECTING.

lens, as represented in Fig. 38. First, with the fork
held as indicated, make it to vibrate. The string will
divide up into segments, all of which can be plainly seen
and counted. Second, turn the fork so that it vibrates

Fig. 38.

in a horizontal plain. The number of segments will
be doubled. Third, hang another button upon the pin-
hook, so that the weight will be doubled. Count the
segments while the fork vibrates, both perpendicularly
and horizontally. In this way some of the laws of
vibrating strings can be demonstrated.

Fasten a small piece of wire to one prong of the tun-
ing-fork, and when the latter is vibrating draw it quickly
across a piece of smoked glass. The undulating line
will show well when projected.

THE OPEIDOSCOPE.

Take a tube, of any kind, that is five or six inches
long and an inch or more in diameter, tie a thin rub-
ber membrane or a piece of tissue-paper over one end,
and on the middle of the membrane glue a piece of
looking-glass that is not more than the eighth of an
inch square. The light from the por^e lumiere falls

ACOUSTICS.

61

upon a mirror a, and is received upon the bit of mirror
upon the end of the tube. The open end of this tube
is to be held at the mouth and various sounds produced,
varying in pitch and intensity. The vibrations of the
membrane will move the mirror, and the beam of light

Fvg, 39.

reflected from it upon the screen will describe various
beautiful and regular curves, depending upon the man-
agement of the voice. It will be easy to find some
pitch and intensity which will give a straight line : then,
while the sound is being made, if the outer end be
swung sidewise at right angles to the line, an undulat-
ing line will appear, m every way like those produced
by the vibrating tuning-fork described on another page.
If there are prominent over-tones in the sound they will
be made apparent by their interference, giving a trace
just like the traces upon a smoked glass by Scott's
Phonautograph. The forms are regular enough for a
tone of a given pitch and intensity, to enable one to
write his music with them for notes ; and if a tune like
"Auld Lang Syne " be footed in the instrument, the ef-
fect is quite amusing. The size of these figures, at the
distance of fifteen or twenty feet, may be six or eight
feet or more.

62 THE ART OF PROJECTING.

CHLADNl'S EXPERIMENT.

A glass plate of any form, if fixed by a clamp, will
give out a musical sound when a violin bow is drawn
across its edge. If the surface of the glass be strewn
with sand, the latter will be arranged in some symmet-
rical form. The glass plate may be prepared as for
the magnetic phantom, and the sand fixed after its
acoustical arrangement, and afterwards projected as an
ordinary transparency. It is generally best to exhibit
this phenomenon during the process of arrangement,
and this will require the fixtures for vertical projection.
The glass to be sounded is to be made fast, and so
placed that as much as is possible of it is over the con-
denser of the vertical attachment ; then the sand sprin-
kled upon it, and the focus adjusted for the upper
surface.

When the bow is drawn, the sand is seen to arrange
itself according as the plate gives out one sound or an-
other, which depends upon the part of the plate that
is bowed, and where it is damped, also upon its form.
It is well to have round, square, triangular, and hex-
agonal pieces, eight or ten inches in diameter.

To show water-waves upon a Chladni plate. Professor
Morton has devised the following way : A plate of glass
about a foot square is so held by its middle that one
corner covers the condenser of the vertical lantern. To
this corner is cemented a thin ring of soft rubber, of
about five inches in diameter, and into this water is
poured to the depth of one-tenth of an inch. Project
the surface of the water and then draw the bow across
the edge of the glass, as in the other cases, so as to
produce a musical sound. The water within the rub-
ber ring is thrown into a system of large waves, which

ACOUSTICS.

63

form a shaded net-work of singular beauty. Drawing
the bow so as to produce notes of different pitch, the
waves will be large or small as the notes are low or
high, and with a mixed note it is possible to get two or
more systems superposed.

If a common tuning-fork be struck and then have
one of its prongs put in contact with the surface of the
water in this tank, a beautiful radiation of ripples may
be seen, resembling somewhat the arrangement of iron
filings about the poles of a magnet. The motion of
water in a shallow bell-glass can be projected by letting
the parallel beam from the vertical lantern go through
it, doing away with the condenser, as the vessel itself
would act as a lens if water were in it. The bow may
be drawn across its edge when it will give out a musical
sound, the water will be thrown into ripples, and a large
objective might be used to project the whole surface.
The bell-glass may be filled with ether or alcohol, and
then sounded. Some of the liquid assumes the sphe-
roidal form, and these are driven over the surface to the
nodal lines. (See Tyndall on Sound.)

MANOMETRIC FLAMES.

The flame of a candle, or lamp, or gas-jet, if a lumin-
ous one, can be projected upon a screen by using a

concave mirror
(Fig. 40). It
will be invert-
ed and magni-
fied. If while
the flame is
projected the
mirror be tilted
Tiij. 40. SO as to swing

64 THE ART OF PROJECTING.

the beam horizontally, the flame will appear drawn out
into a band of light, due to persistence of vision. But
if the flame be not a bright one, the image will be too
dim to be useful, if the screen is ten or fifteen feet
away. The intermittent character of the singing hy-
drogen flame can be shown in ' this way, but it is much
better to use common gas in place of hydrogen, as the
flame is much brighter. The flame of common gas
may be made still brighter by passing it through ben-
zole or naptha, or tow saturated with ether. The room
must be quite dark. (See Tyndall on Sound, p. 223.)
In the American edition of Atkinson's Ganot's Physics
is pictured Koenig's apparatus for observing manomet-
ric flames. In place of the rotating reflector use the
concave mirror, as above, and the same figures will ap-
pear upon the screen.

One can make a tolerable substitute for that apparatus,
if gas be not obtainable, by fastening over the mouth
of a small two-inch funnel, such as is used in chemical
laboratories, a piece of tissue-paper or thin rubber. A
piece of rubber tubing, two or three inches long, may
be drawn over the stem of the funnel, and the other
end drawn over the mouth of a common jeweler's blow-
pipe. A sheet of pasteboard may now be rolled so
large that the broad end of the funnel, which has the
tissue-paper pasted to it, may fit snugly in it. The
whole fixture may now be supported in any way, by
means of retort-stands. A gas- flame from a small
round orifice, or a common candle may be used for the
flame ; the end of the blow-pipe is to be inserted in the
blaze, with the opening upward. If now, either a com-
mon mirror be used to give angular motion to the re-
flected beam, or the concave mirror to reflect the flame
upon the screen, while a sound is made in the large

ACOUSTICS. 65

tube, it will disturb the flame so much as to give a dis-
tinctly serrate image either upon the screen or in the
plain mirror. The annexed figure will give an idea of

Fig. 41.

the arrangement mentioned : a is the tube for produc-
ing sounds, in b is the funnel with tissue-paper over its
mouth, c rubber connection to the blow-pipe //, which
opens upward into the flame from the candle e.

THE ORGAN-PIPE.

The vibrations of the air reed of a sounding organ-
pipe may be shown, by having a small pipe made of
iron gas-pipe and blown by illuminating gas, which may
be lighted ; and when the pipe is sounding the reed will
be seen to swing backward and forward in front of the
embonchure. That it really vibrates may be seen by re-
flecting the light from a mirror upon a screen, and tilt-
ing the mirror, as is done in showing the manometric
flames.

mach's experiment.

The movement of the air within a sounding organ-
pipe has been studied optically by Mach, a German
physicist. His method was to stretch a membrane
across the node of a pipe with glass sides, and in the
open end he ran a fine platinum wire to the membrane,
and thence out to be connected with a galvanic battery.

66 THE ART OF PROJECTING.

A sponge dipped in strong sulphuric acid was drawn
along upon the stretched wire, the acid gathering itself
up into small drops at regular distances apart. When

Fig. 42,

a current of electricity of sufficient strength was sent
through the wire it was heated red-hot, and the acid was
vaporized in dense fumes that, on account of its great
density, sunk down toward the bottom of the tube,
making so many gaseous strings hanging from the wire.
These, of course, were subject to the motions of the
air in the tube, and when the other end of the tube was
sounded by wind from a bellows, the free end partook
of the vibrations. The motions were then observed
through a revolving stroboscopic disk, described further
on. Not only the swaying of these gaseous threads
was observed, but some of the Lissajous's curves were
seen.

I think it highly probable that the motions of the air
in such a sounding-tube can be shown to an audience,
by having the tube with glass sides filled with dense
smoke, and a strong beam of light converged in it, and
having the stroboscopic disk so placed that the focus of
the lens would be in the holes, and so permit a large
amount of light to be used. Where the node was
formed no movement would be visible ; but by giving
the disk a suitable velocity, at any other place than the
node, the vibration might be shown in its different
phases.

ACOUSTICS, 67

LISSAJOUS'S CURVES.

The optical method of studying vibrations is attract-
ive to old and young, to students of science, and to
musicians ; but the apparatus generally used is so
costly that not many can afford to purchase it. The
following directions will enable any one to have a pair
of the tuning-forks made at the nearest blacksmith's
shop, that will be found even more satisfactory for pro-
jections than the more costly ones.

Choose a piece of steel that is an inch broad, one-
fourth of an inch thick, and about four feet and a half
long. Have it made into two large tuning-forks, one of
them to be about fifteen inches long, and the other
twelve inches. Let the tines be two inches apart, and
the flat sides should face each other on each fork. A
stem may be now welded upon the bend ; it should be
about five-eighths of an inch in diameter, three or four
inches long, and made of round steel. When one of
these forks is struck in the manner of common tuning-
forks, it will be seen to vibrate through quite a large
arc, and will continue to vibrate perceptibly to the eye,
for half a minute or more. If, while the fork is vibrating
the stem be held upon a table or floor, or some other
resonator, a deep sound will be heard, and the larger
one will make about fifty vibrations per second, while
the shorter one will probably make seventy or seventy-
five vibrations per second. A stand will be needed for
each of these, and may be made by mortising a post
three inches square, and three or four inches high, into
a board eighteen inches long and ten inches wide
(Fig. 43). This post should have an inch-and-a-half

68

THE ART OF PROJECTING.

hole bored through
it lengthwise, s o
that a smooth stem
may freely turn in
it. This stem
must have a large
head upon it, thro'

which is bored a

-^'*^* ^^' hole to receive the

stem of the fork. Set-screws should be provided, to
fasten the stems in their proper places. These sup-
ports might be made of cast-iron, in which case they
would not need to be nearly so large.

Next make four slides of iron, an inch and a half or
two inches long, and bent so as to
slide upon the fork and be fixed with
a set -screw where it is wanted.
These are for loading the forks and
making them vibrate slower, as they
are nearer the ends.

Lastly, each fork will need a small
mirror fastened to its end. The
Tig. 44. small, round pocket mirrors, about

an inch in diameter, I have found to answer well ; but
care should be taken, in selecting these glasses, to get
plain mirrors. Most of these small ones are on poor
glass, and will spread a beam of light over a large
space. These mirrors may be fastened to the end of
the fork with the cement known as marine glue, and
will adhere strongly enough for all careful work ; but
sometimes these are fitted with a screw in the back, and
screwed into a tapped hole in the end of the fork.

A still better way to fasten this small mirror, is to
cement to its back a piece of rubber as long as the

ACOUSTICS, 69

breadth of the fork, a quarter of an inch thick, and half
an inch broad, this to be cemented to the end of the
fork. The fork will not vibrate at all with this attach-
ment at first ; but if a thin wedge is cut out from each
side of the rubber, until it moves very freely, the vibra-
tions of the fork will not be much interfered with ; at
the same time the amplitude of the vibrations will be
much increased.

When the mirror is fastened to each fork, it will be
advisable to determine their pitch, which may be done
by comparing them with a properly-tuned piano, organ,
or another tuning-fork with known pitch.

EXPERIMENTS WITH THE FORKS.

7", The Sinuous Line. Cut off most of the light from
the lantern or porte lumiere with a diaphragm, so that
the beam is not more than an inch in diameter and
consists of parallel rays. Adjust the fork so that it

^'ig. 4:5.

will vibrate perpendicularly, and place it so that the
beam of light will fall upon the mirror at its end.
This should be again reflected to the screen by a mir-
ror tn held in the hands, to swing the beam around the
room. When the fork is made to vibrate by striking it
with a small billet of wood, if the mirror ni is held still,

70

THE ART OF PROJECTING.

a band of light will appear upon the screen, three to
five feet long, depending upon the amplitude of vibra-
tion and the distance to the screen. If now the mirror
m be turned so as to swing the beam at right-angles to
the band of light, a long sinuous line of light will be

xAAAA,

Fig. 46.

spread upon the wall. It may be seen to be forty or
fifty feet long if the mirror be moved fast enough. At
the time the fork is struck attention may be called to
the sound. If two beams of light, about half an inch
apart, and one above the other, be made to fall upon
the first mirror while it is vibrating, and the mirror m

Fig. 47.

(Fig. 45) be moved as before, two undulating lines will
appear, one above the other (Fig. 47), with phases ex-
actly corresponding. Let the two beams of light be

Fig. 48.

brought side-by-side and they will appear to have op-
posite phases (Fig. 48), and will show as beautiful in-
terlacing lines. A double image prism put in the path

ACOUSTICS. 71

of the beam just in front of the fork, serves well to
give this double line of light.

//. Overtone. If the fork be struck about midway of
its length, a much higher sound will be heard along
with the fundamental. Let the mirror be turning when
the fork is struck, and the large sinuous line seen be-
fore will now be seen covered with ripples due to the
overtone.

III. Interference. In the place of the mirror at m,
place the second fork so that the beam of light from
the first will fall upon it, and be reflected to the middle
of the screen, having both forks to vibrate perpendicu-
larly. Now load the shorter fork with slides until it is
nearly in unison with the long fork. When they are
both made to vibrate, the line of light upon the screen
will be seen to lengthen and shorten with regularity ;
at the same time beats will be heard corresponding with
the lengthening of the line. These beats may be made
to vary in frequency by moving the slides. If the beats
are as many as five or six a second, or more, and the
second fork be swung upon its vertical axis, the inter-

rig. 4 'J.

ference may be noted (Fig. 49) ; the swellings corres-
ponding to the periods of coincidence, and the con-
traction to the periods of interference.

If the forks are now brought to unison and struck,
the resultant figure will depend upon their relative
phases. If they have like phases, so that each one
reaches its limit at the same instant, the line of light
upon the screen will be much elongated, the amplitude

72 THE ART OF PROJECTING.

being equal to the sum of the two amplitudes. If their
phases are opposite, so that one reaches its upper limit
at the same instant that the other reaches its lower
limit, then the spot of light will not be drawn out into a
line at all, but will remain quiescent. These various
relative vibrations can only be obtained by trial, but
usually in four or five strokes one will develop such a
phase as he requires.

IV. Resultants, Keeping the two forks in unison,
turn the second fork so that it vibrates horizontally.
Adjust the light so that it falls upon the second mirror
as before, and thence to the middle of the screen.
Now, if both forks be struck, the resulting figure may
be a straight line, an ellipse, or a circle depending

Fig. 50.

upon the phase of the first fork when the second one
begins to vibrate. Fig. 50 represents these unison
forms. By moving one of the slides so that the fork is
not quite in tune with the other, the figure will move
through each of these phases alternately. When the

Fig. 51.

circle is obtained upon the screen, swing the second
fork through a small arc, and the circle will be drawn
out into a luminous scroll, (Fig. 51). If the forks are

ACOUSTICS,

73

not quite in unison, the same experiment will give the
scroll of irregular amplitude, (Fig. 52).

Fig. 5ti.

Remove the slides from the short fork and fix them
upon the long one near the end, and, if necessary, at-
tach two pairs, and adjust them so that the ratio of vi-
brations is as 2 to I ; that is, their pitch is an octave
apart. The resulting figures are shown in Fig. 53 ; and

Fig. 53.

when the forks are tuned exactly, the figure first de-
veloped will remain, with no other alteration than a
decrease in size, and may be a parabola, an 8> called
a lemniscata, or an intermediate form.

While this figure 8 is upon the screen let the second

J ly

fork be rotated through a small arc, as before with the
unison, and the scroll shown in Fig. 54 will appear.

By trial the slides may be so adjusted upon one of
the forks that all the varying ratios in the octave may
be obtained. The simpler the ratio the simpler the

74

THE ART OF PROJECTING.

Fig. 55,

figure, and such ratios as 2 to 3 (do to soi), and 3 to 4
{do to fa), may be known by their representative figures,

Fig. 56,

55 and 56. The ratio i to 3 {do to sol, in next octave,)
will present such forms as those in Fig. 57.

Fig. 57.

In any case, the figure will remain constant when the
ratio is exact, and the ratio may be known by counting
the number of loops upon the top and one side. Thus,
in the fully developed figure^ with the ratio 2 to 3, there
may be counted two loops upon the top and three loops
upon the side, which indicate that the fork that vibrates
horizontally makes three vibrations, while the other one
makes two.

The overtones may be developed and exhibited upon
each of these compound forms by striking upon the
fork rather lightly, about midway of its length, while it
is giving any particular figure. Thus, if the forks are
in unison and a circle has been obtained, the overtone

ACOUSTICS. 75

developed will cover the circle with ripples which ap-
pear to move around it.

For the exhibition of the Lissajous curves with such
forks as have been described, it is not necessary to use
a lens, but the whole light from the porte lumiere may
be allowed to enter the room, and the first fork placed
with its mirror in the middle of the beam. If, however,
it be desirable to admit less light into the room, a dia-
phragm may be used that admits a beam only an inch
in diameter or less. A lens may be used which will
concentrate the light upon a much smaller space, mak-
ing a much brighter spot, but will very much reduce the
size of the figures. When a lens is used, it must be so
placed as to project the mirror upon the second fork.
Its focal length should be two feet or more.

All of these phenomena can be shown by means of
a lantern, — even an oil lantern will answer. It will be
found best to use a beam of parallel rays, which may
be used in such a lantern as is represented in Fig. 26
by simply removing the front lens of the condenser.
With other lanterns it will be necessary to remove the
objective, and push forward the light until the beam
emerges with parallel rays : then, with a diaphragm cut
off all the light except a beam of the size of the mir-
ror upon the forks. The conditions are then the same
as with sunlight, and a lens may or may not be used.

SYMPATHETIC VIBRATIONS.

Let the two forks be brought to unison and at right
angles, so as to give, when struck, one of the forms of
Fig. 50. If now, but one of the forks be struck, the
straight line due to its vibration will slowly swell into
an ellipse, which will be due to the absorption by the
second fork of the vibrations of the first. This may

76 THE ART OF PROJECTING^

be demonstrated by changing the pitch of one of the
forks, when no change of form of the projected beam
will be observed. One of the conditions for the suc-
cess of this experiment is that both forks should rest
upon the same table, in order that the vibrations may
be conveyed through the solid wood from one fork to
the other. The intensity of the sound-wave in the air
is not sufficient to communicate a motion that will be
perceptible. A voice sounding the same fundamental
note as one of the forks, will set it vibrating, as will be
evident by the spot of light upon the screen being
drawn out into a line.

With one of these forks Melde's experiment may be
shown in the most satisfactory manner. Choose a soft
white cord eight or ten feet long (a silk cord is best, though
a cotton twine will work very well), tie one end to the
fork at a and let the other end hang over a hook driven
in the wall at b. Weights varying from a pound to

Fig. 68.

half an ounce or less may be hung upon this free end
of the string, with which its tension may be varied.
The fork may be struck with a billet of wood, as in the
former experiments, when the string will be made to
vibrate, either as a whole, or in equal segments, the
number of which will be inversely proportional to the
stretching weight. The amplitude of these vibrations
of the string will be considerable, and if the string vi-
brates as a whole it may be eight or ten inches, or even

ACOUSTICS. 77

a foot ; and when the number of segments is as many as
sixteen or twenty, they can all be seen and counted by
a large number of persons at a time. If the string
«, h, is twice as long, and may reach back to a^ the
free end may be held in the left hand while the fork is
struck with the right. It will then be very easy to vary
the tension of the cord while it is vibrating, and the
segments can be made to change through its whole
series of one, two, three, four, etc. The various forms
and motions of the cord may be shown to still better
advantage, by making a strong beam of light from the
porte lumiefe or lantern to fall upon it in the direction
of its length.

Crova's apparatus consists of disks of glass about
fifteen inches in diameter, which may be made to turn
upon a suitable rotator. These disks are at first painted
black, and then curves of various forms are traced
through the paint to the glass. The upper part of the
disk is projected in the ordinary way, and then if it be
rotated, the lines which are drawn upon it will appear
to move or to be quiescent, according as they are con-
centric, eccentric, or some other form. If a diaphragm
with a slit in it, long enough to reach across all the
lines which are drawn upon the disk, be placed behind
it, a series of dots will appear upon the screen, which
will change their positions as the disk turns round.

With properly drawn curves the various wave-motions
in air in organ-pipes, reflection of sound-waves, nodes,
mterference, and so forth, as well as the transverse vi-
brations in light-waves, may be well shown.

AN ATTACHMENT TO THE WHIRLING TABLE FOR PRO-
JECTING LISSAJOU'S CURVES.

Two posts / and /' are made fast to the frame upon
the opposite sides of the inertia plate a. A small

78

THE ART OF PROJECTING.

wooden pulley j, about an inch in diameter, is made to
turn upon an axis that is made fast in the post/, and
with such adjustment that the pulley rests upon the

Fig, 59,

plate a and turns by friction on that plate. It is best
to have a thin india rubber ring upon the friction pulley
to insure it from slipping. Above the pulley the mirror
m is so mounted as to swing in azimuth, and is made
to do this by a wire fastened to it
at its hinge, and bent into a loop /
at its lower end, which is opposite
the face of the pulley s. Another
twist in the wire at o will be needed,
for a pin which is fast in the post
/ ; this will make a lever of the
wire /, with the fulcrum at o, and if
it is properly fastened to the hinge
Fiff. 60. q£ |-jjg mirror will cause it to vibrate

in a horizontal plane when the plate a revolves.

ACOUSTICS. 79

A somewhat similar arrangement is made for the
other side, save that the friction pulley / has its bear-
ing made fast in a separate piece r, which is so fastened
to the end of a long screw d that the whole fixture can
be moved to or from the centre of the plate a. The
piece c is furnished with two guides, which keep it
steady in any place where it is put. The mirror m' is
made to tilt in a perpendicular plane by an arrange-
ment quite similar to the former one, save that the wire
connection has its lower end bent into a horizontal
loop, through which a pin in the face of the pulley / is
thrust This is practically an eccentric, and, being
directly fastened to the hinge of the mirror m\ gives to
it an angular motion proportional to the distance of the
pulley face-pin from the centre. The mirrors should
be not less than two inches square. If then the pin is
an eight of an inch from the centre of the friction pul-
leys, they will have ample angular motion ; much larger
than can ever be got from forks.

Experiments. ^It is evident that if the two friction
pulleys have equal diameters, and they are at equal dis-
tances from the centre of the plate a, they will vibrate
in unison in their respective planes. Now let a beam
of light r, from the porte lumiere, fall upon the mirror
m at such an angle as to be reflected first upon the
mirror w', thence to the screen. If the plate a is now
revolved, the beam of light will describe a circle, an
ellipse or a straight line, either of which can be made
at will by simply adjusting the crank of one of the
mirrors to the required angle. Thus, suppose the
mirror ni is tipped back its farthest by bringing the
pulley pin at the top, as indicated in the drawing, at
the same time that the mirror m is at its maximum an

8o THE ART OF PROJECTING,

gular deviation. The beam of light will describe a
circle.

If it moves slowly, the path and direction of the
moving beam can be nicely observed. These two ad-
vantages are not to be had with forks ; for, first, it is
accidental if one gets a circle or any other desired re-
sultant figures from forks in unison, for the obvious
reason that the phases cannot be regulated, and second,
the vibrations of the fork are so rapid that the analysis
of the motion can only be made in a mechanico-mathe-
matical way.

By moving the fixtures on the left side toward the
centre of the plate a, the pulley / will not revolve so
fast. If moved half-way it will make one revolution
while the other makes two, and the vibrations stand in
the ratio i : 2 represented by forks in octave. Such
ratio is shown upon the screen by a form very much
like the figure 8, and known as the lemniscate.

Between these two places, every musical ratio in the
octave can be got, and the resultant motions projected
in their proper curves. More than that, while the mir-
rors are both vibrating^ any of the ratios desired can be
moved to at once by merely turning the thumb screw dy
which is wholly impossible with any forks which require
stoppage and adjustment of lugs for each different
curve.

Again, if the fixture c is moved still farther toward
the centre than half-way, the curves projected will be
those belonging to the second octave, until the pulley
reaches three-fourths of the way, when the ratio will be
1 : 4, and the resultant figure will be like a much-flat-
tened double eight.

If one would show the phenomenon of beats, it will
be necessary to have the mirror m and its attachment

LIGHT. 8 1

so adjusted as to have it vibrate in a perpendicular
plane like m' . This can be done by fixing its hinge at
right angles, and the rest the same as for mirror m\
The reflected beam from the second mirror may be
received upon a large mirror held in the hands, and
thence reflected upon the wall or screen.

LIGHT

RECTALINEAR MOVEMENT.

Tnat light moves in straight lines can be shown by
admitting the light from the porte lumiere through a
small hole. It goes straight across the room, and its
course can be tracked through the room by the dust
particles, or a little smoke, which it will light up. Also,
by having the room otherwise quite dark, permit the
light to come in the round orifice, half an inch in diam-
eter, as it is reflected from the landscape outside, and
not reflected from the mirror. The room is thus a large
mmera obscura, and an inverted image of the landscape
will be seen upon the walls, or upon a small screen held
a foot or two from the orifice. This image will be par-
ticularly strong if the ground be covered with snow, as
much more light is reflected from that than from grass
or foliage. If persons are passing their forms will be
seen, and appear as if walking head downward.

Parallel rays A will be reflected from the mirror of

82 THE ART OF PROJECTING.

thQ porte lumiereyVfh.i\Q converging b and diverging c rays
will be obtained by interposing a convex lens of any
size in the path of the parallel rays.

Fig. 61.

Transparent substances, like glass, some crystals,
gases, and water permit the rays a to go through them
and appear upon the screen. Translucent substances,
like paper, ground glass, milk, allow but a few scattered
rays to go through them, and a diffused light appears
on the screen. Opaque substances, such as metals,
thick pieces of wood, stones, etc., stop all the light,
reflecting some and absorbing the rest.

INTENSITY OF ILLUMINATION.

When the lens is interposed in the path of the beam
the light appears as a circular disk upon the screen,
and as the rays cross each other at the focus f^ that
point may be considered as the source of light. Cut a
sheet of paper or a board s, one foot square, and hold
it any distance from the focus, say two feet. Its shadow
upon the screen will be bounded by «, e, which may
be measured in square feet. Now move the paper
to /, twice as far from the focus, and again measure
the shadow b^ d^ it will be but one -fourth the size
of the other, proving that at s the paper received

LIGHT, , 83

four times as much light as it did at /. Hence the in-
tensity of light varies inversely as the square of the
distance. Other measures with other distances can be
made for confirmation : a good exercise for scholars.

Fig. G,i.

When a lantern must be used in place of sunlight, it
will be necessary to remove the objective and move the
light backward from the condenser until a sharp focus
is produced in front, and then work in front of that ; or
still better, remove both condenser and objective, the
outlines of shadows will be quite well defined with the
electric light, and witTi the lime light, but not with any
oil light.

REFLECTION.

The reflecting power of various surfaces can be
shown by holding them in the path of the beam from
the reflector. Common mirrors, plain glass, colored
glass, metals polished and unpolished, woods, horn,
polished stones, paper, will all exhibit difference in this
property.

Reflection from the two surfaces of glass is seen
upon the screen when the parallel rays from the first
mirror reach it. Then will always be seen two or
three indistinct images of the sun, side by side. When

^4 THE ART OF PROJECTING.

the sun is near the horizon, so that the porte lumiert
is nearly horizontal, more of these reflections will ap-
pear, due to multiple reflections upon the surfaces of
the mirror. These can be magnified a good deal in
the following way. Place the lens o at about its focal

Fig. 61.

length distant from the orifice, and then hold another
plane mirror r so that it will reflect the beam upon an-
other screen s, moving the mirror r to such a place as
to project the image of the orifice. It will be seen to
be double, and when the images overlap, the light will
be much brighter. Multiple reflections from the two
surfaces of the mirror r may be seen by holding it at a
small angle to the beam of parallel rays. A piece of
plate glass two or three inches square answers for this
experiment.

That the reflected beam moves through twice the
angle of the incident beam, may be shown by holding
the mirror r in the beam without the lens o. If the
mirror be perpendicular to the beam, the light will be
reflected back through the aperture ; turning the mirror
slowly when it is 45° to the incident light, the beam
will be overhead 90° ; when it has been turned 90°, and
is now in the plane of the beam, the reflected part will
have moved through 180°.

LIGHT. 85

Pepper's Ghost is but a reflection from the surface of
unsilvered glass. His fixtures were made upon a large
scale, were costly, and not practicable in every place.
His reflectors were large sheets of glass about five feet

'i<?r

broad and six feet high. The light was a powerful
lime light. Fig. 62 will give an idea of the conditions
employed last year in his traveling lectures. The front
of the stage s s was heavily curtained, except a space
of a few feet in the middle of it, where there was a
recess opening back, and apparently to the back of the
stage c, which could be seen through a large plain glass
reflector ^, twelve or fifteen feet long and six feet high,
placed at an angle of about 45°. This glass is seldom
noticed unless one is looking for it. The lantern for
illuminating the ghost b is behind the curtain on the

86 I HE ART OF PROJECTING.

right, and throws a powerful beam upon it. It being
dressed in white, a good deal of the light is reflected
from it in all directions, and a part of that which falls
upon the glass at r will be agam reflected toward /,
when it will appear as if it came from <r, as far back of
r as ^ is front of it. All of the lights in the room are
turned down except that in the lantern, and none of
that is permitted to find its way into the room save
what is reflected from the ghost. There is black cloth
for absorbing the light back of b. The person who
holds conversation with the phantom is at d^ but of
course he cannot see what those see who are at /, or
near that line, and all his movements are guided by his
knowledge of the necessary position of the reflection.
In his book, Cyclopaedic Science Simpiijied, Professor
Pepper has given several methods for showing such
spectra. The skeleton, the talking head, and others
are thus exhibited.

The extensiveness of the preparation for exhibiting
the ghost will prevent most experimenters from at-
tempting it ; but if the teacher would care to show the
principle, he will find the following a cheap and effect-
ive one, which he can extemporize with what materials

lig. 63.

LIGHT. 87

he is likely to have at hand. The beam of light from
the porte lumiere is directed upon the object 0, which
should be a small one : a doll dressed in white, or even
the outline of one cut in white paper. The light from
it will of course be scattered from it in all directions.
A pane of white glass r will receive some of these rays,
and reflect them toward s, where they will appear to
come from d . If the object ^ is a puppet or a moving
figure of any sort, it can be made quite a good phan-
tom, though diminutive. The glass r can be moved so
as to give every one in the room a view of the phenom-
enon, while the hand put up to 0' will reveal the shad-
owy nature of what is seen.

Of course all extraneous light should be shut out by
having the window curtains tightly drawn, and also
with black cloth about the apparatus to absorb all the
scattered rays, especially about o and 0' .

Obviously, a lantern at / could take the place of the
sunbeam, but the light needs always to be a very strong
one, for but a fraction of the light is reflected from the
object, and this is again largely reduced by transmis-
sion through the glass; nevertheless, as the light is
used at the distance of but a foot or two from the ob-
ject, it can be lighted sufficiently well for a small room
in the night with an oil lantern like Marcy's Sciopticon.

8S THE ART OF PROJECTING,

THE KALEIDOSCOPE.

The very great beauty and variety of the forms seen
in the kaleidoscope makes them very desirable objects
for projection. The following method will be found
efficient :

I St. With porte lumiere.

Fig. 66.

The condenser {: may have a focus for parallel rays
from a foot to eighteen inches or more. Choose an
objective o, with focal length of eight or ten inches. It
does not need to be more than an inch or two in diam-
eter. Now cut two strips of looking-glass two or three
inches broad and an inch shorter than the focal length
of the objective. These may have the same breadth
throughout, or they may taper to an inch broad at the
outer end, as shown in the picture. They may now
have their long edges brought together on one side
and inclined to each other forty-five or sixty degrees,
and secured there by enclosure in a tube ; or, if it be for
temporary use, they may be held in place by a retort
clamp and work just as well. The condenser V may
now be placed close to the orifice and its focus will
then be at some place o. Bring the fixed reflectors
wdthin the converging rays, so that they will receive the

LIGHT, 89

focus just within their outer and narrower end, and at
the same lime be so inclined that the light falls upon
the surfaces of the glasses from the broader end, as
shown above. Everything now depends upon the ad-
justment of the light to these reflectors.

When properly placed, there will appear, high upon
the screen, the sectors of the polygon equally illumi-
nated : six of them, if the reflectors are sixty degrees
apart, and eight, if they are forty-five degrees. No
direct light should fall upon the screen, and will not, if
the end of the reflectors be kept high enough to cover
the focus of the condenser. A few minutes' work with
this will enable one to find the proper position for the
best effect.

When the sectors appear equally illuminated, the
objects to be projected may be placed between the
condenser and the reflectors, — the fingers moved
about, a pencil, a key, a comb, or a strip of paper
with pins in it, or a leaf of a plant, perforated paper,
or the common glass trinkets which are usually put in
kaleidoscopes. If the objective lens be put close to
the outer end of the reflectors, the objects shown will
have a much sharper outline. For the best chromatic
effects flat pieces of colored glass will be found better
than round ones, as they transmit much more light,
but an assortment of the two will make a fine ap-
pearance.

The common kaleidoscopes, which are so abundant
in the market, can be used for this work by removing
the ground glass in front of them and substituting a
piece of plain glass. These are generally provided
with a small lens, which will answer for an objective,
but at the end for the eye there is seldom quite room
enough to permit light to pass in sufficient quantity for

90 THE ART OF PROJECTING.

good illumination. By removing the objective the dia-
phragm of black paper can be removed. As the
objects are all magnified so much, it will be found that
quite small bits of colored glass will look better than
large ones.

2d. With a lantern.

It will be observed that the essential condition for
showing the kaleidoscope with the porte lumiere is,
that all the light that reaches the screen must be the
light that is reflected from the inclined mirrors, and
that the focus of the converging beam must fall just in-
side the outer end of them. Hence the focus needs to
be as small as possible for the best effect. With the
lime light the focus is quite broad at its narrowest
part ; therefore when the kaleidoscope is placed in the
beam it will be necessary to adjust the light by raising;
it, so that the reflectors receive all of the light, and it
also may be necessary to draw it back a little that the
focus may come to the proper place.

The ordinary objective upon the lantern will not
be needed, of course ; but an objective having a focal
length equal to or a little longer than the length of the
kaleidoscope may be used, holding it in a retort
holder or in any other convenient way. Let as much
as possible of the extraneous light be excluded from
the room by black cloth about the front of the lantern.
With these precautions a very good projection of ka-
leidoscopic forms can be shown. Even with the better
forms of the oil lantern it is possible to project them
quite well. As the diameter of the disk is doubled
with this fixture, it will be necessary to move the
lantern much nearer to the screen.

LIGHT.

91

CONCAVE MIRRORS.

Fig. 67,

Concave mirrors, sufficiently good for demonstration,
are fitted to wall lamps, or the reflector generally fitted
to oil lanterns may be used. Such a one held in the
path of the beam from the porte lumiere will reflect
the rays to a focus, where there will be sufficient heat
to ignite wood, paper, etc. If the mirror be tipped,
the beam, after passing the focus, will diverge and
cover the whole ceiling, as the focus is quite clo§e
to the mirror.

Fig. 68.

The image formed in front of the concave mirror
may be seen by letting a strong light fall upon the
object and having the mirror above it, as indicated.
If the object 0 be inverted and hidden otherwise from
view, it will appear upright ; and at 0', to one standing
in front of the mirror, all in a room can be made to
see it by turning the mirror a little, so it will face them.

92

THE ART OF PROJECTING.

A small bunch of flowers, a statuette, the hand, etc.,
are good objects to exhibit this property.

Images can likewise be projected upon a screen by
means of the concave mirror.

Fig. 69.

At a distance six, eight, or ten feet from a screen
hold a lighted candle close in front of the mirror ;
slowly separate them : the image of the light will
appear inverted upon the screen, and much enlarged.
Advantage is taken of this property of the concave
mirror to project some phenomena, such as manome-
tric flames, etc., which see.

CAUSTICS BY REFLECTION.

A concave polished surface, like a strip of tin, two
or three feet long and an inch or two wide, bent into

Fig, 70.

LIGHT. 93

an arc of a circle or any other curve, held in the
divergent beam of light, as shown in the figure, and
resting one edge upon a white wall or a piece of
white paper, will exhibit fine caustic curves, which
will change as the strip is more or less bent. The
brighter the surface that reflects, the brighter will the
curves c c appear. Large rings, silvered and polished
on the inside, are sometimes used for this ; but a strip
of tin will answer well.

CONVEX MIRRORS.

The back of a concave mirror, such as already
mentioned, forms a very good convex mirror. Hold
that in the beam of light, in the same way as the con-
cave mirror was held, and note the result. Objects of
any size are usually much distorted when seen by
reflection in a convex mirror, as witness your own
countenance when looking into one.

These distortions can be projected, though with
much loss of light, by strongly illuminating the object
0, and with an objective focus the reflection upon the

Fig, 71-

screen. In this way very humorous distortions of the
human countenance may be photographed by using
the camera at c.

94

THE ART OF PROJECTING,

TOTAL REFLECTION.

This phenomenon is generally shown by properly
directing a beam of light into a vessel of water.
Perhaps the simplest way is to fill a glass beaker with
water, containing a little milk or a little magnesia
stirred into it for the purpose of enabling the eye to
trace the course of the light through it. Next placing
the beaker in a convenient place, with a bit of looking-
glass direct a small beam of light upwards through
the side of the vessel, so as to strike the under surface
of the water. By trial, the proper incident angle will
be found at which the light will not emerge from the
upper surface of the liquid, but will be totally reflected ;
the course of the beam will be easily traced through
the milky fluid.

With suitable arrangements, very striking and beau-
tiful effects may be produced in a stream of water.

Fig* 70,

The high tanks made for showing the direction and
form of water jets are generally made with a glass
window opposite the orifice H, through which a beam

LIGHT, 95

of light from a lantern or from the sun may be directed
while the water flows. For the success of this experi-
ment it is necessary that the orifice should be round,
smooth, and thin, and the body of water in the tank
must not be disturbed by currents. In the figure,
water is admitted at F, while at G there is a partition
with a good many orifices in it through which the water
flows, keeping it at a constant height I. When, there-
fore, the light is concentrated upon the orifice H, it is
not scattered, but lights up the whole of the curved
stream, giving it the appearance of molten silver. If
colored glasses are interposed back of E, D, the color
of the stream will also correspondingly change, with
very pleasing effects.

Fig. 71 represents still another form of this experi-
ment, in which the vertical attachment to the lantern
is used. A vertical fountain jet is opened in the
ascending beam from the lantern. The falling water
is beautifully illuminated. Plates of colored glass
may be used, as before.

MIRAGE.

Direct the beam from the/(?r/<? lumiere, so that it is
horizontal or nearly so. Put in a diaphragm with a
hole about half an inch in diameter, or less, as the
first condition is to have a small beam of parallel rays.
No lens will be needed. Next heat a brick, or, still
better, a poker or any convenient piece of metal that
is a foot long or more, until it is nearly to a red heat ;
then place it just in front of the diaphragm and parallel
with the beam and about a quarter of an inch below
it or to one side of it. The current of heated air will
so deflect some of the light as to very much elongate
the bright spot upon the screen, or even present
another one some inches distant from the first.

96

THE ART OF PROJECTING,

Fig, 71^

LIGHT,

97

REFRACTION.
1.— Of Glass.

Fig. 72.

Project any object that is three or four inches long, —
a lead pencil or an arrow cut out of paper. A single
lens is all that is needed. Then hold in front of the
object a piece of glass three or four inches long,
half an inch broad, and the thicker the better. If the
glass is held exactly perpendicular to the beam of light
no refraction will be observed \ but turn one end of it
towards the opening, and at once the picture upon the
screen will appear as if a piece of the object had been
cut out and was held to one side of it. The thicker
the glass is the greater will be the displacement ; but a
piece that is an eighth of an inch will quite likely make
as much difference as the thickness of the object pro-
jected.

Two pieces of glass may be put together and held as
before, or turned in various directions with reference
to each other and the object.

2 — Of Watkr.

A hand mirror held at r will reflect the light down-
ward into the chemical tank (Fig. 73), which should be
filled with water in which a little finely powdered resin
7

98

THE ART OF PROJECTING.

has been stirred to give a turbidity to it, as the beam
will be traced much better ; a few drops of milk or
of chalk-dust will answer the same purpose. A little
smoke in the air will serve to mark the course of the

T\g. 73.

beam till it reaches the water, where its direction will
be seen to change, becoming more perpendicular. Re-
flect the beam upon the water at various angles of
incidence and mark the course of the refracted rays.

In place of the small tank take a beaker, or any
vessel with square glass sides, and reflect the beam
upon the surface of the water as before.

The experiment may be varied by filling the vessel
half full of water, then carefully pouring strong alcohol
upon it to the depth of an inch or two, being careful
not to mix them while pouring, and some ether upon
the; alcohol in the same careful way. Sending now the
beam through them as before, the different refractive
powers of the various liquids will be seen.

Heated Air.

Fig. 74,

LIGHT.

99

In the diverging beam from the lens hold a
heated poker or any well heated body at a. The
air that is heated in its neighborhood will so deflect
the rays of light as to make quite a fine appearance
of heat upon the screen. An alcohol lamp lighted
and held there will have its whole projection upon
the screen : the lamp and flame in outline shadow,
while the heated gases rising make an interesting
picture.

To present a still more striking case, project a large
solar spectrum, as described on another page, and about
half-way between the prism and the screen hold the
lighted alcohol lamp, moving it slowly along in the
different colors of the spectrum.

4 — Lenses.

The refractive power of difTerent forms of lenses
may be shown by holding them in the beam of
parallel rays. The refractive power of a lens of
water is seen by taking two
watch glasses or one watch glass,
and a piece of plain glass a little
larger, and bringing the two to-
gether under the surface of water.
The space between these will be
quite filled and they will adhere
tightly enough for the experi-
ment.

Bifj.

The same thing will also be exhibited by placing a
watch glass upon the upper ring of the vertical attach-

lOO THE ART OF PROJECTING.

ment (Fig. 27). When water is poured in to fill it,
the light will be refracted and an object may be pro-
jected with it as with any other lens.

Fig. 76.

The formation of images by lenses is well shown by
holding a lighted candle or lamp in front of the screen
at any distance in a darkened room, and bringing the
lens close to the light, then moving it towards the
screen until the inverted image appears. Try this
with double convex and with plano-convex lenses of
different focal lengths, also with a meniscus and with
a concave lens.

THE SOLAR MICROSCOPE.

This has been described on a former page, and may
be turned to ; but, as nearly all of the art of projection
•depends upon the use of lenses, it will be well, in
giving instruction to dwell upon the conditions for
forming images with single and with compound lenses,
with parallel converging and diverging beams. The
_porte iumiere, the magic lantern, the solar microscope,
the telescope, may be illustrated by methods that have
been already explained.

THE RAINBOW.

This phenomenon in nature is due to refraction and
reflection in drops of water. It is hardly practicable

LIGHT. lOl

to project a rainbow with an artificial shower, although
it can be done by having the beam a widely diverging
one, to fall upon spray from a small fountain that
spreads a thin sheet at right angles to it ; but a bow
that will rival the natural one in the sky may be pro-
jected by using two lenses with short focus, such as
are put into lanterns for condensers.

mg, 77.

Place the first lens in position, as if for common
projection. If the second lens be now brought near
the focus of the former and slowly moved towards the
screen, a luminous disk will appear upon it, having a
red border. Let this disk be made as large as is
desirable, which can be done by moving the lens back-
ward or forward. Now cut a piece of paper ;-, with
a round top a little smaller than the diameter of the
lens, and place it at r. All the light from the lower
part of the screen will be cut off, and nothing will be
left but a bow, with the colors in the same order as
those in the primary bow, and very brilliant. It may
be enlarged to twenty or thirty feet.

A second method requires a conical prism ; and, if
this is not already possessed, one may be made by
taking a thin, clear, glass funnel, with a mouth three
or four inches in diameter. Cut a piece of plain white

I02 THE ART OF PROJECTING.

glass a little smaller than the mouth of the funnel, and
fasten it there with wax or putty ; break off the stem ;
put the prism under water, and when it is filled stop
it with a cork : it is now ready for work.

Next cut out a semicircular slit from a piece of paste-
board. It need not be more than the one sixteenth of
an inch broad, and it may be an inch across, as repre-
sented at A, Fig. 78.

Place the condenser in front of the aperture, and
hold the prism near the focils, and then a little distance
back of it may be placed the pasteboard with the slit s.
The semicircular diverging beam is refracted, and
suffers dispersion ; which gives a very good bow, but
with the red innermost, like the secondary bow.
- Still another way is possible: If a beam of light
fall upon a cylindrical reflector, like a glass rod, or
even a tin tube, like the handle to a tin dipper, the
light is reflected from it in a large nearly complete
circle. Place such a reflector at s above, in place of
the pasteboard slit, and then with the prism a bow will

LIGHT.

103

I04 THE ART OF PROJECTING.

appear, as before. The prism may be an ordinary one,
but the bow will not then be perfect for a semicircle.
The size of this bow will depend upon the size of the
prism, for the ring of light can be indefinitely enlarged
by varying the angle of incidence of the beam.

With a curved slit cut about the size of a common
transparency and projected with the lantern, holding
a common prism in front of the objective, a very good
bow is seen. As the refraction bends the rays down-
ward, it will be necessary to tip the front of the lantern
up considerable in order to get the bow upon the
screen. (Fig. 79.)

Lastly, let a beam of parallel rays, about an inch in
diameter, fall upon a glass sphere filled with water, —
an ordinary small glass flask answers well, but the
larger the flask the greater should be the size of the
beam. If, now, a small white screen be placed between
the sphere and the aperture in the window, a bow will
be seen concentric with the aperture and arranged so
that the red is outside and the violet inside. At a
greater distance from the aperture another bow will be
formed, much fainter than the first, and with the colors
in the inverse order. • If difierent colored glasses are
interposed in the path of the beam of white light, the
bow will be seen to consist mainly of the tint of the
glass.

CHROMATIC ABERRATION.

Caustic curves, due to chromatic aberration in the
lenses, may be projected by taking two rather large
lenses of short focus, such, for instance, as those made
for lantern condensers.

Place the first one as if for common projections.
The second may be held in the hand and brought near
to the focus of the first and then inclined, as shown in

LIGHT. 105

the figure. Moving it towards the screen, beautiful
colored figures will appear, which will change with the
angle of the second lens to the light that is incident
upon it. A comet, a hollow funnel, a mock sun, and

Fig, 80,

other curious forms may be projected, all of them
brilliantly colored.

With a lantern, it will be sufficient to remove the
objective and place a large lens, like one of the above,
near the focus of the condenser, when the same figures
will appear.

DISPERSION

Is usually shown by decomposing a beam of solar light
with a triangular prism. The beam should be a rather
small one, not more than one fourth of an inch in
diameter, if a pure spectrum be wanted. If it be more
than this, there will be more or less white light in the
middle of the band.

±uj. HI.

The smaller the aperture, the purer will be the colors
into which the light has been decomposed ; but if a

To6

THE ART OF PROJECTING,

very small beam be used, the room will need to be
quite dark. When it is desirable to have a large and
brilliant spectrum, the light may be sent through a
condenser, with a focus one or two feet long, and using

^^

— ^

=^

>\

^

V-

H

N

)

Fig. S3,

Fig, 82.

a diaphragm near the focus to cut off the marginal
rays. This will permit much more light to be properly

refracted for a good
spectrum. At a
distance of twenty-
five feet, a common
triangular prism
will give a spec-
trum about five
feet long; but it may be indefinitely lengthened by
inclining the screen to it, as shown above, and it will
usually be quite bright when made ten or fifteen feet
long ; if the room be otherwise, quite dark.

The dispersive power of different substances may be
shown by making a V trough of glass, with an included
angle of sixty degrees. Water, alcohol, ether, spirits
of turpentine, etc., may be put in it, and the beam sent
through them. In this case the spectrum will appear
overhead upon the ceiling. If the glass trough have
three or four tight partitions in it, all the substances
may be used at once, and thus their refractive powers
compared.

LIGHT. 107

COLORS OF THIN FILMS.

Let a soap bubble be held in the beam of diverging
rays, near the focus of the lens (Fig. 74), and in such
a position that some of the light will be reflected from
its upper surface.

As soon as the bubble becomes thin enough, brilliant
colors will appear upon it, which will be reflected to
the walls and ceiling, as they will spread over a
large surface. If the bubble is held quiet long enough,
each of the prismatic tints will appear in turn upon the
walls, and sometimes the series will be repeated.

If the bubble is projected in the way mentioned
upon page 44, three or four of these series may be seen
at the same time.

Instead of blowing a bubble with a pipe, as shown
in that figure, blow a mass of them in the dish con-
taining the solution. Very large masses may be made
and the colors reflected from them in the same way as
above, or with the lantern.

The tension of the bubble film may be shown by
leaving the tube open after the bubble is blown, when
the latter will contract as if it were being drawn into
the bowl of the pipe ; or the bubble may be blown upon
the end of a glass tube
bent twice at right an-
gles, after which the
open end may be put
an inch or two under
the surface of water in
the chemical tank and Fig, 8r>.

projected. The water in the tube will stand below the
level of the water in the tank indicating pressure.

When these colors from thin films appear upon the

lo8 THE ART OF PROJECTING.

screen, pieces of glass of various colors may be inter-
posed between the lens and the bubbles, when dark or
black bands will be seen to take the place of those
colors that have been stopped by the tinted glass.

Yellow light that is nearly monochromatic can be
obtained by interposing a crystal of bichromate of
potash. Let the crystal be a thin and quite clear one.

Colored solutions may be used for the same purpose.

Under the head of Spectrum Analysis other means
for producing monochromatic light will be found, with
colored lights which are appropriate for examining
bubbles.

Bubbles made of common soap-suds will not last
long, and various preparations have been described
for making persistent bubbles, some of which would
last three days.

A piece of glycerine soap about the size of a marble,
sliced and dissolved in water at a iio° Fah., will make
a bubble that will last half an hour. Prof. Cooke gives
the following method for making a still more persistent
bubble : —

" Procure a quart bottle of clear glass, and some of
the best white castile soap (or, still better, pure palm-
oil soap). Cut the soap (about four ounces) into thin
shavings, and having put them into the bottle fill
this up with distilled or rain water, and shake it
well together. Repeat the shaking until you get a
saturated solution of soap. If, on standing, the solu-
tion settles perfectly clear, you are prepared for the
next step ; if not, pour off the liquid and add more
water to the same shavings, shaking as before. The
second trial will hardly fail to give you a clear solution.
Then add to two volumes of soap solution one volume
of pure concentrated glycerine.

LIGHT, 109

Newton's rings.

Choose a piece of white window-glass three or four
inches square, and with clothes-pins or other means
clamp it to the lens with longest focus you have ; a
lens with focal length of two or three feet will answer,
though less curvature is better. Find by rocking the
lens upon the plate with the thumbs where the point of
contact is. This may be seen by a set of rings which
surround it, and which move from place to place when
the lens is rocked. Having found this place where
the rings appear, place it near the focus of the con-
denser having a diaphragm of pasteboard with a hole
in it not more than a quarter of an inch in diameter
just back of the plate. This cuts off most of the
light that would otherwise be scattered in the room, and
prevents the rings from appearing plain. The objective
used may have an inch focus. There will usually
be seen as many as six rings, and the outer ones at
the distance of twenty feet or more may be two or
three feet in diameter. Byinterposing colored glasses
or colored solutions, as with the bubbles, these colored
rings will appear alternately with black rings.

RECOMPOSITION OF WHITE LIGHT.

This may be effected in several ways.

1st. By receiving the decomposed light from one
prism upon the face of another prism like it, but turned
so that the ray will have its original direction.

2d. By a lens. Let the decomposed rays from the
prism fall upon a double convex lens placed so near to
the prism that all of the colors of the spectrum may
pass through it. Bring the screen to the conjugate

no THE ART OF PROJECTING.

focus of the lens, and then the light will appear as a
brilliant white spot. Interpose a piece of colored

glass, and the spot will
at once change its color.
3d. By reflection from
a concave mirror. The
colored rays will be con-
F^O' S6, verged as white light

would be, and appear upon a small screen placed at the
focus as a spot of white light
4th. By reflection from a
series of small mirrors. Let
the spectrum fall upon the
small mirrors, and so incline
them that they will reflect ^^9* 87-

the light to the same place upon the screen or the wall.
5th. By rotating colored disks.
Disks painted with the colors of the spectrum are
sold in the market under the name of Newton's disks.
They are made by pasting sections of colored tissue
paper upon a large, stiff pasteboard disk.

These colors should have the following angular
value : —

Red, 60°, Blue, 55°,

Orange, 35°, Indigo, 35°,

Yellow, 55°, Violet, 60°.
Green, 60°,

This disk may be rotated upon the whirling table,
or, what is much better, a zoetrope rotator, and it will
appear a dusky white. It will be better to have a strong
light thrown upon it while it is turning.

Another good way is to cut disks of properly-colored
papers and make a radial slit in them. When put

LIGHT. 1 1 1

upon the rotator, they can be made to slide by each
other so as to expose a greater or less angle of any
color. By using any two or more of these at a time,
many interesting effects from combined colors can be
exhibited.

One may often find colored stars or rings or other
fanciful designs on posters for advertisements or wrap-
pings on goods of various sorts, which may be utilized
with the rotator in studying color.

fraunhofer's lines.

The solar spectrum as usually projected with around
orifice and common prism, with an included angle of
60°, appears complete, and is often called a pure spec-
trum. If, however, the prism be of flint-glass or, better
still, a bottle prism filled with bisulphide of carbon, it
may be placed in such a position as to present the
absorption lines known as Fraunhofer's.

Fig, SS.

To do this it will only be necessary to place the
prism in the full beam from the porfe lumiere and turn
it so that one side is very nearly parallel with the beam.
A spectrum will be formed containing a number of
dark perpendicular lines known as the C D E F and
G lines. These may be still more marked by placing

112 THE ART OF PROJECTING,

a lens in front of the orifice at about its focal length
distant from it, and placing the prism at its focus, and
inclined to the concentrated beam in the same way as
above. The spectrum will then be very bright and
some lines well marked.

In order to show the Fraunhofer lines to advantage
it is necessary to have the room quite dark ; to use a
very narrow slit and a lens in conjunction with a good
triangular prism of flint glass or of bisulphide of
carbon. The diaphragm containing the slit through
which the light must pass should be placed close to the
opening in order to exclude all the light that is not
directly used.

Fig. 89.

This diaphragm may be made of pasteboard with
a slit cut in it three quarters of an inch long and
the fiftieth of an inch in width ; the edges should be
smooth and parallel. A lens with a focus of five or
six feet is best for sharp definition of the lines, but
one with a focus of only a foot or two may be used to
exhibit the large and more prominent of them. Place
the lens at such a distance from the slit as to project
it sharply upon the screen, at a distance from the lens,
say twenty feet. Then bring the triangular prism dose

LIGHT. 113

to the lens as shown : The light will be deflected and
dispersed, and the screen should now be brought
where the spectrum will fall perpendicularly upon it,
and at the same distance from the lens that it was
before, namely, twenty feet. Turn the prism until the
spectrum has its least deviation, which will be found by
a little trial. The Fraunhofer lines should appear. If
they are indistinct, move both the lens and prism back
or forward in the beam until they are distinct, for it is
now only a matter of focussing.

If the lens has a focus five or six feet distant it will
need to be quite as far from the slit as the length of
its focus, and the screen adjusted as before, but the
lines should appear plainer and in greater number.
With such a lens and a good glass prism the spectrum
should be about five feet long, and with good focussing
the D line should be seen double. These lines may
be seen by a large number by moving the screen edge-
wise an inch or two.

One may use a condenser and converge a large beam
upon the slit. This will make the spectrum brighter
and permit a narrower slit to be used, but the definition
of the lines is not so good as when parallel rays fall
upon the lens. If the object be to project a spectrum
that shall be well defined upon its sides and to show
only the more prominent lines, let the slit be made as
broad as the twentieth of an inch ; a lens with about a
foot focus may be used to project the slit in the ordi-
nary way, and the prism placed at the focus and turned
to its angle of least deviation, which, as before, must be
found by trial. In this way a beautiful and well-defined
spectrum will be produced, which at the distance of
twenty feet would be about five feet long and two feet
broad.

8

114

THE ART OF PROJECTING.

ABSORPTION BANDS.

If a piece of colored glass be held in the path of
the beam of white light before it enters the lens, Fig.
89, a part of the light will be absorbed and black
bands of greater or less breadth will appear upon the
screen. The glass may be held between the prism
and the screen with about the same result. Some
of the pieces of colored glass, which are quite com-
mon, will give very distinct absorption bands. It will
be well to try red, yellow, green, blue, and violet
glasses. If the color is very deep a greater width
must be given to the slit else the spectrum will be seen
with difficulty.

The chemical tank (see page 34) may be used to
hold solutions of various kinds in this place. A wedge-
shaped tank is also very convenient, as it enables one

Fig, 90,

to pass the light through any required thickness of a
solution, and thus to note the effects of thickness upon
absorptive action. This tank may be made five inches
long, four inches broad, and an inch thick at its broad
end. A piece of thick rubber cut as in the figure will
answer for bottom and edges of this tank.

LIGHT. IIS

Each end being bent up at right angles, the glass
may be bound to it by clamps, as in the other tank.

** A solution of alizarin in carbonate of potassium
or sodium, or in ammonia, exhibits a spectrum having
a band of absorption in the yellow, another narrower
one between the red and the orange, and a third very
inconspicuous band coinciding with the line E. Pur-
purine dissolved in carbonate of potassium or sodium
exhibits two dark bands of absorption about the green
part of the spectrum. A solution of the same sub-
stance in aqueous alum exhibits the same peculiar mode
of absorption, but likewise a yellow fluorescence. A
solution of purpurine in sulphide of carbon exhibits
four bands of absorption, of which the first, situated
in the yellow just beyond D, reckoning from the red
extremity, is narrower than the rest. The second is
situated in the green, nearly coinciding with the line
E, The third in the blue, near F, and the fourth,
which js very inconspicuous, in the indigo. Lastly, the
solution of purpurine in ether gives a spectrum giving
two bands of absorption, one narrow and very dark in
the green, nearly coinciding with E. The second in
the blue, broader and less strongly marked, and having
its centre at the line F ; the solution is also slightly
fluorescent." (Stokes.)

The following series of experiments upon Absorption
is taken from an article by A. H. Allen in Nature, vol.
4, p. 346. A lime light may be used if it is desirable
to project these when sunlight is not available : —

" A beam of light from the lantern is passed through
a slit, focussed by a lens, refracted by a bisulphide of
carbon prism, and the spectrum exhibited in the usual
way. A flat cell containing a solution of permanga-
nate of potash is next placed in front of the slit. With

Il6 THE ART OF PROJECTING.

a weak solution and narrow slit a series of black bands
are produced in the green part of the spectrum ; but
with a stronger solution the green and yellow are com-
pletely cut out, allowing only the red and deep blue
lights to pass. On widening the slit these bands of
colored light of course increase in width also, gradually
approaching each other until they overlap, producing a
fine purple by their admixture.

If the experiment be repeated, substituting for the
permanganate an alkaline mixture of litmus and potas-
sium chromate in certain proportions, only the red and
green light are transmitted, the blue, and especially
the yellow^ being completely absorbed.

On widening the slit as before, the red and green
bands overlap and produce by their union a very fine
compound yellow, while the constituent red and green
are still visible on each side. The effect is most strik-
ing when by the widening of the slit a round hole is
exposed in its place, when then appear on the. screen
two circles, respectively green and red, producing
bright yellow by their mixture. This experiment is the
more striking as it immediately follows the process
of absorbing the simple yellow. The mixture above
described (suggested by Mr. Strull) answers better than
a solution of chromic chloride.

Of course, it is a well-known fact that all natural
yellows give a spectrum of red, yellow, and green, and
a common effect illustrating the compound nature of
yellow is noticed when exhibiting a continuous spec-
trum on a screen. When the slit is narrow the green
is very fully developed and only separated from the
red by a very narrow strip of yellow, while on gradually
increasing the width of the slit the red and green are
sure to overlap, producing the brilliant yellow we

LIGHT. 117

generally notice. Thus the purer the spectrum the
less yellow is observed.

If the continuous spectrum be produced with a quartz
prism, a little management and adjustment of distance
of the screen will cause the two spectra to overlap so
that the red of one may be made to coincide with the
green, blue, or any desired ' tint of the other. The
same result is obtained by employing two slits at the
same time, the distance between which can be adjusted.
By this means two spectra are obtained simultaneously,
any portions of which may be made to coincide.

A saturated solution of potassium chromate absorbs
all rays more refrangible than the green, while a solu-
tion of ammonio, sulphate of copper stops all but the
blue and green. These statements may be proved by
placing flat cells containing the liquids in front of the
slit of the lantern, and on placing one cell in front of the
other in the same position, the green light only is trans-
mitted. This experiment serves to explain the reason
that the mixture of yellow and blue generally results
in green, all other rays being absorbed by one or other
of the constituents.

By placing the two cells in front of separate lanterns
and throwing disks of light upon the screen, a beauti-
fully pure white is produced when the blue and yellow
overlap. I employ one lantern only for this exper-
iment, using two focussing lenses side by side to pro-
duce the overlapping circles of light. I also employ
a cell with three compartments, containing solutions of
analine, ammonio, sulphate of copper, and a mixture
of potassium chromate with the last solution, and pro-
jecting images on the screen by means of three lenses
fitted on the same stand but capable of separate ad-
justment.

Ii8 THE ART OF PROJECTING.

I can thus exhibit overlapping circles of brilliant
red, blue, and green light, which produce a perfect
white by their admixture ; while at the same time there
is seen the compound yellow produced by the union of
red and green, the purple arising from the red and
blue, and a color varying from grass green to sky blue
produced by the combination of the green and blue
light. This experiment has the advantage of exhibit-
ing at the same time the three primary colors, — red,
green, and blue, — the compound colors produced by
their mixture, their complimentary tints, and the syn-
thesis of white light."

The flat cells mentioned are made by cutting thin
pieces of board to the desired shape, and cementing
pieces of window-glass on each side by means of pitch.

INTERFERENCE SPECTRA IN REFLECTED LIGHT.

Fiq, 91.

Let a beam of light about an inch in diameter fall
upon a thin piece of mica, M, distant eight or ten feet
from the porfe /umiere. A part of the light will be
reflected, and in that may be placed a slit at Z, and a
lens O may project the slit in the ordinary way. At
the focus of the lens place a good prism so as to have

LIGHT, 119

a spectrum fall upon the screen at S. This spectrum
will be seen to be traversed by a large number of black
bands distributed throughout the whole length of it.
If the plate of mica be very thin and white there may
be as few as eight of these striae, but if it be thicker
their number will be largely increased.

The room will need to be made as dark as possible
for this experiment, as the spectrum will not be very
bright at best, and it therefore cannot be enlarged.
If the length of the spectrum exceeds a foot it will be
quite dim. These lines, however, can be seen to great
advantage by placing the eye close to the prism when
in its place as shown above.

If the spectrum of the light reflected from mica be
received upon a paper screen painted over with a solu-
tion of quinine and thus rendered fluorescent, such
interference striae will make their appearance in the
ultra violet part of the spectrum.

SPECTRUM ANALYSIS.

To project the spectrum of any substance what-
ever it must be heated until its vapor is brilliantly
incandescent. The heat of the electric arc is best
for this work as every substance is vaporized there.
The lime light may be used to exhibit the prin-
ciples of spectrum analysis, but its heat is insuffi-
cient for most of the metals. The characteristic
lines of Sodium, Calcium, Lithium, Barium, Stron-
tium, Potassium, and Copper may be tolerably well
exhibited with a lantern furnished with oxyhydrogen
jet and gases.

I St. To exhibit the spectrum : —

Produce the lime light as you would for common
projection. Remove the objective and place at the

I20

"HE ART OF PROJECTING.

focus in front of the lantern the slit d. The objective
0 may then be so placed as to project a sharp image of
the slit upon a screen in front of it at a distance of
fifteen or twenty feet ; then place the triangular prism

Fifj. 9

close to the objective. The screen will now need to
be moved, that the refracted rays may fall upon it, and
at the same distance from the objective that it stood
in front, otherwise the edges of the spectrum will
appear blurred. This should give a spectrum about
five feet long at the distance of twenty feet, but the
length will depend upon the dispersive power of the
prism. It will be longer with a bisulphide of carbon
prism than with one made of glass. If a still longer
one is needed use two similar prisms close together
and each one turned to the point of minimum devia-
tion.

If a very pure spectrum is needed, all of the con-
densers may be removed and the slit put in their place.
A parallel beam will then fall upon it, and the projec-
tion may then be made in precisely the same way as
for the solar spectrum. In this case the light will be
much less intense.

2d. To project the spectrum of the elements: —
Remove the lime cylinder and its holder, and light
the gases : the tongue of flame will be six or eight

LIGHT, 121

inches long. Now hold a stick of glass like a large glass
stirring rod in the flame at the same place when the
lime cylinder is fixed: It will glow brilliantly with
nearly the monochromatic light of sodium, and if the
prism is in its place the bright yellow line indicative of
that element will appear upon the screen. The glass
will need to be turned slowly, and the attention of one
person will be needed constantly to keep it in place.
Sticks of soda glass may be had in the market, made
especially for projecting the sodium line in this way,
but the spectrum can be obtained from almost any
piece of glass.

Another good method is to soak soft-pine sticks six
or eight inches long and half an inch thick in saturated
solutions of the chlorides of the various elements to be
projected, as the chlorides are more volatile than other
salts. Let the sticks remain in these solutions several
days before they are to be used, as a much larger
quantity of the material will be absorbed. These
solutions ma}' conveniently be made in test tubes six
or eight inches long, remembering to label each tube
by pasting a bit of paper upon it and writing the symbol
of the substance contained in it. The chlorides of all
the substances named above may be prepared in this
way and a stick provided for each one.

The saturated and still wet stick must be put imme-
diately into the flame where the glass and the lime
cylinder are otherwise placed, and, holding one end in
the hand, keep turning it slowly. The stick will glow
and give out the kind of light that is peculiar to the
included element.

The spectrum consisting of bright lines will be
quite bright and sufficiently large to be plainly seen by
an audience of a thousand persons. Sodium, Calcium,

122 THE ART OF PROJECTING.

Lithium, and Copper are especially good for this work
and give satisfactory spectra.

When this monochromatic light froYn the stick of
glass or the saturated solution of sodium chloride is
made to appear, it will be a good time to give atten-
tion to its effects upon other colors. Observe the
faces of individuals, the colors of flowers, of ribbons,
of pictures. It is a good plan to have prepared a set
of strips of bright-colored papers, or ribbons, or the
Newton's disk, for exhibition in monochromatic light.

REVERSED LINE.

The dark sodium line is the only one that is ever
projected, owing to the great difficulty there is in
making the vapors of other substances sufficiently
dense to absorb the powerful rays from the electric arc
or of the lime light. With either, a pure spectrum
must first be projected, and the slit should be nicely
focussed, as described. — Then having provided a
gas jet with Bunsen burner, or an alcohol lamp in
front of the slit, hold in it a small iron spoon con-
taining a lump of metallic sodium as large as a pea.
It will take fire and burn with a yellow blaze and a
white vapor, through which the light from the lantern
must pass. If this vapor is dense enough it will stop
rays from the other light that have the same refrangi-
bility ; and as its own luminousness is not very great,
it will leave a black line upon the screen in the place
where the sodium line would appear if the light came
from it.

It will be best to have a screen a foot square with a
hole through it, to set in front of the sodium flame to
prevent its light from falling upon the large screen and
injuring the effect.

LIGHT.

123

FLUORESCENCE.

Only blue or violet or ultra-violet rays are capable of
producing this phenomenon, and these may be obtained
either by passing common white light from the sun, or
the electric light, or the lime light, through a piece of
blue or violet glass or through a solution of ammonia,
sulphate of copper; or, better still, by producing a pure
spectrum. The best effects are to be observed by using
a prism of great dispersive power, like quartz.

FUj' 93.

When colored glass is to be used to obtain the
violet light, it suffices to place a lens of a foot focus
near the orifice and the glass just in front of it.
Fluorescent solids and solutions may then be examined
at S. A piece of uranium glass or a solution of quin-
ine in a test tube or bottle will exhibit; this property
so that many can see it at the same time. It will be
well to use two bottles or beakers of clear glass, — one
to contain pure water and the other the solution of
quinine — and examine them side by side in this blue
light. The fluorescence will then be more noticeable.

When artificial light is used in a lantern it will only
be necessary to place the colored glass in front of the
condenser, as if to project a picture upon it, and
otherwise use the light as with sunlight.

Pictures are sometimes made of fluorescent material.

124 THE ART OF PROJECTING.

The outlines of flowers, butterflies, letters, etc., are
drawn upon paper with a lead-pencil, and then painted
with substances that exhibit different colors by fluores-
cence. When these pictures are used they may be
pinned to the screen and the light allowed to fall upon
them as before. Examine the pictures or other things
by light transmitted through red, yellow, green, blue,
and violet glass. The kind of light that induces
fluorescent action will then be apparent.

When fluorescent substances are to be examined in
the light of a solar spectrum they may be made to pass
through it from the red towards the violet, and con-
tinuing beyond the visible part, for the ultra-violet rays
are capable of powerfully exciting fluorescence in
some substances. Stokes found this invisible spec-
trum that was competent to induce such action to be
as much as three or four times the length of the visible
spectrum.

The following substances manifest fluorescent ac-
tion : —

Red Fluorescence, Chlorophyl,

Orange " *"

Yellow " Madder mixed with Alum,

Green " Turmeric Stramonium and

Night-shade,
Brazil wood. Uranium glass,

Thallene,
Blue " Quinine — horse - chestnut

bark, Petrolucene,
Purple " Bichloranthracene.

These substances are generally prepared in solutions
or decoctions for this purpose.

Chlorophyl may be prepared by boiling tea-leaves
until water will remove nothing more, and then soak-

LIGHT. 125

ing them in hot alcohol. The chlorophyl will thus be
extracted and the tincture is ready for examination.

A few pieces of the hulls of horse-chestnuts, or of
the inner bark of the horse-chestnut tree digested half
an hour in cold water, will be sufficient for this.

Alcoholic tinctures of madder, stramonium, night-
shade, and Brazil wood will be needed.

A grain of sulphate of quinine may be put into
a pint of pure water and shaken up occasionally.
This substance is sparingly soluble in water. A
little tartaric acid may be added to the water with
advantage : the fluorescence will be more strongly
marked.

A good method of exhibiting this is to have a rather
large glass vessel containing pure water set in the path
of the violet rays. Pour the quinine solution into it :
opalescent clouds at once appear to form, though noth-
ing is precipitated.

Thallene and anthracine are obtained from some of
the products of the distillation of petroleum and coal
tar, and are not in the market.

The Aurora tube and Geisler tubes when lighted by
the electric spark may be used to obtain fluorescent
effects. With the former, writing and drawings made
with proper solutions, may be seen when such markings
would be entirely invisible in common white light.
Geisler tubes are often made to contain some pretty
design in uranium glass, or there is some vessel con-
taining a fluorescent solution surrounded with a jacket
filled with some gas which gives a violet light like
nitrogen.

A very beautiful effect is produced by exposing a
number of highly fluorescent media to the flame of
sulphur burning in oxygen in a dark room.

126 THE ART OF PROJECTING,

DOUBLE REFRACTION.

A piece of calc spar will be needed to show this.
Its size is not very material, though the thicker it is
the farther apart will the refracted figures be. It
should have smooth faces, but the natural faces are
often good enough to permit this phenomenon to be
projected.

Fig> 94.

Make a hole a quarter of an inch in diameter through
a bit of cardboard (unless you chance to have a dia-
phragm with holes of various sizes) and place it at the
aperture ; the small beam of light which comes
through it should be directed horizontally upon the
screen. Next place the piece of spar in front of it,
and then project the hole with an object lens with a
foot or more focus. The two spots will appear upon
the screen, and if the spar be rotated the one spot
will revolve about the other. Instead of the hole in a
diaphragm, it will do as well to make a black spot upon
a piece of glass and project it in the same way.

Either side of the spar may be used for showing
this phenomenon.

A double-image prism may also be used with still
better results as the images will be still further separated.

LIGHT.

127

POLARIZATION OF LIGHT.

Plane-polarized light may be obtained in great
quantity by using for a reflector in the porte lumiere
a plate of glass blackened upon its back surface.
Choose a piece of good window-glass, without bubbles
or striae, and paint it upon one side with lamp-black
mixe4 in Japan varnish. It will be best to lay on two
or three coats in order to completely cover the surface.
Hold it between the eye and the sun, and all the
uncovered and thin places can be seen ; they should
receive another coat of paint. This painted glass
should be of the same size as the plane mirror in the
porte lumiere^ upon which it may now be placed and
fastened by tying about them both a thread or stretching
a ring of elastic cord over them. If the beam of light
which is now reflected from the unpainted surface of
this glass is sent through a double-convex lens and
then received upon the screen, it will be seen to be

Fig, 95.

much less intense than the beam from the silvered
mirror, but some of the most attractive experiments in
the whole domain of physics are possible with this
light.

A Nicol's prism n will be necessary, and the larger
it is the better, but very good effects may be obtained

128 THE ART OF PROJECTING.

with such small ones as come with the polarizing
attachment to common microscopes. With one having
a face three-quarters of an inch upon a side, everything
essential can be shown to a large audience.

1. Place the prism at the focus of the lens so that
all the light will pass through it. Now, if the prism be
rotated upon the beam as an axis, the disk of light upon
the screen will decrease in brightness until it is nearly
or quite invisible; and if the prism be turned still
further in the same direction the light will reappear
and attain its maximum brightness when the prism
has been turned ninety degrees from the position
where the light disappeared.

2. Turn the prism so that the light is cut off from
the screen; and then, holding it in that position,
slowly introduce a thin sheet of clear mica between
the lens and the prism. The light will reappear upon
the screen from that transmitted by the mica. If the
mica is as thin as the fiftieth of an inch, or less, the
light may be colored a beautiful blue or green or red.
Turn the mica round in its own plane, and these colors
will appear in succession. Let the prism be rotated
while the mica plate is held still, the same effects will
be observed.

3. In the same manner experiments with thin plates
of selenite may be tried.

4. Bring the lens forward so as to use it as an objec-
tive, and project a thin piece of selenite or of mica with
varying thicknesses. Hold the prism in the focus as
before. With each different thickness of the plates
different colors will be transmitted which are often
ver}' beautiful indeed. If the pieces of these minerals
are not more than an inch square, a larger lens may be
used for a condenser, and then, with an objective of

Fig. 96.

LIGHT. 129

four or five inches focus, project the piece in the same
manner precisely as you would with the solar micro-
scope. The prism must be held in the focus of the
objective always.

5. Geometrical designs in mica.
Choose a thin plate of mica
that is clear, and three inches
or more square. Hold it in the
polarized light and see if it pre-
sents Hvely colors ; it will if it
is thin enough. It ought not to
exceed the fiftieth of an inch in
thickness for best effect. WTien
the tint appears uniform over
its whole surface, as it will if its thickness is uniform,
it may have drawn upon it with a lead pencil such a
figure as the accompanying one, and then trim to
the edge of the circle with scissors. Afterwards, wth
a sharp pen-knife cut about one fourth of the way
through the mica on all the lines ; then with a needle
point start to split the point on the edge. \Mien a
thin leaf has been raised a little between two points,
carefully move the needle round the edge so as to
separate the same thickness all around the circumfer-
ence. Do not disturb the points of the star more than
at the extreme point, just enough to keep the needle
in the same layer. If the knife has cut through this
layer that has been raised at the edge, the parts 0, 0, 0,
can be removed, leaving the sLx-pointed star a little
raised above the surface 0, 0, 0.

Examine this now with the polarized light, and the

star will appear to be of one color and the cut-away

parts of another. If the interior part c be removed it

is very probable that that part will exhibit still another

9

I30 THE ART OF PROJECTING.

color. If it does not, it is because the part removed
had the same thickness as one of the others, or differ-
ent from it by a wave length. Designs of any kind
that fancy may dictate may be thus made upon sheets
of mica. To project them plainly use an objective, as
in 4, and place the Nicols prism in the focus of it.

Designs in selenite are still handsomer, and figures
of birds, butterflies, flowers, and fruits may be bought
in the market. Selenite is so brittle that a good deal
of skill is needed to work it, and it would be tedious
to a beginner. Such designs had better be purchased.

6. Unannealed pieces of glass when they have a
regular form, as a square, a triangle, or a circle, make
good objects to project by polarized light. They are
generally a quarter of an inch or more in thickness, and
an inch or two in diameter. Pictures of their appear-
ance are often figured in works on physics. Pieces
of thick glass, fragments of glass vessels, and glass
stoppers of bottles often show double refractive power.

7. A good way to exhibit the development of this
double refraction in glass is to take a piece of thick,
plain glass and stand its edge upon a piece of iron,
heated to redness, projecting the whole in the polar-
ized beam with the prism in its place. As the glass is
heated and strained, colors will develop upon the screen
and arrange themselves symmetrically, depending
wholly upon the external form of the glass.

8. It is convenient to have a piece of glass as much
as a quarter of an inch in thickness and an inch
square or more, that is annealed, and consequently
gives no bands or colors. If this is strained by being
pinched in a hand-vice, tufts of light or black brushes
will be seen to start out from the place of pressure if
the whole be projected.*

LIGHT.

131

9. A bar of glass half an inch thick, an inch broad,
and six or eight inches long, may be gently bent with
the fingers while held in place for projection. The
strain induces double refraction, and that manifests
itself by bands of light or dark, or color.

All of these should have their outline sharply pro-
jected by an objective of proper focal length.

10. A small crystal of Iceland spar, having its obtuse
angles ground off and polished so as to present a sur-
face as much as a quarter of an inch square, will pre-
sent a beautiful series of rings and bars when projected*

Fig, 97.

It will not be necessary to use an objective, but
simply to put both crystal and prism in or so near to
the focus of the condenser that as much light as possi-
ble may be transmitted through them. When in place
let each in turn be rotated upon its axis, and observe
the appearance and disappearance of the light and dark
bands. At a distance of twenty feet from the prism the
outer rings should be about four feet in diameter.

11. A crystal of rock candy, with parallel faces, and
not more than the twentieth of an inch in thickness,
will present another system of rings and bands. Pro-
ject it in the same manner as the spar was projected.

12. A piece of a quartz crystal cut at right angles
to its axis will, if projected in the same way as the last,
exhibit colors upon the screen, which will vary as the

132

THE ART OF PROJECTING.

prism is turned. If it be put close to the prism there
may appear a system of concentric circles about a uni-
form-colored field in the centre. The colors which
this central field assumes when the analyzer is rotated
are often superb.

Spectacle glasses that are usually called Brazilian
pebble are made of quartz, and such will exliibit
brilliant colors by projection in plane polarized light.
This serves for a test of their genuineness, as glass
will give no such effect.

1 3 . The system of bands and colored
curves seen in biaxial crystals is not
easy to project, because the angles at
which these are to be seen are so great.
With some crystals of potassium nitrate
it is possible to show both axes at a
time with the same arrangement as was
described for calc spar. A clear crys-
tal about the quarter of an inch in
diameter, and the twentieth of an inch
thick, may answer for this. Such small
crystals are usually mounted in a disk
of cork.

Fig. 98 represents the double system
of rings and brushes seen in a crystal
of nitrate of potash, where the plane
of its axes coincides with the plane of
the polarizer; and Fig. 99 shows the
appearance when the planes are slightly inclined to
each other.

14. There are many minute crystallizations, such as
are prepared for the microscope, that make fine objects
when projected in polarized light. These objects may
.be prepared beforehand ; or the crystallization with the

Fig. 99.

LIGHT,

^11

accompanying development of polarization properties
may be projected. It will be simply necessary to mag-
nify the object by using a lens of short focus as in the
former instruction for the solar microscope. The strip
of clear glass, holding a drop of a saturated solution
of the substance, the objective, and the Nicol's prism
being put near the focus of a condenser of twelve to
eighteen inches focus, that the specimen may be lighted
as much as possible, and also have sufficient light trans-
mitted.

±ig, lOO,

The following list of salts and other substances will
be found to be beautiful objects for polarized light : —

Alum,
Borax,

Oxalate of Ammonia
" Lime,

Carbonate of Lime,

Oxalic Acid,

Carbonate of Soda,

Picric Acid,

Chloride of Barium,

Prussiate of Potash,

" " Copper and

Salicine,

Ammonia,

Sulphate of Copper,

Chloride of Sodium,

« 1,1 a

Chlorate of Potash,

Magnesia,

Citric Acid,

Sulphate of Iron,

Nitrate of Bismuth,

" Soda,

« " Copper,

" Zinc,

« " Potash,

Sugar,

and

134

THE ART OF PROJECTING.

Starch,

Tartaric Acid,
Urea,
Human hair.

Petals of flowers, as of the

Geranium,
Scales of Fishes.

Fig. loi represents
the appearance of
starch grains of the
potato, as seen in
common light with

Fig, 101, Tig. 10?. ihe microscope, and

Fig. I02, the same seen by polarized light.

The following method of preparing double salts for
examination with polarized light is given by Mr. Davies
in the " Quarterly Journal of Microscopic Science " : —

" To a nearly saturated solution of sulphate of cop-
per and sulphate of magnesia add a drop on the glass
slide, and dry quickly. To effect this, heat the slide so
as to fuse the salts in its water of crystallization, and
there remains an amorphous film on the hot glass.
Put the slide aside and allow it to cool slowly. It will
gradually absorb a certain amount of moisture from
the air, and begin to throw out crystals. If now
placed in the microscope, numerous points will be seen
to start out here and there. The starting-points may
be produced at pleasure by touching a film with a
fine needle point so as to admit of a slight amount of
moisture being absorbed by the mass of the salt."

A slide of salicine crystals makes a splendid object
for such projection, and should be in every collection.
Make a saturated solution of the crystals in distilled
water, and place a drop carefully upon a slide that has
been carefully cleaned. Evaporate over a lamp until
it is dried to an amorphous mass. Upon cooling, a

LIGHT. 135

number of "circular crystals will be formed with radiat-
ing forms between them. These circular crystals may
be made larger and regularly disposed by touching the
mass with a fine needle point when crystallization
begins. Such ones will form about each point touched.
Magnify such objects so much that these circular
crystals will appear a foot in diameter upon the screen.
The Nicol's prism will show each one with four arms
that will turn about the centre of the crystal when the
prism is rotated, while the radiating crystals will show
as red, yellow, or purple brushes sweeping over the
screen.

By inserting a sheet of transparent mica or thin
selenite between the reflector and the object, a colored
field will appear as a kind of background, upon which
the minute crystals, such as chlorate of potash and
oxalic acid, will appear more highly colored. The eifect
is usually to heighten the color.

THE DOUBLE IMAGE PRISM.

JFiflr. 103,

With a large lens project the image of the aperture
upon the screen, the light being polarized by the black-
ened reflector. At the focus of the lens place the
prism. Two images of the aperture will appear and

136 THE ART OF PROJECTING.

overlap each other. Turn the prism on the beam as
an axis ; the images will turn about each other.

Place a thin piece of mica between the orifice and
the lens. The two disks upon the screen will appear in
complementary colors, save where they overlap, which
will be white. Turn the prism as before ; the colors of
the two disks will change, always being complementary
to each other.

Again, remove the reflector, and place the lens close
to the orifice. Fix the prism near the focus so that a
large part of the light passes through it ; and then,
with lens and Nicol's prism near it, project the images
of objects placed close to the double-image prism. In
this case the latter acts as a polarizer.

When large Nicol's prisms can be had, one of them
may be substituted for the reflector upon the porte
lumiere. The light passing through it will be polarized.

Fiu. 104.

The object to be examined, o, may be placed near to it
in front, then projected with any convenient lens, in
the focus of which place the other Nicol's prism. This
allows a long amount of light to be used, and is one
method in use with lanterns. The only hinderance to
the use of these larger prisms is the costliness of them.
All of these experiments may be performed with a
lantern, with one of the more powerful lights. The usual
method of polarizing the light is to have an elbow in

LIGHT. 137

front of the condensers that carries a series of plain
glass plates inclined to the beam so that it meets it at
the polarizing angle of glass. Part of the light is trans-
mitted and is absorbed by a piece of black cloth. The
light that is reflected is sufficiently well-polarized for
all purposes of demonstration ; and such a beam may
be treated in every way like the beam from the porte
lumiere and with like results.

DIFFRACTION.

Reflect the beam from the porte lumiere through a
slit like one for showing the Fraunhofer lines. It
ought not to be more than one sixteenth of an inch
wide. Receive this beam, without magnifying it, upon
a second slit in a screen at a distance of four or five
feet from the first slit. Make the room as dark as
possible, and then hold a sheet of white paper behind
the second slit anywhere from a few inches to several
feet. Colored fringes will appear on each side of the
central line, with a series of alternate black and white
bands or lines. These may be received upon a screen
twenty feet away, when they should have a united
breadth of a foot or more, but the light is necessarily
very weak. A lens does not improve them very much.

With a piece of perforated paper or tin or lace, or
still better, with an eidotrope, which consists of two
disks of perforated tin made to revolve in opposite di-
rections, like the chromatrope, a very beautiful exhi-
bition of the phenomenon of diffraction may be given
in the following way : —

Take two large, short, focus lenses, such as form the
condensers in Marcy's sciopticon. Place one close to
the opening to the porte lumiere, as shown in the figure.
The second one may be put so far in front of the other

138 THE ART OF PROJECTING,

lens that the beam is again crossed in front of it, and
the disk upon the screen is of the desired size, six or
eight feet in diameter. Now introduce the perforated
paper or the eidotrope at the place marked e. The

Fig. 10 rt.

screen will appear covered with minute spectra, as
each hole will form one or more spectra ; but if the
paper be held at e, between the lens and the screen, the
projection becomes very gaudy and symmetrical. If it
be the eidotrope, turn it while held in that place, and
the colors will change and will rival the colors pro-
duced by polarized light. Try the effect of a comb,
of wire gauze, of the fingers, or other objects. Very
curious and interesting appearances will appear upon
the screen.

If one has a piece of glass finely ruled with a dia-
mond, it may be projected as is any object with the
Jfor/e /umiere, and the diffraction spectra will appear
upon the screen. With plenty of light for the projec-
tion, and with the room otherwise well darkened, a
number of the Fraunhofer lines may be seen in these
spectra.

Again, let a concentrated solution of alum or cam-
phor be poured upon a glass plate, and allowed to dry
rapidly, so as to cover it with a crust. Put it in the
focus of a lens with a short focus, and a series of

LIGHT. 139

halos will be seen by placing a small screen but a foot
or two from the glass. Fine rulings upon blackened
glass will in the same place give fine colors. These
rulings may be as coarse as fifty to the inch j the finer
rulings will answer better. The rapidly-diverging rays
necessitate the placing of the screen close to the plate,
else the colors will be too faint.

PERSISTENCE OF VTSTON. THE STROBOSCOPE.

Let a disk a foot
in diameter be cut
out of any conve-
nient material, — tin,
copper, zinc, o r|
pasteboard. Near
the periphery cut
out a number of
holes at equal dis-
tances apart, — ten
or twelve will be
enough. They m ay

be cut half an inch pig, lofi.

is diameter. This disk is to be put upon a rotator like
the one used to show the Newton's disk, and may now
be placed so that the focus of the condenser with the
J>orfe lu7niere will be in the holes as the disk revolves,
as in Fig. 106. This permits the light to pass to the
screen only when the holes are at the focus, at which
time a powerful beam will pass and is immediately cut
off. With such a fixture a very great number of amus-
ing and instructive experiments may be made.

I. While one person turns the stroboscopic disk
let another one stand in front of the screen and swing
his arms, or move his body rapidly sideways, or make

I40 THE ART OF PROJECTING,

low courtesies. To spectators he will appear to have
a dozen arms or bodies. There will also be as many
shadows upon the screen.

2. Make a wheel to turn in front of the screen, the

Fig, 107*

larger the wheel the better. A buggy wheel or old
fashioned spinning-wheel make good objects. Let both
disk and wheel be turned at the same time. The ap-
pearance of the wheel will depend upon its velocity.
It may be made to appear as if standing still or moving
slowly forward or backward, or as if it had a multitude
of spokes.

3. While the wheel is turning a little in front of the
screen, look through the wheel at the shadow of it.
Some remarkable curved lines will appear to group
themselves about the axles of the wheel and its shadow.

4. If two wheels of the same size are made to turn
one in front of the other while they are in this inter-
mittent light, curious curves and fixed straight, light
or dark lines and mixed, changing paths can be seen,
according to the position the spectator has with refer-
ence to the wheels.

LIGHT, 141

5. If a small wheel but two or three or four inches
in diameter, and toothed like a cog-wheel in a clock, be
placed within a foot or two of the disk, and so that its
shadow will fall upon the screen, its shadow will not
only be much magnified, but the motions of the wheel
will appear as with the larger one, number i.

6. Let large disks three or four feet in diameter be
made, having various symmetrical figures painted upon
them. When the disks are revolved, many curious
motions may be simulated : as of a girl jumping rope,
a man sawing or chopping wood, boys playing leap-
frog, a man opening and shutting his eyes and mouth,
wind-mill with sails turning, etc. Such designs gen-
erally come with the toy called the thaumatrope,
made to look through into a mirror while turning.
These may be copied upon large sheets of pasteboard
and rotated in any convenient way. The turning-table
may be made to answer for this.

7. Another good and very pretty application of
this is to have a large star with five or six points made,
and the alternate points colored with different tints,
as red and blue. When such a disk is revolved in this
intermittent light it may appear to stand very still, or
to slowly revolve forward or backward, while its points
may be doubled or tripled or quadrupled, and its colors
will apparently overlap and give the tints proper to the
mixture.

8. Such pictures as are sold with the thaumatrope
may be fastened to the front of the disk containing
the holes through which the light passes, as is repre-
sented in Fig. 107, and after the light has passed through
the disk, it may be reflected upon its face by a small
mirror, 7n (Fig. 108), and can thus be seen very well if
the light be strong. When used in this way the disk

142 THE ART OF PROJECTING.

may be made very much larger, as much as two or
three feet in diameter, and the number of holes in-
creased. By
removing the
mirror m a
little farther
away the beam
can be reflect-
ed so as to cov-
er the whole
Tig, 108. face of the disk.

A small toy steam-engine, such as may be bought
for a dollar or two, may have a light paper disk fitted
for it to turn, but if sunlight be used, care must be
taken lest it take fire in the focus of the sun's rays.

An oxyhydrogen lantern may be used for such work.
The objective will need to be removed, and the perfo-
rated disk placed so that the most of the light goes
through the holes when they are in position, and the
unused light cut off from entering the room by black
cloth or some other provision. Otherwise it will be
used just as with sunlight.

THE CHROMATROPE.

This instrument consists of two disks of glass so
mounted that they may be rotated in opposite direc-
tions. Various designs are painted upon the disks,
and fine effects may be obtained by projecting them in
the ordinary way with the lantern or the parte lumiere.
If instead of using disks of glass, disks are made of
wire gauze, perforated tin, or paper or lace, very curious
interference figures are produced, and this form ift
called the eidotrope.

LIGHT.

143

The accompanying figure represents a chromatrope
with an arrange-
ment for quickly
replacing one
disk by another
of different pat-
tern. Rotation
is given by fric-
tion pulleys.
With this form
there is a disk
with the so-called
seven primary
colors to illus-
trate Newton's

theory of colors, i ^ 5

one to illustrate Brewster's theory, two to illustrate
Young's theory, and a chameleon top, designed by
President Morton, of Stevens Institute, Hoboken.

The effects with all the forms of chromatropes are
due to persistence of vision.

Interesting subjective effects may be observed by
projecting in the ordinary way bits of colored glass an
inch or two square, so as to have upon the screen a large
patch of color with a boundary of white light. The eyes
must be fixed attentively upon the colored patch for
about half a minute, when the colored piece must be
quickly removed, the eyes to be kept meanwhile upon
the screen. To prevent the eyes from unconsciously
wandering while looking, it will be found advisable to
pin a large black button or a piece of black paper to
the screen in the middle of the disk. This is to be
kept in the centre of vision. The effects observed will
of course depend upon the color upon the screen, and

144 THE ART OF PROJECTING.

the sensitiveness of the eyes for various colors. Gen-
erally, after looking steadily at a given color, and the
disk is made suddenly white, the outline of the colored
part will be seen in a color complementary to the one
looked at first. Thus, if a square red glass should be
projected the residual image would be a square green
one. If a blue one was projected its complementary
image would be orange, and so on. A great variety of
such effects are obtainable with various colored pieces
of glass, or of films of gelatine, by projecting them
singly, in juxtaposition, or superposed.

Let disks of white cardboard a foot or two in diame-
ter have partial sectors painted black, with India ink,
so that the white and black parts alternate four or five
times in the circumference. This is to be rotated while
a powerful beam of light falls upon it. The persist-
ence of some of the elements of white light being
greater than of others, the disk will appear of various
colors ; purple, green, and yellow being generally well
developed.

HEAT. AIR THERMOMETER.

A bulb blown upon one end of a small glass tube,
five or six inches long, answers for this experiment. A
drop of colored water can be made to enter the tube
by first heating the bulb a little by holding it in the
fingers with the open end of the tube a little below the
surface of the water. A bubble or two of air will be
expelled, and the fingers may be removed from the
bulb. As it cools a drop will be driven into the tube,
and with a little painstaking it can be brought to any
required place by cooling or heating the bulb. These
movements can be shown with the porte bwiiere and a
single lens, as shown in Fig. 17, or it can be put in

HEAT. 145

front of the condenser of the lantern. A touch of the
finger will heat the bulb sufficient to cause the drop to
rise in the tube, and it may be made to descend by
simply blowing upon the bulb, or by dropping a little
water or ether upon it.

Many of the pieces of apparatus for illustrating the
expansion of metals by heat are so small that they may
be readily projected. Thus Gravesand's Ring, Pyrom-
eters, etc. The latter may have a small bit of mirror
fastened to the end of the index, and the light so
arranged that as the index rises, the beam will move
upward. A rise in temperature of only a few degrees
can be then shown, and the alcohol flame may be dis-
pensed with ; the warmth of the hand or a little hot
water answering the purpose.

FORMATION OF CLOUDS.

The condensation of liquid in the form of vapor into
minute globules and in the production of a shower of
rain may be very well illustrated and projected for
class purposes in the following manner : —

Place about an ounce of Canada balsam in a Flor-
ence flask and make it boil. At the top of the flask
clouds of globules of turpentine will be seen hovering
about, altering in shape very much like sky clouds, and
the globules are large enough to be visible by the naked
eye. If a cold glass rod be gradually introduced into
the flask these clouds may be made to descend in
showers. Lawso7i Tait in Nature.

Another : Take a flask of one or two litres capacity ;
rinse it out with distilled water, and attach to the neck
a cork and glass tube of about twenty or thirty centi-
metres length. Place the glass tube in the mouth and

146 THE ART OF PROJECTING.

exhaust, when a dense cloud will be formed ; then on
blowing into the flask the cloud disappears. The cloud
may be produced and dissolved as often as wished,
and if a beam from the oxyhydrogen light be sent
through the flask the experiment becomes very eflec-
tive. C. y. Woodward i7i Nature.

MAXIMUM DENSITY OF WATER.

Take a small test-tube, not more than
two or three inches long and half an inch
in diameter, and through a tight-fitting
cork thrust a small glass tube about three
inches long, allowing it to project as
much as two inches. Fill the test-tube
with water at about 4° centigrade and
cork it tight, so that the water will rise in
the glass tube. See that there are no air
bubbles beneath the cork. Mark the
height of the water in the small tube by
tying a thread about it. Project the
whole with a lantern or with the porte
lumiere. Now, if a small vessel contain-
Tig, 109. ing hot water be brought up under the
test-tube so tliat the latter dips in it, the expansion of
the watLT will be indicated by the rise of the water in
the tube, and the latter will overflow if it be sufficiently
heated. Now, bring up under it in the same way a
freezing mixture of ice and salt, or a mixture of equal
parts of cold water and .nitrate of ammonium. The
water will contract in volume to its minimum, which
should be indicated by the thread ; then it will again
expand until it freezes, the expansion again causing the
water in the tube to overflow. The freezing mixture
should be stirred constantly to hasten the work.

HEAT, 147

THE GALVANOMETER.

As many of the experiments in heat require the
thermo-pile and galvanometer, the latter is treated of
in this place rather than with electrical experiments.

In the "American Journal of Science,"' Vols. II, III,
V, IX and X, are given several ingenious arrangements
for projecting the movements of a galvanometer needle,
and if one desires to do extremely accurate work be-
fore an audience he will do well to obtain some one of
these forms. If, however, it is desirable only to ex-
hibit qualitatively and with no great degree of precision
the relation of heat to electricity, or the law of the
galvanometer, etc., the following method will be found
to answer, with the advantage of being extemporized
in a few minutes : Make 2, fiat coil about an inch square,
of rather fine-covered copper wire having the ends
of the wire a foot or more in length. Upon one side
of this coil stick a bit of beeswax as large as a small
marble, and through both wax and coil thrust half of a
fine cambric needle. Press the wax down upon the
middle of a piece of glass four or five inches square,
and then holding the plate horizontal, suspend upon
the needle point a small compass needle an inch or two
long. This is now ready to place upon the upper con-
denser c (Fig. 27) of the vertical attachment and
then be projected. If a current from a battery or a
thermo-pile be sent through the coil, the needle will be
deflected. The needle will of course point towards
the north, and that place will easily be noted upon the
screen as zero. A small magnet brought into the
neighborhood will serve to bring the north pole of the
needle to any required place. If a circle with in-
scribed degrees should be drawn upon the glass by

.148 THE ART OF PROJECTING.

either of the methods described upon pages 31 or 32,
the movement of the needle can be noted in degrees.
If the needle is too short to reach the numbers upon
the glass, it can have a fine straight bristle made fast
to its ends with a little mucilage.

With the thermo-pile connected with the galvanom-
eter, the sensitiveness of the former may be shown by
presenting the hand to one face of it, or it may be
breathed upon or blown upon with a common hand
bellows. Let fall a drop of water, of ether, and of
alcohol upon the face. The evaporation cools it.

The heat generated by percussion may be exhibited
by providing a leaden bullet which should have at first
the same temperature as the thermo-pile, which may be
known by putting it upon the pile, handling it with a
pair of small tongs. It should not move the needle.
Then strike it once with a hammer so as to indent it
considerably, and with the tongs quickly put it again
upon the face of the pile. It will indicate a higher
temperature.

The heat generated by friction may be shown by
rubbing a stick upon the floor and then bringing it to
the pile as in the other case.

See Tyndall's work on Heat for a method of show-
ing heat from the crystallization of sodium sulphate.
The same thing may be shown with the air thermom-
eter sunk into the solution, which may be projected
with lantern or porte lumiere by preparing the solution
in a beaker, fixing the air thermometer in it with a drop
•of colored water in it, and projecting the whole upon
the screen by means of a large lens. The crystalliza-
tion itself will be seen, as well as the manifested heat,
•when it reaches the bulb of the thermometer.

Mix in a test-tube resting upon the face of the ther-

HEAT. 149

mo-pile, a few drops of water and sulphuric acid about
equal parts : the heat evolved will illustrate the origin
of heat from chemical reaction.

A few crystals of nitrate of ammonium in a test-tube
may have an equal bulk of water poured upon them ;
the cold produced is from the absorption of heat dur-
ing liquefaction.

Interpose between the source of heat and the ther-
mo-pile various things, such as rock-salt, a solution of
iodine in bisulphide of carbon, glass, crystals of various
kinds, tubes filled with gases and vapors of various
sorts. Also, project a solar spectrum with a part of
the same beam that projects the galvanometer by the
method described upon page 112. Move the thermo-
pile through the various colors, and note the degree
indicated by the galvanometer, particularly beyond the
red end of the spectrum. The thermo-pile should be
placed where the Fraunhofer lines are seen best upon
a small screen placed temporarily to receive it.

Many experiments on this subject will be found in
Tyndall's work on Heat, which one will find himself
able to repeat with satisfaction.

CALORESCENCE.

Let the light from the />orfe lumiere, or from the elec-
tric or lime light, be sent through a vessel containing
bisulphide of carbon in which some iodine has been
dissolved : the solution will be jet black and will stop
every light ray, but will permit the rays of greater wave
length to freely traverse it. A lens may now be inter-
posed and the obscure rays treated in every way like
luminous rays. With a very powerful beam platinum
foil may be raised to incandescence in the focus of the

150 THE ART OF PROJECTING.

lens, and with a less powerful one pieces of wood and
paper may be ignited.

A transparent solution of common alum is opaque
to the same rays that are so easily transmitted by the
iodine solution.

A test-tube filled with water placed at the focus of
the obscure rays in a minute or two may be made to
boil ; an air thermometer will scarcely be affected at
that place.

MAGNETISM.

With the vertical attachment to the lantern the
phenomena of magnetism are best shown.

1. Have two or three small magnetic needles
mounted upon needle points thrust through pieces of
cork, so as to turn freely. Place one upon the upper
face of the condenser to the vertical attachment, and
project it sharply upon the screen. A piece of iron or
another magnet brought into its neighborhood will
disturb it, and every motion will be plainly noticeable
as well as the direction of the exciting body.

2. Place two of these needles near to each other,
but not so near as to touch, and give to one of them a
twirl so that it revolves upon its support. It will soon
set the other revolving and it may be stopped itself
after setting the second one going, and afterward be
again started while the other one stops.

3. Place a third, quite small one not more than
half an inch long in the neighborhood of the other
two, and again set the one whirling.

4. The magnetic phantom.

Lay a small magnet an inch or two long upon the
upper condenser ; and upon the magnet lay a piece of
clear glass three or four inches square. Project the
magner, and then scatter from a small sieve, or gently

MAGNETISM.

151

with the thumb and finger, fine iron filings upon the
glass. The fihngs will arrange themselves in the
familiar lines called the magnetic phantom, and the
whole being magnified to ten feet or more in diameter
makes a very striking picture.

5. The elongation of an iron rod when strongly-
magnetized, may be shown by placing a small helix
around the iron rod of the common pyrometer made
for showing the longitudinal expansion of a rod by
heat. To the end of the index finger that sweeps over
the quadrant affix a small bit of plane mirror not more
than one fourth of an inch square. So adjust the light
to this small mirror that the reflection from the latter
will fall upon the most distant part of the room ; the
farther away the better. When the current of elec-
tricity is sent through the helix the rod will be slightly
elongated, but the slight tilting of the mirror may
become a displacement of two or three inches at a
distance of thirty feet.

DIAMAGNETISM.

The electro-magnet for demonstrating diamagnetic
phenomena need not be over three or four inches in
length, and the poles an inch apart.
Objects to be tested may be suspen-
ded by a thread between the jDoles,
and the whole projected either in a
beam of parallel rays or in front of
the focus of a lens. In the latter
case the whole will be seen in pro-
file, but perfectly distinct. The fol-
lowing experiments may be projected
with such a magnet if a battery of j,ig. no,
three or four cells be used : —

152 THE ART OF TROJECTING.

1. Suspend oblong pieces of various metals half an
inch in length, and note whether they set themselves
equatorially or axially between the poles. Iron,
nickel, platinum, bismuth, antimony, zinc, tin, lead,
silver, copper, alum, glass, sulphur, sugar, bread, paper,
charcoal, are good substances to experiment with.

2. Suspend a cube of copper between the poles,
and twist the thread so that the copper will rotate
rapidly by torsion. It will quickly be brought to rest
when the current is made to pass.

3. Fill small very thin tubes with liquids, and sus-
pend them in the same manner. Try solutions of iron,
cobalt, water, alcohol, turpentine, and salt.

4. Place the magnet upon the upper condenser of
the vertical attachment, and upon its poles place a
watch-glass containing a little water or sulphuric acid ;
project the water in the watch-glass, and notice the
distribution of light upon the image of the water.
Now complete the circuit. The water will change irs
form slightly and the light will be differently refracted,
thus making it quite visible. Salts of iron or nickel
will scatter the light like a concave lens.

5. Hold the flame of a candle between the poles.

6. Blow small soap bubbles with oxygen and with
illuminating gas, and hold them as close to the poles as
possible or drop them so they will rest upon both.

7. Heat a coin and place it just beneath the poles,
and then drop a piece of iodine upon the coin. The
heat will volatilize the iodine, and the purple vapor will
be repulsed.

ELECTRICITY.

Most of the experiments in electricity which can be
shown by projection require the use of the galvanom-
eter, such for instance as give evidence of the existence

ELECTRIC! 7 Y. 153

of electrical currents, their direction and strength.
These will only need the arrangement already described
under the head Galvanometer. For other experiments,
such as that of the electric light, there will be needed
some one of the many fixtures for holding the carbons
to be ignited. If this can be put into a lantern the
carbons may be projected at once upon the screen by
removing the objective and drawing the carbons back
until the image appears plainly upon the screen. This
image will be made much sharper by putting a dia-
phragm with about an inch aperture over the conden-
sers, which in this case serves for an objective.

For the projection of spectra precisely the same
conditions need to be observed as for the lime light: —
Some regulator in the lantern, a slit in the focus of the
condensers, an objective to project the slit and the
prism in the focus in front of the objective. The spec-
trum of metals is easy with this arrangement. Make
a small cavity 'n the end of the lower carbon stick,
and place a small bit of the metal whose spectrum is
wanted in it; then bring down the upper carbon upon
it so as to complete the circuit and then raise it a little,
the metal will be at once fused and volatilized, emit-
ting its characteristic light, which will appear upon the
screen as bright bands. Silver, copper, zinc, iron, and
mercury give good spectra among the more common
elements.

For the successful working of this method of spec-
trum analysis, not less than forty cells will be needed,
and fifty are decidedly better than forty.

DECOMPOSITION OF WATER.

This is effected by sending a current of electricity
from three or four cells through water that has been

154

THE ART OF PROJECTING,

slightly acidulated by the addition of a little sulphuric
acid. The terminals of the wires in the water are
usually made of strips of platinum to prevent other
chemical reactions from taking place. For projection,
an excellent way is to insert two test-tubes filled with
the acidulated water, and introduce them into the tank
already described, having previously fixed the two
platinum terminals through the rubber bottom as

Fig. 111.

shown in Fig. iii. When the current is sent through
these wires the bubbles will rise rapidly and soon fill
the hydrogen tube. This tank is of course to be pro-
jected in the ordinary way, either with lantern or parte
luwiere, in which case the bubbles will appear very
large and the water will appear to be in great commo-
tion.

In place of water fill the tank with a solution of
acetate of lead, and without the test-tubes project the
tank and make connection with the battery of two or
three cells as before : the crystallization of the lead will
at once begin and rapidly grow upon one of the termi-
'-^"- reverse the current, and the formed crystals will

ELECTRICITY.

155

dissolve while others will grow upon the other terminal.
The same thing can be done still better by filling the
horizontal tank for the vertical attachment with the
solution of lead acetate, and then bending a piece of
platinum wire or of tin wire around the interior of the
tank. Then, on inserting another wire at the centre of
the solution, and making connection with two or three
cells so as to make the centre wire the negative and the
hoop the positive pole, a beautiful growth of metallic
crystals will shoot out from the centre and spread out
over the entire field. In place of the solution of lead
use a strong solution of the bichloride of tin, using a
tin hoop in the solution. Crystals of tin will shoot out
and appear in great beauty.

These solutions in the horizontal tank should not be
more than an eighth of an inch deep.

HEATING BY THE CURRENT.

Make a small coil of platinum wire, and thrust the
ends of the wire through the rubber of the tank, as

217. tVr

shown in the engraving, Fig. 112. Fill the tank with
water, and having projected the whole, send the current

156 THE ART OF PROJECTING.

through the wire. If the current is sufficiently great
the wire coil will be heated at once, and a convection
current will at once show itself in the water, the heated
water next to the wire rising rapidly to the top. The
effect will be still more marked if a drop or two of
some one of the aniline dyes be let fall from the surface
over the wire. Its greater density will carry it at once
to the bottom ; but when the current is sent through
the wire, the movements in the water will be rendered
very plain. The bichloride of tin or the sulphate of
zinc will also answer the same purpose.

CHEMISTRY.

Most of the chemical reactions that are usually ex-
hibited before classes in the recitation or lecture-room
can be shown in a much more satisfactory way by
means of the apparatus for projection than in the ordi-
nary way. The method is moreover both cheaper and
easier; cheaper, because each experiment requires but
a few drops of the substance in a test-tube or the
tank, instead of the large quantity necessary for many
to see at once, and easier, because the preparation
needed for experiments upon an extended scale is
always tedious and tiresome. One who uses the tank
(Fig. 20) for the first time for projection, say of sil-
ver, in a solution as dilute as two or three drops of the
nitrate to the tank full of water, will be surprised at
the prodigious precipitation brought about by the addi-
tion of a single drop of hydrochloric acid introduced
upon the end of a glass rod. Great heavy clouds roll
and tumble about upon the screen, looking as though
they might weigh tons.

CHEMISTRY, 157

ACIDS AND ALKALIES.

Nearly fill the tank with water and add a few drops
of blue litmus solution ; then dip a glass rod into a
weak acid solution of any convenient kind and gently
stir the litmus solution with it : it will turn red in the
neighborhood of the rod. After washing the rod, dip
it into an alkaline solution of ammonia or potash, and
again stir the solution in the tank. Blue clouds will
form in the red sky upon the screen until the whole is
again a beautiful blue.

In place of litmus solution use a solution made by
boiling purple cabbage. Acid turns this red, and an
alkali turns it green. Such changes may be effected a
number of times in succession in the same solution.

Nearly fill the tank with sulphate of soda, in which
is put either litmus or cabbage solution to color it ; the
latter is the best. After projecting it as a blue solu-
tion dip the terminals of a battery of three or four
cells into it. Decomposition will begin and the acid
and alkaline reactions will be observed about the poles.

REACTIONS AND PRECIPITATION.

Fill the clean tank nearly full of pure water and add
a drop or two of a solution of nitrate of silver and stir
it well. Then dip the glass rod into very dilute hydro-
chloric acid. Very dense clouds of chloride of silver
will form and fall to the bottom of the tank. Add a
few drops of strong ammonia water, and the cloudy
solution will again become clear.

A small piece of carbonate of lime or of soda placed
in the tank containing a very dilute solution of hydro-
chloric acid gives up its carbonic acid in apparently
large quantities

158 THE ART OF PROJECTING.

To water made slightly acid, add enough litmus
solution to turn it red and project it ; then drop a lump
of carbonate of arfimonia into it. It will dissolve
rapidly with effervescence, and the solution will be
made blue about the crystal, and if there is enough of
it the whole solution will ultimately become blue.

The gradual solution of substances in water may be
nicely shown by filling the tank with pure water and
dropping a crystal of alum or sulphate of zinc or sul-
phate of copper into it. Where the substance is dis-
solved the solution will be denser, and its refractive
powers changed, which will be manifest by gently stir-
ring it with a glass rod.

A dilute solution of copper sulphate may be placed
in the tank. With a pipette, force into the solution
some ammonia water : A dense precipitate will at first
be formed, which will afterwards be dissolved if am-
monia enough has been added, leaving the solution a
beautiful blue color. A few drops of sulphuric acid
will reproduce the precipitate and destroy the color ;
and when the solution again becomes clear, a few drops
of ferrocyanide of potassium added will produce a
brownish-red bulky precipitate, which will present a
fine appearance upon the screen.

In like manner all of the characteristic reactions of
inorganic chemistry may be projected, and often with
much less expenditure of materials than would be used
if large vessels were employed to demonstrate the
same things. One who has projected a number of
these phenomena will find no difficulty in projecting
any reaction that may be observed in a test-tube.

Pictures of chemical apparatus, of processes, etc.,
will be very convenient for projection when instruction
is given in chemistry.

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