THE MAGIC LANTERN
_AND ITS MANAGEMENT_.
THE MAGIC LANTERN
_AND ITS MANAGEMENT_
INCLUDING
_FULL PRACTICAL DIRECTIONS FOR PRODUCING THE LIME
LIGHT, MAKING OXYGEN GAS, AND PREPARING
LANTERN SLIDES_
BY
T. C. HEPWORTH
LATE LECTURER ON SCIENCE TO THE ROYAL POLYTECHNIC
INSTITUTION, LONDON.
[Illustration]
London
CHATTO AND WINDUS, PICCADILLY
1885
[_All rights reserved_]
PREFACE.
During the past few years, the introduction of a dry-plate photographic
process of such a nature that it can be practised successfully by
amateurs of both sexes has aroused an amount of interest in the camera
and its capabilities, such as few scientific instruments can command.
The Magic Lantern is now closely allied with the camera, for there
is no means of showing the perfection of a photographic picture so
well as by its aid. But many persons are deterred from adopting the
latter contrivance because they fancy that there must be innumerable
difficulties to surmount before they can hope to master its management.
It is for these that the following pages have been written, in which
I have tried to place clearly before the reader what can be done,
and how to do it. I have also tried to indicate the educational value
of the lantern, and have pointed out in a necessarily brief manner
how various branches of knowledge can be illustrated by its aid. The
numerous personal inquiries addressed to me after my lectures, and
the numbers of letters constantly received, asking for information
regarding the lantern, its management, and the preparation of lantern
slides, have convinced me that there is room for a manual, humble
though it be, dealing with these subjects.
T. C. HEPWORTH.
32, CANTLOWES ROAD,
LONDON, N.W.
CONTENTS.
THE HISTORY OF THE MAGIC LANTERN.
PAGE
The introduction of mineral oil lanterns.--The advance
caused by the aid of photography.--The optical
system of a modern lantern.--Educational value of
the lantern 1-12
THE LIME-LIGHT.
Different forms of lime-jets.--Precautions to be observed
in their use 12-16
DISSOLVING VIEWS.
The old method and the new.--The biunial lantern.--The
dissolving-tap and its management.--Coincident
discs.--‘Effects’--The application of the
double lantern to spectrum analysis 17-22
MAKING OXYGEN GAS.
Necessary precautions.--The purifier.--Explanatory
diagram.--Practical details.--The retort.--The
gas-bag.--Iron bottle for compressed gas.--Pressure-boards.
--Lime cylinders 22-33
THE SCREEN OR SHEET.
Arrangement adapted to a sitting-room.--Public exhibitions.
--Method of hanging a large sheet.--Sheets
on frames 33-37
PICTURES FOR THE LANTERN.
Photographic slides.--Home-made pictures.--Drawing
pictures on ground-glass.--The slide-painter’s
easel.--Mounting slides 37-43
ON THE COLOURING OF PHOTOGRAPHIC TRANSPARENCIES.
The process described.--Oil-colours employed.--Apparatus
required.--Mixed tints.--How to lay on the
colour.--Finishing touches.--Painting in water-colours 43-55
SHOWING SOLID AND OPAQUE OBJECTS ON THE LANTERN
SCREEN.
Chadburn’s opaque lantern.--The aphengescope.--Showing
coins, medals, etc.--The opaque lantern
in a law court.--The physioscope 56-61
THE LANTERN MICROSCOPE.
Photographic microscopic slides.--The solar microscope.--A
sketch lecture 61-65
MECHANICAL OR MOVING PICTURES.
Comic slips.’--Beale’s choreutoscope.--The dancing
skeleton 66-69
CONCLUDING REMARKS.
The management of the lantern in public.--Signals
between lecturer and operator 70-75
THE MAGIC LANTERN
_AND ITS MANAGEMENT_.
There is no optical instrument so well known or so highly held in
popular estimation as the Magic Lantern. It is somewhat unfortunate
that its old appellation, given to it when it was nothing more than a
mere toy, should have stuck to it so long. And more than one attempt
has been made by those conversant with its capabilities as a scientific
instrument, and as an important aid to education, to give it a name
more consistent with its real value. The late Mr. Dalmeyer was, I
believe, the first to re-christen it the ‘Optical Lantern;’ and there
is evidence that others are adopting the term, and that in time the old
name will sink into oblivion.
With regard to the early history of this deservedly favourite
contrivance, but little is known. Its invention has been ascribed to
Friar Bacon, but the evidence on this point is of the most legendary
character. It seems, however, certain that the first to give an
intelligent description of it was Kircher--a Jesuit who lived two and
a half centuries ago, and who published a work entitled ‘Ars Magna
Lucis et Umbræ,’ or ‘The Great Art of Light and Shadow.’ The body of
the lantern, as described by Kircher, consisted of a room several feet
across, furnished with an opening in which a lens was placed. The
source of light was an oil-lamp, and the pictures for exhibition were
of the roughest kind. Indeed, we may presume that the effects produced
fell far short of those attained by the cheapest toy lantern of our
schoolboy days. Even if we search the columns of an Encyclopædia or any
similar book of reference of forty years ago, we shall not find any
great improvement on the description afforded by Kircher. The magic
lantern will there be found described as ‘an optical toy, by which
glass pictures executed with coloured varnishes can be thrown upon a
wall or screen.’
But from this time a gradual improvement began. First, the old oil-lamp
was replaced by the argand burner; then came gas; finally the
brilliant lime-light--and some time in the near future, perhaps, will
come electricity. The improved means of obtaining light have naturally
led to larger pictures being thrown on the screen; for the size of
such pictures is governed only by the amount of light available. Thus,
supposing we own a modern lantern constructed with one of the improved
three or four-wick lamps, and that it will give us a good picture of
about six feet diameter, by using the same lantern, and without any
change whatever in its optical arrangements, save fitting it with a
lime-light jet, the disc can be increased to fifteen feet diameter.
A picture of the same size could be produced with the original lamp,
provided that the operator retired to the necessary distance from the
screen, but the light would be so attenuated that the picture would be
but a ghost of what it ought to be. It will therefore be understood
that the enlarged image formed by the lantern lens can be made to fall
at any point in front of that lens, and may be of any size; but to
render it practically available its size, and therefore its distance
from the lens, must be regulated by the amount of light at disposal.
During the past few years two circumstances have combined to render the
optical lantern more popular than ever: one being the introduction of
photographic transparencies representing scenes from the remotest parts
of the earth, and the other the construction of metal-bodied lanterns
of improved character, and burning mineral oil. As a good paraffine
lamp is compared with a farthing rushlight, so is one of these lanterns
to the toy of our boyhood. The light given, although it falls far short
of the brilliance of the lime-light, is much more intense than could
have been hoped for some years ago from oil. But its brilliance is not
all due to the paraffine, but quite as much to the careful ventilation
and general construction of the flame-chamber. There is, too, no danger
in the use of this form of lantern, for the reservoir containing the
paraffine is far below the combustion-chamber, and therefore it cannot
become unduly heated.
The lenses, too, in this new form of lantern, although placed in the
same position as those in the old-fashioned magic lantern, are of a
very different stamp. And this leads me to a brief consideration of the
optical system comprised in these instruments.
[Illustration: FIG. 1.
L, Light; _c_, _c_, Condensing lenses; _o_, _o_, Objective lenses.]
There are two sets of lenses: one being known as the condenser, and
the other as the objective. The condenser is made up of two or more
glasses, the usual form consisting of two plano-convex lenses placed
face to face. The duty of this condenser is to take up as much as
possible of the light afforded by the lamp, and to change the direction
of the rays so that they are cast through the picture or slide placed
in front of it. A glance at Fig. 1. will show how this is accomplished,
and how the rays indicated by the dotted lines would be lost if not
turned to account by the condenser. The use of the objective, on the
other hand, is to magnify the image of the picture, and to present
it in as perfect a form as possible on the screen placed for its
reception. Its distance from the picture is governed by the distance
of the lantern from the screen, and can be regulated to a nicety
by the focusing screw attached to it. It is generally acknowledged
by all acquainted with the requirements of the lantern that the
photographic camera lens (known as the quarter-plate portrait lens)
fulfils admirably this duty; and this particular form of lens, which
is by no means expensive, is generally fitted to modern lanterns of
the mineral-oil type. The condensers vary from four to three inches
in diameter; perhaps the best size is three and a half inches. The
relative positions of condenser, light, slide, and objective are
indicated in the diagram to which attention has just been called, Fig.
1.
The optical lantern always inverts the image of any picture projected
by it, an inconvenience readily obviated by placing the slide on the
stage upside down. At the same time the picture must have its front
turned towards the condenser, not away from it, otherwise every part
of the slide suffers reversal on the screen. In some cases this
does not matter, but when any wording, such as that on a signboard,
for instance, is contained in the picture, it reads backwards.
Such accidents can be prevented by a white label, easily seen in
semi-darkness, placed in a certain position on the picture. If such
a precaution had been observed at a certain exhibition at which I
was present, General Garfield would not have been shown standing on
his head--not a dignified position for the President of the American
Republic.
The first lanterns, constructed to burn mineral oils, were very
different to those of modern make, and took their pattern from the
cumbrous instruments which before them were provided with colza-oil
lamps. The reservoir for the oil was carefully kept in a cistern at
the back of the lantern, and bubbled through a pipe to the lamp on
the same principle that water is supplied to an earthenware poultry
fountain. It was doubtless thought necessary in those days to maintain
the paraffine in the cistern at a height equal to or above the wick,
as in the case of colza and other heavy oils. But paraffine is more
of the nature of a spirit--for it volatilizes very readily--and in
modern lamps for lanterns the reservoir is kept some inches below the
point of combustion. By the time that the liquid reaches this point,
it has become so heated in the course of its passage along the metal
wick-holders that it issues as a gas. This can be readily proved by
turning down the wicks after the lantern has been burning some time,
when the flames will remain above the wick-holders, although the wicks
themselves have been turned down quite out of reach. In the older
lanterns, too, the glass chimney was thought to be indispensable, but
now this has been done away with, and with it the cylindrical wick has
also gone. The modern combustion-chamber is made of charcoal iron, and
is closed at one end with a thin pane of glass, just outside which the
condensing lenses are placed, and at the other end by a silver-plated
reflector. The necessary air to promote combustion is admitted by
suitable orifices below, and a draught is established by a long iron
chimney above.
There is no great difficulty in managing one of these lanterns. The
loosely plaited cotton wicks should in the first instance be carefully
cut parallel with the wick-holders. After this they do not require
much attention. It is a far better plan to remove the charred end of
the wicks by scraping them along with the back of a knife, than to be
always cutting them afresh. However, any loose strands that may project
from the cotton should be certainly snipped off with scissors.
After lighting the lantern--which is best done with a long
wax taper--each wick should be turned down quite low, and the
combustion-chamber closed. In about two minutes, by which time the
front glass and the lantern generally will have become warmed, the
wicks should be turned up to their highest; that is to say, to a pitch
just short of smoking-point. If the wicks are turned low for any length
of time the lamp will be sure to smell. It will also smell if, in the
process of charging with oil, some of the liquid has been smeared
against any part of the lantern which may afterwards become heated. The
best oil only should be used.
I have already hinted at the educational value of the optical lantern,
but those who have not experimented with it have but a faint idea of
its capabilities in this respect. I am of the opinion that every school
and college should possess one, and that both pupils and teachers would
soon discover its many advantages. Now that it is manufactured in such
a portable and efficient form, and can be so easily managed, there
is no possible hindrance to its adoption, unless it be its old name
of ‘magic lantern,’ which is associated with things too childish for
consideration. But this objection cannot hold good when it is pointed
out how many different branches of knowledge can be illustrated by its
use.
The schoolmaster of a London Board School once told me of some of
his difficulties in teaching such a subject as geography. Most of
the children had never seen the sea, and many of them had never even
travelled so far as the Thames. What notion could such waifs have of
a mountain, a valley, a cliff, or even of a rock? The value of the
optical lantern at once becomes apparent here. But better informed
children in high-class schools, although they see many places beyond
the London streets, can yet reap much advantage from the mode of
instruction which I am advocating; and as the range of knowledge
increases, so will some such means of illustration be forced upon
teachers. The labour and expense of tracing a dozen diagrams on glass,
in the way hereafter explained, is considerably less than that involved
in drawing a single large diagram for the schoolroom wall. By this easy
method the illustrations in any book can be roughly copied and rendered
available for an entire class.
In the higher branches of knowledge the lantern can also give its help.
Botany, zoology, and natural history generally can be illustrated by
diagrams, photographs, and in many cases by natural preparations. The
revelations of the microscope can be transferred to the lantern, and
the most complex organisms can be seen clearly defined and magnified
many hundred times their natural size. It is possible, too, to show by
the lantern many interesting experiments in chemistry and electricity
which cannot be shown to a large number of persons in any other way.
Let me cite for instance the formation of crystals, which can actually
be watched in progress, highly magnified, by the simple expedient of
smearing a piece of glass with a solution of sal ammoniac and placing
it in the lantern. There are several adjuncts which may be fitted to a
first-class lantern which I have not space to notice in detail. With
the lantern-microscope ordinary microscopic slides can be utilized,
and tanks for containing the living denizens of our ponds and ditches
may be employed. The marvels of polarised light may be demonstrated
with the lantern-polariscope. By another special device called the
‘aphengescope’ and which is made to fit on the lantern, opaque objects
can be thrown on the screen. It is possible, for instance, to utilize
ordinary photographs or diagrams on card, to show the moving works of
a watch, sections of fruit, and many other objects as explained in a
subsequent chapter.
The lantern, at the time of exhibition, should stand firmly. It can,
if it be a small one burning oil, be placed on a box standing upon
a table. I myself prefer to place it on a photographic tripod-stand,
and this I effect by screwing on to the top of the stand a base-board
upon which the lantern firmly fits. For large lanterns, such as the
lime-light biunial, a far more solid arrangement is requisite. My own
method is this. The lantern screws on to the top of its travelling-box,
at the lower corners of which are sockets to admit four wrought-iron
legs. These legs are bent outwards, and what I may call their toes are
turned out, and have a hole into which a screw may be inserted and
driven into the floor. The lantern is fastened to the box by two hinged
pieces at the back, so that the nozzles can be raised to any extent
desired.
THE LIME-LIGHT.
The lime-light consists of a jet of mixed hydrogen and oxygen gases
under pressure, ignited, and forced upon a cylinder of lime, which it
renders white-hot. The heat given by these gases is second only to the
heat of the electric arc, and will melt that most refractory of metals,
platinum. Even the lime block cannot withstand the great heat to which
it is exposed, but is quickly pitted under the action of the flame.
Hence lime-light jets are furnished with an arrangement by which the
lime cylinder can be turned at frequent intervals, so as to offer a
fresh surface for the gases to play upon.
There are three kinds of jet used for the lantern. Firstly, a jet in
which a stream of oxygen is forced through the flame of a spirit-lamp
on to a cylinder of lime. This form is perfectly safe, and although it
presents an immense improvement upon any form of oil arrangement, does
not afford sufficient light to illuminate a screen more than about nine
feet in diameter. Secondly, there is what is known as the blow-through
jet. In this case the jet is connected with the house gas, and the
oxygen meets it at the point of ignition, and is _blown through_ it on
to the lime. This form of jet is also safe; indeed, it is often called
‘the safety jet,’ and affords plenty of light even for professional
use. It is the one that I recommend the amateur to work with. Thirdly,
there is the mixed jet, in which the two gases in separate bags are
both under pressure, and mix together before reaching the external
orifice. This form of lime-light gives the most light of any; but such
care is requisite in dealing with it, that I shall say no more about
it, considering it unfit for casual acquaintance.
A beginner might fancy that it would be a simple thing to mix the two
gases in one bag, put it under pressure, and use them thus. So it
would, and a beautiful light would be the result. Probably a beautiful
explosion would be another result, for the two gases mixed form a most
terrible compound, and a bag so charged would be almost equal in danger
to a live shell.
With the spirit jet, or the ‘blow-through,’ the operator need have no
fear of danger. Oxygen is not an explosive, and, although the best
supporter of combustion, is not itself capable of being ignited. The
spirit jet I should not recommend, except in situations where coal gas
is not obtainable, so in my directions for using the lime-light, let it
be understood that I am considering the employment of the blow-through,
or safety jet.
Let us suppose, then, that we are preparing for an exhibition. The gas
is made, and is at hand in the gas-bag; our lantern--and I will, for
simplicity’s sake, consider it a single lantern only--is raised on its
stand-table, or other support, at a convenient height from the ground,
and we wish to have a private rehearsal in order to see that all is
right. The first thing is to attach the hydrogen terminal of the jet,
marked H, by flexible tubing to the nearest gas-bracket. Now take a
cylinder of lime from its box, clear out the hole in the centre with a
bit of wire, or a match, and place it on the pin provided for it above
the jet. See that it turns freely, and so adjust the pin that the lime
is distant from the jet nozzle about the sixteenth of an inch. Light
the gas, and turn it down so that the flame is about an inch high. This
can be done with advantage an hour or more before the lantern is really
required, for the lenses and body of the lantern will by this means get
thoroughly warmed, and any moisture upon the glasses, which would show
as a blemish on the sheet, will be removed.
In all cases the hydrogen should be lighted first, if only to
thoroughly warm the lime. Now the oxygen-bag can be put between the
pressure-boards, and connected by another tube to the tap marked O.
A half-hundred-weight having been put on the boards, the gas-bag tap
may be turned on to the full. We may now attend to the jet. Turn on
the hydrogen so that it flames up some inches over the lime, then
gradually turn on the oxygen. At first it is air only that comes away,
but presently with a characteristic little snap the two gases come
together, and the brilliant lime-light is produced. A little care in
adjustment of each tap alternately will soon show us the amount of gas
from each which will give the best result.
It will soon be apparent to the operator that the spot of light on
the lime must be exactly in the axis of the system of lenses, or the
effect upon the sheet will be spoiled. The vertical movement of the
jet is governed by a little screw, which holds it to the post on which
it is fixed, which post stands upon an iron tray gliding between
grooves. We must raise or lower the jet until the right place is found,
when the screw can be brought home, and the jet is fixed. Even now,
most probably, we shall find that the outer margin of the disc is
ill-defined. This shows that the light is either too near or too far
from the condenser. By moving the jet on its tray bodily backwards and
forwards, we shall soon be able to find its correct position, and when
found, that position will hold good to the end of the exhibition. We
can now put a picture on the slide-stage, and focus it by means of the
screw on the front lens. If the lime is properly adjusted and centred,
our picture will be illuminated equally well in every part.
DISSOLVING VIEWS.
So far, we have considered the production of a brilliantly illuminated
picture with a single lantern. For many years after the lime-light
came into use, this was all that was required of it. But suddenly the
beautiful effect known as dissolving views was contrived--an effect
which, at the time of its introduction, made a wonderful sensation--and
the method of producing which was for a long time kept secret. The
old way of dissolving one picture into another, and the way which
must still be adopted if oil-lanterns are in question, was to use
two lanterns, side by side, and by a kind of see-saw arrangement in
front of the lenses, to gradually uncover one nozzle whilst the other
was being closed. This was easily done by furnishing each end of the
see-saw with a screen of tin, the edge of each being cut into teeth
like a comb. The one picture was thus caused to mingle with the other
until the first lantern was quite closed, when the new design became
perfectly disclosed. It soon became evident that the same effect could
be produced more simply and economically with the lime-light lanterns,
by contriving a special form of gas-tap which would slowly turn off
the gases supplied to one lantern, while it as slowly admitted the
gases to the other. By this arrangement nearly half the gas supply is
saved, and therefore very little more is wanted for a dissolving-view
apparatus than for a single lantern.
Dissolving view, or biunial lanterns, as they are generally termed, are
now made in very compact form. The two systems of lenses, one above
the other--not side by side as of old--are fitted into one mahogany
case, lined with tin, and furnished with doors, so that the lights can
be tended when necessary. At the back of the arrangement is placed
the dissolving-tap, which is connected by indiarubber tubing with
the lime-jets, and has two nozzles by which the hydrogen and oxygen
respectively can be supplied to it. The dissolver is also furnished
with by-passes, so that when the gases are turned from one lantern,
just sufficient remains to keep the jet turned down ‘to the blue.’
The management of a pair of lanterns like this is, of course, much
more onerous than that of a single lantern; but when once understood
presents no difficulty. Before lighting up, be quite sure that all
connections--and there are many--are quite secure, and that each place
where indiarubber tubing fits on to metal is secured with twine. Now
light the hydrogen in both lanterns, which you can do by placing the
dissolving-tap in an upright position. Move the dissolver until the
upper jet seems on the point of going out; but prevent it doing so by
opening the hydrogen by-pass, and adjust this little tap so that the
flame remains about one inch high when the other lantern is being used.
Move the dissolver backwards and forwards a few times, so as to see
that it works well, and that the hydrogen flares up in each lantern
alternately.
We can now pay attention to the oxygen supply. First see that the
bag is properly adjusted between the pressure-boards, and that the
weight--one 56 lb. weight is sufficient to begin with--is in its
place, above the upper board. Attending to the lower lantern first,
turn up the hydrogen, and very gently admit the oxygen by turning
the tap attached to the jet. When the light has been satisfactorily
established, the oxygen by-pass must be turned in the same way that the
other by-pass was just now treated, so as to admit a small amount of
gas passing to the lantern not in use. If this be not done, the sudden
inrush of the oxygen is sure to cause a small explosion, which will
blow the light out. This does not indicate any danger whatever, but is
inconvenient and undesirable during an exhibition. The upper lantern
may now be attended to with the same precautions; and if all has been
done well, the light will move from jet to jet alternately, as the
dissolver is worked.
In using a double lantern, it is necessary to make both pictures
coincident on the sheet. If one picture overlaps the other at every
change, it has a most slovenly appearance. Such an error can be avoided
by manipulating certain screws fitted to the metal front of the
lantern, by which the two discs thrown upon the sheet can be adjusted
until they quite coincide. These screws allow the lower nozzle to be
pointed slightly upwards, and the upper one to point downwards, so that
the images cast by each may be made to meet at the screen.
The double lantern is mainly employed for dissolving views; but its use
does not stop here. A great many beautiful “effects” can be compassed
by its means, a few of which I may here describe. Statuary slides form
very beautiful pictures if photographed direct from the marble; but
their effect is much enhanced if, by means of the auxiliary lantern,
a glow of colour is thrown upon the screen at the same time. A few
squares of differently tinted glasses, each mounted like an ordinary
slide, with a mask of oval or round shape, are all that is needed.
Again, a wintry scene in one lantern may be much improved by the effect
of falling snow, produced by working a special form of slide in the
other lantern. This slide consists merely of a frame containing a
roller at the top with the handle projecting outside. As the handle
is rotated, a long ribbon of black paper pierced with needle-pricks
is rolled up upon it, and each prick makes a descending spot of light
upon the screen, which together look exactly like falling snow. Sunset
and moonlight effects, windows lighted up in night-scenes, ripples
upon water, can all be managed by specially-devised ‘effect’ slides in
the second lantern. These effects can be much extended when a triple
lantern is employed; but as this instrument is not often found--out of
the hands of professional operators--we need not further allude to it.
But the double lantern can be used with great advantage from an
educational point of view, in a manner that was first suggested by
the present writer. I mean in the demonstration of the main features
of Spectrum Analysis. Let me cite one example. A slide prepared and
coloured so as to represent the continuous solar spectrum, marked
with the principal _Frauenhofer_ lines, is placed, say, in the lower
lantern. We now wish to show the bright lines given by an incandescent
metal, say _sodium_. A slide, all blackened out but the double D
line due to sodium, and so placed on the glass that it will exactly
register with the D line in the coloured spectrum slide, is placed in
the upper lantern. The dissolver is now brought into play, and can be
so adjusted that while the continuous spectrum has all but faded away,
the two brilliant sodium lines stand out boldly in their proper place.
The spectra of all the other metals can be treated in exactly the
same manner, keeping the continuous spectrum in the lower lantern for
constant comparison and reference.
MAKING OXYGEN GAS.
There is very little danger incurred in the use of the lime-light
if only the operator be intelligent in its employment. But there
are certain dangers connected with the preliminary making of the
oxygen gas, so that that part of the work should only be entrusted
to a careful worker, and one who, knowing where the greatest care is
required, will take every precaution against disaster. I have myself
made several thousand feet of oxygen on different occasions, and have
never yet met with any mishap; but as I know of cases where serious
accidents have occurred, I have endeavoured to find out why they
have happened, and I now consider myself forewarned, and therefore
forearmed, against their repetition. The articles required for making
the gas comprise a retort in which to generate it, a gas-stove to
furnish the necessary heat, a wash-bottle or purifier, an indiarubber
gas-bag, and several feet of tubing. Beyond these is wanted the mixture
of chlorate of potash and peroxide of manganese in powder, which forms
the charge of the retort, from which the gas is generated on the
application of heat.
I have found that the best proportions of chemicals to use are four
parts (by weight) of chlorate to one of manganese. Before being mixed
together, both should be most carefully picked over, and most probably
both will yield a small crop of bits of straw and wood-chips, which are
not only injurious, but would in sufficient quantity prove actually
dangerous if permitted to remain. The larger crystals of the chlorate,
and any lumps which may be found in the manganese, should be rubbed
down to powder by gentle pressure with the wooden spoon used for mixing
the compound, about two pounds of which will yield sufficient gas for
an evening’s show. The mixture can now be funnelled into the retort,
the nozzle of which, after being blown through to see that the passage
is clear, can be screwed into its place.
The purifier, or wash-bottle, is made of tin, or may consist of
a wide-mouthed bottle with an indiarubber cap, fitted with inlet
and outlet tubes of metal or glass. In either case the vessel is
three-parts filled with water, the inlet tube dipping some inches below
the surface. This tube is subsequently connected by three or four
feet of indiarubber tubing with the retort. As the gas is given off,
it bubbles through this water, which not only cools it, but catches
the particles of solid matter which are sure to be blown from the
retort. The outlet tube is connected with the gas-bag. In the annexed
illustration the relative positions of the retort, purifier, and
gas-bag are shown, but in practice certain modifications are desirable.
It is as well, for instance, to stand a chair between the retort and
purifier, upon which the connecting rubber tube may rest, otherwise the
water evolved from the crystals of potash will condense in the tube
and obstruct the passage of the gas. Again, the gas-bag, instead of
being on the same level, should be laid on a table, so that any water
accidentally driven from the purifier may not be forced into it. The
rubber tubes should have an orifice of nearly half an inch, and should
on no account contain an inside coil of wire. Previous to making gas
the bag should be warmed for an hour or so in front of a fire, so as to
make it soft and pliable. The cock should then be opened, and the bag
rolled tightly up so as to expel any air. The tap should then be closed.
[Illustration: FIG. 2.
Diagram showing relative positions of _g b_, Gas-bag; _w b_,
Wash-bottle; and _r_, Retort.]
When all is ready, and every junction made except that between the bag
and the pipe leading from the purifier, the gas-stove may be kindled,
but turned down almost to its lowest point. If this precaution be not
observed, the oxygen will come off with such rapidity as to be quite
out of control, and will probably blow the connections away. In two or
three minutes the gas will commence to bubble through the purifier,
at first intermittently, but the bubbling will gradually become more
regular, until it is evident that oxygen is coming over in earnest. The
brightening of a spark at the end of a blown-out match, held to the end
of the purifier-tube, will announce the fact that all the air has been
driven out of the pipes, and that gas has taken its place. Now is the
time to connect the gas-bag with the purifier. With a quick movement
the indiarubber tube should be slipped over the nozzle of the bag with
one hand, while the tap is turned on with the other. The gas should
now come over regularly until the bag is nearly half full, and this
will occupy about fifteen minutes. The action will then probably flag a
little, when the flame feeding the stove may be increased. When the bag
is nearly full the flame can be raised, if occasion should require, to
its utmost.
Some care is requisite in finishing the operation. When the bag is
quite full and as tight as a drum, it should be disconnected before
anything else is done. Of course at the moment of taking the tube from
it, the tap must be turned. The next thing is to detach the retort
from the purifier _before the gas is extinguished in the stove_. This
is a most important point, for if the retort were cooled by putting
out the gas first, the water might be sucked into the retort from the
purifier, and a small steam-boiler explosion would be the result. In an
hour or so the retort will be cool enough to handle, when the nozzle
can be unscrewed and the vessel washed out with several changes of hot
water. It can then be dried and put away for the next occasion. The
retort-nozzle and its indiarubber tube should also be washed out, or it
may get clogged.
Particular care must be taken to purchase the chlorate and manganese of
some reliable dealer. Accidents have been reported owing to lamp-black,
bone-black, etc., having been supplied in error for the manganese,
which is a black powder much resembling them. I find that the best
plan is to buy several pounds of each at a time, and to test a small
quantity of the mixed ingredients in a test-tube over a spirit-flame.
If the gas comes off with a slight sparkling of the ingredients all
is right; but if there is anything approaching to an explosion, the
manganese is at fault. One accident which I heard of was due to
connecting the retort to the wrong side of the purifier; the gas had
no outlet, and the retort exploded, breaking all the windows of the
room in which it was.
Retorts are made either of copper or iron. I have tried both, and much
prefer the latter, which have also the advantage of being cheaper than
the copper ones. A good retort should serve for at least fifty charges,
and even then a new bottom can be put into it, and it will do duty for
another fifty. But no retort will last long unless properly treated. I
am quite satisfied that the spent charge if allowed to remain has the
effect of eating into the metal; it should, therefore, be washed out
soon after use. The thickness of a retort after much use can be gauged
by tapping it with the back of a knife when empty.
The oxygen gas is most usually stored in an indiarubber bag. These
vary very much in quality, and the cheapest kinds are to be avoided.
The rubber should be soft and pliant, and should be covered outside
with stout twill. A good bag will almost last a lifetime if it be only
used occasionally. I have had one for many years, which looks shabby
enough with constant use, but seems to be as gas-tight and as efficient
generally as the first day I had it. Even then it had been in use for
some months by a friend.
[Illustration: FIG. 3.
Iron bottle for compressed oxygen gas.]
A bag full of gas is not the most portable thing that could be named,
and various attempts have been made to replace it by some contrivance
of a more convenient character. There is, too, the danger of accidental
injury, which can readily occur if the bag is carelessly handled. In
very cold weather, for example, some bags get quite stiff and hard,
and have to be almost thawed before they can be used. Neglect of
this precaution will lead to cracks, which will eventually turn into
holes. Metal gas-holders--miniature copies of the big reservoirs seen
at gas-works--which serve as travelling-boxes for the rest of the
apparatus--are recommended by some, but are seldom used. Perhaps the
most convenient arrangement--which, however, is only suitable for a
single lantern--is to employ an iron bottle in which the oxygen is
greatly compressed. Such a bottle charged with gas can be obtained
from several dealers, and I have had some little experience of their
use. A bottle measuring three feet in length, and about seven inches
in diameter, will hold about eight feet of compressed gas--sufficient
for an ordinary evening’s work. At the end of the bottle is an opening
closed with a tap which can only be opened by a special form of key.
Into this orifice is screwed a nozzle, upon which an indiarubber tube
can be readily fitted. When a bottle of this kind is used, the amount
of oxygen which reaches the lime must be directly controlled by this
tap, and not by the tap attached to the jet, which must remain fully
opened. The reason of this is that the gas issues with such force,
that, unless controlled in the way I have stated, it will blow off
the tubing by its pressure. I found it so difficult to regulate the
supply, that I had an independent screw-tap made to fit on to the
bottle. This screw-tap is of the same pattern as those used on steam
engines to control the supply of steam to the cylinder, and with it
I can adjust the oxygen supply with the greatest nicety. My chief
objections to the bottle system--which presents many advantages in the
matter of portability--are, firstly, that you cannot charge the bottle
yourself--it must always go back to the dealer for that purpose--and
you are therefore dependent for one of your first requisites upon
others; and secondly, you do not know how much gas you are using,
for you cannot see it diminish as you can when you employ a bag. For
these and other reasons I still prefer bags to store my gas, and shall
continue to do so until some improvement comes about.
[Illustration: FIG. 4.
Gas-bag and pressure-boards.]
The gas-bag is wedge-shaped, and the thin edge of the wedge has in its
centre a gas-cock for the entrance and exit of the gas. The object
of the bag being in this form is, that it can conveniently be placed
between the pressure-boards. These, in their simplest form, consist
of two boards strongly put together--the size of the bag which is
used--which are hinged together at one end. At the centre of this
end, and between the hinges, is cut an oval opening through which the
gas-cock is thrust. A shelf on the upper board serves as a support for
the weights. At least two weights are required--one being sufficient
at first, when the bag is full; the other being added later on when
the pressure becomes reduced. The weighted gas-bag should always be
placed as near the lantern as circumstances will allow--in order that
it may be under the eye of the operator--and the tube from it which
supplies the lantern should be guarded, so that it is not accidentally
stepped upon.
The lime-cylinders are sold in air-tight tin canisters holding one
dozen each. They are of two qualities--hard and soft. For work where
large pictures are required--necessitating the use of the mixed jet
with both gases under heavy pressure--the hard limes must invariably
be used. But where only a small sheet has to be covered, and one gas
only is subjected to the unusual pressure, the soft limes will afford
perhaps a better light than the harder ones. Occasionally the lime, be
it hard or soft, will split. In such a case it ought to be immediately
replaced by a fresh one, otherwise the flame may possibly be diverted
towards the condensing lenses, and will fracture that nearest to the
light. It is advisable always to keep a spare lime within the lantern,
lying on the tray to which the jet is attached. By this means it gets
warmed, and ready for use if required. Lime is most absorbent of
moisture, and will, if kept where air can get to it, swell to double
its former size, and finally crumble to powder. For this reason the tin
canister containing the cylinders should have stretched over it, when
once opened, a broad indiarubber band which will cover the junction
between lid and box. If this precaution be neglected, the limes will
swell with a force sufficient to break the tin-containing vessel
asunder. A compressed lime cartridge, to which water is applied by
artificial means, has lately been successfully introduced to supersede
explosives hitherto used for mining purposes. This will show the
immense power of expansion of which the substance is capable.
THE SCREEN OR SHEET.
And now a word about the best form of screen or sheet for showing
lantern pictures upon. The best possible form of surface is a smooth
whitewashed wall; but as this is not commonly found among the
appointments of a sitting-room, where the lantern will be wanted, we
must find some substitute which will most nearly resemble it. A sheet
of cartoon paper, which can be bought of any length, and measuring more
than four feet across, will do well if only a small disc is desired.
The paper can be rolled up out of the way at a minute’s notice. If a
larger screen is wanted, it can be made of stout calico, faced with
white paper, and can be made to roll up and down like a school map of
large dimensions. A map-mounter, or even an upholsterer, would soon rig
up such an arrangement.
There is a very effective way of showing small pictures and diagrams
with an oil-lantern, which I have more than once adopted, where a
room has been long in proportion to its breadth. This is to make a
wooden frame just large enough to take the full width of a sheet of
tracing-paper, and to put this screen between the lantern and the
spectators. Tracing-_cloth_ should be avoided, as it is so transparent
that the light streaming from the lens makes a blotch in each picture
when seen through it.
If the lantern is brought into regular use--in a schoolroom, for
instance--it might be thought worth while to have a canvas sheet
whitewashed, and hung in the same manner that a stage drop-scene is
fixed. In making such a screen, the canvas should be tacked on a frame,
and should have a coating of thin glue, which should be allowed to
dry before two or more coats of whitewash are applied. It will be
understood that a really opaque screen, such as this represents, is the
more effective, because the light, instead of being partly lost--as
it must be in penetrating an ordinary calico sheet--is reflected and
utilized.
But for public exhibitions on a large scale, the calico sheet is the
screen commonly in vogue, for it is conveniently put up and taken down,
and can be rolled into a bundle for easy transport. A sheet properly
hung should be as flat as a board and perfectly free from wrinkles
of any kind, and this can be accomplished without much difficulty by
adopting the following method, which is applicable to screens from
twelve feet square upwards.
[Illustration: FIG. 5.
Showing method of hanging a sheet.]
The sheet should have at its top edge a strong cord sewn into its hem,
which cord should terminate at each side with a loop. On its sides and
bottom edges, the sheet should have brass curtain rings sewn on to it
at intervals of two feet. Having chosen the position of the screen, a
couple of screw-eyes are screwed into the roof, or cornice near the
top of the hall, at such a distance apart that the sheet can easily
go between them. Through each of these screw-eyes is passed a thin,
strong line, having at its end a clip like that commonly attached to a
dog’s chain. This clip is to clutch the loop of rope on each side of
the sheet. The sheet can now be pulled up into position: the free ends
of the cord being fastened to screw-eyes in the floor. It now merely
remains to lace with string the curtain rings on the side of the sheet
to the adjacent cord, and the arrangement is complete. The annexed
diagram shows the upper corner of such a sheet, with its attachments,
as just explained. Where the hall is of such a height that this plan is
not available, or where the stern custodians faint with horror at the
thought of their walls or ceilings being pierced with a screw-hole, the
same method can be carried out by fastening the screw-eye to a timber
upright on each side, supported by light struts.
Some persons prefer a sheet stretched on a frame, and such a frame of a
very portable character can be easily devised. The frame itself is best
made of lengths of pine, about the size of broom-handles. These can be
joined together so as to make a frame of any reasonable dimensions,
by six-inch lengths of brass tubing. The same tubing, mitred into
L-pieces, will serve for the four corners. The sheet is furnished
with tapes to secure it to the frame, and the woodwork is kept in an
upright position by strong cords fastened by screw-eyes to the floor.
The appearance of such a screen is much enhanced if the plain woodwork
be hidden by a little drapery in the shape of narrow curtains at each
side, and festoons of the same material above.
PICTURES FOR THE LANTERN.
Before the advent of the art of photography, pictures or slides for
the lantern were traced on glass and painted by hand; and this art
was carried to very great perfection. But no handiwork can equal
the productions of the photographic camera; so that when it was
found possible to produce transparent positives on glass, that could
be magnified on the sheet to any extent without deterioration, the
occupation of the artist on glass was almost gone. Photographs are now
almost exclusively used, and form, when properly executed, the finest
pictures for lantern use. The manner of their production is fully
described in my little book ‘How to Photograph;’ but as there may be
many would-be lanternists who do not care to dabble in photography, I
will here give a few plain directions by which lantern slides can be
extemporised.