THE OPTICAL SYSTEM OF A LANTERN
[Illustration: FIG. 33A.--Optical System of Lantern.]
As previously noted, the essential parts of an Optical Lantern are, in
order from rear to front: (1) The illuminant; (2) the condenser; (3) the
slide and slide stage; (4) the objective, to which must be added, (5) the
body or framework which holds the whole together. Fig. 33 is a diagrammatic
representation of the entire optical system and Fig. 33A shows all the
various parts _in situ_: A being the illuminant, shown in Fig. 33 {58} as
an arc lamp, B the condenser, C the slide stage, and D the objective. The
foundation, so to speak, of the whole instrument is of course the slide,
which, as made in this country, consists of a square of glass 3¼ inches
diameter, the slide itself being somewhat less than this on account of the
binding, &c.; in making calculations it is usually taken as a 3-inch
circle. Slides are usually made by binding together with strips of paper or
cloth two such squares, on one of which is the photographic film or
painting forming the picture, the other being simply a plain cover glass
placed over the slide surface to protect it, and between the two being
placed a paper mask with an aperture of whatever size or shape is required,
that of the aforesaid 3-inch circle being usually taken as the standard or
normal dimension for this aperture.
The slide being illuminated by one of the various methods discussed in the
previous chapters, is focussed on the screen by the objective, which must
be selected according to the size of picture required and the distance
between lantern and screen.
These points will be gone into later, and also details as to various types
of objectives and their respective advantages; but it may be said here that
a lantern objective consists usually of a combination of lenses of 2 inches
or 2½ inches diameter mounted in a rackwork focussing system at a distance
from the slide of 6 inches to 18 inches, according to the length of its
'focus.' As our slide is from 3 to 3¼ inches diameter, it is evident that
all the light radiating from this cannot possibly get through the objective
unless it is _converged_ upon it, and to do this is the function of the
condenser. The following two diagrams, Figs. 34 and 35, will make the
matter clear.
S represents our glass slide of 3 inches clear diameter, R the radiant or
illuminant, and L our objective, shown here for the sake of simplicity as a
single lens.
The slide is well illuminated by the light emanating from {59} R, but it is
obvious that the bulk of this light will never pass through the lens, and,
in fact, only the very centre of the slide will under these circumstances
appear upon the screen at all.
[Illustration: FIG. 34.--Optical System without Condenser.]
[Illustration: FIG. 35.--Action of Condenser.]
What is evidently wanted is to _converge_ these outer rays, or in other
words to bend them in so that they also pass through the objective, and
this is the function of the condenser as illustrated in Fig. 35. The
condenser is here represented also by a single lens, but in practice it
also is invariably constructed of two or even three lenses, for both
optical and mechanical reasons. It is evident from the above diagrams that
the condenser must be somewhat larger in diameter than the slide itself,
and condensers for ordinary lantern work are usually 4 inches to 4½ inches
diameter. The former size {60} will suffice if the condenser is placed very
close to the slide, but it is often advisable to leave a little intervening
space, especially if the illuminant is a powerful one, in order to allow
any condensation of moisture readily to evaporate and escape. Hence
lanterns for long range work (which involve, of course, good illumination)
are usually made with condensers of 4½ inches diameter. Lantern condensers
of to-day usually take one of the two forms shown in Fig. 36, but the exact
curve must be left to the manufacturer, as the focus of the condenser must
have a definite relation to that of the objective. Taking, however, the
design of E, the most common of all, the two lenses should not be exactly
similar unless the objective is pretty short in focus, or, in other words,
unless the distance of the illuminant on the one hand and that of the
objective on the other are approximately equal. If the lantern is intended
for long range work, that is equipped with a long focus objective, the
front component of the condenser should also be constructed longer in focus
(that is to say, with a shallower curve) than the rear one, and it is
amazing how careless manufacturers are in this respect. If, as is often the
case, the lantern is fitted with several objectives of different foci, it
is usually necessary to supply alternative condensers also, or at least to
supply an interchangeable front component.
[Illustration: FIG. 36.--Forms of Condensers.]
If the entire condenser is too long in focus, light is lost; if too short,
it is impossible to obtain an even disc, as there is invariably a dark
patch either in the centre or round the edges.
The mounting of the condenser also varies with different makers; but it
must be remembered in any case that it gets {61} extremely hot, especially
the back component, and hence the glass must be mounted _loose_ in its
cell, otherwise there is great danger of it cracking. Also the space
between the components should be well ventilated, in order to provide for
the escape of moisture which usually at the start of a lantern exhibition
is deposited upon the glass, and should be got rid of before the actual
lecture commences.
Even with all care, the back component of a condenser will sometimes crack,
though such an accident should be a rare occurrence; and hence a
professional operator will usually provide himself with a spare lens, and
the condenser should be so constructed that it can readily be changed, and
with as little delay as possible.
Condenser lenses as made in this country are usually ground from the glass
known as 'English Crown,' and comparatively rarely crack; but they are very
slightly green in colour. French condensers, on the other hand, are whiter,
but the glass is more brittle, and a fracture a more common occurrence. The
French variety are (or were before the war) cheaper and generally met with
in cheaper instruments. More expensive lanterns are usually fitted with
English condensers, as the tinge of green is almost imperceptible, and the
advantage as regards greater security pretty considerable.
THE SLIDE CARRIER AND SLIDE STAGE.--Taking still the optical system of the
lantern in order from back to front, we now come to the slide, slide
carrier, and slide stage. The slide itself has already been described, and
the carrier is simply a mechanical contrivance, usually of wood, designed
for the purpose of readily changing the pictures and which in its turn fits
into the stage of the lantern. It may be asked why, if slides are now
always made to a standard size, the slide carrier should not itself be
built into the lantern and form the stage; but the answer is, in the first
place, that slides of a different size, _i.e._ American or Continental,
_may_ be met with, {62} and also that there are various mechanical slides
on the market--for example, chromotropes or scientific models, such for
instance as are made to illustrate the movements of the planetary
bodies--and these slides are permanently mounted in wooden frames which
could not be put into a carrier. The commonest form of carrier is that
known as the 'Double Sliding' pattern (Fig. 37), which consists of a frame
with two apertures for the slide, and an outer frame through which this
itself slides and which fits the stage of the lantern.
[Illustration: FIG. 37.--Double Sliding Carrier.]
This carrier, as will be seen, allows the next picture to be placed in
position in the second aperture while the former one is being projected,
and at a signal from the lecturer, the inner frame slides smoothly through
the outer, and the slides are thereby changed. This carrier is simple,
cheap, and quiet in its action; its one disadvantage is that each alternate
slide has to be inserted from opposite sides of the lantern, and unless the
operator is fairly tall this almost necessitates an assistant.
Nevertheless, the carrier is the most popular of any, its other advantages,
especially as regards price, being so great. It is usually constructed in
such a way that the slide, as it moves out from the central position,
automatically rises in its groove in order to facilitate quick removal.
Another pattern deservedly popular is that known as {63} 'Beard's
Dissolving Carrier' and is shown in Fig. 38. In this ingenious carrier all
the slides are inserted from the same side, the oncoming slide being pushed
_in front_ of its predecessor, and being therefore somewhat out of focus it
produces a blur on the screen.
The movement is performed by pushing in a projecting handle, and on
withdrawing this the slide which is finished with comes with it, and the
finish of the movement presses the new slide back until it is in its proper
position and in focus.
[Illustration: FIG. 38.--Beard's Dissolving Carrier.]
The entire action is simpler than it sounds, and the temporary blurring of
the image on the screen during the process of changing is supposed to give
somewhat the effect of 'Dissolving Views,' and hence the name 'Dissolving
Carrier.'
This appliance is three times the price of the 'Double Sliding' pattern,
but the fact that it is worked from one side only is a decided advantage,
though on the other hand it is not (unless great care is used) quite so
silent in its action as the 'Double Sliding' type.
A further modification of this carrier adapts it to take any of the
recognised 'foreign' sizes of slides, so that if a few American ones, for
instance, are met with among a collection of English manufacture, there is
no need to change the carrier. {64}
There are other varieties of carriers on the market which there is no need
particularly to describe, such as, for example, carriers fitted with roller
curtains to give the effect of a curtain rolling up, magazine carriers to
hold twenty-four or more slides and exhibit them in rotation, and other
patterns too numerous to mention. Of these the reader must be left to judge
for himself, but, generally speaking, _simplicity_ in a carrier is the most
important point to be looked for, and complications, however ingenious,
should be avoided.
[Illustration: FIG. 39.--Focussing Action of Lens.]
The lantern stage must also receive consideration, but it will be better to
discuss it as part of the mechanical construction of the lantern.
THE OBJECTIVE is really the most vital part of a lantern, as the definition
of the picture almost entirely depends upon the excellence or otherwise of
this lens. This will be obvious at once when it is realised that the
objective has to project on to the distant screen a greatly magnified image
of the comparatively small lantern slide, and the intending purchaser is
strongly advised to economise almost anywhere rather than on this item.
The action of a lens in focussing the image is perhaps best explained by a
simple diagram (Fig. 39), from which it will be seen that all the rays
proceeding from any one point on the object are re-converged (when the lens
is in focus) to a definite point on the image, and the perfection of the
picture depends upon the lens performing this function accurately. {65}
The imperfections are chiefly two, viz. those known as chromatic and
spherical aberration respectively. Chromatic aberration simply means that
all the colours composing the original beam of, say, white light are not
equally refracted or converged, and therefore do not meet again at the same
spot (the well-known prism or lustre effect), and reveals itself by
coloured fringes round the edges of the various details in the picture.
[Illustration: FIG. 40.--Achromatic Lens.]
By spherical aberration we mean that the light falling upon the centre of a
lens is not brought to a focus at exactly the same spot as the marginal
rays, and a general want of definition is the result, usually accompanied
also by a want of 'flatness' in the image, that is to say the edges of the
picture do not focus at the same time as the centre.
Chromatic aberration is easily cured by using an achromatic or compound
lens made by cementing together two lenses of crown and flint glass
respectively, as in Fig. 40.
It will be seen that the flint glass component by itself is a _concave_
lens and therefore neutralises in part, or in whole, the convex crown lens.
Flint glass has both greater dispersive power and also greater refractive
power than crown glass, but fortunately not to the same _degree_; hence a
compound lens made in this way and with curves carefully worked out may
have its chromatic effect entirely neutralised while retaining very
considerable refractive or 'focussing' power, and simple achromatic
objectives of this type are quite inexpensive.
In lanterns intended for Science demonstration, as distinct from the mere
projection of slides, lenses of this pattern are very frequently used, as
they will project the latter when required reasonably well, and for the
demonstration of {66} experiments or of apparatus on the screen have
advantages that need not be discussed here.
For very long focus lenses also they are sometimes employed, as the trouble
from spherical aberration is much less apparent with lenses of long focus
than with short, and the difference in expense is much more in the former
case than in the latter. For short focus lenses, however, as used in
moderate-sized halls, they are not good enough, and the type of lens almost
universally employed is that known as the 'Petzval' combination (Fig. 41).
[Illustration: FIG. 41.--Petzval Combination.]
This lens really consists of two achromatic combinations, the pair at the
front being cemented together, and that at the rear having an air space
between. The combination is so designed that the spherical aberration of
the one pair neutralises that of the other, and the result is or should be
a lens corrected both for chromatic and spherical aberration.
These lenses, however, vary very much in the perfection of their results,
and as they are at present usually imported in bulk from France, the
customer does well to insist upon a demonstration of his own particular
lantern before acceptance.
The magnifying power of a lens depends upon its 'focus' multiplied by its
distance from the screen, and the focus in the case of a simple lens is
easily determined by the familiar 'burning-glass' experiment, that is by
focussing an image of the sun upon a piece of paper and measuring
accurately {67} the distance the lens must be away to produce the most
concentrated spot.
In practice it is sufficiently accurate to focus a distant window, or other
luminous object, upon the paper, any error obtained by this method being
for ordinary purposes a negligible one.
With a compound lens, such as a 'Petzval' combination, this method does not
hold good, as the optical centre of such a lens is not necessarily midway
between its two components.
The actual focus can be got pretty approximately by focussing a window or
other object as before and measuring the distance from one definite point
(say the front edge of one of the lens cells) to the paper, then turning it
round and taking a second measurement from the _same_ point, the mean
between the two measurements giving the actual focus.
In practice the 'simple equivalent focus,' as it is termed, of a lantern
lens is usually determined by measuring the magnification of the image
thrown upon the screen, when, by knowing the original size of the slide (a
'standard' slide of 3 inches diameter is usually taken) and the distance
between lantern and screen, we get the focus from the following very simple
equation:
Diameter of picture on screen Distance between lens and screen
(in feet) (in feet)
----------------------------- = ---------------------------------
Diameter of slide (in inches) Focus of lens (in inches)
or perhaps more simply still:
Distance between lens × Diameter of slide
and screen (in feet) (in inches)
------------------------------------------ = Focus of lens in inches;
Diameter of picture (in feet)
or, if we know the focus of the lens but want to know how far from the
screen we must go to produce a given-sized picture, the formula will be:
{68}
Diameter of picture × Focus of lens
(in feet) (in inches)
------------------------------------ = Distance required (in feet).
Diameter of slide (in inches)
It is handy for the lanternist to remember that, dealing with a standard
3-inch slide, a 6-inch lens will _always_ give a picture whose diameter is
_one-half_ the distance from lens to screen, a 12-inch lens half this again
or _one-quarter_, and a 9-inch lens half-way between the two.
Bearing these simple figures in mind, the approximate distance can usually
be _guessed_ sufficiently near for the first trial, and then the lantern
shifted a little nearer or the reverse as required.
The following table may, however, be useful, as showing readily the
magnification produced at different distances by lenses of given foci:
+------+---------+--------+--------+--------+--------+--------+--------+
| Disc | Focus | Focus | Focus | Focus | Focus | Focus | Focus |
|wanted| 4½ in. | 6 in. | 8 in. | 10 in.| 12 in.| 15 in.| 18 in.|
+------+---------+--------+--------+--------+--------+--------+--------+
| feet.| ft. in. | ft. in.| ft. in.| ft. in.| ft. in.| ft. in.| ft. in.|
| 9 | 13 6 | 18 0 | 24 0 | 30 0 | 36 0 | 45 0 | 54 0 |
| 12 | 18 0 | 24 0 | 32 0 | 40 0 | 48 0 | 60 0 | 72 0 |
| 15 | 22 6 | 30 0 | 40 0 | 50 0 | 60 0 | 75 0 | 90 0 |
| 18 | 27 0 | 36 0 | 48 0 | 60 0 | 72 0 | 90 0 |108 0 |
| 20 | 30 0 | 40 0 | 53 4 | 66 8 | 80 0 |100 0 |120 0 |
| 25 | 37 6 | 50 0 | 66 8 | 83 4 |100 0 |125 0 |150 0 |
| 30 | 45 0 | 60 0 | 80 0 |100 0 |120 0 |150 0 |180 0 |
+------+---------+--------+--------+--------+--------+--------+--------+
THE DIAMETER OF THE OBJECTIVE.--The diameter of the objective must depend
to a certain extent upon its focus in the case of a double combination such
as a Petzval. These lenses consist, as has already been said, of two
achromatic components some distance apart, and for technical
considerations, which need not be discussed here, the _distance_ between
these components is usually about two-thirds of the focal length. This is
not a universal rule, as the lenses of different makers vary a good deal;
but it is generally a fact {69} that the longer the focus of the lens the
greater is usually the separation between the two lens systems.
The entire lens therefore mounted in its tube resembles a _tunnel_ of
varying length according to its focus, and through this tunnel a _cone_ of
light rays have to be passed. It is plain, therefore, that a lens of long
focus, which in practice means a long tube length, must be made also of
large diameter, or a portion of the cone will be cut off, with a consequent
loss of light.
In practice lenses up to 6 inches focus are usually made of 2 inches
diameter, and there is no advantage in a larger size. With a lens of 8
inches focus there is a slight gain in increasing the diameter to 2-3/8
(the next 'standard' size), and lenses of longer focus than this should
certainly be 2-3/8 inches up to, say, 12 inches focus, when a lens of 3
inches diameter is preferable. These large lenses are, however, very
expensive, both in themselves and also on account of the fact that their
weight entails heavy and expensive brass mounting, and hence lenses up to
14 or 15 inches focus are often supplied in the 2-3/8 size for reasons of
economy.
To sum up, _short-range_ lanterns, as they are called, are usually fitted
with lenses of 2 inches diameter, and _long-range_ instruments either with
3-inch lenses or the intermediate size of 2-3/8 inches. If a lantern is
purchased for either long or short-range work, it is usually fitted with a
brass front for a large lens, and so arranged that a shorter focus lens of
2 inches diameter can easily be interchanged, utilising the same brass
mounting.
Lenses of _variable_ focus have also been designed, in which an additional
lens can be added or subtracted to increase or decrease the focal length;
but nothing very practical has yet been achieved in this direction, and
therefore these 'Omnifocal' lenses have never come into general favour.
Objectives like condensers want cleaning at times, and care must be taken
not to scratch the glass, as the concave lens of each component is of flint
glass, and very soft. A {70} clean chamois leather is the best thing to
use, but a soft cloth, or even a handkerchief, may be employed with care.
It is very important that a lens be reassembled, after cleaning, the
correct way, as a single lens reversed would utterly spoil the definition.
The front component is usually balsamed together, and therefore all that is
needed is to see that the whole combination is not reversed. In the Petzval
system this lens should have its convex constituent towards the screen
(Fig. 41). The back combination is usually loose, and the two lenses are
sometimes separated by a thin brass ring. In the Petzval lens the concave
element should be inside, with its concave surface outwards, the deep curve
of the other lens should fit into this concavity, and the flatter curve
face towards the condenser. One or two makers, however, have introduced a
modification of the Petzval system in which the whole of this back
combination is reversed, and the exact arrangement should therefore be
noted very carefully when taking the lens to pieces.
* * * * *