THE BODY OF THE LANTERN
We now come to the mechanical construction of the optical lantern, and a
great variety of design presents itself, according to price, type (_i.e._
short range or long range), and the individual ideas of the various makers.
Lantern bodies as a rule are now made of metal, although up till quite
recently the better class instruments were more usually made of polished
mahogany lined internally with iron; but there has of late been a consensus
of opinion in favour of metal only.
In the cheaper lanterns this metal body is usually made either of Russian
iron or of sheet-iron tinned and japanned, {71} there being little to
choose either in price or quality between the two varieties, and in all but
the very cheapest instruments the front is usually of brass.
In better lanterns the body is more often made of enamelled steel, the
front as before being of brass; but brass, copper, or aluminium are also
used occasionally for the body of the lantern.
[Illustration: FIG. 42.--Hughes' Short-Range Lantern.]
In deciding upon the type of body to be purchased the main considerations
to be borne in mind are: (1) The type or types of illuminant to be used, a
powerful arc lamp for example requiring a larger body than is necessary for
a weaker radiant; (2) the size and position of the lens to be carried, a
Petzval objective of say 3 inches diameter which has to be supported at the
end of a long brass mount for long-range work obviously demanding a body of
greater strength and rigidity than is required with a 6-inch focus lens of
2 inches diameter; (3) price.
Fig. 42 shows an extremely good lantern body for short-range work made by
Messrs. Hughes, the illustration depicting the instrument complete with a
'Luna' methylated spirit lamp, though, of course, any other illuminant
suitable for a small lantern could be used instead. {72}
This lantern illustrates well one point that has already been emphasised as
important, viz. the ventilation of the condenser. It will be noticed that
this is placed _outside_ the body of the instrument instead of inside as is
usual with larger bodies, and that wide slots are cut in the condenser
mount to allow free escape of steam.
Other points of this excellent design are the screw adjustment to the slide
stage (facilitating the use of special slides, such, for example, as those
illustrating the movements of the planetary bodies which sometimes involve
the use of extra thick frames) and a simple but efficient tilting
arrangement to the base.
[Illustration: FIG. 43.--Long-Range Lantern.]
Such a lantern is hardly suitable for a powerful arc lamp or limelight jet,
or for heavy long-range lenses, but is a very good typical instrument for
use in moderate-sized halls, and a lantern of this general type is usually
found in lantern catalogues, though, of course, the exact designs vary
according to the ideas of the manufacturer. Of lanterns for long-range work
a good example is perhaps Messrs. Newton & Co.'s 'Intermediate' pattern
(Fig. 43).
This again is only typical of many others by the various makers, but the
principal points are common to all. These are: (1) The large and
well-ventilated body; (2) the long {73} baseboard; (3) the strong and
massive brass front necessary to carry the large long-range lenses; (4) the
velvet curtain at the back to close in any stray light from a powerful arc
lamp.
[Illustration: FIG. 44.--Connections for a Bi-unial Lantern.]
The two foregoing designs are perhaps sufficiently typical of lantern
bodies in general to make further detailed description of individual
designs unnecessary; but reference should be made to features which special
requirements may render advisable.
Under this heading mention must be made of _Bi-unials_ or Double Lanterns,
as used for the once famous 'Dissolving Views.'
A bi-unial lantern consists essentially of two different instruments, each
complete with its limelight jet or other illuminant--front, condensers,
objective, &c., usually mounted on one body--and with some arrangement for
'dissolving' or turning the light in each lantern gradually on and off.
Fig. 44 shows the back view of such a lantern with two limelight jets and
dissolving tap, this piece of mechanism (shown below in the illustration)
being so arranged that when the lever is horizontal _both_ lanterns are on
full, but moving the lever either way cuts off the gas supply to one
lantern. In the case of limelight the tap should always operate by cutting
off the oxygen supply in advance of the coal gas (in order to avoid a
'snap'), and the latter should never be cut off entirely, but a small bead
of flame left to keep the jet alight, until the lantern is required for the
next slide. {74}
This is usually arranged for by means of a bye-pass, and a bye-pass is
sometimes provided on the oxygen side as well, but is usually discarded in
practice.
A bi-unial lantern can be worked in the same way with acetylene gas, but
with the electric arc it is impossible to turn the light on and off
gradually, and in practice dissolving must be done by keeping both lanterns
fully alight, and using a dissolving shutter, that is a movable shutter
that covers each objective alternately. The same arrangement must be used
with other illuminants, such as oil, only in this case the lanterns must be
mounted side by side, on account of the tall chimneys. With oil lamps the
arrangement answers fairly well, the dissolving fan, as it is termed, being
made with serrated edges which give the _gradual_ obliteration required;
but with the electric arc the extremely sharp definition becomes a serious
difficulty, and a good dissolver for this illuminant has never yet been
found, though, in view of the fact that dissolving views are more or less a
thing of the past, the matter cannot be regarded as important.
The advantages claimed for a double lantern are two: first, a 'Dissolving'
effect by which one picture fades gradually into the next, and which is
supposed to be more pleasing than the movement of a carrier; and second,
'Dissolving Effects' can be shown, such as exhibiting a landscape by day
and changing it into a moonlight scene, or bringing on the appearance of a
snowstorm, which can easily be done by means of a roller slide, with minute
perforations shown in motion by the second lantern while the landscape
remains on the screen from the first. In the days when dissolving views
were all the vogue, a third or even a fourth lantern has been added for
more complicated effects, and at the famous Polytechnic demonstrations of
years ago, I believe that as many as six were sometimes employed.
In these days of the cinematograph it is doubtful how far interest in such
effects could be revived, and a lantern has {75} gradually come to be
looked on more as an instrument for showing illustrations as required by
the lecturer rather than as a pleasing exhibition in itself, and as
dissolving views have lost their attraction, the double or triple lantern
has been relegated to the limbo of antiquity.
[Illustration: FIG. 45.--Beard's Circulating Water Tank.]
Among other 'special' lanterns should be mentioned models made with
water-cooled stages, for use with very delicate slides. This elaboration is
not necessary with ordinary slides and illuminants of moderate power, but
where very delicate slides, such as specimens of natural colour
photography, have to be shown, it is an advisable precaution to pass the
beam of light first through a tank of water in order to absorb the heat.
Lanterns intended for this work are usually constructed with a kind of
double stage, a glass trough of water fitting into the rear aperture and
the slide-carrier into the front one. Such an arrangement answers quite
well for most purposes, but for extreme cases lanterns are equipped with a
trough connected to a large outside tank and complete circulatory system,
after the manner of the cooling tank of a gas engine.
Such a lantern, constructed by Messrs. Beard, is illustrated in Fig. 45,
and it will be seen that in this instrument the water trough is placed
between the lenses of the condenser. {76} This is a very good position, as
the beam of light at this point is, or should be, parallel, whereas between
the condenser and the slide it is convergent, and therefore a condenser of
a larger diameter than the slide must be employed in the latter case if the
trough is of considerable width.
While dealing with 'Special' lantern bodies, we should perhaps just mention
here the numerous pattern lanterns made for the demonstration both of
lantern slides and of Scientific Phenomena, such as the projection of
insect life or other microscopic objects, polarised light experiments,
electrical apparatus, opaque objects, &c. A detailed description of these
lanterns and how to use them belongs to the second part of this work, as
also does the popular cinematograph; but educational institutes, and even
boys' clubs, when considering the purchase of a lantern, might well reflect
whether it would be advisable to spend a little more money in the
acquisition of an instrument which can be utilised for a variety of
purposes.
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