CHAPTER VIII
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,
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.
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.
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.
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
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.
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.
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
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.
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.
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.