Optical Projection. Part 1: The Projection of Lantern Slides

by Lewis Wright; Russell Stuart Wright

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.