PROJECTION LANTERNS 784. Parts of the Lantern. A projection lan consists essentially of a light-source of high power enclosed in the lantern proper or light-box, a condenser, a slide carrier, enabling the slides tb be changed rapidly, and a lens. The lantern is usually placed on a stand, bringing the lens opposite the centre of the screen, the stand being fitted with a shelf for the slides before and after projection. The table and shelf should be edged with strips of wood so as to form trays, in order to prevent slides and acces sories from falling off accidentally, as may easily happen when working in almost total In addition to these normal parts, projection lanterns fitted with very powerful light-sources are equipped with cooling devices similar to those required in cinematograph projectors. Some lanterns, too, are fitted with tinting discs, usually held in front of the lens on an eccentric axis, and permitting of filters of coloured glass or gelatine being placed in the beam issuing from the lens.
A certain number of stereoscope-viewing cabinets (§ 827) can, by the addition of a sim plified light-box and condenser, be used as projectors. In order to obviate having to reverse the views, one of the oculars is replaced by a lens between the glasses of which a prism for the reversal of the image is fitted.
Instruments for projecting the images of opaque bodies will be dealt with later.
785. The Light-box. The reader will find in the catalogues of makers and suppliers of pro jection instruments more complete descriptions than can be given here of the numerous models of projection lanterns. Some resemble enlarging lanterns, but with more ample ventilation, per mitting the use of more powerful light-sources and not requiring such careful light-trapping, since any escape of light is of small account, particularly when the lantern is placed behind the spectators.
Within the limits of the money which can be spent, it is best to choose lanterns the light-boxes of which are of very large size, as this gives great latitude in the choice of the illuminant. It is
also advisable that the condenser be capable of insertion and removal from the side without having to take out the lamp, and that the spring platen under which the carrier slides should allow the use of other carriers than the one supplied with the lantern. Finally, the lens board must be capable of being moved out to a sufficient distance from the slide carrier to permit lenses of considerable focal length being employed. Such lenses are necessary in long halls unless the lantern is placed among the audience—an inconvenient arrangement for both spectators and lanternist.
Lanterns of simple construction are usually fitted with condensers of 4 in. diameter, which, however, allows a slide area of only 21 X in. being projected.' It is well to choose a lantern with condensers of 41 in. diameter, which per mits of slides 31X 3 in. or 21 x 31 in. being shown. Better still, if the cost is not prohibitive, is the choice of a lantern with a condenser of 6 or 6i in. diameter, allowing of the projection of transparencies of size 3k 41 in. on the special plates for direct colour photography.
For lantern work in very large halls where it is not always possible to place the lantern at mid-height of the screen, it is very desirable to have lanterns in which the various parts can be de-centred. This avoids the necessity of tilting the lantern, such angling of the optical axis causing a certain amount of distortion, unless the screen be also tilted so as to be at right angles to the axis of the projection lens. A brief refer ence may be made here to the almost obsolete double or bi-unial lanterns, consisting of two bodies, one above the other or side by side, and allowing of one picture being gradually " melted" into another (dissolving views). This was done by a pair of linked shutters or mechanism for turning on the light in one lantern whilst that in the other was turned off.