796. Slide Carriers. Slide carriers of the usual type comprise a fixed frame which is slipped under the spring platen of the lantern stage and within which a light holder for two slides can be pushed to and fro. When one of the apertures in this holder is centred on the optical axis, the other aperture is outside the lantern stage, so that slides can be put in or taken out. Stops at each end limit the travel of the holder. To facilitate the removal of the slides, levers are usually fixed under each half of the frame, being raised on inclined planes when the respective aperture has emerged from the fixed frame. The slide is thus raised about in. in its grooves.
In some carriers, the openings of the go-and return holder are fitted with adapters in which the slides are placed. In this way slides of various sizes, with the picture horizontal or vertical, can be shown by means of one carrier, provided a suitable assortment of adapters is available. There are also carriers with two pairs of apertures, one behind the other, e.g. for slides of the British and continental sizes. It is neces sary to re-focus when using successively a front aperture and a rear one. There are also slide carriers in metal in which each of the holders is mounted on a revolving plate, so that the slide can be placed horizontally or vertically as may be required.' Mention may also be made of the magazine carrier which the lecturer loads beforehand with slides placed in the order in which it is intended to show them, and which is actuated from a dis tance (G. Massiot, 1909). There are also numer ous automatic devices (used for advertising) by which a picture is thrown on a screen.
797. Projection Lenses. The lenses generally used for projection work are of the Petzval type (§ 98 and Fig. 69), in rack mounts for focussing. Mounts are also used in which may be placed interchangeable tubes, each carrying a lens of different focal length.' (See § 807 for notes on the choice of the focal length for projections under given conditions.) Owing to the light diffused by the lantern the luminosity increases a little when use is successively made of lenses with a relative aperture increasingly larger beyond the size necessary to pass the beam of " directed " light, but the contrast of the image decreases.
The requirements of cinema projection and of projection of colour slides have led some makers to produce special lenses with the same relative aperture as the Petzval lens, but better corrected, especially as regards curvature of In some of these lenses the distance from the focal plane to the pole of the rear glass is not more than half the focal length. The rear
glass is thus quite close to the slide and can utilize the whole of the beam without the need of such a large diameter of lens.
For lanterns used in halls of very different sizes the use has been suggested of lenses con structed on the principle of variable-focus telephoto lenses (§ 1°8), in order to avoid the carriage of a series of lenses of different focal lengths (A. Brouquier, 1901). Such an arrange ment also enables the size of the pictures to be changed during one and the same lecture, and thus to show on a magnified scale the details of a subject after showing it as a whole.
It is easy to improvize cheaply a projection lens of large aperture, but incompletely corrected, yet nevertheless quite suitable for projection in a hall of very large size where the spectators nearest to the screen are some yards away. To do this, two plano-convex lenses are assembled so as to form a very large " Rarnsden ocular " (L. Lumiere, 1924). To obtain a lens of focal length F, two identical plano-convex lenses of focal length f = liF are obtained and mounted with their convex surfaces facing each other, at the ends of a tube (or box) of length equal to 8/9F. It should be borne in mind that the nodal points of the whole lens are crossed, their separation being equal to 4/9F.
For instance, two lenses of 56 in. focal length and 8 in. diameter, mounted at 37 in. from each other, will form a lens of 42 in. focal length working at F/5•3.
Testing a projection lens is best done under normal conditions of use, employing as a test plate a thin plate of metal in which holes with clear-cut edges have been pierced.
798. Cooling Devices. Until recent years, overheating of the lantern slide (or of the pre paration, in cases where anatomical or other sections are directly projected) was avoided by a water trough placed in front of the condenser. This trough, the sides of which are formed of glass, contains either pure water or certain salt solutions the efficiency of which has sometimes been over-stated.