OPTICAL LANTERN An appliance, popularly known as the " magic lantern," by means of which transparencies are optically projected by artificial light upon a screen, the diameter of the image being thereby increased by from 24 to 15o times. The enlarging lantern (which see) is an optical lantern, but the increase in the size of the image rarely exceeds from 20 to 3o times, and is generally much less. An optical lantern comprises a body, an illuminant, an optical system, and means for holding the transparency (slide) in the path of the light. The body may be of wood or metal, the former (generally mahogany) being the stronger, and the latter being the lighter, and therefore having advantages from the point of view of portability. A typical high-class lantern having a wooden body is here illustrated. There should be a door on one or both sides, and a close-fitting cowl or crinkle as light-tight as possible. When an oil-lamp is the source of light, the chimney projects at the top. The illuminant (see the next article) is supported by the body in line with the optical axis of the lenses, and the lamp or jet is mounted on a metal plate or in a metal tray, which slides in grooves formed in the lantern body, it being necessary to provide for adjusting the distance between the condenser and the illuminant. The condenser, consisting in its commonest form of two plano-convex lenses mounted in a brass cell with the convex surfaces facing each other (see " Condenser "), collects the light rays and causes them to illuminate the transparency evenly ; thence the rays pass to the objective lens, which projects them upon the screen. (For a diagram explaining the optical principle involved, see " Condenser.") The objective is a lens of the Petzval portrait type (see " Lens " ), but it is not wise to use it for photography, as its visual focus may not coincide with the actinic focus. The draw-tubes in the front of the lantern, and the rack and pinion on the jacket of the objective tube, allow of proper focusing. Frequently, in front of the lens mount is a " flasher," consisting of a hinged or pivoted disc serving as a lens cap. The transparency, carried in a wooden slide carrier having a to-and-fro movement, is inserted into the stage of the lantern imme diately in front of the condenser, the carrier being held in place by a spring plate.
To shield the audience from stray light rays, it is usual to fit a heavy curtain to the back of the lantern.
A " single " lantern has but one illuminant and optical system ; a " biunial " has two, one above the other ; and a " triunial" (now but rarely used) has three. When oil is the illuminant it is necessary to have the systems (actually separate lanterns) side by side. Biunial
and triunial lanterns were much used in earlier days for the production of the then popular dissolving views (which see).
The screen may be a white sheet, or a plastered wall painted " flat " or " dead " white. Col lapsible frames for sheet screens afe commercial articles. When the lantern is behind the screen and the audience in front, the screen should be wetted to make it more translucent, and the slides should be inserted into the carrier with the fronts facing the screen. Ordinarily, when both lantern and audience are in front of the screen, the slides are inserted with their fronts facing the operator, who stands to one side of and slightly to the back of the lantern stage. Always, the slides are inserted upside down, but unless they have been carefully " spotted " beforehand (see " Lantern Slides, Masking, Binding, and Spotting "), some of them are sure to be shown upside down on the screen. The lanternist should accept unspotted slides only at the lecturer's risk.
In connection with the calculation of the size of the disc on the screen, of the distance of the lantern from screen, or of the focal length of the lens to be employed, it should be said that the use of a set of interchangeable objec tives renders the lanternist largely superior to the difficulties frequently caused by par ticularly small or large halls, and by the in convenient placing of pillars, lights, etc., in the building.
The size of the disc of light projected by a lantern upon n screen depends upon the focal length of the lens and the distance between lantern and screen. Assuming the diameter of the slide to be 3 in., the size of the disc under certain conditions is found by multiplying 3 by the distance (in feet) between lantern and screen and dividing by the focal length of the lens in inches. Thus, at a distance of 4o ft., a 7-in. lens gives a picture more than 17 ft. in diameter, as 3 x 40 — 17+. To determine the focal length 7 of lens necessary under certain conditions, mul tiply 3 by the distance and divide by the diameter of the desired disc. Thus, at a distance of 24 ft., and to produce an 8-ft. disc, a lens of 9-in. focal length would be necessary, as 3 x 24 = 9. To determine the distance at 8 which a given lens will produce a given disc, multiply the focal length by the diameter of the disc and divide by 3. Thus, a 9-in. lens would produce a io-ft. disc at a distance of 3o ft., as 9 3 x io = 3o. A simple calculation on the lines of the above saves the lanternist the trouble of trying different lenses or of shifting his apparatus.
The " opaque " lantern—that is, a lantern for projecting images of opaque objects—is described under the heading " Aphengescope."