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Bausch & Lo mb Optical Company lens, or lenses, being set in an inner rim within the larger rim of the mount. The intervening space of one-quarter inch around the circumference is left open, except at the connecting points, permitting a nearly even circulation of air on both sides of the lens. Either of these condensing systems will give satisfactory results with lantern slides. With most of our outfits we supply the triple system. Better optical corrections are possible with three elements than with two, enabling a sharper focusing of the light rays in the diaphragm plane of the projection lens. The second plano-convex lens is of the exact focal length to do this, 1. e. same focus as that of the projection lens. Screens White Opaque and Aluminum The brilliancy of the picture on the screen is largely dependent upon the per- centage of light rays that are reflected back from the screen to the eye of the observer. The character of the screen, then, is an important element to consider. The reflecting quality of a screen is determined by its opacity and surface finish. A well finished white wall serves the purpose admirably, but as such a wall is not often available a good, heavy cloth such as shade cloth, properly treated and coated with either a white opaque or an aluminum finish, makes a very desirable screen. This screen should be mounted on a spring roller so that it may be rolled up, when not in use, and thus protected from dust and dirt. The aluminum finished surface has about twice the reflecting value of the ordinary white surface, but a narrower field. Thus, while an aluminum screen is very advantageous, particularly in the projection of opaque objects, it should only be installed where the audience can be seated within a total angle of 60 from the center of the screen. For portable screens, which can be readily folded up into a small space, white sateen is perhaps the most efficient, because of its highly finished surface. Sateen 20