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FOR MANAGERS AND OPERATORS 75 is controlled by moving the lamp toward or away from the condenser. If the spot is edged with light, bright blue and has a sort of "puckered" appearance the lamp is too far from the condenser. This is a condition which will usually only appear when using long focus condensers. It is pos- sibl'e with short focus lenses, but it is not likely you will ever get the lamp that far back with them. THE PRINCIPLES OF OPTICAL PROJECTION. Reproduced by courtesy of Bauscli & Lamb, Rochester, N. Y. The following brief review of the leading principles of projection is designed for the reader who, having no knowl- edge of the science of optics, wishes to understand the operation of projection apparatus. With the aid of the apparatus we throw or project upon a screen an enlarged image of a transparent object (a slide or film). The process is almost the reverse of ordinary photography. For example, in photographing a scene by means of the photographic objective or lens we obtain a reduced image of that scene on the ground glass. This glass is replaced by the sensitized plate and by the use of chemicals the image is fixed thereon. Now in projection we reverse this process. From the picture made with the lens we make a transparent slide, or we use the film negatives, and by means of a condensed light we strongly illuminate these, and with an objective lens an enlarged image is pro- jected upon the screen, and this screen image corresponds with the real objects first photographed. From this illustration it will be seen that the first essen- tial in projection work is the lens or objective. Just as in photography the quality and tone of the picture depend to a very great extent upon the quality and character of the lens, so in projection the objective is the factor which determines the excellence of the screen image. The condensing lenses must be of a diameter slightly greater than the diagonal of the slide or film in use. The size most commonly used is 4^ inches in diameter. As the condensing lenses are in close proximity to the arc or other source of light, they arc, of course, subjected to considerable heat and will expand and contract accordingly as they are heated or cooled. Some arrangement should, therefore, be made for this expansion and contraction so that it will be as even as possible. We have done this in our