Optic projection : principles, installation and use of the magic lantern, projection microscope, reflecting lantern, moving picture machine, fully illustrated with plates and with over 400 text-figures (1914)

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CH. IX] MICRO-PROJECTION FOR LARGE CLASSES 225 For a larger audience and a correspondingly larger room the screen distance might be made 15 to 20 meters (50 to 65 ft.), and the magnification raised from 250 at the lower limit up to about 5,000 diameters at the upper limit. The smaller room enables one to get more brilliant screen images, and to use a wider range of objects (see table of magnifications § 391). In the smaller room the screen should be at least 4 meters (12-13 feet) square, and in the larger room 5-6 meters (15-20 feet) square. B FIG. 123. DIAGRAMS SHOWING THE CONSTRUCTION OF OBJECTIVES FOR MICRO-PROJECTION AND FOR PHOTOGRAPHY. (From the Catalogues of Zciss, Leitz, and the Bausch & Lomb Optical Co.). A Microsummar of Leitz. B Microplanar of Zciss. C Microtessar of the Bausch & Lomb Optical Co. When used for micro-projection the diaphragm is wide open and no ocular is employed. In the diagram of the Microtessar, F represents the front lens, d the diaphragm, and B the back combination of the objective. The arrow indicates the direction of the light. In articles and books upon projection, it is advocated sometimes, that oil or water immersion objectives as high as i .5 or 2 mm. should be used for class demonstration. There is no doubt that brilliant images with short screen distances can be obtained with high power objectives, but such projection is only applicable for small numbers ; and if fine details are to be seen, the observer must be very close to the screen. Furthermore, no screen image in its finest details is equal to that which one gets in looking directly into a compound microscope. (For high power projection sec § 401). If it is high magnification that is desired, it is vastly better to use lower objectives with an amplifier (§ 356, fig. 126). The lower