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224 MICRO-PROJECTION FOR LARGE CLASSES [Ca. IX
looking directly into a compound microscope supplied with a low ocular and a 16 mm. objective. For a small audience near the screen higher powers are satisfactory (see § 401).
§ 354. Size of specimens for projection. — To meet the requirements of teaching and demonstration the modern scientific man and public lecturer should be able to commence with the projection microscope where the magic lantern leaves off, and carry the projection to the smallest size adapted to micro-projection; that is, from a specimen 60 mm. in diameter to one of half a millimeter or less in size. This requires an opening in the stage slightly larger than the largest specimen, that is, at least 65 mm. in diameter.
CHARACTER AND RANGE OF PROJECTION OBJECTIVES FOR DEMONSTRATION TO LARGE CLASSES
§ 355. Objectives from 125 mm. to 4 mm. equivalent focus are especially useful in micro-projection. The powers of 125, 100, 75, 50, and 25 mm. equivalent focus, and in some cases those of 20 and 1 6 mm., are constructed on the plan of photographic objectives (fig. 123). These are always to be used without an ocular, and their iris diaphragms are wide open.
At the present time the low objectives used in ordinary microscopic observation are also used in projection. The field is not flat, as with the micro-planar and other forms of photo-micrographic objectives, but they are much cheaper and the screen images are very brilliant. Formerly many of the objectives used in projection were made especially for that purpose. They gave very brilliant, flat fields over a narrow angle, but they were neither satisfactory for ordinary microscopic observation nor for photography.
Most of the projection with the microscope is, however, accomplished with objectives of about the following range: 50 mm., 1 6 mm., and 8 mm. With these in a triple nose-piece or revolver, the projection microscope can accomplish great things, especially if assisted occasionally by amplifiers. For an audience of 2 50 to 500 and a screen distance from 7.5 to 10 meters (25 to 33 ft.) the magnifications will range from about i 50 to 3000 diameters (§ 391).