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 require- 

 ments 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 pro- 

 jection 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 DEMON- 

 STRATION 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 micro- 

 scopic observation are also used in projection. The field is not 

 flat, as with the micro-planar and other forms of photo-micro- 

 graphic 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, accom- 

 plished 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 mag- 

 nifications will range from about i 50 to 3000 diameters ( 391). 



