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 dia- 

 phragm, 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 dis- 

 tances can be obtained with high power objectives, but such pro- 

 jection is only applicable for small numbers ; and if fine details are 

 to be seen, the observer must be very close to the screen. Further- 

 more, 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 



