CH. IX] MICRO-PROJECTION WITH DIRECT CURRENT 



241 



falling upon them, and hence, following the law of the conserva- 

 tion of energy, all this absorbed light is transformed into heat. 

 The darker the specimen the more light is absorbed, and the quicker 

 it will be spoiled by overheating. The stage water-cell against 

 which the specimen rests conducts this heat away, in part, and 

 makes it possible to exhibit the specimen a longer time (see 852). 



FIG. 135. MECHANICAL STACK OF GREAT RANGE. 



(Cut loaned by the Spencer Lens Co.). 



This can be clamped to any rectangular microscope stage and as no part of 

 the clamp extends above the stage the full range of 85 by 65 mm. is available 

 and slides 50 x 75 mm. (2 x 3 inches) can be examined to the edges. This is 

 of the greatest convenience in examining serial sections, and also in projecting 

 them on the screen. 



368. Microscope-tube, and focusing device. If a tube 

 for receiving the objective is used it should be a large one, (fig. 

 121, 145). The small tubes used on most microscopes, and on all 

 when using an ocular, cut down the field too greatly (fig. 137, 147). 

 The tube should be short, that is, about 9 to 10 cm. (4 in.) long, and 

 4 to 5 cm. (2 in.) in diameter. There should be coarse and fine 

 adjustments as for the ordinary microscope (fig. 121). 



369. Mounting of objectives of low power. For the lowest 

 powers (125 to 75 mm. equivalent focus) it is better to have no 

 tube at all, but to have a black shield about 15 cm. (6 in.) in diam- 



