THE PROJECTION MICROSCOPE 211 



oxy-hydrogen light, upon thinness and good staining ; the 

 more transparent the section, the less cone can be used. 

 Biologists will therefore find it best to prepare their own 

 slides, employing stains which, like carmine, gold, and silver, 

 give density to the marking. German professors, I believe, 

 are as a rule in advance of English in these matters, and in 

 overcoming this difficulty, which is the principal one. In 

 projecting blood-corpuscles, for instance, when the images are 

 quite sharp on the screen, there is so little colour in them, 

 that they are not easy to observe. Professor Strieker gets 

 over this by treating the preparation with a 0-6 per cent, 

 solution of salt in which is dissolved some fuchsine solution. 

 In this red ground the amreboid corpuscles stand out white, 

 and their movements are readily seen, while the red discs 

 appear light yellow. Any such expedients are a great help 

 to the screen image ; and doubtless as screen demonstration 

 becomes more common, both such methods, and others of 

 differentiating details of structure such, e.g., as the striations 

 in voluntary muscular fibre will be perfected or improved. 

 The limit of power will be found, not in mere magnification, 

 or mere brightness on the screen, but either in too little con- 

 trast of the image in its shades, owing to want of density, or 

 woolliness owing to thickness of the section, or to the lens. 

 Bearing these considerations in mind, few objects except mi- 

 crobes will be found out of the range of projection, and some 

 even of these can be shown with a good oil-immersion lens. 



106. Management of the Electric Microscope. This is 

 perfectly simple. The lamp is first adjusted at a distance 

 from the back lens of the condenser which gives a nearly 

 parallel beam from that lens alone : usually this is about 

 6 inches, but the exact distance may be obtained with each 

 instrument. The arc is centred by the screw slides of the 

 table or stand, and the front lens of the condenser is racked 

 in or out, or the lamp screwed backwards or forwards, until 

 when the objective fitting and sub stage condenser are re- 



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