ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



613 



(d) ; and the heat-gnide (/). The polariser b is rotatory about an 

 excentric axis and can be swung into, or out of, the ray path. The 

 position of its polarising plane can be read off on the graduated scale 

 which is shown in the figure. At insertion the tooth n engages in the 

 notch k The polariser is a Grosse Air-Nicol, whose side is twice as 

 large as its length. It is rectangular in cross section, and has a side 

 length of 25 mm, ; its shortness and large aperture make it very useful 

 here on account of the small available space. The iris diaphragm is 

 placed immediately above the polariser, and is provided with a handle g 

 for regulating the aperture of illumination ; during observations it is 

 in general to be kept fully open. With a central stop in the objective 

 the iris secures dark ground illumination. Immediatelv over the iris is 



Fig . 90. 



the unspherical illuminating lens (not visible in figure), which induces a 

 good combination of the illuminating beams and thereby secures an 

 increased brightness, a matter of great importance in projection. 

 Between the illuminating lens and the preparation the controllable gas- 

 burner d is applied and is pushed in and out by the lever h, being- 

 passed in through the aperture o until the aperture is closed by the 

 plate p ; the effect of the plate is to exclude lateral air-currents. After 

 insertion the burner stands over the middle of the illuminating lens. 

 The burner is adapted for heating the preparation to comparatively low 

 temperatures (100°-200^), as well as to high temperatures (about 700°). 

 It is constructed with two tubes, one for gas and one for air. In the case 

 of low temperatures, however, only one tube is used, viz. that one whose 

 nozzle is marked G-. The gas supply is regulated by the handle m, 

 which operates an index moving over the outer graduated cu'cle. The 



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