ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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chamber can be replaced by a small stage for the examination of solid 

 bodies such as ruby glass. Illumination is by arc-light, or by mirror- 

 reflected sunlight, and the light after passing through a diaphragm tube 

 is concentrated by a lens on a slit arrangement, which is adjustable on 

 both sides ; the length and breadth of slit are both measurable by drum 



graduations. The slit can be rotated through 90 . A stronger, adjust- 

 able objective focuses the slit into the field of view. The optical axis 

 with lenses, etc., can be arranged vertically or horizontally. 



For obtaining dark-ground illumination, a special objective and 

 diaphragm are used (figs. 94, 95). Behind the optical part of an 

 objective a spring stamp-diaphragm is screwed, which presses against 



