3 6 4 



PHYSIOLOGICAL PHYSICS. [Chap, xxvin. 



can be brought under the opening in the stage. It 

 thus acts as a stop, and regulates the quantity of light, 

 aiding in definition with high powers by cutting off 



the outside rays. 

 Under the stage is 

 fitted A66, a con- 

 denser, an arrange- 

 ment of convex 

 lenses for concen- 

 trating the light 

 from the mirror on 

 the object and so 

 increasing the il- 

 lumination. The 

 form used in Zeiss's 

 microscopes is 

 Abbe's, and can be 

 removed or replaced 

 at pleasure. It is 

 specially serviceable 

 for high powers. 

 There are two 



All 



focussing 



arrange- 



Fig. 



ments in such an 

 instrument. The 

 coarse adjustment is 

 made by grasping 



ejge ^ 



leS.-Compom^Microscope (Zeiss's 



with finger and 



thumb of one hand , the other hand steadying the foot of 

 the instrument, and, by means of a slightly turning 

 movement, slowly moving t down or up the split tube 

 as may be desired, thus bringing the lenses nearer to or 

 taking them farther away from the object. The object 

 having been brought into view, accuracy of definition 

 is obtained by a slight turning, in one direction or 

 in another, of the fine screw F, the fine adjustment. 



