112 SELECTION AND USE 



it, the latter may now be adjusted exactly over the centre of 

 rotation. With common objectives this answers very well, but 

 first-class objectives are apt to show the effect of being thrown 

 out of centre. 



Where rotation in regard to the illumination alone is required, 

 several plans may be employed. One of the oldest is that 

 found in the large microscopes of Hartnack and others, and 

 recently adopted by Mr. Browning. This device consists in 

 forming the stage in two pieces, the lower part being connected 

 with the foot, and the means of illumination, while the upper 



Fig. 25. 



part, which rotates on the lower, is rigidly attached to the arm 

 which carries the body. In this way the body, arm, stage and 

 object may all be simultaneously rotated in relation to the 

 illumination, and for ordinary purposes this answers very well. 

 When polarized light is used, however, it is obvious that it 

 is impossible to rotate the object in relation to the polarized 

 ray, without also rotating the analyzer, and, as every one that 

 has worked much in this direction knows, it is often of great 



