ZOOLOGY AND BOTANY, MICBOSCOPY, ETC. 



433 



section is large enough to fill, or nearly fill, the field. For this and 

 other purposes, a micrometer can be pushed into the eye-piece. In the 

 instrument now described, with a B eye-piece, 50° of the scale between 



Fig. 57. 



the optic axes are equal to an apparent angle in air of 69^° for muscovite. 

 Exact measurements must, of course, be made by means of a stage 

 goniometer. 



