130 MICROSCOPIC ILLUSTRATIONS. 



which the tracing is made with day-light, this inconve- 

 nience is entirely remedied. 



SECT. 11. Micrometer Eye-pieces. The micrometer eye- 

 piece is made to slide into the body of the microscope, 

 like the ordinary eye-pieces. This eye-piece is designed 

 either for drawing or measurement. When we employ 

 it for drawing, a plate of glass, with divisions similar to 

 those represented in the plate, fig. 13, must be used, in 

 the manner described by Mr. Bauer. When for mea- 

 surement, a slip of pearl, with divisions of 100 or 200 

 to an inch, answers best for all practical purposes. It 

 must be remarked, however, that the same divisions re- 

 present different measurements according to the power 

 of the object-glass and length of the body. When, there- 

 fore, they are once ascertained by the rules laid down 

 in the Micrographia, page 48, they must not be altered. 



For either of the above purposes, the micrometer must 



be placed at c, page 80; in the focus of the ey^-lens, a. 



Now as the distance of distinct vision differs in different 



persons, and even in the same person, according to his 



state of health, the adjustment must be made so as to vary 



the distance between a and c. This is ordinarily done 



by moving the lens , with a screw. On consideration, 



however, it is evident that by this means the achromatism 



of the eye-piece is disturbed. I have obviated this defect 



with my eye-pieces by giving the adjustment to the 



field-bar, c, which carries the micrometer. 



