Ch. V] 



MEASURING WITH THE MICROSCOPE 



143 



Ocular Micrometer and its Valuation 



§ 238. This, as the name implies, is a micrometer to be used in 

 connection with an ocular. It consists of rulings on a cover-glass of 

 fixed or of movable lines. 



This form of micrometer is placed at the level where the real image 

 is formed, i.e., at the level of the ocular diaphragm of all oculars. 

 With positive oculars it would therefore be outside the ocular (fig. 22) 

 and with negative or Huygenian oculars between the lenses (fig. 23- 

 24). The image of the object under the microscope appears to be 

 directly upon or immediately under the ocular micrometer, and hence 

 the number of spaces on the ocular micrometer required to measure 

 the real image may be read off directly. This, however, is measuring 

 the size of the real image, and the actual size of the object can only 



