212 



USE OF THE MICROSCOPE. 



point, at some known distance apart, say from ^io or t-oVo ^^ 

 an inch, as shown by fig. 136 : on this the object is placed, and it 



Fig. 136. 



is necessary that both it and the lines be seen at one and the 

 same time : the number of lines which the object occupies 

 wiU give the exact measurement. This method, however, is 

 very inconvenient and can only be eifected with a single 

 lens, or with a compound microscope of low power, for 

 with higher powers the focal point is so precise, that the 

 plane in which objects can be distinctly defined is almost a 

 mathematical one, and the lines and the object, therefore, 

 cannot be in focus, or be distinctly seen together ; besides, if 

 the object be immersed in a fluid, the lines will become indis- 

 tinct from being filled with it, and thus the operation of 

 measuring will be rendered almost, if not quite, imprac- 

 ticable. It is also inapplicable to opaque objects of any thick- 

 ness, and even to transparent ones after they are mounted, 

 but is of great use for other purposes, and will be again 

 alluded to. 



Eye-piece Micrometer. — This consists of an eye-piece, having a 

 divided glass placed in its focus: if the positive eye-piece be used, 

 the divided glass is placed below the field-lens ; but if the nega- 

 tive, then the point selected is between the two lenses. The 

 positive eye-piece is the invention of Eamsden, and has been 



before alluded to in pages 73 

 and 176. Mr. Eoss, who 

 employs it for his microme- 

 ters, adopts the form re- 

 presented by figs. 137 and 

 138. Fig. 137 exhibits the 

 external appearance, and fig. 

 138 a section of the same = 



Fig. 137. 



