66 



THE MICROSCOPE 



optical combinations should be similarly made and 

 recorded. 



To measure any object subsequently it should be 

 first focussed carefully in the ordinary way. 



The eikonometer should then be applied to the eye- 

 piece and the size of the object read off on the eiko- 

 nometer scale as millimetres, and the actual size calcu- 

 lated by dividing the observed size by the magnification 

 constant for the particular optical combination em- 

 ployed in the observation. 



(c) By means of the filar micrometer. 



The Filar or cobweb Micrometer (Ramsden's microm- 



FiG. 58. Ramsden's Filar micrometer. 



FIG. 59. Ramsden's 

 micrometer field, a, fixed 

 wire; .b, reference wire 

 (fixed) ; c, travelling wire. 



eter eyepiece (Fig. 58) consists of an ocular having a 

 fine "fixed" wire stretching horizontally across the 

 field (Fig. 59), a vertical reference wire : fixed ad- 

 justed at right angles to the first; and a fine wire, paral- 

 lel to the reference wire, which can be moved across 

 the field by the action of a micrometer screw; the 

 drum head is divided into one hundred parts, which 

 successively pass a fixed index as the head is turned. 

 In the lower part of the field is a comb with the inter- 

 vals between its teeth corresponding to one complete 

 revolution of this screw-head. 



