146 



MEASURING WITH THE MICROSCOPE 



[Ch. V 



ocular and the ocular micrometer is mounted properly it may be 

 introduced through the opening in the side. This was a common 

 method with the older microscopes. When there is no side opening 

 the eye-lens may be unscrewed and the ocular micrometer on a 

 cover-glass laid upon the ocular diaphragm. 



Ocular Micrometer with Movable Scale 

 § 241. This form is a Huygenian ocular with a five millimeter 

 scale divided into twenty one-fourth millimeter intervals. The 



pitch of the screw 

 moving the scale is \ 

 mm. ; therefore one 

 complete revolution of 

 the drum moves the 

 scale one-fourth of a 

 millimeter, or one in- 

 terval. The drum is 

 divided into 100 equal 

 divisions, thus ena- 

 bling one to measure 

 TT0- of an interval on 

 the micrometer scale. 

 This ocular microm- 

 eter combines the ad- 

 vantages of the ocular 

 micrometer with a 

 fixed scale and the 



Fig. 



91. Ocular Micrometer with Movable 

 Scale and Recording Drum. 



(From the Catalogue of the Spencer Lens Co.). 



The recording drum is divided into 100 equal 

 divisions. 



filar micrometer. To complete the measurement of an object not 

 exactly included between any two lines of the scale, the drum need 

 be revolved only partly around. 



§ 242. Valuation of the movable scale ocular micrometer (fig. 91). 

 — Use a 4 mm. objective and proceed exactly as for the micrometer 

 with fixed lines, except that a partial stage micrometer space can be 

 measured by rotating the drum until the ocular micrometer exactly 

 coincides with the stage micrometer. Make sure that the lines of the 

 two micrometers are correctly related, as shown in fig. 90 and 98. One 



