136 



MAGNIFICATION AND MICROMETRY [CH. IV 



animal (frog, hen, turtle, etc.). Arrange the micrometer ocular so 

 that the long axis of the corpuscle will coincide with the cross line 

 in the micrometer scale (Fig. 121). Get one end of the corpuscle 

 exactly level with one end of the micrometer scale. Note the posi- 

 tion of the drum, and then rotate it until the other end of the 

 corpuscle is exactly against the nearest line of the micrometer. 

 Count up the entire intervals required and the partial interval on 

 the drum. Suppose it requires 5 entire and 0.60 intervals (see 

 explanation of Fig. 121) then the whole corpuscle must be 5.60 

 intervals multiplied by 4^. the value of one interval: 5.6X4= 



21. 



FIG. 120. Screw Ocular 

 Micrometer with moveable 

 scale. This is a Huygenian 

 ocular zvith a 5 nun. scale 

 divided into 20 % nun. in- 

 tervals. The pitch of the 

 screw moving the scale is 



1 4 mm. therefore one com- 

 plete revolution of the drum 

 moves the scale one interval 

 or ^4 mm. The drum is 

 divided into 100 gradua- 

 tions thus enabling one to 

 measure woth of an interval 

 on the micrometer scale. 

 This ocular micrometer combines the advantages of the ocular micrometer 

 with fixed scale and the filar micrometer. To complete the measurement of 

 an object not exactly between any two of the micrometer lines the drum need 

 be revolved only partly around. (Cut loaned by the Spencer Lens Co. ) 



o 12345 FIG. 121. Figure of the scale of the screw ocular 



micrometer, showing the divisions and the cross line. 

 At the left is shown an object on the scale not quite fill- 

 ing fo of the intervals. To measure this the drum need 

 be revolved only sufficiantly to measure the part of the interval filled by the 

 object being measured. 



'Originally the scale was divided in 50 ^ mm. spaces, and no cross line 

 was present. In 7905 the present form of scale was specially prepared from 

 the writer's specifications, and has since that time been regularly supplied. 

 (Cut loaned by the Spencer Lens Co.) 



194. Varying the Ocular Micrometer Valuation. Any 

 change in the objective, the ocular or the tube-length of the micro- 

 scope, that is to say, any change in the size of the real image, pro- 



