MICROMETRY MTCROMETRIC MICROSCOPES 



187 



eyepiece; what fraction of a division should be added to the 

 number of whole divisions is ascertained by turning the microm- 

 eter screw so as to displace, to the right, the scale in the eyepiece 

 until the end of the object just touches the scale division beyond 

 which it originally extended. Read the drum, and add this 

 fraction of a division to the reading first obtained. 



Instruments of the type illustrated in Fig. 121 have a fixed 

 scale within the eyepiece across which travel cross-hairs moved 



FIG. 121. Bausch & Lomb Optical Co. Filar Micrometer. 



by a micrometer screw provided with a graduated drum. As 

 in the type just described one complete revolution of the drum 

 is equivalent to i division of the scale within the eyepiece. The 

 object to be measured is moved until the image fall under the 

 scale and one edge in contact with one of the rulings. The 

 number of whole divisions included within the image is recorded 

 and the fraction of a division is ascertained by moving the cross- 

 hair and reading the drum. 



For ordinary objects the first type described is more rapid 

 but for very tiny objects such as pigments, etc., the second 

 type is more convenient and in the hands of the author some- 

 what more accurate. 



Method 4. Projecting a scale of known value into the field of 

 mew by means of substage condensers. This ingenious and prac- 

 tically universal method appears to have first been suggested 

 by Goring about 1820, and was rediscovered by Pigott in 1870, 

 and employed by Sorby in refractive index determination in 



