MICROMETRY MICROMETRIC MICROSCOPES 



155 



hairs CC, and the parallel indicator lines C. One complete 

 revolution of the drum is equivalent to one of the small divisions 

 of the fixed scale. Fig. 100 shows a Bausch & Lomb microm- 

 eter ocular whose fixed scale consists of a black comb of equi- 

 distant teeth; the center of each tooth is marked by a sharp, 



FIG. 99. Scale of the Zeiss 

 Filar Micrometer. 



FIG. 100. Scale of the Bausch & Lomb 

 Filar Micrometer. 



short line. One complete revolution of the drum is exactly 

 equivalent to one division of the comb; turning the drum 

 causes the line C to move across the field. The zero line O and 

 the horizontal cross-hair HH are fixed with respect to the comb, 

 but the entire system of comb and zero point may be displaced 

 at will, by a set screw, to the right or left, for convenience in 

 measuring. 



Before filar micrometers may be used for micrometry the value 

 of one division of the ocular scale must be ascertained by means 

 of a stage micrometer with the draw-tube of the microscope in a 

 recorded position. 



When using micrometers in which the diameter of the image 

 of the object is measured by the movement of a micrometer 

 screw, a number of observations should be made, always moving 

 the cross-hairs in the same direction to eliminate " back-lash." 



Method 5. Projecting a scale of known value into the field of 

 mew by means of sub stage 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, 



