THE MICROMETER 39 



done with the naked eye or with a very low- 

 power magnifier. 



The magnification should be carefully ascer- 

 tained and marked on each negative. For 

 lower powers this is easily found by using a 

 thin piece of metal with, say, a Jth inch hole 

 bored through it. When the photograph is 

 taken and the microscope slide removed, the 

 piece of metal is placed on the stage of the 

 microscope. The image of the hole falls on 

 the ground glass screen, and its amplification 

 can be easily ascertained. 



With high powers a micrometer must be 

 used. This is a glass slide with lines ruled 

 on it, the y^th and y^th of an inch apart. 

 This is focussed as in the former instance on 

 the ground glass screen. If the lines of the 

 T ^th divisions appear 2 inches apart, then 

 its magnification is 200 diameters. If the 

 lines of the nnny*' 1 division appear, say, 3 inches 

 apart, the magnification will be 3,000 diameters. 



A convenient method is to have the support 

 of the camera graduated in inches and parts 

 of an inch. In this way the amplification 



