156 MEASURING WITH THE MICROSCOPE [Ch. V 



If now one focuses a 16 mm. or other objective upon this real image, 

 it will appear very clearly in the field of the microscope. In order to 

 utilize the image for micrometry the valuation of the spaces must be 

 determined by the use of a stage micrometer as with the ocular microm- 

 eter (§ 240). Place a stage micrometer under the microscope and 

 focus the hnes sharply. Then with the screw or rack of the substage 

 condenser focus the condenser up and down until the image of the 



Fig. 97. Net Scale for Use in Micrometry with the Condenser 



Image. 



lines or net on the lantern slide are also sharp. Arrange the stage 

 micrometer so that the lines are parallel with the hnes of the con- 

 denser image. Make any two of the hnes coincide. Count the 

 number of spaces in the condenser image included between- any 

 two of the lines of the stage micrometer, and divide the value of 

 the space in the stage micrometer by the number of spaces of the 

 condenser image included, and the quotient will represent the valua- 

 tion of the spaces of the condenser image in millimeters. For ex- 

 ample, suppose the stage micrometer is ruled in o.i mm. and that 

 12 spaces of the condenser image are included in 9 spaces of the stage 

 micrometer; then each space of the condenser image has a valuation 

 of 0.9 mm. -^ 12 = 0.075 irini- 

 As the size of the image varies with the distance of the object from 



