64 THE MICROSCOPE 



vision. Special eyepieces are also obtainable having 

 a sledging adjustment to the eye lens for focussing 

 the micrometer. 



The value of one division of the micrometer scale 

 must first be ascertained for each optical combination 

 by the aid of the stage micrometer, thus : 



1. Insert the eyepiece micrometer inside the ocular 

 and adjust the stage micrometer on the stage of the 

 microscope. 



2 . Focus the scale of the stage micrometer accurately ; 

 the lines will appear to be immediately below those of 

 the eyepiece micrometer. Make the lines on the two 

 micrometers parallel by rotating the ocular. 



3. Make two of the lines on the ocular micrometer 

 coincide with those bounding one division of the stage 

 micrometer; this is effected by increasing or diminish- 

 ing the tube length; and note the number of included 

 divisions. 



4. Calculate the value of each division of the eye- 

 piece micrometer in terms of /*, by means of the fol- 

 lowing formula : 



x= 10 y. 



Where x = the number of included divisions of the 



eyepiece micrometer. 



y = the number of included divisions of the 

 stage micrometer. 



5. Note the optical combination employed in this 

 experiment and record it with the calculated microm- 

 eter value. 



Repeat this process for each of the other combina- 

 tions. Carefully record the results. 



To measure an object by this method read off the 

 number of divisions of the eyepiece micrometer it 

 occupies and express the result in micro, by a refer- 

 ence to the standard value for the particular optical 

 combination employed. 



