TESTING THE MICROSCOPE ITS USES. 11 



count off the number of degrees which the instrument has 

 passed over. Suppose, that, in a given case, the number be 

 seventy ; then revolve the instrument in the opposite direction 

 and count as before. The number of degrees will of course be 

 the same. 



Add the two figures together, and the total number of de- 

 grees (viz., 140) will represent the angle of aperture. 



