I] THE USE OF THE MICROSCOPE 9 



if considered as a single lens. This can be obtained by dividing 250 

 by the equivalent focal distance in millimetres, or 10 by the focal 

 distance in inches. Thus if the focal distance is 4 mm., the initial 

 magnifying power is 62-5, and if the focal distance is inch, the 

 initial magnifying power is 40. The compensating oculars of the 

 apochromatic objectives are named 6, 8, 12 etc. according to their 

 magnifying power. Thus the magnifying power of no. 6 ocular with 

 objective 4 mm. is 375. 



By working distance is usually meant the distance between the 

 front of the objective and the cover-slip, when the objective is focussed 

 for an object on the lower surface of the cover-slip (this being of 

 medium thickness). 



