PRACTICAL PHOTOMICROGRAPHY. 



35 



the nature of which explains itself to anyone looking at our 

 cut. The manufacture, however, is by no means so simple, 

 and the purchaser ought to see that the interior of the aperture 

 is as nearly circular as possible, and that, in closing, the seg- 

 ments do not " jam," or work very tightly, a very common 

 fault. The segments should not " lock," that is should not 

 interlace with each other, for that construction generally 

 means a " jam." 



Another accessory usually accompanying a condenser is a 

 set of " black-ground discs," the nature of which will be under- 

 stood from figure 6 and the use of which will be treated later. 



FIG. 5. POWELL AND LEALAND'S APOCHROMATIC CONDENSER N. A. t, 4 



Frequently also other " stops " accompanying the condenser, 

 we figure three of them here, so that their description may be 

 recognised when we come to mention their uses. 



FIG. 6. STOPS. 



There are various other accessories frequently fitted to the 

 substage of a microscope, sometimes to be used along with the 

 condenser, sometimes independent of the condenser. Thus a 

 polariser, with or without selenite discs, is often made 

 to fit the substage ; colored glasses, technically called " light 

 modifiers" are also common accompaniments of an achro- 



