106 PHOTOMICROGRAPHY 



micrographs obtained only show superiority in excep- 

 tional cases over those obtained when good single lenses 

 are employed. The introduction by Messrs. Zeiss of a 

 small aplanatic condenser in place of their older simple 



Fig. 32 

 NELSON'S APLANATIC CONDENSER 



lenses is, however, evidence of the growing popularit}^ 

 of that form of condenser, and the purchase of one of the 

 small aplanatic doublets is certainly to be advised. 

 Failing that, a double convex lens of about 4" diameter, 

 such as that supplied by Leitz, and shown in Fig. 7, 

 should be obtained. 



Condenser Mounts. Any of these condensers can be 

 obtained mounted on a suitable stand for the optical 

 bench, and fitted with centring screws. Some of the 

 simpler stands have only a vertical motion, others a 

 horizontal as well (Fig. 31). An iris diaphragm is 

 desirable on the optical bench, and is either fitted to the 

 condenser or carried on an independent centring stand. 



Flasks as Condensers. Spherical flasks about 6" in 

 diameter, filled with water (Fig. 27), may be used in place 

 of a bull's-eye, and have the advantage that by the 

 addition of suitable colouring matters to the water the use 

 of separate colour-filters is obviated, but they are neither 

 so satisfactory nor so convenient as lenses. 



