104 



PHOTOMICROGRAPHY 



consequently be adequate to give the illumination 

 required by the objective, and increase as the power of the 

 latter increases and its field of view decreases. 



BuWs-eye Condensers. The most commonly used con- 

 denser is a plano-convex uncorrected lens known as a 



Fig. 31 

 BULL'S-EYE CONDENSER WITH mis ON ADJUSTABLE STAND 



" bull's-eye." Some of the cheaper forms have such large 

 errors that they cannot be satisfactorily used for photo- 

 micrography, but the better ones can be used quite well 

 for most purposes. Their spherical error is least when the 

 convex side faces the parallel beam, or cone of light of 

 smaller angle. That is, a bull's-eye is placed with its 

 flat side towards a radiant at its focus when a parallel 



