ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



323 



issuing from the objective-focus quits the rear focal plane as an almost 

 parallel beam. The basis of this light-cone appears on the rear objective- 

 plane as a bright circular spot ; thus the iris and the light-circle appear 

 in the same plane. The magnitude of the light-circle is dependent on 



Fig. 44. 



the aperture of the objective, and the aperture angle of the objective 

 corresponds to an equal aperture angle of the condenser ; but the 

 aperture angle of the condenser is, again, dependent on the opening of 

 the iris diaphragm. The corresponding aperture of the condenser is 



Fig. 45. 



calculated -according to the opening of the iris, and the aperture of the 

 objective is determined by this known magnitude. The property which 

 especially adapts the condenser as an apertometer is not only the cor- 

 rection of ^spherical and chromatic aberration, but the attainment of 



