282 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



1 • 40 mm. aperture, or a specially constrticted achromatic condenser of 

 1 • mm. aperture can be used. To obtain a successful photomicrograph 

 it is necessary that the illumination should be limited to that part of the 

 object which it is desired to photograph, because otherwise the light 



coming from the surrounding parts has the effect of fogging the picture. 

 A sharp image of the source of light must therefore be projected upon 

 the object, and to this end the condenser is provided with an arrange- 

 ment for cross-centering and for fine-adjustment. The limitation of the 

 illuminating cone is effected by an iris-diaphragm. 



