CRITICAL ILLUMINATION 123 



14. Use the Flat of the Oil Flame. The field of view is 

 evenly covered, and so far as can be judged the back lens 

 of the objective is filled with light, but such a large light 

 source is never of even intensity over its surface. And 

 the condition for critical illumination is far from attained ; 

 instead of a solid axial cone there are many cones, both 

 axial and oblique, overlapping. 



An Evenly Illuminated Field with Critical Light, The 

 process adopted to obtain critical light has now been 

 outlined, and the consequent appearance of the object 

 noted. In certain circumstances, particularly with high- 

 power objectives, the best conditions of illumination for 

 photomicrography are attained, but more frequently 

 an image of the radiant quite precludes the possibility of 

 taking a photomicrograph, and it is necessary to consider 

 how the illumination may be made even over the entire 

 field, without destroying the perfection of the image. 

 The use of a larger radiant surface and diffused light have 

 been mentioned, and their inapplicability to critical 

 illumination pointed out. There are, however, two 

 courses that may be pursued to get even light, without in 

 any way depreciating the image formed. 



The first is based on a slight departure from absolute 

 focus of the image of the radiant. 



The second depends on the amplification of that image 

 by means of an auxiliary condenser, or the replacement of 

 the image of the radiant by that of the illuminated disc 

 of the auxiliary condenser. 



Slight Departure from Strict Focus of the Radiant. 

 Exp. 7 has shown that an evenly illuminated field may 

 be obtained by racking the substage condenser up or 

 down from its position of focus, and that, in many cases, 

 this can be brought about by a very slight movement, 

 involving no curtailment of the aperture of the objective. 

 Many experienced workers are of opinion that no loss of 

 definition results from a slight upward motion of the 



