CRITICAL ILLUMINATION 115 



three-quarters of the N.A. of the observing objective " 

 (Journal of the Royal Microscopical Society, June, 1910). 



Critical illumination is the condition under which an 

 objective will work to its best advantage, and produce the 

 finest image of which it is capable, but the procedure by 

 which it is obtained may seem rather complex to a be- 

 ginner, and is here led up to through simple experiments. 

 The adjustments are really not difficult, and if the prin- 

 ciples underlying them are studied at the outset and always 

 put in practice they soon become almost mechanical, and 

 the worker who has mastered them is in a position to 

 tackle any kind of photomicrography, from the lowest to 

 the highest power. 



In the next chapter, dealing with the use of small 

 photographic lenses, these principles seem to be abandoned. 

 But there is then no question of great resolution, and the 

 object field to be evenly illuminated is, as a rule, so great 

 that the apex of the solid axial cone of light, if employed, 

 would only appear as a bright spot in its centre. The 

 difficulties introduced by the appearance of this bright 

 spot or image of the radiant across the object are so great 

 as to overshadow any gain produced by that method of 

 illumination, and consequently another course is usually 

 adopted under these circumstances. On the other hand, 

 where ordinary microscopic objectives are used covering 

 a much smaller field, critical light should always be 

 aimed at, and means to employ it are generally to be 

 found. 



Preliminary Experiments with a Condenser : 



1. Hold a bull's-eye condenser or double convex lens 

 in such a position that it throws an image of a radiant of 

 low intensity on to a white card. Remove the card and 

 place the eye in the position of the aerial image, the 

 entire disc of the condenser appears evenly filled with 

 light. 



2. Move the condenser into other positions between 



