CRITICAL ILLUMINATION 125 



Ramsden circle should show the back lens of the objective 

 full of light. But if the image of the radiant is not 

 sufficiently large to cover the back lens of the substage 

 condenser, the full aperture of the latter is not made use 

 of, and the effect produced by a partially closed substage 

 iris may be noted on the back lens of the objective. If 

 black shadows appear on the Ramsden circle their cause 

 may be found in bad centring of the bull's-eye, or in its 

 spherical aberrations. Any falling off in brightness to- 

 wards the edges of the image or disc of light projected, 

 has the same effect on the aperture of the substage con- 

 denser as a disc of light too small to cover its back lens 

 or an iris diaphragm restricting it. 



The next diagram, Fig. 39, is intended to demonstrate 



Fig. 39 



DIAGRAM OF ILLUMINATION BY MEANS OF TWO AUXILIARY CONDENSERS 



a slight weakness inherent in this method of illumination. 

 A second single condensing lens is shown close behind an 

 iris diaphragm in the upper part of the figure. The lower 

 half indicates the course of the pencils of rays when this 

 secondary lens is not introduced, and if the substage 

 condenser is imagined as situated in the position of the 

 secondary lens (the conditions of Fig. 38), and if its full 

 aperture is to be employed, the diagram shows that the 

 outer parts of the image of the radiant covering it are 

 largely ineffective, owing to the pencils of light forming 

 them, more or less missing the substage condenser. On 

 the other hand, when the auxiliary lens is introduced, as 

 shown in the upper part, all the light is directed into the 

 substage condenser. 



^ Parallel Light. A beam of parallel light, obtained by 

 placing the radiant at the focus of the condenser, may be 



