124 PHOTOMICROGRAPHY 



condenser, provided the back lens of the objective re- 

 mains full of light, but the method should only be used 

 with caution, and an examination of the back lens of the 

 objective never omitted in critical work under these 

 conditions. 



Use of the Auxiliary Condenser. The use of the aux- 

 iliary condenser is most readily studied with the micro- 

 scope in a horizontal position on the optical bench, 

 employing a piece of white card on which to focus the 

 light. 



Magnified Image of the Radiant. Place a bull's-eye or 

 double convex lens 2" to 4" in diameter in such a position 

 that it projects a magnified image of the radiant across 

 the diaphragm of the substage. Fig. 38 shows in dia- 





Fig. 38 



DIAGRAM OF ILLUMINATION WITH AUXILIARY CONDENSER 



grammatic form the course of the rays when a Watson- 

 Conrady condenser and Nernst lamp are used. The 

 condenser is placed about 3J" from the lamp, in such 

 a position as to project a sufficiently large image of the 

 radiant to cover the maximum aperture of the substage 

 condenser that it is desired to make use of. The substage 

 condenser is now in the position of the eye in the first 

 experiment with the condenser page 115. The bull's-eye 

 is full of light, and may be considered as the source of 

 light to be focussed by the substage condenser across 

 the image plane. This is effected either by holding a 

 pencil point against its surface and focussing it by the 

 substage condenser, or by closing down the attached iris, 

 if there is one, until the aperture is so small that it can 

 be focussed in the object plane. If the auxiliary con- 

 denser is suitable and properly centred the field of view 

 should be evenly illuminated, and an examination of the 



