146 PHOTOMICROGRAPHY 



and extend the camera or move the card so as to obtain 

 the desired magnification, or to make the image fall with- 

 in the limits of the plate. 



Adapt the substage condenser to the objective in use 

 by removing the front lens if necessary. 



5. Obtain Critical Illumination. Open the iris and 

 focus the object on the ground glass with a low or 

 medium objective. Now rack or slide the condenser (with 

 its front lens removed, if that is not to be used) down or 

 up until the image of the illuminant appears in focus at 

 the same time and across that of the object. If the image 

 of the flame is not quite central move the lamp or mirror 

 until centrality is obtained ; do not alter the adjustment 

 of the condenser. 



6. To obtain Even Illumination. Focus the object on 

 the screen. Move the slide on the stage till a clear 

 space is in front of the objective, and very carefully 

 examine the disc on the ground-glass or white screen, and 

 see that the illumination is perfectly even. 



If a change of objective has been made it may be 

 necessary to slightly alter the centring of the condenser, 

 but the movement should be at the most very slight. If 

 the image of the radiant does not cover the field, slightly 

 rack up the condenser till the illumination is even. Re- 

 move the ocular and examine the back lens of the objec- 

 tive ; if not full of light it is better to use an auxiliary 

 condenser as described next. 



7. Adjust the Substage Iris. Carefully examine the 

 image on the ground, glass while gradually closing the 

 iris diaphragm. Use the aperture which gives the best 

 result in regard to clearness of image, and sufficient depth 

 of focus without any signs of diffraction. Examine the 

 back lens of the objective to find what portion of its 

 aperture is utilised. 



8. Focus. Bring the image of the object into position 

 over the marked area on the ground glass corresponding 



