134 PHOTOMICROGRAPHY 



Approximate positions for the camera and object, and 

 the bellows extension to produce the desired magnification, 

 are found from the enlargement table given on page 276. 

 A convergent beam of light from the condenser is 

 arranged to cover the object evenly, as shown by the 

 dotted lines in the figure, and fill the aperture of the lens. 



Some of the problems connected with illumination, 

 which are among the chief difficulties to be surmounted 

 in photomicrography, are now encountered. There is no 

 question of critical light, but even illumination must be 

 obtained without an image of the radiant appearing on 

 the photograph. At first it is difficult to decide whether 

 the disc of light on the ground glass is even or not, and, 

 if a means for focussing on a card is not provided, it is 

 a good plan to remove the ground -glass focussing screen 

 and place a piece of white card some little distance behind 

 the camera. Inequalities of illumination can as a rule 

 be more easily detected in this way ; if shadows 

 appear on any portion of the disc, either the lamp or 

 condenser is moved until the light is perfectly even and 

 white. The defects caused by slight errors of centra tion 

 of the lamp, or in its distance from the condenser, should 

 be noted, and the steps necessary to get rid of shadows 

 in any part of the circle determined. If an image of the 

 radiant appears on the screen it can be eliminated by 

 interposing a piece of finely ground glass between it and 

 the condenser. 



Fig. 44 shows Zeiss' Horizontal and Vertical Camera 

 arranged for the photography of large transparent objects, 

 and a second method of illumination is there indicated, 

 involving the use of tw r o condensers. Those shown in the 

 illustration are two of the Kohler single lens condensers. 

 The collecting lens is No. 1, a large condenser, 8 cm. 

 in diameter and 16 cm. focal length, "is suitable for 

 incandescent gas. A somewhat smaller lens la, 6 cm. 

 diameter and 12 cm. focus, is used with smaller radiants, 



