150 PHOTOMICROGRAPHY 



sity to introduce a cooling trough if care is taken to keep 

 the light on no longer than is necessary, but its use is to 

 be recommended whenever valuable preparations are on 

 the microscope. The construction of small troughs is 

 described in the section devoted to light filters, for 

 which purpose they are most frequently used. The 

 large ones illustrated here and at Fig. 48 are made of good 

 flat glass in order to avoid distortion of the light passing 

 through them. 



Illumination by Convergent Beam of Light. The method 

 now to be outlined is that which gives the greatest in- 

 tensity of illumination from any given light source, and 

 depends on the formation of an enlarged image of the 

 radiant on the substage condenser as described on 

 page 124 and illustrated at Figs. 38 and 39. 



A figure of the optical bench forming part of Zeiss' 

 large photomicro graphic apparatus is given (Fig. 48) to 

 show the arrangement of the illuminating system for 

 high -powers with a horizontal microscope. The camera 

 is carried on a similar table to that forming the optical 

 bench. The condenser used varies with the size of the 

 radiant, and may be one of the Kohler double convex 

 lenses referred to on page 105, and shown in the figure, or 

 the aplanatic lens Ic which is particularly suitable for 

 a 4-ampere arc or Nernst lamp. 



In place of these, a Watson-Conrady condenser (Figs. 38 

 and 39), a Nelson, or other aplanatic condenser, or a 

 simple double convex lens such as is shown at Fig. 7 may 

 be used. 



The microscope and lamp are arranged for critical light 

 as described above (operations 1 to 5). 



66. The condenser is then placed on the optical bench 

 in such a position that it projects an image of the radiant 

 on the substage diaphragm. Small aplanatic condensers 

 require to be placed much closer to the radiant (about 1 J") 

 than the Kohler lens figured. 



