60 ELEMENTARY PHOTO-MICROGRAPHY. 



position can be selected that passes much more 

 light than any other position. Perhaps the position, 

 however, may not give the most desirable colours. 

 Should this be so, try a change of mica or selenite, 

 and if not now satisfactory, adopt such a medium 

 position of the prism as shall be found to combine 

 the greatest possible amount of light with satis- 

 factory contrasts of colour. With the polariscope 

 the time of exposure is about doubled. 



In landscape photography, colour screens are 

 used to obtain correct colour values, but in photo- 

 micrography they are used for quite a different 

 purpose, viz., to secure contrast. For instance, 

 bacteria and many anatomical and vegetable sec- 

 tions are so transparent that they have to be single 

 or double-stained before it is possible to differentiate 

 their form and structure. In their original colour- 

 less state they would never give a well-defined 

 photograph, hence artificial contrasts have to be 

 produced by staining. The general rule for deter- 

 mining the particular colour screen to be used for 

 insertion between the light and the object during 

 exposure is that it should be the complementary 

 colour of the one on the object. Thus if the stain 

 be red, a green screen must be used, because a red 

 object viewed through a green glass will appear 

 black, and blackness or absence of light will give 

 more or less transparent places on the negative, 

 an essential element in the production of vigorous 



