ELEMENTARY PHOTO-MICROGRAPHY. 75 



Orthochromatic plates, preferably backed, are 

 used for this class of work, and in rotating the prism 

 a 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. 



The following substances will be found partial 

 larly suitable for a display of colour and pattern. 



Sugar. Coumarin. 



Chlorate of potash. Aspartic acid. 



Brucine. Sulphate of copper. 



Salicine. Distillation from 



Sections of rock. vapour of coke. 



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 



