ELEMENTARY PHOTO-MICROGRAPHY. 37 



colours. It will be found that sober tints yield 

 better negatives than blue and yellow; the mica 

 or selenite, therefore, can be selected accordingly. 

 A dead black body tube is essential, and the most 

 suitable powers are 2in. and lin. objectives. The 

 eyepiece is dispensed with to compensate for the 

 loss of light through the prism. 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 

 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 satisfactory contrasts 

 of colour. With the polariscope the time of expo- 

 sure 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 sections 

 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 photo- 

 graph, hence artificial contrasts have to be produced 

 by staining. The general rule for determining the 

 particular colour screen to be used for insertion 

 between the light and the object during exposure 



