1 68 PHOTOMICROGRAPHY 



matic objectives are used, but an apochromat should give 

 a sharp photomicrograph if the focus is found in white 

 light and the screen then inserted before exposure. A 

 colour screen that does not pass sufficient light to focus 

 by must be rejected ; this difficulty arises more especially 

 when using blue filters. 



Exposure ivith Screens. The increase of exposure 

 necessary when using colour screens is rather difficult 

 to decide, except by experiment. It depends both on 

 the colour and amount of light transmitted, .and on the 

 plate used. The matter is discussed in the next chapter. 



