38 ELEMENTARY PHOTO-MICROGRAPHY. 



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 two comple- 

 mentary colours superposed produce blackness, and 

 blackness, or absence of light, will give more or less 

 transparent places on the negative, an essential 

 element in the production of vigorous pictures. 

 Experiment alone will teach the student just the 

 right tint to employ. He should, therefore, have a 

 selection not only of colours, but of light and dark 

 shades. When colour screens are used, the acety- 

 lene light reduces exposure, and gives better results 

 than the oil lamp. Orthochromatic plates must be 

 used in every case. 



