7 6 ELEMENTARY PHOTO-MICROGRAPHY. 



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 

 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 acetylene light reduces exposure, and gives 

 better results than the oil lamp. Orthochromatic 

 plates must be used in every case. 



Exposure with green screen is lengthened about 

 three times, and with red five times. 



The use of a signal-green glass between the 

 lamp and microscope will vastly improve the de- 

 finition of any achromatic objective. Note that 

 the colour screen should be placed in position 

 before the focussing of image on the camera screen 

 is completed, and not after. 



Monochromatic light is often essential for the 

 very best results. This is light of one colour and 

 one uniform wave length. Images formed by waves 



