ELEMENTARY PHOTO-MICROGRAPHY. 6l 



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. 



