USE OF SCREENS 41 



Isochromatic plates are generally most useful, 

 as with the object stained purple, which is 

 usually the case, a yellow screen has to be 

 used between the light and the object to 

 make the purple come out much stronger. 

 At the same time it increases the exposure 

 four to six times, according to the density 

 of the yellow glass screen. The yellow screen 

 can also be used to reduce contrast where part 

 of the image is yellow. Take as an example, 

 a transparent insect containing yellow eggs ; 

 in the ordinary way of photographing, the 

 eggs would come out dark, but with a yellow 

 screen the eggs would come out full of detail. 



The illustrations have been, for the most 

 part, taken on 12 by 10 plates. For this size 

 a comparatively long camera had to be used. 



The objectives can be used without any eye- 

 pieces, or with varying eye-pieces, according to 

 the magnification required. 



Approximate Exposures. 



If an object has a normal exposure, say, of 

 10 seconds when amplified to 20 diameters, 



