COLOUR SCREENS 167 



most effective way, a radiant deficient in blue rays should 

 be used with a screen transmitting light at the red end of 

 the spectrum, and a plate sensitive to red. And to a 

 certain extent control over contrast may be had by a 

 suitable selection of the illuminant. In practice this is 

 not usually nearly so convenient as control by colour 

 screens, but the effect of a change from a white to yellow 

 light should not be overlooked, and in this connection 

 the following considerations should also be kept in mind, 

 although fairly obvious. 



A screen capable, for instance, of cutting off both 

 yellow and green rays, would have no advantage over 

 one cutting off the yellow only, if the radiant did not 

 emit green rays, or if the plate were insensitive to green ; 

 and a screen that is able to cut off all the yellow from a 

 light of low intensity might be quite inadequate to do so 

 with a brighter light. 



Choice of Screen. The readiest way to decide what 

 screen to use is to examine the preparation under the 

 microscope with various colours, and select the one which 

 gives the best result, the greatest contrast, the greatest 

 amount of detail, or the best resolution, as may be de- 

 sired. Inspection in this way is, however, not always a 

 satisfactory guide, as the transmissions of screens are not 

 always such as their colour would suggest. Many yellows 

 and greens, for instance, have incomplete absorptions 

 in the blue and violet, and blue dyes often give a red 

 transmission band. Methylene blue, for example, shows 

 two such bands, one from the red end to 6800, and another 

 between 6200 and 6500. The photographic result 

 obtained when such screens as these are used would not 

 be quite what was desired, and in order to determine 

 their transmissions with certainty a spectroscope must 

 be employed. 



Focussing when Screens are Used. It is essential to 

 focus the object with the screen in position when achro- 



