COLOUR SCREENS 163 



mentary," and one will absorb all light transmitted by 

 the other. 



Such accuracy in the adjustment of object and filter 

 can hardly ever be attained, and the opacity of the object 

 is often of much more importance than its colour in 

 determining the absorption of light. 



Decrease of Contrast and Increase of Detail. Increase 

 of detail is particularly required in preparations trans- 

 mitting yellow or red light, which, being non-actinic, 

 may in extreme cases appear uniformly black in the 

 photograph. Small differences in these colours give 

 details which are easily discerned by the eye, but they 

 may be entirely missed by a plate which reacts mainly to 

 the blue rays, as these are absorbed practically to the 

 same extent by all parts of the object. In order to 

 obtain detail the photograph must be taken by light 

 approximating in colour to the transmission of the pre- 

 paration, and this involves the use of an orange or red 

 screen and a plate sensitive to orange or red. If there 

 are any colourless or faintly coloured parts, in the pre- 

 paration they may be so over-exposed as to be entirely lost 

 before the detail in the yellow part is registered. By using 

 a dark orange or red screen the difference in actinic 

 property of the light passing the yellow and colourless 

 parts of the preparation is so reduced that it is possible 

 to get detail in the one before the other is hopelessly 

 over-exposed. In any case the excessive contrast is 

 much reduced, and whether the two parts will print suffi- 

 ciently well when a mean exposure is made, depends upon 

 the latitude of the plate, and the amount of over- and 

 under-exposure given to the two parts. If a filter can 

 be found that reduces the errors of exposure in proportion 

 to that actually given, sufficiently to bring them within 

 thje latitude of the plate, a print giving detail in all parts 

 of the specimen can be obtained. 



Correct Colour-rendering is not often of great import - 







