1 64 PHOTOMICROGRAPHY 



ance in photomicrography as it does not necessarily 

 tend to give the contrast that is so essential. In colour 

 photomicrography, on the other hand, it is of the first 

 moment, and whenever it is required a screen, usually 

 yellow, must be very carefully selected to counteract 

 the greater actinic effect of the blue ; and in this connec- 

 tion the proportion of blue to other colours in the light, 

 and relative sensitiveness of the plate used to the 

 different colours, must be carefully considered. These 

 factors are discussed on page 165. 



The following rules governing the use of filters can be 

 stated ; their application will be made clearer by a study 

 of the examples given on Plates 17, 19, and 20. 



RULES FOR THE USE OF COLOUR SCREENS 



1. To Improve Resolution and Definition. (a) Use a 

 monochromatic screen with a transmission in the yellow- 

 green corresponding to best corrections of the objective. 



(b) In certain cases, when light of very short wave 

 length is desired, use a monochromatic blue screen ; 

 this is generally not so satisfactory as (a) with achromatic 

 objectives. 



2. To Increase Contrast. Use a screen complementary 

 in colour to the object, or to the part of the object that it 

 is desired to make darkest in the photomicrograph. 



3. To Decrease Contrast or Increase Detail. Photo- 

 graph by light corresponding in colour to the transmission 

 of the object, or the part in which detail is required. This 

 is most useful for yellow or red preparations. 



4. To get Correct Colour-rendering. Use a screen pro- 

 perly adjusted to the light and plate, to absorb just tl 

 correct amount of the more actinic rays, in order tlu 

 all the colours of the spectrum may act on the plate 

 the proportion of their visual luminosities. 



Colour of the Illuminant. In adjusting the colour oi 



