210 PHOTOMICROGRAPHY 



colour system, and it offers a means of adapting the 

 plate to any source of light. Paler yellow screens, or 

 sometimes even a green one, are used with artificial 

 illuminants to compensate for the smaller proportion of 

 blue rays or over-abundance of red rays. Suitable 

 screens can be obtained in several cases from the plate 

 maker, and accurate colour-rendering cannot be obtained 

 unless a correctly adjusted one is used. The Kj and K. 2 

 screens of Wratten and Wainwright are also useful for 

 this purpose. 



" Single " Processes. Autochrome, Dufay, or Paget. 

 The glass support is coated with the colour screen, and a 

 panchromatic emulsion then spread on top of that, the 

 whole forming a single plate. When loading the dark 

 slide, in darkness or by one of the special safelights, the 

 plate must be placed with the emulsion surface to the 

 back, so that exposure is made through the glass and 

 colour screen. Apiece of card is placed behind the film 

 to protect its delicate surface from abrasion. To focus 

 accurately the ground glass is reversed, and used with 

 the ground side away from the microscope in register 

 with the film side of the plate. The result of exposure 

 and development is the formation of a negative con- 

 sisting of dots corresponding to the different coloured 

 elements of the screen. Red light, passing through the 

 red elements only, produces (after development) a black 

 spot under each, and, if the negative so produced be held 

 up to the light, these spots prevent light passing through 

 the red parts, and the plate appears a complementary 

 blue-green from a blending of the light passing through 

 the blue and green elements. 



A transparency showing the colours of the original is 

 obtained by removing the black dots of the negative, and 

 replacing the clear parts of the negative by black. Thus, 

 in the example given, red light only can pass through the 

 plate after the removal of the black dots under the red 



