TWO-COLOUR PHOTOMICROGRAPHY 209 



red negative with the green dye, and that from the green 

 negative in the red dye, binding them together as de- 

 scribed in the three-colour process. 



If a section is stained blue and yellow, or blue and green, 

 satisfactory negatives may be obtained on orthochro- 

 matic plates ; it is only when an orange or i^d colour has 

 to be reproduced that it is absolutely necessary to use 

 panchromatic plates. Generally, however, the reproduc- 

 tions only approximate to the colours of the original, 

 but for the purposes for which the process is suitable this 

 is immaterial. 



Screen-Plate Processes. The Autochrome, Dufay, and 

 Paget colour plates are typical of the " Screen-plate " 

 processes, which differ from the "Three-colour " in that 

 only one negative is taken. There are other processes 

 besides these, but only the three of which examples are 

 given are here described. Instead of the three-colour 

 filters previously described, only one is used, consisting 

 of minute elements of red-orange, green, and blue-violet 

 entirely covering the plate. The elements are so small 

 that at the normal distance of distinct vision the eye 

 can hardly detect them, and the screen should appear of 

 a neutral grey colour. They consist in the Autochrome 

 plate of irregularly scattered grains of starch suitably 

 dyed, while the screen of the Paget and Dufay plates is 

 made by dyeing a collodion or gelatine film in such a way 

 that it is covered with a mosaic of regularly arranged 

 square or rhombic elements. 



The colours used in the screen are not the same as those 

 of the three-colour screens. Each element fulfils the 

 double purpose of a taking and viewing screen, and con- 

 sequently they are mutually adjusted for that purpose. 

 The exposure through each element is the same, and no 

 allowance for a variation in the nature of the illuminant 

 can be made in that way. A yellow i: ~,ht filter properly 

 adjusted to the screen and plate is an essential part of the 



