204 PHOTOMICROGRAPHY 



given as a frontispiece and on Plates 3, 10, 18, 22, 

 30 and 37. 



The processes mentioned may be classified according 

 to the method of application of the light filter into 

 " Screen-plate " and " Three-Colour " processes. Both 

 ultimately depend on analysis of the image-forming rays 

 by their passage through three light filters blue- violet, 

 green, and red each one of which absorbs the colour 

 complementary to it, and transmits only a definite portion 

 of the image-forming rays to give a negative on a panchro- 

 matic plate corresponding to one of the primary colours 

 which make up the object. They differ fundamentally in the 

 methods of employing the screens, and the process by which 

 the coloured positive is built up, in one case, from three 

 negatives taken independently through the three filters, 

 and in the other, from one negative taken through an 

 ingenious combination of the three filters on one plate. 

 In the latter case the single negative combines in itself 

 the three elementary images intimately blended. 



It is obvious that three screens which will divide day- 

 light into three components, so accurately balanced that 

 a coloured positive can be obtained by suitable methods 

 from the three negatives, will not do for artificial light 

 relatively poor in blue rays, unless a variation can be 

 made in the three exposures which will compensate for 

 the incorrect colour analysis. If this is not possible 

 the resulting negative formed by the blue rays would be 

 relatively too weak by an amount depending on the 

 deficiency of blue rays in the artificial light, and the 

 positive would consequently be quite false to the original. 

 This difficulty is nearly always encountered in photo- 

 micrography, as daylight is not often used, and the 

 methods for overcoming it are detailed below in connec- 

 tion with the description of both processes. 



Three-Colour Processes. The analysis of the image- 

 forming rays, and the subsequent synthesis of the coloured 



