SCREEN-PLATE COLOUR PHOTOMICROGRAPHY 211 



elements and the subsequent blocking out of the green 

 and blue. This process is called " Reversal," and is a 

 conversion of the negative to a positive. 



The whole process of development and reversal is very 

 quickly carried out, the principle of the reversal being 

 an oxidation of the black parts of the negative by potas- 

 sium permanganate or bichromate, and a subsequent 

 darkening of the parts not hitherto developed, by immer- 

 sion of the plate in the original developer in bright light. 

 The plate is finally fixed in hypo (although this is stated 

 by Messrs. Lumiere to be unnecessary), washed, dried, 

 and varnished. What is judged to be the right quantity 

 of varnish is poured on to one corner of the plate, and is 

 caused to flow evenly over the entire surface by judicious 

 tilting of the plate towards each corner in turns. Any 

 excess is run back into the bottle. 



General formulae suitable for any screen plate are given 

 in the appendix (pp. 273-4) with an outline of the process, 

 but each maker advises some method particularly suited 

 to his plates ; or some special developer, such as Quinomet. 

 It is unnecessary to give these, as the makers supply the 

 details, but advantage undoubtedly arises from care- 

 fully following the instructions accompanying the plates 

 when they differ from the general formulse given. 



A true colour-rendering depends very largely on correct 

 exposure ; an idea of what is necessary can be gathered 

 from the descriptions accompanying the colour plates. 

 Over-exposure gives a too dense negative, and conse- 

 quently on reversal a thin positive, very poor in colour. 

 Too short exposure leads to a dense positive, through 

 which the colours can only with difficulty be seen. The 

 Howard Farmer reducer and silver or mercury intensifiers 

 can be used to improve defective positives. 



" Duplicating " Process. A drawback of the single 

 process is that each plate embodies a screen, which is an 

 expensive item, and the Thames Plate Co. overcame this 



