212 PHOTOMICROGRAPHY 



by making the screen on a separate piece of g] 

 so that it is still of use even if the plate is spoilt, 

 and exposing a panchromatic plate in the camera behind 

 the screen film to film. The Thames plate is not now on 

 the market, but the Paget Prize Plate Co. make a screen 

 of this sort and supply it together with suitable plates. 



The negative is made as in the .single method, but it is 

 fixed, and not reversed. From it clear black positives 

 are made on lantern plates ; metol-quinol is a good de- 

 veloper for these, and they are bound up in register face 

 to face with a viewing screen similar to that used for 

 taking the photograph. The process of binding up in 

 exact register requires a considerable amount of care. 

 Some practice, however, makes it fairly rapid and easy. 

 The first appearance when the screen and positive are 

 put together is somewhat like a Scots plaid,, and by 

 moving the two plates slightly the size of the pattern is 

 varied. When it is as large as possible the plates are 

 gripped with a bull-dog clamp, and a very slight move- 

 ment brings them into exact register, when correct colour- 

 rendering is seen. The plates are then bound up together 

 with lantern-slide binding. It is very easy to get a 

 reproduction in colours complementary to the original by 

 a slight error in register, and care must be taken that the 

 adjustment is such that the colours are correctly rendered. 



Comparison of the Colour Processes. The best results, 

 the most brilliant colours, and clearest slides with sharpest 

 definition are undoubtedly produced by the three-colour 

 process, but it is at the same time the most tedious to 

 work. Any number of slides can be made from one set 

 of three negatives. 



The single-screen process is the simplest, but has the 

 drawback that only one positive is obtainable from each 

 exposure. The duplicating-screen process enables as many 

 positives to be made as may be desired from one negative, 

 and in that respect is very superior to the single process. 



