CHAPTER XIV 



PHOTOGRAPHIC OPERATIONS 



Plates. There is no need to describe in detail the 

 ordinary routine of photographic processes, but the points 

 of chief importance in photomicrography require more 

 than passing mention, particularly on account of the great 

 use that is made of colour sensitive plates. The advan- 

 tages of panchromatic plates have been mentioned in 

 the chapter devoted to colour screens, and they would 

 always be used were it not for their cost and the diffi- 

 culties of manipulation though these are really not 

 great after a little experience due to the necessity of 

 developing them either in darkness or in a very dull 

 green light. For these reasons orthochromatic plates 

 are much more frequently used than panchromatic, and 

 for many purposes their lack of sensitiveness to the 

 orange and red rays is of no great moment. Non-colour 

 sensitive plates are as a rule not nearly so satisfactory 

 for the reasons which have already been given. 



Panchromatic Plates. There are not many makers of 

 panchromatic plates, but the following are well-known 

 to us. Wratten and Wain wright's " Panchromatic " and 

 " M " plates, the latter specially made for photomicro- 

 graphy with a fine grain. Ilford's Bathed plates, Paget's 

 ' k Process Panchromatic " and " Hydra Panchromatic," 

 Gem " Tricol," and Sanger-Shepherd Colour plates. 

 Some workers prefer to sensitise ordinary plates to 

 yellow and red by bathing them in certain dyes as 

 required, both on account of the expense of panchro- 

 matic plates and their liability to lose their colour 

 sensitiveness in time. It is well to select fine-grained 

 ordinary or medium plates for this purpose, and the 



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