THREE-COLOUR PHOTOMICROGRAPHY 



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picture, are most simply explained by reference to the 

 Sanger-Shepherd three-colour process. Three negatives 

 are taken on panchromatic plates, one through each of 

 three screens of the primary colours, blue- violet, green, 

 and red. Sanger-Shepherd, and Wratten and Wainwright 

 include filters suitably adjusted for their panchromatic 

 plates in their sets of photomicrographic screens, and they 

 are used in the way previously described for any other 

 colour filter. Three negatives are taken, one after another, 

 focussing carefully with each screen. Apochromatic objec- 

 tives and compensating eyepieces are naturally most useful 

 here, as any considerable error in the colour correction 

 might produce images of unequal size on the plate ; but 

 good-class achromats are quite satisfactory. No other 

 special apparatus is necessary, although a triple-repeating 

 back to hold the three plates is a convenience. 



The exposures necessary depend on the plate and the 

 screens in conjunction with the illuminant used. In the 

 case of Sanger-Shepherd tricolour screens and colour 

 plates the exposures with daylight are the same through 

 each screen. 



With Wratten and Wainwright's screens in conjunction 

 with their " M " plate, the three exposures are in the 

 following ratios for various illuminants. 



The exposures must be carefully calculated in accord- 

 ance with these factors, or any variant from them given 

 in the plate box, and the plates should be from one batch 

 to ensure their being of the same speed. This variation 



