208 PHOTOMICROGRAPHY 



prints with better half-tones. (Formulae are given on 

 p. 273.) 



The blue-green transparency may be made on an 

 ordinary lantern plate in the usual way for black tones, 

 and toning in one of the Leto toners (the blue one), or by 

 immersing it in a bath of Potassium ferri-cyanide and 

 Ferric ammonium citrate (p. 273). It should be borne in 

 mind that this procedure intensifies the image, so that 

 the black transparency should not be quite so dense as 

 is required in the final blue image. 



The mouse's head on Plate 30 is an example of the 

 three-colour process, and the particulars given with it 

 will give some idea of the exposures and other working 

 data. 



Two-Colour Process. It is not so essential that the 

 colours of stained sections should be as accurately re- 

 produced in a photograph as the colours of a naturally 

 coloured object. The chief object of differential staining 

 is the emphasis and discrimination of the salient features 

 of the section. The various stains are of use principally 

 on account of their selective properties, and the depth of 

 tint produced can be varied at will. This gives great 

 latitude in colour photomicrography, and renders un- 

 necessary a very nice adjustment of the dyed positives 

 in the majority of cases, and even permits of the use of 

 a very simple " Two-colour process." Plant or animal 

 sections, for instance, which have been double-stained in 

 approximately complementary colours, say red and green, 

 or blue and yellow, can be satisfactorily photographed 

 through two screens made with the identical dyes used 

 for staining the section, by soaking cleared gelatine plates 

 in dilute solutions, as described on page 156. 



To photograph a green and red section, for instance, 

 take two negatives, one through the green and the other 

 through the red screen. Make positives by means of the 

 bichromated gelatine process, and stain that from the 



