THE CAUSES OF DIFFERENTIAL DYEING 109 



corpuscles are particularly impermeable. It is possible to dye 

 collagen in one colour, cytoplasm and contractile substance in 

 another, and red blood-corpuscles in a third, by the use of three 

 different anionic dyes. The result is achieved without any reliance 

 on different chemical affinities. 



The three qualities in the tissue-constituents that make possible 

 the differential action of dyes — chemical affinity, density, and 

 permeability — may act together or may antagonize one another. 

 Chromatin, for instance, is chemically reactive (basiphil), dense, 

 but permeable : all these characters act together to render it easily 

 coloured by cationic dyes. Red blood-corpuscles, on the con- 

 trary, are chemically reactive (acidophil), dense, but very im- 

 permeable. A microscopical preparation coloured with two or 

 three dyes owes its variegated appearance to complex inter- 

 actions, and it is not always easy to disentangle the effects of the 

 three factors. 



Those anionic dyes that penetrate readily tend to colour all 

 acidophil and amphoteric tissue-constituents about equally 

 deeply, and they are therefore diffuse in action. They are useful 

 'background' dyes, giving colour-contrast to a cationic dye used 

 for chromatin. If methyl blue or a similar anionic dye were used 

 instead of a diffuse one, it might spoil the effect by giving some of 

 its colour to chromatin. 



It is best to choose background dyes of colours near the middle 

 of the visible spectrum (yellowish or green), since these appear to 

 the human eye as 'unsaturated' ; that is to say, they stimulate all 

 the colour-receptors in the retina to some extent and thus appear 

 pale, as though mixed with white. Orange G, being yellow, is suit- 

 able. For contrast the chromatin should be dyed in a 'saturated' 

 colour near one end of the spectrum (red, blue, or violet). Violet 

 gives the best contrast with yellow, because the colours are 

 complementary. 



If chromatin be dyed black or grey, any background dye will 

 necessarily reduce the contrast. 



