CHAPTER 12 



The Differential Action of Dyes 



The purpose of dyeing in microtechnique is nearly always to 

 obtain contrast between the constituent parts of an object. If a dye 

 were perfectly diffuse in action, the whole of a section or other 

 microscopical preparation would be uniformly coloured by it. 

 This result is not produced by any dye, for certain parts of the 

 specimen are always somewhat more strongly coloured than others, 

 even by the most diffuse acid dyes. The mere production of con- 

 trast between the specimen as a whole and its surroundings is 

 seldom useful, though plankton organisms (for instance) may 

 sometimes be dyed with no other intention than this, w^hen the 

 desired end is recognition by external characters rather than study 

 of internal structure. 



We have seen in chapter lo that the different objects in a 

 preparation may take up different amounts of the same dye, and 

 that different dyes (a typical basic and a typical acid dye, for in- 

 stance) may attach themselves differently to the same object; that 

 is to say, one dye may dye it deeply, another slightly or not at all. 



It is now necessary to consider in greater detail the way in 

 which dyes may be used to give striking contrasts and thus exhibit 

 clearly the diversity of the parts of a microscopical preparation. 



There may be chemical or physical reasons for the stronger 

 coloration of a particular object. It may be dyed strongly either 

 because it possesses many chemical groups capable of reacting 

 with the particular dye used ; or because there is a lot of colourable 

 matter in it per unit volume (that is to say, because it is very dense 

 in the physical sense) ; or because it is easily permeable by the dye 

 used, while other tissue-constituents are more difficult to penetrate. 

 In short, depth of coloration is affected by chemical affinity, 

 density, and permeability. 



The matter is complicated, because these three factors may 

 either act together or antagonize one another. Thus chromatin is 

 chemically reactive (basiphil), dense, and permeable, and there- 



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