THE DIFFERENTIAL ACTION OF DYES 229 



fore easily dyed; red blood corpuscles are chemically reactive 

 (acidophil) and dense, but relatively impermeable and therefore 

 not easily dyed except by easily diffusing dyes. Also, different 

 dyes are differently affected by the chemical affinity and perme- 

 ability of the various tissue-constituents. The matter will be 

 analysed by a separate consideration of each of the three factors 

 affecting intensity of colouring. 



Chemical Affinity 



The simplest way of getting sharp colour contrasts is to take 

 advantage of different chemical affinities by using a basic and an 

 acid dye in succession. Not every pair of colours is suitable, how- 

 ever. It is an interesting fact that yellow basic dyes are scarcely 

 ever used. This has come about by a process of natural selection, 

 and no one seems to have mentioned the subject. The reason 

 is curious. Basic dyes are above all dyes for chromatin, and 

 chromatin exists in the form of separate objects in cells and never 

 forms a background against which other cellular constituents are 

 viewed. It is therefore desirable to stain it darkly, and to use a 

 light dye for the background. Now when the colour-receptors in 

 our eyes receive light near the middle of the spectrum, in the 

 region of the yellow and greenish yellow, they are stimulated in 

 such a way that the colour appears highly 'unsaturated' ; that is to 

 say, in this region of the spectrum there is an appearance of the 

 adulteration of the light by whiteness and thus the colours appear 

 pale, while the regions towards the ends of the spectrum are not 

 diluted in this way. It would never enter anyone's head, there- 

 fore, to stain chromosomes yellow and surround these objects on 

 all sides with cytoplasm stained blue, for the blue would make it 

 difficult to see the chromosomes. We therefore choose our basic 

 dyes from the regions towards the ends of the spectrum, while 

 we usually avoid the ends when choosing our background dyes. 



When a black dye is used to colour chromatin (iron haemat- 

 ein, for instance), any acid dye used merely to colour the back- 

 ground must somewhat reduce the contrast. 



It might be supposed that we only needed one basic and one 

 acid dye. It is true that we could dispense with many that are used, 

 without detriment; nevertheless we could not limit ourselves to 

 two. Differences of chemical affinity among acid dyes and among 

 basic dyes account in part for this fact, though differences in 

 capacity to penetrate are more important (p. 234). 



