THE CLASSIFICATION OF DYES 179 



able to penetrate red blood-corpuscles and other close-textured 

 components of tissues. This is so despite the large size of the dyeing 

 ion. The capacity to penetrate well must be ascribed to the tend- 



Br Br ^ 



Til Jx J\^Br 



^O- Na+ 



Eosin Y 



ency of the ions to remain separate instead of aggregating (p. 236). 

 Eosin Y can be made to stain red blood-corpuscles most power- 

 fully, and show them up so vividly that the course of the capillaries 

 can sometimes be traced as easily as in an injected specimen. 

 Mann's methyl blue/eosin (the long method) is particularly well 

 adapted to this purpose. ^^^ 



Phloxine and erythrosine B are background dyes closely re- 

 sembling eosin Y. The former has chlorine atoms as well as 

 bromine; er}'throsine B is exactly the same as eosin Y except that 

 four iodine atoms are substituted for the four bromines. In both 

 of these, especially phloxine, the greatest absorption of light is 

 shifted towards the longer wavelength, with a resulting bluer (less 

 yellow) shade. 



AZINE AND RELATED DYES 



(quinone-imines) ' 



These dyes include some of the most valuable in microtechnique. 

 The chromophore is a quinonoid ring associated with a substi- 

 tuted imino-group (HN=). They may be regarded as derived from 

 quinone-imine by the substitution of an aryl ring for the H of the 

 imino-group, and they are therefore sometimes called quinone- 

 imine dyes. In the simplest of these the two rings are connected 



I i ^^11 



Quinone-imine Indophenol 



by a single link only (-N=). The indophenol dyes, which are 

 briefly mentioned on p. 288, are of this nature. In all the quinone- 

 imine dyes that are important in microtechnique, however, there 



