310 DYEING 



must unquestionably be called dyeing, yet we do not follow this up 

 in the way described in the last paragraph. On the contrary, we do 

 not allow the chromophore to act in this way, for we deliberately 

 prevent visible light from reaching the dyed object. A brief 

 mention must be made of this technique, in which dyes are used 

 but their most obvious character is not. 



Quite a number of dyes are fluorescent, including some very 

 common ones, such as eosin. The typical 'fluorochromes', how- 

 ever, have been chosen on account of their powerful fluorescence. 





Skeleton formula of a typical acridine dye 



and are not particularly adapted for use with visible light. Several 

 of these, such as coriphosphine, are acridine dyes, but others 

 belong to various other groups (the anthraquinonoid, for instance). 

 The fluorochromes, in fact, possess ordinary chromophores and 

 auxochromes, and attach themselves as basic or acid dyes. 



When a fluorochrome has attached itself to the tissue, the pre- 

 paration is illuminated by ultraviolet light. New rays of visible 

 light originate wherever the dye is present. Different fluoro- 

 chromes give light of different colours. The object has now be- 

 come self-luminous. It is almost as though we were examining the 

 abdomen of a glow-worm in the dark. The microscope uses the new 

 rays originating in the fluorochrome. The image is produced in an 

 unusual way, for there is no interference of direct and diffracted 

 rays such as is always concerned in image-formation when an 

 object is illuminated from below with visible light. 



Since the intrinsic colour of a fluorochrome is irrelevant, some 

 very feebly coloured ones are usable. Fluorescein itself is an ex- 

 ample. This is an acid xanthene compound, too pale to be used as a 

 dye. One could go further still and use a fluorescent substance 

 that lacked a chromophore, provided that it could attach itself to 

 the tissues. Quinine sulphate could be used in this way. This is a 

 colourless, salt-like substance, the cation of which attaches itself 

 like a basic dye and therefore interferes with subsequent dyeing 

 by basic dyes.^*^ It is fluorescent, but since it is not a coloured 

 substance it cannot properly be called a fluorochrome. It might 

 almost be said of substances such as this, whether fluorescent or 

 not, that they lack chromophores but possess auxochromes. 



