DYEING AND OTHER PROCESSES OF COLOURING 31I 



Fluorochromes have proved of practical value in the rapid 

 recognition of certain bacteria, ^^^ because the latter can be made to 

 shine brilliantly in the dark and therefore can be seen easily with 

 only moderate powers of the microscope. It would not appear, 

 however, that the use of fluorochromes has resulted in many 

 important discoveries in biology. This is not surprising, since they 

 are not in general more selective than non-fluorescent dyes. 



Fluorochromes are said to exhibit 'secondary' fluorescence. 

 'Primary' fluorescence is the direct origin of visible rays from 

 tissue-constituents when irradiated with ultraviolet light. This can 

 give valuable histochemical information. It does not fall within 

 the scope of this book, since no colouring agent is used. 



The foregoing analysis of the various processes of colouring 

 used in microtechnique is summarized in table 1 1 . 



TABLE II 

 A nalysis of the processes of colouring in biological microtechnique 



Group D is not very satisfactory, since few processes can be 

 referred to it with certainty and the reactions involved are not 

 understood. The other six groups are vaHd and clearly distinguish- 

 able, and many examples of all of them except F could be quoted. 



The process of dyeing (G) is distinguished from all the others 

 by the fact that the tissues are exposed to the action of a solution 



