1 62 



DYEING 



to add the two densities together to obtain the density of two 

 filters used one behind the other. As a result of this, the absorptive 

 capacity of coloured substances is often given in the form of 

 density-curves, such as that shown in fig. 19. It is necessary to 

 understand this method of representation, because it is so com- 



5- 



° 4 



to 



to 



2 



to 



< 



ct 3 



< 



o 

 o 2 



O 



UJ 



QC 



V 

 I 



b 

 I 



g 

 I 



o 



I 



400 



450 



500 550 



WAVELENGTH, m^ 



600 



650 



FIG. 18. Graph showing the reciprocals of the transmission of Hght 

 through a layer i cm thick of basic fuchsine, 0-00062*^0 aqueous. ^^ 



monly used; but in fact one seldom makes practical use of the 

 ease of addition of densities, if one's main interest is in the use of 

 dyes in microtechnique; and transmission gives a much more 

 direct statement of what is actually the important point when one 

 looks through the microscope — the colour of the dye. 



Any change in structure that destroys the quinonoid linkage 

 results in the loss of colour. Many dyes can be changed to colour- 

 less or Z^wco-compounds by the action of reducing agents such as 

 sodium hydrosulphite (Na2S204). The leuco-representative of 

 pararosaniline may be regarded as a triaryl derivative of methane: 

 that is to say, as methane in w^hich three of the four hydrogen atoms 

 have been replaced by aryl rings. (An aryl ring is either a simple 

 phenyl ring or else a phenyl ring with a special group or groups (in 



