THE CHEMICAL COMPOSITION OF DYES l6l 



centration that will give a transmission of about 20% at the wave- 

 length of maximal absorption. It will be observed from the graph 

 that basic fuchsine transmits most of the red and orange light, and 

 also a considerable amount of violet and blue, but absorbs strongly 

 in the green, especially at about 540 m/x. Different specimens of 

 this mixed dye naturally differ somewhat in the w^ave-length of 

 greatest absorption. 



100 



80 



60 



o 



m 

 in 



to 



z 

 < 



40 



20 



400 450 500 550 600 650 



WAVELENGTH, mp. 



FIG. 17. Graph showing the transmission of light through a layer i cm 

 thick of basic fuchsine, o-ooo62°o aqueous. ^^ The following abbreviations 

 are used in this and subsequent figures: — v, violet; b, blue; g, green; y, 



yellow; 0, orange; r, red. 



Since the active property of the dye is to absorb, not to transmit 

 light, it is for some purposes desirable to express its absorptive 

 capacity as the reciprocal of the transmission ; for instance, instead 

 of saying that the transmission at a particular wave-length is y^ 

 of the incident light, we may turn the fraction upside down and 

 use the number 5 to represent the absorption. A curve of re- 

 ciprocals for the same solution of basic fuchsine as before is shown 

 in fig. 18. Alternatively, one may use the logarithms of the re- 

 ciprocals. Such logarithms are called 'densities'. This method of 

 expression is particularly useful with light-filters, because if one 

 know^s the density of each at any w^ave-length, it is only necessary 



L 



