colors, in the usual solvents and under the usual conditions, show 

 but one absorption band in the visible spectrum. The curves are 

 seldom perfectly symmetrical, however, and usually give indica- 

 tions of localized secondary absorption in some portion of the band. 

 It has been shown that this secondary absorption is due, in numer- 

 ous instances, to a tautomeric form of the dve. It should never be 

 accepted as evidence of the presence of a second dye unless it has 

 been ascertained that it is not found with a pure sample of the dye 

 under conditions of examination. 



The absorption curve of a green dye is recorded in Fig. 4. It has 

 a principal band in the red and a secondary band in the violet. Both 

 the absorption curve and the color of the dye could be matched 

 closely by mixing a suitable blue and a yellow dye in the correct 

 proportions. All green dyes absorb appreciable amounts of violet 

 light as well as of red light. 



In Fig. 5 is given the absorption curve of a dye mixture, together 

 with the curves of the component dyes. The mixture is reported to 

 have been marketed in good faith as asymmetrical dimethyl 

 thionin, a dye which is intermediate in constitution and in color 

 between thionin and methylene blue (see methylene azure p. 48). 

 The absorption curve plainly indicates the presence of two dyes, 

 and suggests their pro})able identity. (It would be advisable to 

 effect the separation of small amounts of both dyes, if their positive 

 identification is desired.) The color of the mixture is very similar 

 to that of dimethyl thionin. The absorption curve of that dye, 

 however, is a simple and well defined curve resembling those of 

 thionin and methylene blue, but occupying an intermediate posi- 

 tion in the spectrum. 



This illustration shows how valuable the spectrophotometric 

 analysis may be in determining whether a given product is a simple 

 dye or a mixture of two or more dyes. This fact, together with its 

 use in determining the exact shade of any dye, makes it the most 

 valuable test to apply to a stain, other than to determine by actual 

 use whether the sample will prove satisfactory to the micro.scopist 

 or not. 



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