upon the absorption of light in varying degrees, over an extensive 

 spectral range. Whereas the unaided eye is able to register only 

 the composite effect, it is possible to resolve this effect into its 

 component factors with the aid of a spectrophotometer. Althothe 

 eye is unable to distinguish between a violet dye and a suitable 

 mixture of a red and a blue dye, the heterogeneous character of the 

 mixture is readily apparent upon spectrophotometric examination. 

 Pure dyes may have simple absorption spectra, in that their light 

 absorption is all at one part of the spectrum, or they may be more 

 complex, showing two or more points on the spectrum at each of 

 which light is absorbed to greater extent than on either side of it. 

 Thus even in the instance of pure products of identical color to the 

 eye, the spectrophotometer frequently reveals decided differences 

 when the character of the light absorption is considered in detail. 

 The essential principle of spectrophotometric analysis may be 

 understood by reference to Fig. 2, which is a diagram of a spectro- 

 photometer. Two parallel beams of light of equal intensity enter 



Diagram of Spectrum 

 Showing; complementary colors 



Wave lenatk: ^°° <" 500 550 600 650 ^^ 



*^ ' ' L 



> I 



Col or: violet blue green yellow orange re d 



Complementary color! yellow orange red violet bh 



lue 



Fig. 1. Diagram of spectrum 



the photometer box by separate orifices, pass thru a prism where 

 they are resolved into visible spectra, and then reach the eye in 

 contiguous fields so that very accurate comparison between the 

 two spectra is possible. The arrangement is such that one beam 

 passes directly to the prism whereas the intensity of the second 

 beam may be reduced in any desired proportion by revolving the 

 photometer circle. , A glass cell containing a dilute solution of the 

 dye to be exainined is interposed in the path of the first beam and a 

 similar cell containing water (or whatever solvent is used in the 

 case of the dye) in the path of the second beam. The spectrum of 

 the beam which has passed thru the dye solution will be found 

 deficient in those portions which have been absorbed by the dye: 

 and the degree of the deficiency at any position in the spectrum 

 may be measured by determining the degree to which the intensity 

 of the light of the second spectrum must be reduced in order to 

 obtain an equal intensity in the two fields observed by the eye. 



,26 



