54 



Biological Stains 



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. Although 

 the 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. 



TABLE 1 



SPECTRUM 



(Showing Complementary Colors) 



While it may be necessary or advisable in some instances to 

 determine absorption in the infra red or ultra violet, the measure- 

 ment of absorption within the visible spectrum, upon which color 

 is dependent, is adequate for most purposes. The essential prin- 

 ciple of operation of a typical spectrophotometer is best under- 

 stood by considering first the principles of a visual instrument, 

 although such instruments are now practically obsolete because 

 of the greater efficiency of photoelectric measurements. These 

 principles may be understood by reference to Fig. 1. Such instru- 

 ments comprise essentially two beams of light from a common 

 source, one to pass through a glass cell containing the sample to 

 be examined, the other to pass through a similar cell containing 



