COLOR 83 



band spectrum by means of a slotted diaphragm. Such a spectrum is 

 formed automatically when a mixture of wavelengths, such as sunlight, is 

 passed through a narrow slit and then through a prism. Since the refrac- 

 tive index of the glass is different for each wavelength, being highest for 

 violet and lowest for red, the colors are sorted out of the mixture and 

 can be caught on a screen, all in order, as a spectrum (Fig. 29a). If 

 the light reflected or transmitted by a colored object is concentrated and 

 passed through a prism, the spectrum formed will naturally have lightless 

 regions in it corresponding to the wavelengths whose removal from the 

 sunlight, through absorption by the object, gave the latter its color. Such 

 a spectrum is an 'absorption spectrum', and is the basis of spectral anal- 

 ysis, that powerful weapon of chemistry and astronomy with which sub- 

 stances are detected by means of their specific fingerprints on sunlight. 



With a little practise, a normal person can learn to distinguish about 

 one hundred and sixty distinct hues in the sunlight spectrum.* If we now 

 let any two of these hues escape through narrow slits and aim them with 

 mirrors at the same piece of paper or ground glass, or look at one with 

 each eye, or present them in rapid alternation to one or both eyes, we 

 will obtain a sensation different from that given by either hue alone. 

 In most cases, the sensation will be that afforded by some other pure 

 hue, lying between the chosen two in the spectrum. If however the latter 

 are far apart in the spectrum, and lie diametrically opposite each other 

 on the 'color circle' (Fig. 29b), they are 'complementaries' and their 

 mixture will produce white light. Thus any hue in the spectrum (and 

 white) can be produced by mixtures, made by one means or other, of some 

 two other hues. Some white light may need to be added to the spectral 

 hue in order to make it an exact match for the mixture. We are not of 

 course discussing here the subtractive mixtures which one obtains by stir- 

 ring pigments together — the artist's complementary, primary, and second- 

 ary colors have nothing to do directly with those of the physiologist. 



The physiologist often terms red, green, and violet 'primary' colors, 

 because in none of them can any other hues be seen. Yellow is also con- 

 sidered a primary by psychologists, as is blue for that matter. Yellow 

 sensations can be produced by means of simple apparatus which presents 

 red to one eye and green to the other, but yellow is not reddish green or 

 greenish red. Yellow, in this instance, is obviously synthesized in the 



'''Actually, 160 complexes of hue-plus-whiteness. No one has ever yet determined the (much 

 smaller) number of hues which would still be discriminable, were saturation eliminated 

 as a variable. 



