92 THE VISUAL PROCESS 



between them the practical identity of this 'scotopic brightness curve' 

 with that of the photopic totally color-blind eye, the absorption spectrum 

 of rhodopsin, and the curve of the rhodopsin-bleaching power of mono- 

 chromatic lights (Fig. 33, c/. Fig. 35). The rods are completely insen- 

 sitive to deep red because rhodopsin absorbs nothing beyond X650mp,, 

 and they are most sensitive to green because this kind of light is more 

 avidly absorbed by rhodopsin than any other. 



As the intensity of the spectrum is now increased, there is a range of 

 intensity — called the photochromatic interval — within which the spec- 

 trum remains colorless. This interval is not the same for all regions. 

 For red, it is of course non-existent, for as soon as wavelengths longer 

 than 650m|X are seen at all they are seen by cones, and are seen as red 

 light. In succession toward the violet end, the other hues appear as the 

 thresholds of the cones for them are crossed. The now fully colored 

 spectrum has its brightest part moved (the Purkinje shift) to around 

 A557m[X, and extends from A,390m^ to ^760m^l. Beyond A,650m|X lies 

 the pure red. At A,600m[l is orange. The exact center of yellow is at 

 A,582m^, of green at A,515m[X, of blue at A,476m[X. Beyond the indigo 

 of X424-455m[i lies the true violet (see Table I, p. 4). 



In the neighborhood of yellow and blue the change in hue for a given 

 change in wavelength is greatest. To be exact, the two maxima lie at 

 A,580mp, and A,490m(l. Around these values, we can discriminate more 

 different hues, closer together in the spectrum, than we can elsewhere. 

 This is because these wavelengths are maxima in the graph of the in- 

 trinsic pallidity or tinsaturation of the spectrum: as we pass from one 

 side of such a maximum through it to the other side, the appearance of 

 the stimulus changes rapidly with a change in wavelength because the 

 ratio of chroma to whiteness in the sensation is changing so rapidly. 



The blue maximum, and the minor peak of brightness in this region, 

 may be lowered somewhat by absorption in the yellow pigment of the 

 macula lutea of the retina (see Chapter 8, section D) . As the intensity 

 is raised however, yellow and blue stand out more and more. The hues 

 on either side of each of these actually change, gravitating toward which- 

 ever of the two is the nearer — that is, yellow and blue appear to spread 

 more widely in the spectrum at the expense of their neighbors, until at 

 very high intensities yellow and blue alone, greatly unsaturated, fill up 

 the whole spectrum. At dazzling intensities even these lose all chroma 

 and a sensation of whiteness is then evoked by any visible wavelength. 



