320 



THE SPECIAL SENSES. 



all. Just as in late twilight it may be noticed that the sky remains 

 distinctly blue after the colors of the landscape become indis- 

 tinguishable. With a very feeble illumination the dark-adapted eye 

 becomes practically totally color blind. 



Qualities of Visual Sensations. The different qualities of our 

 color sensations may be arranged in two series: an achromatic 

 series, consisting of white and black and the intermediate grays, 

 and a chromatic series, comprising the various spectral colors, to- 

 gether with the purples made by combination of the two ends of 



Fig. 137. Diagram showing the distribution of the Intensity of the spectrum as de- 

 pendent upon the degree of illumination. Tin spectrum is represented along the abscissa, 

 the numerals giving the wave lengths from red, 670, to violet, 430. The ordinates give 

 the luminosity of the ilitYerent colors. Bight curves are given to show the changes in 

 distribution of relative brightness with changes in degree of illumination. _ With the 

 greatest illumination the maximum brightness is in the. yellow (605425); with weaker 

 illumination it shifts to the green (535). (Konia.) 



the spectrum, red and blue, and the colors obtained by fusion of 

 the spectral colors with white or with black, such, for instance, as 

 the olives and browns. 



The Achromatic Scries. Our standard white sensation is that 

 caused by sunlight. Objects reflecting to our eye all the visible 

 rays of the sunlight give us a white sensation. This sensation, 

 therefore, is dm 4 primarily to the combined action of all the visible 

 rays of the spectrum, each of which, taken separately, would give 

 us a color sensation. White or gray may be produced also by the 



