PROPERTIES OF THE RETINA. 



345 



spectrum lose their quality, the blue colors being perceived last of 

 all, just as in late twilight it may be noticed that the sky remains 

 distinctly blue after the colors of the landscape become indistin- 

 guishable. It should be added that the "Purkinje phenomenon" 

 is true only for the parts of the retina lying outside the fovea, 

 that is, for the peripheral field. As the light grows dimmer the 

 perception of blue is lost first in the fovea, so that with a certain 

 feebleness of illumination the central field becomes blue-blind. 

 With a very feeble illumination the dark-adapted eye becomes 

 practically totally color blind. 



Fig. 146. — Diagram showing the distribution of the intensity of the spectrum as de- 

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

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

 the luminosity of the different colors. Eight 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 (605-625); with weaker 

 illumination it shifts to the green (535). — {Konig.) 



Qualities of Visual Sensations. — The different quahties 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, 

 together with the purples made l^y combination of the two ends 

 of 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 ohves and l^rowns 



