Theory of Colour Vision. 403 



constitution of white light and of spectral lines. Conse- 

 quently, no matter how monochromatic the incident light is, 

 the vibrations in the retina are not monochromatic. If 

 sodium light is incident, the vibrations in the retina contain 

 light on both sides of sodium, i. e. orange and yellowish 

 green, or even red and green. 



(5) Thus if we attempt to mix a monochromatic red and 

 green, subjectively we are mixing two wide regions of the 

 spectrum which overlap in the yellow ; and the effect is the 

 same as if we allowed monochromatic yellow to fall on 

 the eye. 



(6) The more irregular the vibrations in the eye which 

 correspond to monochromatic stimulation, the worse is the 

 colour vision of the eye. 



(7) The quality of a light impression on the retina is very 

 conveniently represented by a curve such as fig. 1 — the energy 



Ffr. 1. 



400 500 600/*/* 



curve, as I call it. The abscissse denote wave-lengths in //,/*, 

 but instead of wave-lengths any arbitrary spectrum-scale 

 may be used. The area of the curve represents the intensity 

 of the light impression, and the hue and degree of saturation 

 depend on the position of the centroid and the shape of the 

 curve. To find how much of the luminosity corresponds to, 

 say, the region included between 500 ////, and 505//,//,, it is 

 only necessary to erect ordinates there, and measure the 

 area included between the ordinates. 



If two curves of the type shown in fig. 1, but the one 

 displaced along the spectrum from the other, are super- 

 imposed, the character of the resultant impression is obtained 

 by adding the ordinates. Thus if the separation is small, 

 the character of the resultant curve is not much altered. 

 This is the case of yellowish green superimposed on bluish 

 green to give pale green. If the separation is greater, the 

 breadth of the resultant curve becomes greater in comparison 

 with its height. This is the case of yellow superimposed on 

 blue to give white; white is represented by a curve the 

 breadth of which covers a large part of the spectrum. If 



