286 Report of the Committee on Colour-Vision. 



The Young- 

 Helmholtz 

 theory of 

 colour- vision. 



The lines with the letters B, C, D, &c, below the curves indicate certain fixed 

 lines in the solar spectrum whose wave-lengths have been determined. 



The different degrees of the stimulation given to each of the 

 three sensations by every part of the spectrum is shown in the 

 diagram by the heights of the curves above the horizontal base 

 line. Thus in the middle of the spectrum, near E, each of the 

 curves is to be found of a different height, and these degrees of 

 stimulation of the three sensations, combined together, give the 

 sensation of spectral green. It may be remarked that, on the 

 scale adopted, the three sensations are supposed to be equally 

 stimulated when white light is perceived. The areas of the 

 three curves are therefore equal, and at the places in the 

 spectrum where the curves are of the same height, the stimula- 

 tion of the sensations is also the same. At the extreme red and 

 extreme violet of the spectrum the curves of the red and violet 

 sensations are alone to be found, hence at those parts the sensa- 

 tions are simple. 



According to this theory, the two types of complete red-green- 

 blindness are attributed to the absence of either the red, or else of 

 the green sensation, the absence of the former corresponding to 

 red-blindness, and of the latter to green-blindness. Where the 

 violet and green curves cut, the red-blind person will see 

 what to him is white, and where the red and violet curves 

 cut the green-blind will also similarly describe his sensation 

 of colour. To the normal eye these parts of the spectrum 

 appear as bluish-green and green, as there is a stimula- 

 tion of the green and violet sensations, or of the green alone, 

 over and above that necessary to produce with the red sensation 

 the mixed sensation of white. 



