476 SCIENCE PROGRESS 



simple and non-composite sensation. If the eye be fatigued with 

 pure yellow spectral light the spectrum will appear to have lost 

 its yellow, and though yellowish red or yellowish green will 

 appear less yellow, the terminal red of the spectrum will not 

 be affected (5). If the terminal portion of the red end of the 

 spectrum be isolated in my spectrometer, it will appear as a 

 faint red upon a black background. If the eye be fatigued with 

 red light, even by looking through a red glass held against a 

 light for one second, the red will not be visible for some con- 

 siderable time, but the eye may be fatigued for twenty minutes 

 with yellow light without interfering with the visibility of the 

 red light. 



It is known that if the intensity of a number of coloured 

 lights be reduced in the same proportion all the colours do not 

 disappear at the same moment. If, therefore, spectral yellow 

 were a compound sensation, it should change colour on being 

 reduced in intensity. If, however, spectral yellow be isolated 

 in my spectrometer, and the intensity be gradually reduced by 

 moving the source of light away, the yellow becomes whiter 

 and whiter until it becomes colourless, but does not change 

 in hue. 



The eye may be fatigued with red or green without altering 

 the hue of spectral yellow. Spectacles glazed with red or green 

 glass of a kind which is permeable to the yellow rays may be 

 worn for a considerable time without altering the appearance 

 of spectral yellow. If yellow were a compound sensation a 

 wearer of red spectacles should see the yellow through them as 

 green, because the yellow would fall on a portion of the retina 

 which had been fatigued for red. 



III. The Facts of Colour-Blindness 



Cases of colour-blindness may be divided into two classes, 

 which are quite separate and distinct from each other, though 

 both may be present in the same person. In the first class 

 there is light as well as colour loss. In the second class the 

 perception of light is the same as the normal-sighted, but there 

 is a defect in the perception of colour. In the first class certain 

 rays are either not perceived at all or very imperfectly. Colour- 

 blind individuals belonging to the second class can be arranged 

 in a series. At one end of the series are the normal-sighted, 

 and at the other the totally colour-blind. I have classified the 



