438 



KEPORT — 1886, 



determines the rate at which the colour changes. A sUght inspection of 

 our fundamental curves shows that in the normal division of the third 

 class there are two places in the spectrum at which the ratio of the 

 fundamental sensations changes most rapidly, the one near the line D, the 

 other near the Hne F. 



Now what is the simplest experimental method for determining the 

 places in the spectrum at which the change of colour is the most rapid ? 

 If we take a light of known wave-length, and match it to a part of 



Fig. 7. 



another spectrum, simply by subjectively judging the colour, the 

 difference in wave-length will give us the error of our judgment. 

 Repeating this process with the same light at the same place ia gx'eat 

 number of times we shall obtain a mean error, which our judgment is apt 

 to make in regard to this colour. It is evident that the smaller the 

 change of colour at a given place of the spectrum the larger will be that 

 mean error. This is the way in which Mr. Brodhun has determined 

 experimentally the places in the spectrum at which the colour changes 

 most rapidly. The curve A A A is the result of this sort of experimental 

 investigation on a person of the third class. We see that the places of 

 most rapid change of colour are about the lines D and F, and this agrees 

 perfectly with what we have predicted from the inspection of our curves 

 of fundamental sensation. 



The curve B B B, which was obtained in the same way from a person 

 belonging to the second type of the second class, shows that there is one 

 place of most rapid variation of colour, and this agrees perfectly with the 

 inferences which we can draw from the inspection of the two curves of 

 fundamental sensation for this type. 



These are the pi-incipal features of our investigation on this subject. 

 Its result seems to prove that the views of Thomas Young, slightly 

 modified by modern experimental research, are perfectly correct. 



Thomas Young's theory of colour -vision, one of the most beautiful 

 tmgs in his laurel crown, after lying as it were buried in the darkness of 

 oblivion for more than half of a century, was brought to light again by 

 Maxwell and Helmholtz, and, as we have seen, modern science seems 



