ON THOMAS TOUNG's THEORY OF COLOUK-VISION. 



437 



Green 



special case of the third class, namely, a case in which the curve @ has 

 BO far altered its form as to coincide with Si, then the abnormal division 

 would be a transition form. Are there any facts in our experience that 

 could lead us on to make such an assumption ? Before answering this 

 question I must call your kind attention to the following circumstance : — 



If we construct Newton's colour-diagram (fig. 6), we find that the 

 colours of the three fundamental sensations are : — 



For ift, red (somewhat more purple than the colour of the long- 

 waved end of the spectrum). 



For ®, green (about the wave-length 505 ^yu). 



For 9S, blue (about the wave-length 470 fxfi). 



Assuming now that the colour of the fundamental sensation of ® 

 remains the same, whereas the form of the curve is altered in such a 

 manner as to coin- 

 cide with St, it is Fig. 6. 

 evident that the 

 sensation belonging 

 to this curve would 

 be a resultant of the 

 sensations belong- 

 ing to 01 and ®i, 

 that is, a yellow of 

 about the wave- 

 length 575 fxfi. 



I need hardly 

 mention that co- 

 lour-sensations are 

 entirely subjective, 

 and that in general 

 the colour-sensa- 

 tions of two classes 

 cannot be compared 

 to each other. For- 

 tunately Professors 

 von Hippel and 



A,a,B 



Red 



&,H 



Blue 



Holmgren have met with a young man who with respect to his right eye 

 belonged to the first type of the second class, and with respect to his 

 left eye belonged to the normal division of the third class. 



And this is the only person who can assist us in answering our 

 question. His fundamental sensations for the first-mentioned eye were 

 yellow and blue as compared to the sensations of the other, that is, the 

 normal eye. 



This circumstance, therefore, justifies us in assuming that the first 

 type of the second class is a special case of the third class. But whether 

 the second type is also a special case of the third class future experience 

 only will show. 



§ 5. The following experiments will serve as an additional evidence 

 that our results are correct. These experiments were made at my insti- 

 gation by Mr. Brodhun, a student in our laboratory. 



Before entering upon the details of these experiments I must premise 

 a few observations. According to our theory the colour at any part of 

 the spectrum is determined by the ratio between the fundamental sensa- 



tions, whose resultant produces that colour. The 



change 



of this ratio 



