AS PERCEIVED BY THE EYE. 287 



colour. Hence all colours appear to the colour-blind as if composed of blue and 

 yellow. By measurement on the diagram, we find for red 



Measured, . . -138 Y + -123 B + -749 Bk = 100 R j 



Observed by N., . -15 Y+ -11 B+ -74Bk = 100Rl (8). 



X., . -13 Y+ -11 B+ •76Bk = 100Rj 



• For green we have in the same way — 



Measured, . . -705 Y + -295 B = -95 G + -05 Bk -j 



Observed by N., . -70 Y+ -30 B = -86 G + -14 Bk I . . . (9). 



X., . -70 Y+ -30B = -83G+-17Bk J 



For white — 



Measured, -407 Y + -593 B = -326 W + -674 Bk 



Observed by N., -40 Y+ -60 B= -33 W+ -67 Bk 



X., -44 Y+ -56 B= -33 W+ -67 Bk 



The accuracy of these results shows that, whether the hypothesis of the want 

 of one element out of three necessary to perfect vision be actually true or not, it 

 affords a most trustworthy foundation on which to build a theory of colour- 

 blindness, as it expresses completely the observed facts of the case. They also 

 furnish us with a datum for our theory of perfect vision, namely, the point D, 

 which points out the exact nature of the colour-sensation, which must be added 

 to the colour-blind eye to render it perfect. I am not aware of any method of 

 determining by a legitimate process the nature of the other two sensations, al- 

 though Young's reasons for adopting something like green and violet appear to 

 me worthy of attention. 



The only remaining subject to which I would call the attention of the Society 

 is the effect of coloured glasses on the colour-blind. Although they cannot dis- 

 tinguish reds and greens from varieties of gray, the transparency of red and green 

 glasses for those kinds of light is very different. Hence, after finding a case such 

 as that in equation (4), in which a red and a green appear identical, on looking 

 through a red glass they see the red clearly and the green obscurely, while through 

 a green glass the red appears dark and the green light. 



By furnishing Mr X. with a red and a green glass, which he could distinguish 

 only by their shape, I enabled him to make judgments in previously doubtful 

 cases of colour with perfect certainty. I have since had a pair of spectacles con- 

 structed with one eye-glass red and the other green. These Mr X. intends to use 

 for a length of time, and he hopes to acquire the habit of discriminating red from 

 green tints by their different effects on his two eyes. Though he can never ac- 

 quire our sensation of red, he may then discern for himself what things are red, 

 and the mental process may become so familiar to him as to act unconsciously 

 like a new sense. 



VOL. XXI. PART II. 4 H 



