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, . 2 138 Y+-123 B+-749 Bk=100 R 
Observed by N., 15 Y+ :11B+ -74 Bk=100R (8). 
XLOres 13 Y+ ‘11 B+ -76 Bk=100R 
* For green we have in the same way— 
i Measured, : “705 Y +:295 B=:95 G+-:05 Bk 
(4 Observed by N. ‘70 Y+ -30 B=:86 G+-14 ns | (9). 
+4 = KX, . 70+ 30 B=83 G+-17 Bk 
4 ‘ : 
For white— 
Measured, 407 Y+:593 B=326 W +-674 Bk 
| Observed by N., ‘40 Y+ ‘60 B= 33 W+ ‘67 Bk 
Ke, 44Y¥+ 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 Youne’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 
F 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 
