82 
PEOFESSOE J. CLEEK MAXWELL OX 
between six coloured papers as seen by him. The experiments were made with the 
colour-top in the manner described in my paper in the Transactions of the Eoyal 
Society of Edinburgh, vol. xxi. pt. 2. p. 286 ; and the colour-equations were arranged 
so as to be equated to zero, as in those given in the Philosophical Magazine, July 1857. 
The colours were, — Vermilion (V), ultramarine (U), emerald-green (G), ivory-black 
(B), snow-white (W), and pale chrome-yellow (Y). These six colours afford fifteen 
colour-blind equations, since four colours enter into each equation. Fourteen of these 
were observed by Mr. Simpson, and from these I deduced three equations, gi'^lng the 
relation of the three standards (V), (U), (G) to the other colours, according to his kind 
of vision. From these three equations I then deduced fifteen equations, admitting of 
comparison with the observed equations, and necessarily consistent in themselves. 
The comparison of these equations furnishes a test of the truth of the theoiy that the 
colour-blind see by means of two colour-sensations, and that therefore eveiy coloui' may 
be expressed in terms of two given colours, just as in ordinary Gsion it may be expressed 
in terms of three given colours. The one set of equations are each the result of a single 
observation; the other set are deduced from three equations 
theory, and the two sets agree to within an average error =2T. 
Table b. 
V. U. G. B. 
in accordance with this 
W. T. 
1. Observed . . . 
0 
0 
-100 
+ 45 
+22 
+ 33 
= 0. 
Calculated . . 
0 
0 
-100 
+ 37-5 
+26-5 
+ 36 
= 0. 
2. Observed . . . 
0 
+ 58 
0 
-69 
-31 
+ 42 
= 0. 
Calculated . . 
0 
+58-3 
0 
-67-3 
-32-7 
+41-7 
= 0. 
3. Observed . . . 
0 
+ 32 
-100 
0 
+ 12 
+ 56 
= 0. 
Calculated . . 
0 
+ 32-3 
-100 
0 
+ 8-3 
+ 59-4 
= 0. 
4. Observed . . . 
0 
+ 38 
- 89 
-11 
0 
+ 62 
= 0. 
Calculated . . 
0 
+40 
- 85 
-15 
0 
+60 
= 0. 
5. Observed . . . 
0 
+ 32 
+ 68 
-60 
-40 
0 
=0. 
Calculated . . 
0 
+ 34 
+ 66 
-63-5 
-36-5 
0 
= 0. 
6. Observed . . . 
-100 
0 
0 
+ 82 
+ 5 
+ 13 
= 0. 
Calculated . . 
-100 
0 
0 
+ 83-9 
+ 4-5 
+11-6 
= 0. 
7. Observed . . . 
+ 47 
0 
-100 
0 
+22 
+ 31 
= 0. 
Calculated . . 
+ 44-7 
0 
-100 
0 
+24-5 
+ 30-8 
= 0. 
8. Observed . . . 
-100 
0 
+ 20 
+77 
0 
+ 3 
= 0. 
Calculated . . 
-100 
0 
+ 17 
+ 77-5 
0 
+ 5-6 
= 0. 
9. Not observed. 
Calculated . . 
-f 96 
0 
- 31 
-69 
+ 4 
0 
= 0. 
