| Observer (K.) | (24) | (44) | (68) |
| 44·3 (20) = | 18·6 | + 0·4 | + 2·8 |
| 16·1 (28) = | 18·6 | + 5·8 | - 0·1 |
| 22·0 (32) = | 18·6 | + 19·3 | - 0·1 |
| 25·2 (36) = | 12·2 | + 31·4 | - 0·8 |
| 26·0 (40) = | 3·3 | + 31·4 | - 0·2 |
| 35·0 (46) = | - 1·2 | + 31·4 | + 0·3 |
| 41·4 (48) = | - 2·6 | + 31·4 | + 3·5 |
| 62·0 (52) = | - 3·4 | + 31·4 | + 17·5 |
| 61·7 (56) = | - 3·1 | + 21·0 | + 30·5 |
| 40·5 (60) = | - 1·9 | + 7·7 | + 30·5 |
| 33·7 (64) = | - 1·1 | + 1·1 | + 30·5 |
| 32·3 (72) = | + 0·6 | + 0·2 | + 30·5 |
| 44·0 (76) = | + 1·1 | + 0·7 | + 30·5 |
| 63·7 (80) = | + 0·3 | - 1·8 | + 30·5 |
Mr. James Simpson, formerly student of Natural Philosophy in my class, has furnished me with thirty-three observations taken in good sunlight. Ten of these were between the two standard colours, and give the following result:—
33·7 (88) + 33·1 (68) = W.
The mean errors of these observations were as follows:—
Error of (88) = 2·5; of (68) = 2·3; of (88) + (68) = 4·8; of (88) - (68) = 1·3.
The fact that the mean error of the sum was so much greater than the mean error of the difference, indicates that in this case, as in all others that I have examined, observations of equality of tint can be depended on much more than observations of equality of illumination or brightness.
From six observations of my own, made at the same time, I have deduced the “trichromic” equation—
22·6 (104) + 26 (88) + 37·4 (68) = W (2)
If we suppose that the light which reached the organ of vision was the same in both cases, we may combine these equations by subtraction, and so find
22·6 (104) - 7·7 (88) + 4·3 (68) = D (3)