For my own part, whatever theory of colour sensations may prove to be the right one, I lean strongly to the idea that the cause of vision will be found in chemical action, induced by the impact of the different wave-lengths of light falling on sensitive matter. A white substance may absorb all the wave-lengths found in the spectrum, and if it have three sets of molecules, one of which has an atom or atoms vibrating with the same period as the waves of light which show a maximum for one sensation and another for another, and so on, the requirements for the colour sensations are met. It may be that the sensitive part of the retina is like a photographic plate, but with this essential difference—that the sensitive material is constantly changing. A photographic plate receives an impression which is not recognisable by the eye, though it can be shown that a change in the material does take place during the impact of light, by electrical and other means. When the eye receives an impression of light, Dewar has shown that in this case also a current of electricity is generated. Recent published experiments of my own have demonstrated that with a low intensity of light, the chemical change that occurs in a photographic salt is by no means proportionate to that which takes place with a greater intensity. In the eye, too, there is a limit of sensibility to very feeble light. Again, the curves of the stimulation of the colour sensations to the spectrum are closely of the same form as the curves of sensitiveness of the various sensitive salts used by photographers. These are analogies and, of course, must not be pressed too far. There must be such a complexity in the sensitive material in the eye, both chemical and physiological, that it may be that the changes induced by light on the sensitive surface of the retina have to be considered from both aspects. The purely chemical change is naturally that to which a physicist is most prone to incline, and his bias must be discounted, as must also that of the physiologist.
APPENDIX.
The following is extracted from Maxwell’s paper.
The following table contains the means of four sets of observations by the same observer (K.):—
Table IV. (K.)
44·3 (20) + 31·0 (44) + 27·7 (68) = W.
16·1 (28) + 25·6 (44) + 30·6 (68) = W.
22·0 (32) + 12·1 (44) + 30·6 (68) = W.
6·4 (24) + 25·2 (36) + 31·3 (68) = W.
15·3 (24) + 26·0 (40) + 30·7 (68) = W.
19·8 (24) + 35·0 (46) + 30·2 (68) = W.
21·2 (24) + 41·4 (48) + 27·0 (68) = W.
22·0 (24) + 62·0 (52) + 13·0 (68) = W.
21·7 (24) + 10·4 (44) + 61·7 (56) = W.
20·5 (24) + 23·7 (44) + 40·5 (60) = W.
19·7 (24) + 30·3 (44) + 33·7 (64) = W.
18·0 (24) + 31·2 (44) + 32·3 (72) = W.
17·5 (24) + 30·7 (44) + 44·0 (76) = W.
18·3 (24) + 33·2 (44) + 63·7 (80) = W.
X.—Reduction of the Observations.
By eliminating W from the equations above by means of the standard equation, we obtain equations involving each of the fourteen selected colours of the spectrum, along with the three standard colours; and by transposing the selected colour to one side of the equation, we obtain its value in terms of the three standards. If any of the terms of these equations are negative, the equation has no physical interpretation as it stands; but by transposing the negative term to the other side it becomes positive, and then the equation may be verified.
The following table contains the values of the fourteen selected tints in terms of the standards. To avoid repetition, the symbols of the standard colours are placed at the head of each column:—
Table VI.