kind has actually reached the eye. I think you will admit that the theory is modest in demanding only three sets of nerves, for in the ear, as it seems, there are three thousand nerve-fibrils for the perception of the separate notes. In the eye it would not have been practicable to have employed a separate nerve-fibril for each different tint, for a reason which a moment's thought will render manifest.
But to resume: according to our theory, the first set of nerves responds powerfully to the action of the longer waves, or to that kind of light which we call red; the second set is arranged for waves of medium length, it is strongly set in action by what we call green light; and, finally, the third set is stimulated into action by the shortest waves, or by violet light. Let us for the present call them the red, green, and violet nerves. This diagram shows their relation to the colors of the spectrum (see Fig. 1). As I have just intimated,
Fig. 1.
these nerves can be set into action by electricity or pressure, and other causes besides light. Taking this into consideration, the next point in the theory will not seem so singular to you: it is, that each set of nerves is capable of being acted on, to a lesser extent, by waves of light not properly belonging to it; so, for example, the set adapted for green light can, to some extent, be stimulated by red light. In a case like this, the sensation will still remain that which we call green, though actually produced by red light. The theory demands this, and the results of experiments on persons who are color-blind to red light are in accordance with it, and presently I hope to give some experimental illustrations of it. The red and violet nerves also have this property, and can be partially set into action by light which does not belong to them, but in each case the sensation remains the one that properly appertains to them.
The last point of the theory is, that, when by any cause all three sets of nerves are excited into action with about the same intensity, the resulting sensation is that which we call white.
We are now in a condition to take up the explanation of the sensations which we call yellow, orange, and blue. Let us suppose for a moment that the eye is acted upon by waves of light shorter
