It must have struck many of those who have thought about the matter at all as a most remarkable coincidence that sunlight should be white. White light, as we have seen, consists of a mixture of variously-coloured rays in very different and apparently arbitrary proportions, and if these proportions were a little changed the light would no longer be quite colourless. No ordinary artificial light is so exactly white as that of the sun. The light of candles, gas, oil, and electric glow-lamps is yellow; that of the electric arc (when unaffected by atmospheric absorption) is blue, and that of the incandescent gas burner green. It is exceedingly convenient that the light which serves us for the greater part of our waking lives should happen to be just so constituted that it is colourless. But on a little further reflection it will, I think, appear that this is not the right way to look at the matter. It is precisely because the hue called white is the one which is associated with the light of our sun that we regard whiteness as synonymous with absence of colour. We take sunlight as our standard of neutrality, and anything that reflects it without altering the proportions of its constituents we consider as being colourless. There can be little doubt that if the sun were purple instead of white, our sentiments as regards these two hues would be interchanged; we should talk quite naturally of "a pure purple, entirely free from any trace of colour," or perhaps describe a lady's costume as being of a "gaudy white." Even as things are, the standard of neutrality is not quite a hard and fast one. We have a tendency to regard any artificial light which we may happen to be using, as more free from colour than it would turn out to be if compared directly with sunlight. If in the middle of the day we go suddenly into a gas-lit room, we cannot fail to observe how intensely yellow the illumination at first appears; in a few minutes, however, the colour loses its obtrusiveness and we cease to take much notice of it. The effect may be partly a physiological one, depending upon unequal fatigue of the various perceptive nerves of the retina; but I believe that it is to a large extent due to mental judgment. The standard of whiteness, or colour-zero, can apparently be changed within certain limits in a very short time, and, as we shall see later, this is only one of many instances in which our organs of vision seem to be incapable of recognising a constant standard of reference. And now let us consider how it comes about that each elementary portion of the retina--at least in its central region--has the power of distinguishing so many hundreds of different hues. It is incredible that every little area of microscopic dimensions should be furnished with such a multitude of independent organs as would be necessary if each of the many colours met with in nature required a separate organ for its perception; and it is not necessary to suppose anything of the kind.