fixed, sections of which, as may be seen from the figure, have been painted with the colours which are to be combined. The images of the colours are so rapidly super

Fig. 29.

have black and white sections in the disc, the result is a grey colour; and by making use of other colours, we can produce mixtures of every possible shade, which can be made darker by the addition of black.

The following is a very simple method of mixing two colours together, which can be done without any instrument. Two coloured wafers, b and c (fig. 30), are placed on the table a certain distance apart, and a small plate of glass is placed before the eye, so that the wafer b is seen directly through it, and at the same time a reflected

Fig. 30.

image of the wafer c, so that both images are superposed, and their colours combined. In this case also we produce a true mixture of colours.

Now all these methods give concordant results, although the second method rests on a different principle to the two others. In the latter the different colours were mixed objectively before they entered the eye; by the coloured top, however, the differently coloured rays. were not mixed, but they only fell upon the retina in very rapid succession, the combination of the colours thus taking place on the retina. This, however, shows us

that a mixture of colours does not produce a special excitement on the retina, but one composed of the excitements, which each colour creates independently.

If, now, all the colours of the spectrum are reproduced upon the colour top with the greatest possible exactness, and in the proportion of their brightness, then their mixture will reproduce white, which, on account of the imperfect clearness of the colours employed, will have more or less of a grey tinge. Exactly the same grey, however, can be produced on a colour top, by the combination of black and white; and since black is nothing more than the absence of light, the combination of the colours corresponds exactly to the white which is contained in this grey.

The combination of two simple colours of the spectrum has produced a very remarkable result. It has been found that there are several pairs of colours the combination of which produces white. We include amongst them yellow and indigo, though a mixture of two colouring matters of this tint would produce green. Such pairs are called complementary colours, and besides yellow and indigo, red and greenish-blue, orange and blue, greenish-yellow and violet are complementary colours.

Now if we look for these complementary colours in the spectrum, we find that two complementary colours always lie a certain distance apart. Two adjoining colours can never be complementary, neither can the two ends of the spectrum, red and violet, the combination of which produces purple. It seems, moreover, that one of the complementary colours must always be situated somewhere near green, as greenish-blue, sky-blue,

yellow and greenish-yellow. On the other hand, pure green, as it appears in the spectrum, has no simple complementary colour, and it is necessary to mix with the green two colours, red and violet, to produce the sensation of white light. These three colours, red, green and violet, are now received as primary colours, because they are the only three pure colours in the spectrum which, when combined, produce a nearly perfect white.

We will now proceed to determine what influence is to be ascribed to these three primary colours in the action of the sensation of colour.

The vast number of colours and shades of colour which we meet with in nature have, through the discovery of the colours of the spectrum, been reduced to a certain number of simple colours. But between the simple colours of the spectrum there are a great number of transition colours, which produce many different shades. Each point of the spectrum differs in colour from the point next to it; this change being due to difference in wave-length of the light-vibrations, and therefore it seems as if the number of colours and the transitions between them, which can be seen in the spectrum, must be endless. The question has, therefore, been asked, how is it possible that the retina of the eye should be irritated in so many different ways, for each colour must produce a particular irritation?

We

To consider this question more closely, we must return to our conceptions of the physiology of the nerves. know that the irritation of light takes place in the rods and cones of the retina, and that they transmit the irritation to the fibres of the optic nerve. If, for example, a ray of light falls upon a cone, then some kind of irri

tation must take place in the fibre which is in connection with the cone, to provide us with some means of deciding that we have to do with red light, else the brain would be unable to recognise the sensation of red. When a blue ray of light falls upon the cones, the process of irritation must clearly be of a different kind in the same nerve-fibre, so as to inform the brain that the light causing the irritation is blue. With green light the process in the nerve-fibre must again be different; in short, we must assume that in one and the same nerve-fibre, the irritation produced by each colour is special and different in kind.

This result is, however, directly at variance with our conception of the process of the irritation of the nerves. According to this conception all nerves were the same in their nature, and in the action of their irritation. It. is possible to join together a sensory nerve and a motory nerve, so as to form one nerve, and in this case an irritation of the sensory nerve is directly transmitted to the motory nerve, and causes a contraction of the muscles belonging to it. In both kinds of nerves the process is the same, and if the irritation of a motory nerve during life produces motion only, and that of a sensory nerve sensation only, the sole reason is that the former is connected with the muscles, the latter with the centre of sensation in the brain, producing in this complicated. organ different kinds of phenomena. We have already learnt that the optic nerve differs in no respect from the other nerves of the body. If it were possible, we might place any motory nerve between the eye and the brain, and the perception of light would not be destroyed in the least. The irritation of such a nerve would only