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impulse, which causes the sensation of vision. The movements of. after-images show that the stimulus in vision is fluid and situated outside the cones. The rods are not percipient elements but regulate the formation and distribution of the visual purple.

The decomposition of the visual purple by light stimulates the ends of the cones, and a visual impulse is set up which is conveyed through the optic nerve fibres to the brain. The character of the impulse differs according to the wave-length of the light causing it. Therefore in the impulse itself we have the physiological basis of the sensation of light, and in the quality of the impulse the physiological basis of the sensation of colour. The impulse being conveyed along the optic nerve to the brain stimulates the visual centre, causing a sensation of light, and then, passing on to the colour-perceiving centre, causes a sensation of colour. But though impulses vary in character according to the wave-length of the light causing them, the colour-perceiving centre is not able to discriminate between ad- jacent impulses, the nerve cells not being sufficiently developed.

Even with the normal-sighted there is room for much further de- velopment in the discrimination of colour, but when the develop- ment is not up to the normal standard or there is a defect in any portion of the apparatus diminishing the power of discrimination, colour-blindness is the result.

Evolution of the Colour-Sense. There can be no doubt that an evolution of the colour sense has taken place. The only point is, how, and when, did this occur? It is obvious that in those low forms of animal life in which the most rudimentary sense of sight exists there can be no sense of colour. The animal which can only perceive light and shade, can only discriminate in a rough way between varying intensities of the stimulus. It is obvious, therefore, that the sense of light must have been developed first and then the sense of colour. The sense of sight must have been first developed for those waves which produced their maximum effect upon the sensi- tive protoplasm.

The next process of development would be for the protoplasm to become sensitive to the waves above and below those which first caused an effect. In the physical stimulus which produces the sensation of light there are two factors to be considered, the length of the wave and its amplitude; the greater the amplitude within certain limits the greater the intensity of the sensation.

The wave-length of the physical stimulus is the physical basis of the sensation of colour. How did the sensation of colour first arrive? Let us suppose that the physiological effect of the physical stimulus differed according to the wave-length of the physical stimulus. At a certain stage the eye had become sensitive to a fair range of the spectral rays, that is to say, evolution had proceeded to the extent of making the protoplasm sensitive to rays of light considerably above and below those which first caused a sensation of light. There was then an eye which was sensitive to the greater part of the rays which form the visible spectrum. It was, however, an eye which was de- void of the sense of colour, no matter from what part of the spectrum the rays were taken. The only difference appreciated was one of intensity. Let us now suppose that a fresh power of discrimination was added to the eye and that it became able to discriminate be- tween different wave-lengths of light. What would be the most probable commencement of development of the sense of colour? Most probably the differentiation of the physical stimuli which were physically most different. That is to say, the eye would first dis- criminate between the rays which are physically most different in the visible spectrum, the red and the violet; that is, presuming that the eye had become sensitive to this range. We have examples of cases of defective light-perception in which there is shortening of the red or violet end of the spectrum.

Let us now work out the evolution of the colour sense on the as- sumption that the rays which are physically most different, namely red and violet, were those which were first differentiated. We know that the various rays differ in their effects on substances : the red rays are more powerful in their heating effects, whilst the violet are more active actinally, as is well known by the readiness with which they act upon a photographic plate.

We should now have an individual who would see the spectrum nearly all a uniform grey of different degrees of luminosity but with a tinge of red at one end and a tinge of violet at the other. There is a great deal of evidence to show that this is how the colour-sense was first developed. For instance, in the degree of colour-blindness just preceding total the spectrum is seen in this way.

It will be noticed in the first evolution of colour that the added power of discrimination is something distinct and separate from light-perception. It can be destroyed as by mixing the two colours without interfering with the perception of light. Here we have the foundation for the distinction between light and colour-perception, the proper recognition of which is so essential in physiological optics. As the colour-sense developed it was not necessary that the rays should be so far apart before a difference was seen, so the two colours red and violet gradually encroached on the grey band until they met in the centre of the spectrum.

We have now a series of cases each of which only sees two colours, red and violet, with a varying degree of grey band in the centre of the spectrum. We should expect that those who had the smallest white region left in the centre of the spectrum would have the' best colour-perception, because they belong to a later stage of evolution.

Cases of colour-blindness are found corresponding to all these degrees, from almost total to those bordering on the trichromic.

In all the dichromics a mixture of the two colours which they perceive, namely red and violet, will form white, and so we have the foundation of complementary colours.

The next stage in the evolution of the colour-sense was when a third colour appeared at the third point of physiological difference, that is in the centre of the spectrum in the position of the green.

The colour-sense now assumed a trichromic form, red, green, and violet being seen in the spectrum.

As green replaced the grey which existed in the spectrum of the dichromic we should expect that green should be complementary to the other two colours combined, and this we find to be the case.

We have now reached the stage in which three distinct colours were seen in the spectrum, namely red, green, and violet, and the vision has assumed the trichromic character which must remain.

When the green was first developed it was a comparatively un- important colour. As evolution proceeded the power of differentia- tion affected the regions between the red and the green and the violet until a stage was reached in which a fourth colour, yellow, was seen at the next point of greatest physiological difference.

The next step in the process of evolution occurred when the retino-cerebral apparatus was able to differentiate a fresh colour between the green and the violet, namely blue, five definite colours being seen in the spectrum. It will be obvious that in any further evolution the intermediate portions will be still further differentiated, and so we arrive at those who can see six and seven colours in the spectrum respectively.

It is not necessary to consider the further evolution of the colour- sense because it is not known that any person can distinguish more than seven definite colours in the spectrum.

Colour-Blindness. Colour-blindness is not really a good term for the defect so named. Though in certain varieties there is actual blindness to colour, in the ordinary varieties colours are clearly seen and seen as colours, but there is a lack of power to differentiate between them: for instance, reds are confused with greens and greens with reds. A colour-blind man picked up a red-hot coal, remarking as he did so, " What funny green thing is this? " The case which first drew general attention to the subject of colour-blindness was that of Dalton, the famous chem- ist. After Dalton had received the scarlet gown of a doctor of civil law at Oxford, he actually wore it for several days in happy unconsciousness of the effect it produced in the street. When he was asked what the bright scarlet gown which he wore re- sembled, he pointed to some evergreens outside the window and said the colours were exactly similar to him. The lining of the gown, which was pink, he stated appeared to him sky-blue.

A soldier in the days when they wore scarlet coats took off his coat and put it on a hedge, and was quite unable to find it when he wished to put it on again, though it was the most con- spicuous object in the landscape to other people. Many colour- blind golfers find great difficulty in recognizing the red flags on the greens at a distance.

Those who are colour-blind often first discover their defect as children by finding great difficulty in picking cherries or straw- berries, because of the similarity in colour to their leaves. A colour-blind man has bought a bright green tie under the im- pression that he was purchasing a brown one; an artist has painted the face of a portrait green and trees red. A colour-blind man has written half of a letter in black ink and half in red ink, under the impression that the whole was written in black ink.

At first sight it would seem a very easy thing to detect persons who make such errors. Though this is true in certain cases it is not so in others. In fact, cases have been submitted to experts "who have failed to detect them after an hour's examination.

A musician's wife informed the writer that she had tested her husband again and again and was quite sure that he was not colour-blind, and that he was able to see colours as well as she could ; she was only convinced when she found that he was quite unable to read any of the letters on the card test.

Cases of colour-blindness may be divided into three classes, which are quite separate and distinct from each other though one or more may be present in the same person. In the first class there is light as well as colour loss. In the second class the perception of light is the same as in the normal-sighted, but there is a defect in the perception of colour. In the first class certain rays- are either not perceived at all or very imperfectly. Both these classes are represented by analogous conditions in the