if we reduce the number of fibres to three, in accordance with the number of primary colours, red, green, violet. In fact, all the phenomena of the sensation of colour may be perfectly explained on the supposition that, in each point of the retina, three kinds of nerve-fibres terminate, one of which is sensitive to red, another to green, and the third to violet.

Exactly as white light is produced by the combination of red, green and violet, all other shades of colours may be formed by the combination of these primary colours. If white light falls upon the retina, then all three kinds of fibres, those sensitive to red, green and violet, are irritated, and this simultaneous irritation produces the sensation of white. If the retina is illuminated by red light, then the fibre sensitive to red is irritated most strongly. It is, however, very probable that the two other kinds of fibres are irritated at the same time, though in a less degree; first, the fibre sensitive to green, because green lies nearer to the red in the spectrum, and then that sensitive to violet.

According to this theory, yellow light irritates equally the fibres sensitive to red and to green, and only slightly that sensitive to violet. Yellow, therefore, is not a primary colour, but, physiologically speaking, a compound colour; because it is due to a combination of the sensations of red and green.

Green light irritates principally the fibres sensitive to green, and very slightly those sensitive to red and violet. Blue light irritates simultaneously the fibres sensitive to green and violet in an equal degree, and very slightly those sensitive to red. Blue, therefore, physiologically considered, is also a compound colour.

Violet light irritates very strongly the fibres sensitive to violet, and the other two only slightly.

This theory of the perception of colours was first put forward by Thomas Young, and has more lately been developed by Helmholtz. In accordance with this theory, the action of the spectrum colours upon the retina has been illustrated by Helmholtz by the accompanying fig. 31.

The horizontal lines, 1, 2, 3, represent the three kinds of nerves:-1, the fibre sensitive to red; 2, to green; 3, to violet. The letters under the line 3 correspond to the colours of the spectrum in their natural order, red, orange, yellow, green, blue, violet. A curve is described above each line, showing the strength of the irritation exercised by all the colours of the spectrum upon each kind of nerve. Curve I attains its greatest elevation between red and orange; curve 2, at green; curve 3, between blue and violet. If, now, we draw a perpendicular line through the three curves from each colour of the spectrum, then the perpendicular section of the curves will show us the relative strength of the irritation of the

nerves. We see that yellow exercises a moderately strong irritation upon 1 and 2; green irritates 2 strongly, and 1 and 3 only slightly; blue again produces an irritation of medium strength upon 2 and 3; and violet irritates 3 almost exclusively.

A very interesting observation has proved the fact, that in the retina there must be particular elements sensitive to red. Thus it can readily be proved, that the outer edges of the retina are insensitive to red. If we take a red body, for instance a stick of red sealing-wax, in the hand, and move it to one side beyond the field of vision, looking straight before us all the time-if we now move it slowly forwards till it is just distinguishable on the edge of the field of vision, it will no longer appear red but black, and the red colour suddenly, reappears if we move it forward. This is not the case with blue, which is perfectly distinguishable at the edge of the field of vision.

The retina, at its edges, is, therefore, blind to the colour red, a fact most simply explained by supposing that the fibres sensitive to red are here wanting. Since these parts of the retina are perfectly sensitive to blue, they are not devoid of the fibres which are apportioned to the other parts of the retina for the sensation of colour. If there were but one nerve-fibre for the transmission of all sensation of colour, it is incomprehensible why it should not be sensitive to red at the edge of the retina. It follows, therefore, that there must be particular nerve-fibres for the sensation of red.

This peculiarity of the edge of the retina in the normal eye passes, in the eyes of many, into red-blindness. That is to say, there are a great many persons, almost one in twenty, who are incapable of distinguish

ing red colours distinctly. They know, indeed, from ordinary conversation, that a certain colour is called red, and by experience are enabled to use this expression. They call blood red, because they know it is generally called so, and other objects in the same manner; so that it often happens that they themselves are unconscious of their imperfect sensation of colour. But sooner or later an occasion arises which shows their inability to select a red object from similar objects differently coloured. They confuse especially red with dark green and yellow. If, now, a spectrum is shown to people suffering from this red-blindness, they will distinguish two principal colours, which they will call blue and yellow. They imagine that the spectrum is shortened, especially at the red end, and the extreme red they do not see at all.

This peculiarity may be explained by supposing that in such persons the nerve-fibres sensitive to red are either wanting or insensible to irritation. The world must appear to them quite differently coloured to what it appears to us. What looks to us white, must to them have a greenish-blue appearance, because red is wanting in it, and yet they call it white, because it comprehends the whole of their series of colours. There are, moreover, many degrees of red-blindness, so that sensitiveness to red is present in a greater or less degree.

This fact is a proof that red is one of the primary colours, since it can be wanting altogether, and it is also a substantial confirmation of the theory of Young and Helmholtz, according to which these three primary colours have special nerve-fibres in each point of the retina.

CHAPTER VI.

Incidental Images-The Phantascope-Positive and Negative Incidental Images—Incidental Colours-Harmony of Colours.

IN ordinary vision a picture disappears as soon as the object seen is withdrawn, or it has ceased to be illuminated. In some well-known phenomena, however, it may be observed that the impression of light lasts for an appreciable time after the light is withdrawn. If a burning stick is whirled quickly round in a circle, we have the impression, not of a point, but of a fiery circle. Similarly, a rising rocket and a falling star produce the impression of a line of light. Moreover, if, in the night, the darkness is illuminated by a flash of lightning, we always have the impression that the flash has lasted for some time; although, in reality, it is of such momentary duration, that if a railway train, as it rushes by, were illuminated by lightning it would appear to be standing still. The persistence of these impressions is unpleasant if we have looked at a powerful light or the sun. We then perceive spots of light for some time, even when the eyes are closed, which greatly inconvenience our sight.

se prolonged impressions of light are called