THE author entitles his work, “An essay discussing existing theories, explaining views hitherto incompletely published, and comprising illustrated descriptions of important new experiments.” We shall now proceed to see how the promise conveyed by the title is fulfilled. In the early pages of the book he makes the statement that there are five colours which are distinct sensations, viz., red, yellow, green, blue, and purple. The last, however, he is rather less certain about as conveying his meaning, but finally adopts the name after explaining what he designates by purple. These are the five colours of a 5-colour theory which he propounds, all other colours being mixtures. He has felt, however, that it is no use to bring forward a theory unless he demolishes those other theories which block the way. His examination of these last is chiefly confined to that of Young, which accounts for colour-vision on the assumption that there are only three colour sensations—a number which is a minimum when the fact is remembered that all colours can be produced by a simple colour, or by a mixture of two or three of the colours which are considered to be primary colours. His criticism of the theory is mostly confined to a paper published by Clerk Maxwell thirty years ago, and he scarcely refers to any evidence in its support which has been brought to light in more recent years. Mr. Hunt has entangled himself in mixing up colour and colour sensations together, and has forgotten that in Clerk Maxwell's papers three colours were chosen empirically as approaching the colours which are perceived when the three fundamental sensations are stimulated. Later work has shown that the colours thus shown are not representative of the fundamental sensations. No one, for instance, would say that any green in the spectrum was the colour evoked by the stimulation of the green fundamental sensation, for it is well known that, according to the theory, at every part of this region of the spectrum all three sensations are stimulated, and the nearest approach that a retina possessing normal sensations could make to perceiving this one sensation, would be when the colour evoked was mixed with a percentage of white, rendering the colour impure. We may here parenthetically remark that it is too late for Mr. Hunt to quarrel with the designations of the colour constants, for they are accepted terms. “Impure,” for instance, may be an objectionable term to apply to a colour when mixed with white, but as what is meant by it is understood, it can only be used in that sense. The position of the colour which stimulates only the red fundamental sensation is fairly well known, being near to the red lithium line. The position of the colour which stimulates the violet fundamental sensation is still not absolutely settled, but it cannot be very far from the G line of the solar spectrum. Moreover, recent researches show that at the extremities of the spectrum only a red or a violet sensation is stimulated, any change in colour observed being due to a slight admixture of white light, which is derived from the imperfect transparency of the prisms or reflecting surface of the grating. The colour, for instance, near G, when mixed with a small percentage of white light, in excess of that already mixed with it, takes a violet hue, a colour which is associated with the most refrangible part of the spectrum. As the luminosity of this part is very much less than that near G, the extra percentage of white light required to form this hue is always present. The colour of any ray of the spectrum can be almost entirely freed from the white light derived from the prism by placing another prism in the path of such rays, after passing through a second slit. In an eye-piece or on a screen, the ray will be seen as a well-marked line lying in a faint continuous spectrum. Again, the references to the sensations stimulated in the various types of colour-blind people are not described in any detail, though the evidence which is derived from an examination of their vision is of the greatest importance for the Young or any other theory. The author gives most undue weight to colour diagrams. Colour triangles or circles are not intended to be the basis of a theory, but simply as illustration of it. It is quite possible that Clerk Maxwell's diagrams would not tally with those based on Kœnig's observations, nor should they do so. In fact a diagram may be drawn to illustrate any theory, as the author himself has done to illustrate his own.
By E. Hunt. (Glasgow: John Smith and Son, 1892.)
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