824 EYE Newton. The angles of the sectors were thus given by him : Red 60 45-5 Orange 34 10 5 Yellow 54 41 Green ...60 45 5 Blue 54 41 Indigo 34 10-5 FIG. 20. Diagram of the Colour Disc of Sir Isaac Newton. Violet 60 45-5 With sectors of such a size, ivhite will be produced on rotating the disc rapidly. This method has been carried out with great efficiency by the colour-top of Clerk Maxwell. It is simply a flat top, on the surface of which discs of various colours may be placed. Dancer has added to it a method by which, even while the top is rotating rapidly, and the sensation of a mixed colour is strongly perceived, the eye may be able to see the simple colours of which it is composed. This is done by placing on the handle of the top, a short distance above the coloured surface, a thin black disc, perforated by holes of various size and pattern, and weighted a little on one side. This disc vibrates to and fro rapidly, and breaks the continuity of the colour-impression; and thus the consti tuent colours are readily seen. (3.) Physiological Characters of Colours. All colours have three special characters : (1) Tone, depending on the number of vibrations per second ; (2) Intensity, depending on the extent or amplitude of the vibrations, and passing from the most sombre to the most brilliant shades; and (3) Saturation, which depends on the amount of ivhite the colour contains; thus, it is saturated when there is no white, as in the pure colours of the spectrum, and there may be an infinite number of degrees of saturation from the pure colour to white. (4.) The Geometric Representation of Colours. Colours may be arranged in a linear series, as in the solar spectrum. Each point of the line corresponds to a de terminate impression of colour ; the line is not a straight line, ^^ ^., ^Vr/- as regards luminous R ReA ~Jurhle Violet effect, but is better re- presented bv a curve 21. Geometrical representation of the , . relations of colours as shown by Newton. passing from the red to the violet. This curve might be represented as a circle in the circumference of which the various colours might be placed, in which case the complementary colours would be at the extremities of the same diameter. Newton arranged the colours in the form of a triangle, as shown in fig. 21. If we place three of the spectral colours at three angles, thus, green, violet, and red, the sides of the triangle include the intermediate colours of the spectrum, except purple. The point S corresponds to white, consequently, from the inter section of the lines which join the complementary colours, the straight lines from green to S, R S, and V S, represent the amount of green, red, and violet necessary to form white ; the same holds good lor the complementary colours ; for example, for blue and red, the > = the amount of blue, and the line S R = the amount of red required to form white. Again, any point, say M, on the surface of the triangle, will represent a mixed colour, the composition of which may be obtained by mixing the three fundamental colours in the proportions represented by the length of the lines M to green, Blue Indi(jo theory R O v M V, and M R. But the line V M passes on to the yellow Y ; we may then replace the red and green by the yellow, in the pro portion of the length of the line AI Y, and mix it with violet in the proportion of S V. The same colour would also be formed by mixing the amount M Y of yellow with M S of white, or by the amount R M of red with the amount M D of greenish blue. (5.) The Theory of Colour-Perception. The generally accepted was first proposed by Thomas Young and afterwards re vived by Helmholtz. It is based on the as sumption that three kinds of nerve fibres exist in the retina, the excitation of which give respec tively sensations of red, green, and vio let. These may be regarded as funda mental sensations. Homogeneous light excites all three, but with different , FIG. 22. Diagram showing the irritability of the three kinds of retinal elements. Wave. Thus long 1. Red; 2. Green; 3. Violet. R, 0, Y, G, B, V, waves Will excite initial letters of colours. most strongly fibres sensitive to red, medium waves those sensitive to green, and short waves those sensitive to violet. Fig. 22 shows graphically the irritability of the three sets of fibres. Helmholtz thus applies the theory : " 1. Red excites strongly the fibres sensitive to red and feebly the other two sensation : Red. 2. Yellow excites moderately the fibres sensitive to red and green, feebly the violet sensation : Yellow. 3. Green excites strongly the green, feebly the other two sensa tion : Green. 4. Blue excites moderately the fibres sensitive to green and violet, and feebly the red sensation : Blue. 5. Violet excites strongly the fibres sensitive to violet, and feebly the other two sensation : Violet. 6. When the excitation is nearly equal for the three kinds of fibres, then the sensation is White," This theory explains some of the phenomena of what is called colour blindness or Daltonism. All individuals appear to have some kind of colour-sensation ; in some, however, there may be no sensation for particular colours. The most common defect is insensibility to red (Daltonism properly so called). The spectrum to such an eye is deficient in red, and the sensation corresponding to all compound colours containing red is that of the complemen tary colour only. Thus, white is bluish -green, and intense red appears green, so that red poppies in a green cornfield do not appear of a different hue from the green by which they are surrounded. If we suppose in such cases an absence or paralysis of the red-fibres, the phenomena are accounted for. Blindness to green and violet is rare. Young s theory also explains the appearance of the conse cutive coloured images already referred to. Suppose, for example, that we look at a red object for a con siderable time ; the retinal elements sensitive to red become fa- ti<med. Then (]) if the eye be kept in darkness, the litres affected by red being fatigued do not act so as to give a sensation of red ; those of green and of violet have been less excited, and this excita tion is sufficient to give the sensation of pale greenish blue ; (2) if the eye be fixed on a white surface, the red fibres, being fatigued, are not excited by the red rays contained in the white light ; on the contrary, the green and violet fibres are strongly excited, and the consequence is that we have an intense complementary image ; (3) if ve look at a bluish-green surface, the complementary of red, the effect will be to excite still more strongly the green and violet
fibres, and consequently to have a still more intense complementary