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COLOUR SENSATION] the same size. This is due to irradiation. The borders of clear surfaces advance in the visual field and encroach on 16.—Illustrating the Effect of Irradiation. obscure surfaces. Probably, even with the most exact accommodation, diffusion images form round the image of a white surface on a black ground, forming a kind of penumbra, thus causing it to appear larger than it really is.

(f) Intensity of Light required to excite the Retina.—Light must have a certain intensity to produce a luminous impression. It is impossible to fix the minimum intensity necessary, as the effect will depend, not only on the intensity of the stimulus, but on the degree of retinal excitability at the time. Thus, after the retina has been for some time in the dark, its excitability is increased; on the other hand, it is much diminished by fatigue. Aubert has stated that the minimum intensity is about 300 times less than that of the full moon. The sensibility of the eye to light is measured by the photometer.

(g) Consecutive Retinal Images.—Images which persist on the retina are either positive or negative. They are termed positive when the bright and obscure parts of the image are the same as the bright and obscure parts of the object; and negative when the bright parts of the object are dark in the image, and vice versa. Positive images are strong and sharply marked when an intense light has acted for not less than ⅓ of a second. If the excitation be continued much longer, a negative and not a positive image will be seen. If, when the positive image is still visible, we look on a very brilliantly illuminated surface, a negative image appears. Negative images are seen with greatest intensity after a strong light has acted for a considerable time. These phenomena may be best studied when the retina is very excitable, as in the morning after a sound sleep. On awakening, if we look steadily for an instant at the window and then close the eyes, a positive image of the window will appear; if we then gaze fixedly at the window for one or two minutes, close the eyes two or three times, and then look at a dark part of the room, a negative image will be seen floating before us. The positive image is due to excitation of the retina, and the negative to fatigue. If we fatigue a small area of the retina with white light, and then allow a less intense light to fall on it, the fatigued area responds feebly, and consequently the object, such as the window-pane, appears to be dark.

1. General Statement.— (q.v.) is a special sensation excited by the action on the retina of rays of light of a definite wave-length. On the most likely hypothesis as to the physical nature of light, colour depends on the rate of vibration of the luminiferous aether, and white light is a compound of all the colours in definite proportion. When a surface reflects solar light into the eye without affecting this proportion, it is white, but if it absorbs all the light so as to reflect nothing, it appears to be black. If a body held between the eye and the sun transmits light unchanged, and is transparent, it is colourless, but if translucent it is white. If the medium transmits or reflects some rays and absorbs others, it is coloured. Thus, if a body absorbs all the rays of the spectrum but those which cause the sensation of green, we say the body is green in colour; but this green can only be perceived if the rays of light falling on the body contain rays having the special rate of vibration required for this special colour. For if the surface be illuminated by any other pure ray of the spectrum, say red, these red rays will be absorbed and the body will appear to be black. As a white surface reflects all the rays, in red light it will be

seen to be red, and in a green light, green. Colour depends on the nature of the body and on the nature of the light falling on it, and a sensation of colour arises when the body reflects or transmits the special rays to the eye. If two rays of different rates of vibration, that is to say, of different colours, affect a surface of the retina at the same moment, the effects are fused together and we have the sensation of a third colour different from its cause. Thus, if red be removed from the solar spectrum, all the other colours combined cause a sensation of greenish yellow. Again red and violet give purple, and yellow and blue, white. Yellow and blue, however, only give white when pure spectral colours are mixed. It is well known that a mixture of yellow and blue pigments do not produce white, but green; but, as was explained by Helmholtz, this is because the blue pigment absorbs all the rays at the red end of the spectrum up to the green, while the yellow pigment absorbs all the rays at the violet end down to the green, and as the only rays reflected into the eye are the green rays, the substance appears green. Finally, if colours are painted on a disk in due proportions and in a proper order, the disk will, when quickly rotated, appear white, from the rapid fusion of colour effects.

When we examine a spectrum, we see a series of colours merging by insensible gradations the one into the other, thus:— red, orange, yellow, green, blue and violet. These are termed simple colours. If two or more coloured rays of the spectrum act simultaneously on the same spot of the retina, they may give rise to sensations of mixed colours. These mixed colours are of two kinds: (1) those which do not correspond to any colour in the spectrum, such as purple and white, and (2) those which do exist in the spectrum. White may be produced by a mixture of two simple colours, which are then said to be complementary. Thus, red and greenish blue, orange and cyanic blue, yellow and indigo blue, and greenish yellow and violet all produce white. Purple is produced by a mixture of red and violet, or red and bluish violet. The following table by Helmholtz shows the compound colours produced by mixing other colours:— This table shows that if we mix two simple colours not so far separated in the spectrum as the complementary colours, 17.—Form of Double Slit for the Partial Superposition of Two Spectra. the mixed colour contains more white as the interval between the colours employed is greater, and that if we mix two colours farther distant in the spectrum than the complementary colours, the mixture is whiter as the interval is smaller. By mixing more than two simple colours, no new colours are produced, but only different shades of colour.

2. Modes of Mixing Colour Sensations.—Various methods have been adopted for studying the effect of mixing colours.

(a) By Superposing Two Spectra.—This may be done in a simple way by having a slit in the form of the letter V (see fig. 17), of which the two portions ab and bc form a right angle; behind this slit is placed a vertical prism, and two spectra are obtained,