823 necessary, as the effect will depend, not only on the inten sity 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 photometers, instruments which will be de scribed under the article LIGHT. (</) 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 J- 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 nega tive image appears. Negative images are seen with greatest intensity after a strong light has acted for a con siderable 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 conse quently the object, such as the window pane, appears to be dark. Many curious experiments may be made to illustrate the laws of consecutive images. Thus, if we look at a black figure on a white ground for, say, one minute, and then gaze into a dark part of the room, a gigantic white figure, of corresponding shape, may make its appearance. A white figure on a black ground will produce a black image, a green figure will produce a red, and a red a green, the reproduced colour being always complementary to that of the figure. 4. SENSATIONS OF COLOUR. (1.) General Statement. Colour is a special sensation ex cited by the action on the retina of rays of light of a definite wave length. Thus we have a sensation of red when a certain number of waves of light impinge on the retina in a unit of time, and with about twice the number of waves in the same time the sensation will not be of red but of violet. 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 termad 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. When white light falls on a sur face, the surface may absorb all the rays except the red. If the red rays are alone reflected, then the object will be Violet Indigo- blue Cyanic- blue Greenish- blue Green Yellowish- Yellow green Red Purple Deep- White- White Whitish- Golden Orange Orange Deep- rose White rose K/iite Whitish- yellow Yellow yellow Yellow rose rose velloir Yellow White White Whitish- Whitish- Yellowish- rose green green green Yellowish- White Green Green Green green Green Blue Water- Greenish- blue blue Greenish- Water- Water- blue blue blue Cyanic : Indigo- blue blue red ; if the green rays are reflected, then the object will appear to be green. Again, if we look through red glass, all the rays are absorbed except red, and consequently the world beyond appears to be red. So with regard to the other transparent coloured media. 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, the mixed colour contains more a white as the interval between the colours employed is greater, and that if we mix . two colours further 3 distant in the spec- FIG. 17. Form of double slit for the partial trum than the com- siiperposition of two spectra, plementary 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 study ing the effect of mixing colours. (a) By Super posing two Spectra (Helmholtz and Clerk Maxwell). This may be done FlG. 18. Diagram of double spectrum partially superposed.
in a simple way by having a slit in the form of the letter V (see fig. 17), of which the two portions a b and b c form a right angle ; be hind this slit is placed a vertical prism, and two spectra are obtained, as seen in fig. 18, in which bfea is the spectrum of the slit a b, and c ef d that of the slit b c ; the ^.-ssfea coloured spectra are con tained in the triangle FlG - 19. Diagram showing Lambert s gef, and, by arrange- method of mixing sensations of colour, ment, the effects of mixture of any two simple colours may be observed. (b) By Lambert s Method of Reflection. Place a red wafer on b, in fig. 1 9, and a blue wafer on d, and so angle a small glass plate a as to transmit to the eye a reflection of the blue wafer on d in the same line as the rays transmitted from the red wafer on b. The sensation will be that of purple ; and by using wafers of different colours, many experiments may thus be performed. (c) By the Use of Rotating Discs tvhich quickly super pose on the same Area of Retina different Impressions of Colour. Such discs may be constructed of cardboard, on which coloured sectors are painted, as shown in fig. 20,
representing diagrammatically the arrangement of Sir Isaac