COLOUR-BLINDNESS AND COLOUR-PERCEPTION. CHAPTER I. HISTORICAL. THE first definite step towards an appreciation of the science of colour was made by Newton, by his researches in optics and his demonstration of the compound character of the sun's light. Kepler was the first to discover the compound nature of solar light, but comparatively little attention was paid to his observations. The experiments of Newton attracted more notice, and his theories were accepted by the scientific men of his time. Newton was of opinion that white light is made up of seven different colours: red, orange, yellow, green, blue, indigo and violet, each differing in refrangibility. Brewster was of opinion that there are three different colours in the spectrum-red, yellow, and blue. He believed that the simple spectrum is composed of three different spectra-red, yellow, and blue. Helmholtz pointed out, in a very able paper, the defects in Brewster's methods of experimenting, and, except with artists, this theory has been very generally rejected. B The first definite record we possess of a case of colourblindness, is Huddart's account of the shoemaker Harris, in 1777. In 1794, Dalton described his own case. His attention was drawn to his affection by finding that the flower of the Geranium zonale was of a different colour by artificial light from that which it appeared to him by daylight. The flower was pink, and appeared to him skyblue by day, and without a trace of blue by candle-light. His friends assured him that the colour of the flower was not materially altered when viewed by artificial light. In 1810, Goethe* applied his theory of colours to colour-blindness. He believed that the affection was due to a defective perception of blue, and called it acyanoblepsia. Seebeck † was the first to systematically classify the colour-blind, his observations being based on the examination of twelve cases. His method of examination consisted in letting the colour-blind classify about three hundred pieces of coloured paper. He also used pieces of coloured glass. Szokalski, Purkinje, and Elie Wartman have also made classifications of the colour-blind. Wilson was the first to point out the dangers of colourblindness, though very little attention was paid to his remarks. The only theories of importance are the Young-Helmholtz and the Hering theories of colour-perception. Nearly all the tests and papers on colour-blindness have been based on one or other of these theories, which will be dealt with fully in a later portion of this book. * Goethe, "Farbenlehre," pp. 126-190. +"Ueber den bei manchen Personen vorkommenden Mangel an Farbensinn," von A. Seebeck. Poggendorff's "Annalen der Physik und Chemi;" bd. xlii. (1837), n. 100, p. 177. I will conclude this chapter by alluding to those, not previously mentioned, to whom we are chiefly indebted for a knowledge of colour-blindness and the dangers arising from this defect. It is hardly necessary for me to allude to the work of Holmgren, who, following in the footsteps of Wilson, has done so much to draw general attention to the subject. Amongst foreign contributors Stilling, Donders, Cohn, Phlüger, Joy Jeffries, Favre, Magnus, Burnett, and Hischberg must be mentioned; and in England, Brewster, Herschell, Tyndall, Maxwell, Pole, Abney, Raleigh, Galton, Nettleship, Bickerton, Frost, and Hogg. CHAPTER II. THE PHYSICAL BASIS OF COLOUR. COLOUR is a sensation, and not an unalterable physical quality of bodies. The same substance may vary in colour according to the conditions under which it is viewed. The rays of light which give rise to the sensation of colour, are unalterable, and it is the suppression or admixture of certain of these rays that gives rise to changes in the colour of an object. If we examine, with a prism, a beam of sunlight admitted through a slit in a shutter, we notice that the slit appears much broader than before, and variously coloured. This is due to the unequal refraction of the constituent rays of white light. A beam of sunlight, examined in this way, gives a continuous spectrum; but if we take precautions to prevent overlapping of the different rays, by using a very fine slit, and obtaining a very sharp and defined image of the spectrum, we see that it is not continuous, but subdivided by a number of fine black lines. These lines, which were first discovered by Wollaston, and twelve years later more fully described by Fraunhofer, are due to the absence in sunlight of the particular rays which would, when refracted, occupy this position. Fraunhofer marked out 576 of these lines, and named the principal of them after the letters of the alphabet. Light, according to Huyghens and Young, is |