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theoretical predictions are verified by facts. In examining colour-blind persons, I avoided all leading questions, so that no bias should be given to them one way or the other. I have given a numerous series of cases which, even if the theory be not accepted, are definite facts of colour-blindness to which any future theory must conform.

The practical portion of this book is the outcome of the work which I did at the request of the Board of Trade.

I must here express my great indebtedness to the officials of the Marine Department of the Board of Trade, and especially to the late Mr. Gray, for the assistance which they have afforded me in making my investigations. Mr. Gray took the greatest interest in colour-blindness, and spared no trouble to obtain efficient tests. It seems remarkable to me that criticism has been almost confined to the colour-tests of the Board of Trade, whilst those used by the Navy and Railway companies, which are incomparably less efficient, have scarcely been noticed.

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.

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