CHAPTER XV. MISTAKES MADE BY THE COLOUR-BLIND. As the reader who is not practically acquainted with colour-blindness would find great difficulty in making out to which class a colour-blind person belonged, until he had thoroughly mastered the preceding chapters, I will deal with the subject from another aspect—namely, that of the actual colours themselves. In this way an examiner, by referring to this chapter, will be able to make out the class to which a colour-blind person belongs, even if only one or two facts be known. I. Non-perception of Certain Colours.-The confusion of any colour with black is an indication that there is either a shortened spectrum or a neutral band. 1. Confusion of Red and Black.--This indicates shortening of the red end of the spectrum. It may be met with in otherwise normal-sighted persons. When persons only find difficulty in distinguishing red from black at a distance, it is probable that the shortening is not very great. The following letters appeared in the Times: "SIR, "As one partially colour-blind, perhaps you will allow me to say a word in connection with the interesting leader in the Times of to-day, on practical visiontesting. I have been aware of a personal visual defect, which I have tested on innumerable occasions for something like thirty years. But to me the coloured skeins of the wool-test is no test at all, for both reds and greens in all shades are unmistakable; and, although I have undergone this examination whenever an opportunity has presented itself, I have never tripped. Close at hand, reds and greens are to me as to other people; at a distance, however, my sense of colour, in regard to red especially, is all astray. Standing on the edge of a large field glowing with poppies, I see them up to about thirty or forty yards as other people see them; but beyond that distance they gradually merge into a neutral tint and become lost. Again, what is commonly called the pink may is glorious with colour when near at hand; but at some distance I see nothing but green leaves. A distant red light over a chemist's door appears to be a dull yellow, and the same applies to a red railway-signal. This seems to prove that sight in regard to correctly distinguishing colours and their shades may be normal close at hand, and defective at a distance. Had my vocation been that of a locomotive driver, I have no shadow of doubt that I should have passed the examination with coloured skeins of wool; and I am equally sure that, on the first opportunity, I should have driven my train and passengers to destruction. "I am, sir, your obedient servant, "January 23, 1890." << SIR, "A. W. T. "As the question of colour-blindness is so important, and as the main point as to distance was so completely missed in the late discussion at the Society of Arts, will you allow me to corroborate the statements made in the letter of A. W. T.'? |