I have already referred to the fact that the red rays are the most penetrating of the visible spectrum; but from this it appears as if the ultra-violet rays were still more penetrating. Total Colour-blindness. I have not personally examined a totally colour-blind man; but the following case was related to me by an architect: - X. A., Architect's Assistant.-He did not understand in the least what was meant by colour, and said that colours appeared to him simply shades of white and black. He had to use colours in making plans, but was guided by the name on the colour. One of the clerks, being aware of his defect, scraped off the names of the colours, and so, in a plan, he put Indigo where Crimson Lake should have been, and vice versa. His friend had a house in which there were dark oaken timbers and light-orange coloured plaster. He asked, when looking at the house, why the plaster was so much darker than the wood. His friend told him that the plaster was very much lighter than the wood, but he would not believe it. In a photograph, which was subsequently taken of the house, the plaster came out much darker than the oaken timbers. This case may or may not be one of total colourblindness. From the facts given it is evident that he had very considerable shortening of the red end of the spectrum. This shortening must have extended nearly to the yellow to make a light orange appear darker than a deep yellow. The red element in Crimson Lake was not perceived, so this colour was mistaken for a blue. The following case of total colour-blindness, described by Donders, is quoted by Joy Jeffries. He says— "Professor Donders reported an interesting case of congenital total colour-blindness at the Heidelberg Ophthalmological Society, 1871. An educated young man of twenty-one years of age was totally colour-blind. Strong light blinded him: in moderate light he saw very well. He was myopic one-eighth, and read for hours without glasses. Out of doors all glasses which absorbed light without difference-even the brightest-coloured oneswere pleasant to him, because they reduced the light. In the dioptric spectrum of a gas-lamp his brightest part was between the spectral lines D and E, close to E: hence in greenish yellow. From here outwards towards the red end the light faded rapidly; towards the violet, at first slowly, then rapidly. By moderate illumination he lost less of the brightness of the spectrum on this side than towards the red end. With the polariscope, the complementary colour through the quartz plate appeared to him of the same colour. In turning the double-refractive prism he had a maximum of brightness at every ninety degrees, or equality of brightness, as if the quartz-plate was not there. He had the greatest difference when Donders himself saw purple and green; equality, when he saw yellow and blue. Trials were also made with Chevreul's chromatic circle." The following is an epitome of the different classes of the colour-blind: Five-units see only five definite colours. They confuse orange-red and red, orange-yellow and yellow, rosered and red, purple-violet and violet. They also confuse bluish green and green. Four-units see only four definite colours. They confuse red, orange, and rose-red: greenish blue and green. They also confuse pure blue and violet. Three-units see only three definite colours. They confuse red, orange, rose-red, and most yellows on the one hand, and blue, violet and purple on the other. They have a tendency to mistake rose, gray, and green. They confuse many browns and greens, and bluish greens and greens. Two-units see only two colours. They confuse red, orange, yellow, and green on the one hand, and blue and violet on the other. They also confuse blue-green, purple, and gray. One-units see only one colour. CHAPTER XIV. ACQUIRED COLOUR-BLINDNESS. As a lesion in any part of the track from the eye to the brain may interfere with colour-perception, cases of acquired colour-blindness may be divided into two classes: A. Defects of colour-perception due to disease of the eye or optic nerves. B. Defects of colour-perception, due to some affection of the brain. It is obvious that in those cases in which the visual acuity is markedly affected, the loss of colour-vision is only of secondary interest. We are chiefly interested from a practical point of view in those cases in which the visual acuity is not affected to such a degree as to compel a man to give up his employment. Acquired colour-blindness has been described as an accompaniment of many diseases of the retina and optic nerves. The cases of acquired colour-blindness which are most common are those of central scotoma for colours, associated with atrophy of the optic nerve, or met with in cases of tobacco amblyopia. The following case illustrates the condition of central scotoma for colours. Y. A., Male, aged 41.-He had been a very heavy smoker, and had given up work for eighteen months on account of his sight, which grew worse and worse. said that he could not tell a half-sovereign from a sixpence He by the colour. He had often found difficulty in telling a half-crown from a penny by the colour. There was central scotoma for both red and green in each eye. The scotoma for green was more definite than that for red. I. Examination with Holmgren's Wool Test (Daylight).I examined him in the ordinary way with Holmgren's Wool Test. He picked out the correct greens to match I. with ease. II. Additional Examination with Wools (Daylight).— I then told him to pick out all the greens. This he did, but put several light blues with the group. I then gave him a violet skein (No. II. Classification and Pocket Tests) to match, and he put only violets with it. I then gave him a red (No. III. Classification and Pocket Tests) to match, and he selected only reds to put with it. III. Examination with Pocket Test (Daylight). I. Orange. 1, 7, 8, 12, 13, 19, 25, 76, 78, 67, 54, 89, 104, 99, 109, 123, III. 1, 7, 8, 12, 13, 19, 67, 70, 76. 78, 94, 99, 111, 116, 123. Red, brown, IV. Gray. VI., 73, 91, 124, 125. Green and gray. V. Purplish Gray. 3, 31, 5. Gray and rose. VII. Yellow. 70, 76, 98, 99, 100, 113. Olive-green, orange, green, and yellow. It will be noticed that the mistakes made are those of an ordinary two-unit colour-blind. The following case, which is recorded by Samelsohn,* plainly points to the brain as being the seat of disease. It is an example of complete hemiopic colour-blindness. He says "A man, aged sixty-three, complained of affection of *Lancet, Nov. 26, 1881, p. 918. Р |