# The wonders of optics/The appreciation of colour

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THE APPRECIATION OF COLOUR.
Most people understand each other sufficiently to agree in their ideas about various colours. Thus every one agrees in saying that poppies are red, that the sky is blue, and the leaves green; but if any one were to assert that the sky was red, that the leaves were blue, and poppies green, who could possibly contradict him?
This statement may appear a paradox, and an absurdity to many of our readers, but it is really a problem that has engaged the attention of many of our greatest philosophers. Who can prove that what I see as yellow may not appear blue to you, or that what you see red is not green to me? You would possibly explain the doubt by saying that because we both agree in calling a buttercup yellow, that we see the same colour. I call a buttercup yellow, because I have learnt since my childhood to give this name to the particular sensation I experience when I look at one of these flowers; but that is no proof that the sensation I feel is similar to that felt by everybody else, and it is not merely possible, but probable, that our personal sensations of colour are essentially different, although the arbitrary words we use to designate them are the same.
It may be remarked in parenthesis, that colour is not an entity, but is simply the effect of certain properties of surface or interior structure possessed by every sub-​stance with which we are acquainted. The old saying, that "all cats are black in the dark," is really a profound philosophical truth, which is not only true of cats but of the reddest rose that ever grew in a garden, the bluest violet that ever was plucked, the prettiest girl that ever was kissed under the mistletoe. It is a sad thing to think of, that when we put the candle out, and step into bed, we become blacker than the blackest negro that was ever emancipated. But without light there can be no colour, for there is no material, so to speak, from which to manufacture it. White light, as we have said before, is made up of red, blue, and yellow, and it is by the absorption of one or all of these that all tints are formed. The surface of a poppy leaf has the power of absorbing all the blue and a little of the yellow, reflecting the whole of the red and the remainder of the yellow, the mixture of the two forming scarlet. The surface of a marigold acts differently; all the blue is absorbed, as in the case of the poppy, and a good deal of the red with it, leaving just a little to brighten up the yellow which is reflected with it. Some substances, white marble for instance, have no power of splitting the light into colours, absorbing some and reflecting others, but reflect the whole of it in its integrity. Others again, like black velvet, absorb nearly the whole, just reflecting sufficient to enable us to see its surface.
We began this chapter by speculating on the probability of our seeing different colours to our neighbours, and we shall now proceed to show that our speculations in that direction are not so absurd as they appear to be at first sight.
The phenomenon of colour blindness, or the insensibility of the eye to certain colours, has been for many years past a puzzle both to the physiologist and the philosopher. Perhaps the most remarkable case of the ​sort is that mentioned first by Huddart, and quoted by Sir David Brewster, of a shoemaker named Harris, living at Maryport, in Cumberland, who was utterly incapable of distinguishing any colour at all, and saw everything white, grey or black. The first time that Harris noticed this defect, was when he was about four years old; having found the stocking of a playmate in the street, he returned it to him at his cottage, and noticed that every one said it was a red stocking, but he could not understand why they should call this particular stocking red, as it seemed to him to be like every other. This circumstance remained in his mind, and a few more similar observations confirmed his suspicions that he had some defect of sight that prevented him from seeing as others did. He also observed that other children pretended to distinguish cherries from their leaves by what they called their colour, whilst he could see no difference between them, except those of shape and size. He also noticed that by means of the difference of colour, others could distinguish cherries on a tree at a much greater distance than he could; whilst he, on the contrary, could see other things at greater distances than his companions. Harris had two brothers, whose eyes were similarly defective; one of these, that Huddart examined, mistook green for yellow constantly, and orange for light green.
In the Philosophical Transactions Scott describes a similar defect in his own powers of vision. He states that he was unable to distinguish green, and that the colours known as crimson and pale blue presented no difference of hue. He further confesses his inability to see any difference between bright green and bright red, although he could distinguish between red and yellow, dark blue, and almost every shade of blue, except sky-blue. He goes on to relate how he married his daughter to a worthy young man of his acquaintance, and that ​the day before the wedding the bridegroom came to his house in a full suit of black, as he thought. He was greatly displeased to see him appear in mourning on such an occasion, and took an opportunity to remonstrate with him on the subject. But what was his surprise to hear his daughter exclaim, in loud tones of counter remonstrance, that she had rarely seen her lover in a coat of such a pretty colour, and that her father's eyes must deceive him on this as on many other occasions. Scott's father, his maternal uncle, one of his sisters, and two of his sons had the same defect of sight. Dr. Mitchell mentions the case of a naval officer who for his ordinary uniform chose a blue coat and waistcoat and red trousers, fully believing that they were all of the same colour. A tailor of Plymouth, also mentioned by Dr. Mitchell, mended a black silk waistcoat with a piece of crimson, and another put a red cloth collar to a blue coat. Several celebrated men have suffered from colour-blindness. Amongst them may be mentioned Dugald Stewart, the great philosophical writer; John Dalton, the originator of the atomic theory; and Troughton, the philosophical instrument maker. Dugald Stewart first discovered the defect on hearing a member of his family admire the contrast of colour between the leaves and fruit of a Siberian crab-tree, while he could see no difference between them, except in point of form and size. John Dalton could not distinguish blue from crimson, and he could only see two colours, blue and yellow, in the prismatic spectrum. Troughton could see no difference between dark crimson, bright orange, and yellow—in fact, he could only distinguish blue from yellow.
In an article on this subject, published in the Magasin Pittoresque for 1846, a Swiss physician gives some interesting examples, which are worth repeating. In the solar spectrum obtained by passing a ray of light through a triangular prism, and which is composed of ​the following colours,—red, orange, yellow, green, blue, indigo, and violet,—Dalton could only see yellow, blue, and violet. Rose-colour by day appeared to him a pale blue, but at night it seemed to take an orange hue. By day crimson seemed to be dirty blue, and red cloth dark blue. Dr. Whewell having asked him one day to describe the colour of the doctor's scarlet gown, Dalton pointed to the trees around them, and declared he could distinguish no difference in their colour; and one day having dropped a stick of red sealing-wax in the grass, he had the greatest difficulty in finding it again. Since Dalton's time over five hundred distinctly marked instances of this imperfection have been noticed, and Professor Prévost, of Geneva, has named it Daltonism, an extremely unphilosophical piece of pathological nomenclature, which has unfortunately received the sanction of too many great physiologists to be abolished. Blindness might just as well be called Homerism or Miltonism.
Colour-blindness is much more frequent than is generally supposed, for those who are afflicted with it are mostly ignorant of the defect, and frequently practise trades or professions in which perfect sensibility to the different hues of colour is quite indispensable. An instance of this occurred some time since in the case of an engine-driver, who allowed his engine to run into a luggage train, through not noticing the red danger signal. At his examination it was proved that he was colour-blind, and could not distinguish red from green. Partial colour-blindness is, no doubt, the cause of the frequent disputes that we hear about the tints of certain objects; to say nothing of the glaring instances of bad taste in the arrangement of colour that are now-a-days so common. Out of forty boys at a school at Berlin who were examined by Leebech, he found five who were quite confused in their notions of colour, and ​could not distinguish between ordinary shades of the same hue. This affliction is in many cases hereditary, descending from father to son. It is singular that instances of colour-blindness are much more common amongst men than amongst women, for out of over five hundred cases there were only four in which females were the sufferers. It seems also that persons with grey eyes are more frequently colour-blind than those whose eyes are blue or brown. To the list of great men who were colour-blind, we must not forget to add the celebrated Italian historian, Sismondi.
Physiologists consider that there are two kinds of colour-blindness,—one where only two colours are seen, the other where more than two are perceptible. Daubeny Turberville, an oculist of Salisbury, mentions a case of the former, in which a young girl, like the Maryport shoemaker mentioned by Brewster, could only distinguish between black and white, everything between the two being of different shades of grey. This girl, singularly enough, could see to read in twilight a quarter of an hour after her companions. This sharpness of sight appears to be not at all uncommon amongst those who are colour-blind. Spurzheim mentions the ease of a whole family who were afflicted in the same way as Turberville's patient. All the male members of Troughton's family were equally incapable of distinguishing any colours but blue and yellow.
The cases of colour-blindness where more than two colours are distinguishable, are much more common. Goethe, the great German poet, who dabbled a great deal in optics, knew two young men who, although they possessed powerful sight, and could distinguish between white, black, grey, yellow, and orange, were at a loss when the shades between dark red and rose colour were in question. A piece of dried carmine appeared bright red to them, and a faint carmine hue on a white shell, ​and a rose-leaf, light blue; the leaves of trees and grass appeared yellow, and they confounded rose-colour, blue, and violet together. Goethe supposed them to be incapable of perceiving blue and its several hues, and called their defect by a high sounding Greek name, akyanoblepsy, or blue-blindness. Péclet mentions two other persons, also brothers, who likewise were incapable of distinguishing between blue, violet, and rose-colour. Like Professor Whewell, they confounded the dull scarlet of the trousers of the French infantry with the leaves of the trees. Yellow appeared to them more brilliant than any other colour. Doctor Sommer and his brother could not distinguish between red and its derivatives and other colours; they could only distinguish between yellow, blue, white, and black. Doctor Nicholl mentions a child that could only see red, yellow, and blue, in the spectrum. It could distinguish green, but called it brown when it was dark, and pink when it was pale. The same physiologist knew a man who called red green, and brown dark green. A young lady who was an amateur artist, could not perceive a piece of scarlet cloth hanging on a hedge that was close to her, although others could see it plainly half a mile off. One day she gathered, as a great curiosity, a lichen which she supposed to be of a bright scarlet hue, but which was in reality of a beautiful green. Another time she could see no difference between carmine and prussian blue. A gardener living at Clydesdale, who began life as a weaver, was compelled to give up his first trade because in daylight he confounded all light colours; yellow and its varieties he could distinguish perfectly, but he was incapable of seeing any difference between red, blue, pink, brown, and white. Another man, who was a silk-weaver, had to change his trade, because he could not distinguish between red, pink, and sky blue. A Genevese artist whom circumstances compelled to paint ​a portrait by candle-light, used yellow for pink in laying on his flesh tints, with a pleasing result that may be readily imagined. In fact, the instances of colour-blindness mentioned by physiological writers are almost innumerable, and I should only weary my readers if I related all the authentic cases of this singular affliction. One instance, however, which was very carefully observed by Wartmann, a distinguished German oculist, merits our attention. The afflicted person, whom Wartmann speaks of as D., was thirty-three years old. Those of his brothers and sisters whose hair was fair suffered from the same infirmity, but those whose hair was dark were exempt from it. Like so many others who are colour-blind, he could not distinguish between cherries and their leaves, and confounded a sea-green piece of paper with a scarlet ribbon placed near it. A rose of the ordinary hue appeared greenish-blue. Being anxious to see if reflected, refracted, and polarized light exercised a different action on his retina, Wartmann tried him first with the prismatic spectrum, but he could only distinguish four colours,—blue, green, yellow, and red. He could distinguish perfectly the peculiar black lines seen crossing the spectrum in certain places, and known by the name of Fraunhofer's lines. He then placed in his hands thirty-seven pieces of differently coloured glass, but he could only distinguish four varieties. The colours produced by polarized light seemed to give the patient quite as much trouble as those produced in the ordinary way. Chocolate brown appeared reddish brown; purple, dark blue; and violet, a dirty blue. When colours were illuminated by sunlight, they seemed to him to be redder than usual, even green and blue appearing red.
In considering cases of colour-blindness, it is very difficult not to be misled into using wrong terms, as applied to colour, for we have no possible means of knowing what colour it is that is really seen by the patient. ​Thus, for instance, Dr. Whewell could not distinguish between red and green. But what colour did he really see? Did he see the leaves and cherries both red or both green, or was it some colour between the two that was impressed upon his retina? Again, great care must be exercised in placing implicit reliance on the statement of persons who are colour-blind, for we must recollect that their only means of conveying the results of what they experience is by the use of an organ that is confessedly defective, and which is quite likely to deceive them, and us too, without their being parties to the deception.
The cause of colour-blindness is completely unknown; philosophers and physiologists are still in the realms of hypothesis concerning this peculiar optical defect. As yet, the most careful observation has failed to detect any difference between the eyes of those who are colour-blind, and the eyes of ordinary persons, that could in any way account for this singular affection of the sense of sight.