The peculiar defect of vision known as colour-blindness to which many people are subject, is due to various causes; but very little is known of its real nature. In different persons it has a different effect, being in some a complete inability to distinguish between the commonest colours; while in others it is merely a temporary confusion of the impressions conveyed by different hues, or a tendency to give the wrong names to colours, which can be perfectly distinguished from each other, though the mind cannot verify, so to speak, the distinction.

To take the first case first. A man who is perfectly 'colour-blind' cannot detect the slightest difference between the stripes on the 'red, white, and blue' flag; to him the red and green lamps of the railways are the same; and the leaves and flowers of the most variously stocked garden are more uniform in tone, in the clearest sunlight, than they would be to an ordinary eye by moonlight. (The effect of moonlight, it is well known, is to give a monochromous appearance to the most varied colours.) In the other case, a man who has, say the three cardinal colours, red, blue, and yellow, placed before him, can tell that there is a difference between them, but is unable to identify them; and while perhaps one day he is able to sort a number of pieces of glass of these three colours, he will be unable to perform the operation the next day.

Persons who are thus afflicted—for it is an affliction, though often they do not actually know of the defect to which they are subject—may possess in every other way the keenest eyesight; and it by no means follows that a man who is colour-blind has in any other way less perfect eyesight than an artist or any other person whose calling requires nicety of distinction in the matter of colours and hues. The question occurs, To what is colour-blindness due? In certain cases, to a want of education of the eye in this particular service; but more generally to local causes and diseases, and to hereditary defect. Instances occurring under the first-named class are not real cases of colour-blindness. It is really no more true to say that a man is colour-blind because he calls red 'green,' or blue 'yellow' persistently, and with a perfect appreciation of the difference, simply because he has never been taught, than it is to call a man blind who calls an oval 'round,' because he has learned no better. But in the other instances the colour-blindness is a true defect. In Egypt, China, and other countries where ophthalmia is prevalent, colour-blindness is common; and the peculiar light which exists in certain localities where there is a large expanse of flat sandy soil, and which is known to be very trying to the eyesight, is very often found to produce this defect where it does not otherwise impair the vision. Hereditary cases of colour-blindness are common. The painter Turner has been said by some of his critics to have been colour-blind; and we believe that one of his sisters had a defect of vision which caused her to confuse one colour with another in such a way as to prevent her from describing accurately a picture placed before her.

In reference to the theory that the recent disastrous railway accident at Arlesey was owing to a mistake of the engine-driver as to the colour of the signal displayed against him, a correspondent of the Times points out that colour-blindness may be acquired. 'A few years ago,' he says, 'I was investigating colour appreciation, and the first instance of the acquired defect that came to my knowledge was in the person of an engine-driver. This man confessed, after an accident through his not distinguishing the red signal, that he had gradually lost his colour-power, which had been perfect; and so sensible was he of his loss and its disadvantages, that before the accident he had determined to give up the situation. The manager of the Company, who told me the circumstance, assured me that this driver had been carefully examined but a few years back and passed as possessing perfect sight.'

If a person with perfect sight will look steadily for a few moments at any object, of one of the three primary colours, whether a lamp or anything else, and then close his eyes, and watch so to speak, with his closed eyes, he will find the object reproduced in a kind of cloudy representation, or rather retained on the eye; but its colour will be changed from the primary to its corresponding (complementary) secondary colour. Thus the impression of a red object will present itself as green; yellow as purple; and blue as orange. Vice versÂ, if the object is one of those secondary[A] colours, the reproduction on the retina will be of the corresponding primary colour. In this way, it is quite possible for a man, who has been looking for any length of time at a red light on a railway at night, to remove his eyes for a moment or two; and, on looking again at the lamp, to find that—in the course of the natural relief afforded by the impression on the eye resolving itself into the secondary colour—his sight is for a moment impeded by the floating image (now green instead of red) before his eyes, and the actual lamp (still red) covered, as it were, by the retained figure, so that it appears to be green. This curious effect is no fault of vision, and might easily mislead an engine-driver who, having first actually seen the red light, has, after withdrawing his eyes, immediately afterwards imagined it changed to green or white, in indication of the removal of the obstacle to the progress of his train. In this way, by continual straining of the eye in search of a particular signal, especially at night, with no light beyond that of the glaring furnace of the engine—in itself detrimental to the eyes—it is quite possible that colour-blindness may be acquired, and that a man who was once perfectly able to distinguish the most delicate tints may become insensible to the effects of widely different colours.

Whatever its cause, it is a fact that colour-blindness does exist to a very considerable extent. In Egypt this is so well recognised a fact, that engine-drivers and others employed on railways are obliged to undergo a special examination before they are allowed to proceed to their duties. Many curious stories are told concerning the attempts made by men suffering under this infirmity to escape the penalty of detection; they will often rather run the risk of bringing themselves and others to sudden death in a collision, than lose the coveted post by admitting their defective sight. Sometimes a man will successfully guess at the red, white, and green lamps or flags held before him; but, if the examiner is as astute as the examinee, he will balk his calculations by holding out a cap, or some other article not usually classed among the list of railway signals, and an unguarded 'Red' or 'Green' from the lips of the candidate will send him ruefully off about his business.

Researches lately made in Sweden shew that this peculiar defect of sight is prevalent in that country. Out of two hundred and sixty-six men examined recently by Professor Holmgren, eighteen were found to be colour-blind; and in our own land statistics prove that Englishmen are not free from the infirmity. The late Professor George Wilson, who made a special investigation into the subject in Edinburgh some years ago, stated that out of one thousand one hundred and fifty-four persons of various professions examined in 1852, no less than sixty-five were colour-blind; and of these, twenty-one specially confounded red with green. A gentleman employing a number of men, writing to the Times, states that recently he directed an upholsterer to cover some article of furniture in green leather, and that the man used a skin of bright red leather, not knowing the difference. He could only distinguish colours in their intensity, all appearing to him as different shades of gray.

But instances could easily be multiplied. The practical part of the question is its bearing on the employment of men upon whose sight and power of distinguishing colours many lives are dependent. Engine-drivers and signal-men, railway guards and sailors, often have nothing but a red or green speck of light between the safety and the death of themselves and perhaps hundreds of their fellow-creatures. How many of the 'missing ships' that have set forth in hope, with scores or hundreds of souls on board, and never been heard of again, have gone to their fate through the colour-blindness of the 'look-out,' who can tell? How many disastrous railway collisions have been owing to the same defect on the part of the engine-driver or stoker? The necessity of a rigid examination of all men employed on our railways, in order to ascertain their power of distinguishing the colours of the signals upon which so many lives depend, is being recognised by the directors and other officials. The same precaution ought to be adopted in the case of sailors, and not only once, but frequently. Periodical tests of their eyesight should be made at regular intervals; for in a physical infirmity of this kind, so apt to be overlooked and remain unrecognised even by those who are subject to it, lurk more dangers than in the lack of many other strictly enforced requirements.

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