Color-Blindness, a curious defect in vision, depending on a want of sensibility in the eye, or perceptive capacity in the brain, in consequence of which certain colors are not distinguished, or all colors are alike invisible as such. It is believed that attention was first called to this defect by the publication by Dr. Dalton, the distinguished chemist, in 1794, of the particulars of his own case. The name given to the affection is that proposed by Dr. George Wilson, from whose work on the subject (Edinburgh, 1855) the following summary is chiefly condensed. It has also been called Daltonism. A cause for the lateness of the discovery of this phenomenon may be found in the fact that while the ignorant would not investigate a disability of the kind under which they might labor, the educated and intelligent would learn to compensate for it by the use of other senses. No mention of color-blindness has been found in ancient or modern writers up to the period named; but the examples of the affection already collected are numerous, and among its subjects were Dugald Stewart and Sismondi, contemporaries of Dalton. The difficulty shows itself in three forms or degrees: 1, in an inability to distinguish nicer shades and hues, such as grays and neutral tints; 2, in inability to distinguish certain primary colors from each other, as red from green, or these from secondary or tertiary hues, as scarlet, purple, etc.; 3, in inability to discern any color as such, the person seeing only white and black, lights and shades.

In the first degree, this affection is, among males, rather the rule than the exception. Dr. Wilson found that of 1,154 persons examined by him in Edinburgh, more than one in 18 were in a greater or less degree color-blind; and that of 60 persons in the chemical class of the Edinburgh veterinary college, the majority declined to name any colors beyond red, blue, yellow, green, and brown; while they failed entirely in attempting to arrange nearly related hues of yarns or stuffs, or those of varying shades of the same hue. He found that pink and other pale colors, especially pale yellow and blue and green, were often confounded. The same thing happened with orange and yellow, lilac and bluish gray, etc. In the second degree, in the less marked cases, red and green, or these with olive and brown, fail to be distinguished. And it is apparently singular that colors among the most distinct to a normal eye are in these cases the most easily confounded, red and green being more readily so than yellow and purple; while green is in these respects the most delinquent of all the colors. Dugald Stewart could not distinguish the red fruit of the Siberian crab from the green color of its leaves. Three brothers, Harris, mistook red for green, orange for grass green, yellow for light green.

A tailor at Plymouth regarded the solar spectrum as consisting only of yellow and light blue; while indigo and Prussian blue he pronounced black. Dr. Dal-ton could not by daylight tell blue from pink; he scarcely saw the red of the spectrum, and considered the remainder of it as showing but two colors. But a failure to perceive the more refrangible rays is most common. Seebach concludes that all eyes, however imperfect otherwise, see yellow; and that the sensations of complementary colors are inseparable, so that if the eye be sensible or insensible to either it will be so to both, the eye that fails to see orange also mistaking blue, etc. In the third degree, however, admitted by other observers, all colors are recognized only as giving certain degrees of light or shade. This form is rare. Dr. Wilson found but one case; and in this some of the colors could be named by gas light or transmitted light, none by reflected daylight. - The color-blind very often do not know their own defect; and in the lower walks of life their lack incapacitates them for certain employments, or may even imperil life.

These evils particularly befall weavers, tailors, gardeners, railway attendants, sailors in the steam service, and others dependent on the use of colored articles or the perception of colored signals. The importance of a correct perception of colors, in the present modes of signalling upon railways and shipping, cannot be overestimated. For example, the English admiralty orders require at night a green light on the starboard, a red light on the port side of vessels; and by the color the steersman must know which side of the vessel is toward him, consequently whether it is going to right or left, and whether to starboard or port his helm. Although no case of accident has yet been traced to color-blindness in the attendants, yet such a result is easily conceivable, especially as their powers of vision are not tested; and the most doubtful complementaries, red and green, are much in use as signals. Practical inferences are that the ability in this respect of candidates for the posts of sailors and railway men should always be first carefully tested; but, better still, that form and position of signals should, as far as practicable, be substituted for color, as the former are qualities less liable to be mistaken, and the color-blind generally perceive form even more correctly than other persons. - The cause of color-blindness probably lies somewhere between the eye, as an organ, and the mind; or more correctly, in a want, partial or total, of a certain perceptive faculty, that of color, as an element of active mind.

Dalton thought the retina or humors of his eyes must be colored, and probably blue; a nice post-mortem dissection of his eyes revealed no abnormal coloration or appearance whatever. Dr. Trinchinetti proposed as a cure the extraction of the crystalline lens; but Wilson gives a case of cataract, in which color-blindness supervened on the extraction of the lens. In one instance, the latter found the difficulty to follow permanently on concussion of the brain; sometimes it was temporary, and dependent on congestion, dyspepsia, or hepatic derangement; most frequently it was congenital. Color-blindness is generally hereditary. Leber, who examined many cases, found it a frequent sympton of atrophy of the optic nerve, and of scotoma (muscae volitantes, &c). Dr. Argyll Robertson (Edinburgh "Medical Journal," February, 1869) found it accompanying a case of spinal disease. Dr. Chisholm of Charleston, S. C, observed it in a case of inflammation of the optic nerve. It is often, however, unaccompanied with any impairment of vision. It has been observed during pregnancy, and Lawson met with a case which was produced by over use of the eyes in sorting colors.

As has already been noticed, the ethereal waves of light in the different colored rays vibrate in different times, the number of vibrations in the middle red ray being about 477,000,000,-000,000, while the number in violet light is 699,000,000,000,000 times in a second. There are also waves on either side of these limits which are too slow on the one hand, and too rapid on the other, to be perceived by the human eye, just as some vibrations in the air may be too slow or too rapid to be perceived by the ear. That some persons can perceive a lower tone of red, or the more extreme rays of the violet spectrum, as well as that some can perceive lower or higher notes in music, is a matter of observation, as also the fact that the perception of the depth and tone of various colors varies in different individuals. It is then a matter of no great surprise that in some persons the retina should fail to perceive the difference between the vibrations which take place in some of the colored rays. In the congenital cases, and in some others, the attempt at cure by medicines has been found utterly hopeless; of a cure through education no case is established. The want may be alleviated by carrying about a chromatic scale, named, for purposes of comparison; but little help is thus derived.

It is strange that the substitution of artificial for solar light seems as yet to offer decided relief in the largest number of cases; and a draper has been known to keep his shop lighted with gas during the day for this purpose, and with success. Very white light is less useful in these cases; the best being a light yellow by passing through glass stained with preparations of silver, uranium, or iron. Dr. Wilson found that a good test for persons confounding red and green, and who may be unaware of the fact, was obtained by placing before their eyes a red glass; the beholder is at once astonished at the difference which he discovers in looking at the two colors.