Page:The Cornhill magazine (Volume 1).djvu/433

 Colour Blindness.

If there is one infirmity or defect of those five senses with which we are most of us blest, which more than any other attracts sympathy and claims compassionate consideration, it is blindness—an inability to know what is beautiful in form or in colour, to appreciate light, or to recognize and comprehend the varying features of our fellow-men—a perpetual darkness in the midst of a world of light—a total exclusion from the readiest, pleasantest, and most available means of acquiring ideas.

And yet who would suppose that there exists, and is tolerably common, a partial blindness, which has hardly been described as a defect for more than half a century, and of which it may be said even now that most of those who suffer from it are not only themselves ignorant of the fact, but that those about them can hardly be induced to believe it. The unhappy victims of this partial blindness (which is real and physical, not moral) are at great pains in learning what to them are minute distinctions of tint, although to the rest of the world they are differences of colour of the most marked kind, and, after all, they only obtain the credit of unusual stupidity or careless inattention in reward for their exertions and in sympathy for their visual defect. We allude to a peculiarity of vision which first attracted notice in the case of the celebrated propounder of the atomic theory in chemistry, the late Dr. Dalton, of Manchester, who on endeavouring to find some object to compare in colour with his scarlet robe of doctor of laws, when at Cambridge, could hit on nothing which better agreed with it than the foliage of the adjacent trees, and who to match his drab coat—for our learned doctor was of the Society of Friends—might possibly have selected crimson continuations as the quietest and nearest match the pattern-book of his tailor exhibited.

An explanation of this curious defect will be worth listening to, the more so as one of our most eminent philosophers, Sir John Herschel, has recently made a few remarks on the subject, directing attention at the same time to other little known but not unimportant phenomena of colour, which bear upon and help to explain it.

It is known that white light consists of the admixture of coloured rays in certain proportions, and that the beautiful prismatic colours seen in the rainbow are produced by the different degree in which the various rays of colour are bent when passing from one transparent substance into another of different density. Thus, when a small group of colour-rays, forming a single pencil or beam of white sunlight, passes into and through the atmosphere during a partial shower, and falls on a drop of rain, it is first bent aside on entering the drop, then reflected from the inside surface at the back of the drop, and ultimately emerges in an opposite direction to its original one. During these changes, however, although all the colour-rays forming the white pencil have been bent, each has been bent at a different angle—the red most, and the blue least. When therefore they