According to the observations of Szokalski, this defect of vision especially obtains among nations having a Germanic origin, as the Germans, English, Swiss and Belgians ; the French, Italians, and Spaniards being com paratively free : it seems to be common in the United States, but this does not militate against the proposition, the Americans being descendants of the old British stock. How far this peculiarity may be attributable to the greater sensibility of the inhabitants of southern climes, where the more brilliant sun shine develops colours in a degree unknown in the northern latitudes, is a question which cannot be decided in the present state of our knowledge : nor can we affirm with con fidence, as stated by the same writer, that achromatopsy occurs most frequently in ro bust constitutions, combined with a bilious and melancholic temperament. A difference of opinion exists as to whether achromatopsy is indicated by any visible signs ; Szokalski, Ruete, and Minty affirm that there is no diagnostic mark, and Rau considers that a yellowish tinge of the iris, which has been considered by some to be indicative of it, is far too common to be so regarded. Professor Wartmann has in his first memoir drawn attention to a peculiar golden lustre of the eyes, which presents itself in cases of achro matopsy, where the iris is hazel. Miss Sedg wick t says of the historian Sismondi, that he had brilliant hazel eyes ; he was a Daltonian. At least five other cases are known to have presented the same peculiarity, but the num ber is yet too small to admit of this being regarded otherwise than as a coincidence, for in truth there appears to be as many Dalto Mans with blue, black, and grey eyes, as with hazel, and as many eyes without a yellow pupillary margin as with it.
There is sufficient evidence before us to warrant our considering achromatopsy under two distinct forms : congenital and genital. The former is always persistent : the latter may be divided into permanent and tent porary.
Writers have classified the defect according to degree; but the simplest and most practical arrangement is that of Wartmann, who recog nises two classes only : the Dichromatic and the Polychromatic. This we think advantageous, as avoiding unnecessary sub-classification, the varieties of the defect being endless.
Congenital Achromatopsy. — This form is most common, and the majority of recorded cases of insensibility of the eye to colour are examples of it. It presents the best marked illustrations of both varieties, which we shall proceed to consider.
Class I. (Dichromatic Daltoniem of Wart mann).— Black, white, and the intermediate shades of grey are the only tints recognized by patients of this class. Such persons dis tinguish with facility the forms of objects and the gradations of light and shade, but to them all the charms of nature and of art, as ex-. pressed by colour, are unknown ; their retinm are rather sensitive than otherwise, and they not only see objects at a great distance, but can read with facility in an obscurity amount ing to darkness.
The first of these cases on record was pub lished by Dr. Dawbeny Tubervile, an oculist of Salisbury,* being that of a young woman who consulted him about her sight, which, though excellent in every other respect, incapacitated her from distinguishing any other hues than black and white ; it is especially mentioned that she could read " for nearly a quarter of an hour in the greatest darkness." The next, best marked case has been recorded by M. D. Hombres Firmas.t M. —, of A nduze, of a bilious and melancholic temperament, but lead ing a very active life, had arrived at an ad vanced age without its being known, except to a very few individuals, that there was any thing uncommon in his sight; but all colours appeared to him as tints of grey, between black and white. Like several others having this infirmity, he was fond of painting, and had painted in his apartment two friezes and a pannel between the windows: of these he was proud ; but some of his visitors inquired why he had represented the ground, the trees, houses, and persons all blue? He replied that he wished them to match the furniture, he being quite unconscious that this was red. He had a collection of engravings, some coloured and others plain, but the only dif ference he could perceive was, that some were clearer than others. When criticising a picture he would discuss the composition of the design, the light, shade, and perspective ; but, as to the colours, he was silent. When walking with others in a garden, he affected to speak of the beauty and size of the flowers, their regularity and perfume, but to his eyes they, like the pictures, were all grey. The case of a shoemaker named Harris, described in the Philosophical Transactions,* has been ge nerally quoted as a well-marked example of this form of achromatopsy, but on carefully considering the remarks made upon it by Dr. Dalton t, we are doubtful whether it really was as supposed, and are inclined rather to refer it to the second class.
The case of M. Collardeau I has been described as of this nature. This gentleman was an amateur artist, but the imperfection of his vision gave rise to the strangest pro ductions. So long as he confined himself to the pencil, or to sketching in one colour, he designed with much skill, but his paintings in colours were the reverse of happy. For in stance, wishing to work up a scene in which he had drawn a dog, he unfortunately mistook the colour, and painted red all those parts which should have been deep blue. He has been known to confound on the canvass yel low with blue, and red with green, regarding his work with the complaisance of a man who felt that he had achieved success.