[30] See Dr. Prichard's "Researches into the Physical History of Mankind," vol. i. p. 366.
Among the lower animals, albinism appears to occur most frequently in the race of tame rabbits; they are known by their white fur and red eyes; breeds of these may be propagated; the same observation applies to white mice.
Human albinos, at least, and probably albinos in the animal grade, are affected by a certain degree, more or less decided, of day-blindness. The characters of the albino result from a deficiency in the colouring principle, common to the skin, hair, and eyes. The eyes, for example, are entirely destitute of the dark pigment; in fact, the pink, or red colour, so observable, depends on the fine vessels which are very numerous in the composition of the iris, and on the still greater number of minute arterial ramifications which almost entirely form the choroid membrane, and which, under this condition, are seen through the pupil. To Albinos the state of the eyes is their greatest source of inconvenience. The absence of the black pigment, which has the important office of absorbing superfluous portions of light, renders the eye preternaturally sensible of this stimulus. Strong lights affect the organ painfully; even the glare of open day produces a partial blindness, and the admission of light is intolerable. These evils are balanced in some measure by superior power of vision in dusk, or imperfect darkness.[31]
[31] On this subject see Buffon, Supplement, tom. iv. p. 559; Pallas in his "Novæ Species Quadrupedum," pp. 10, 11, n., and also Blumenbach, etc.
Let us now turn to night-blindness. "Nocturnal blindness," says Scarpa, "is properly nothing but a kind of imperfect periodical amaurosis, most commonly sympathetic with the stomach. Its paroxysms come on towards the evening, and disappear in the morning. The disease is endemic in some countries, and epidemic, at certain seasons of the year, in others. At sunset, objects appear to persons affected with the complaint, as if covered with an ash-coloured veil, which gradually changes into a dense cloud, intervening between the eyes and surrounding objects. Persons thus affected have the pupil, both in the day and night-time, more dilated and less movable than it usually is in healthy eyes. The majority of them, however, have the pupil more or less movable in the day-time, and always expanded and motionless at night. When brought into a room faintly lighted by a candle, where all the bystanders can see tolerably well, they cannot discern objects at all, or at most in a very feeble manner, or they only find themselves able to distinguish light from darkness, and by moonlight their sight is still worse. At daybreak they recover their sight, which continues perfect all the rest of the day till sunset."[32]
[32] Cap. xix. p. 322. Ed. 8vo.
Mr. Bamfield observes that the abolition of eyesight by night has occurred in all ages, and is a common disease of seamen in the East and West Indies, the Mediterranean, and in all hot and tropical countries and latitudes, and that it affects more or less the natives likewise of those regions of the globe. It also occurs frequently among soldiers in the East and West Indies, but, according to the information he has received, it is by no means prevalent amongst sailors employed on shipboard. It is not an uncommon complaint among the Lascars employed in the East India Company's ships, trading between India and Europe. Celsus has remarked that females in due health are exempt from this strange affection of the orbs of vision.[33] It may further be observed, that the inhabitants of northerly latitudes are less subject to night-blindness than are the natives of tropical countries in theirs; but that when the natives of cold climates visit the tropics, they are peculiarly susceptible to this morbid condition of the eye.
[33] Lib. vi. cap. vi.
But we must now pass from the eye to the ear. Deafness from infancy, that is, total deafness, involves the loss of speech, or of the utterance of a definite language. The deaf do, indeed, utter wild cries, and, as if impelled by instinct, scream with rage, howl with pain or fear, and mutter when pleased; but they hear not the sounds which they themselves utter, and which are often harsh, and even terrible.