Day-blindness most probably never occurs as a separate disease; it is immediately the result of a peculiarly irritable state of the retina, insomuch that the stimulus of daylight cannot be borne; and not of the retina only, but of the iris also, which, in order to exclude the rays of light, closes the pupil of the eye to a mere point. All have heard of those singular beings termed _Albinos_. Albinos are found among all nations, and are noted, from contrast of colour, in African and not unfrequent among the dark tribes of Africa; indeed, it would seem that there are families of negroes in which there is an hereditary tendency to the production of Albino children.[30] [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.