EYE AS A CAMERA OBSCURA. 335 
¥ noes, where the pigment of the choroid is altogether absent, and choroidal reflec- 
> tion is at a maximum, vision is imperfect, as well as painful. The 2d, that 
those animals which have the pigment of the choroid at the bottom of the eye 
covered or replaced by a tapetum lucidum, or metalline mirror, are impatient of 
the sun, and, like albinoes, see comfortably only by faint twilight. The 8d, that 
the known laws of luminous reflection make it impossible that vision should be 
perfect, if light is free to cross the chamber of the eye in all directions. 
Now, to take those objections in order :—1. It is not to be denied that albinoes 
of our own race experience great intolerance of bright light; see with difficulty, 
unless it is faint; are generally short-sighted; and exhibit, unless in darkness, 
a continual tremulous motion of the eyeballs, as if vainly seeking to remove from 
the influence of light each dazzled point of the retina on which it falls. But it 
appears to have escaped the observation of physiologists, that it is otherwise with 
many of the albino varieties which occur among the lower animals. In thinking 
over this matter, I called to mind, that in watching in earlier days the habits 
of pink-eyed white mice and white rabbits, I detected no appearance of their 
vision being less perfect, or exercised with less readiness and pleasure than that 
of their dark-eyed brethren; and all to whom I have mentioned this opinion, 
familiar with the ways of those animals, have confirmed my conclusion. Nor is it 
difficult to explain why animal and human albinism should differ. There are, 
in truth, two kinds of albinism, the one showing itself suddenly, or, as it were, per 
saltum, in the immediate offspring of parents who are not themselves albinoes; 
_ the other appearing in every individual of a particular race or variety of animals, 
and having been transmitted to it as a hereditary peculiarity, which has descended 
through hundreds or thousands of generations. 
In the lower animals both kinds of albinism are frequent. A white crow, 
_ for example, or a white blackbird (as we are compelled to call it), is known occa- 
_ sionally to make his appearance among his darker brethren, although unques- 
tionably himself of sable lineage; but a permanent albino variety of either bird 
_ isnot known. In the mouse, on the other hand, the rat, the rabbit, and a few 
_ other animals, albinism has become permanent in certain varieties, probably 
assisted in some by the interference of man in mating the animals. The ferret, 
_ Moreover, appears to be a natural variety of permanent albinism. 
_ The albinoes of our own race are all, so far as I am aware, accidental, or per- 
__ haps it would be better to say, incidental or sporadic cases. The older writers, 
indeed, acknowledged a distinct race of Leucaethiopes, or White Ethiopians, and 
a modern tradition, still sacred among travelling exhibitors, ranks all European 
_ Albinoes among Circassians. There is probably some foundation for both opi- 
 nions; but though albinism is certainly liable to become hereditary among our- 
_ Selves, I am not aware that a case is on record where the parents of a human 
4 albino both displayed this structural peculiarity ; or when it has descended to a 
VOL. XXI. PART II. 4x 
