. Animal physiology. Physiology, Comparative; Physiology, Comparative. ALBINOS. SPHERICAL ABERRATION. 409 being reflected from one part of the interior of the globe of the eye to another, which would cause a great confusion and indis- tinctness in the picture. Hence it is that, in those individuals (both among Man and the lower animals) in whose eyes this pig- ment is deficient, vision is extremely imperfect. The eyes of those individuals (termed Albinos), derive, from the absence of pigment, a very peculiar appearance. The iris does not possess its ordinary colour ; but, owing to the large qu
. Animal physiology. Physiology, Comparative; Physiology, Comparative. ALBINOS. SPHERICAL ABERRATION. 409 being reflected from one part of the interior of the globe of the eye to another, which would cause a great confusion and indis- tinctness in the picture. Hence it is that, in those individuals (both among Man and the lower animals) in whose eyes this pig- ment is deficient, vision is extremely imperfect. The eyes of those individuals (termed Albinos), derive, from the absence of pigment, a very peculiar appearance. The iris does not possess its ordinary colour ; but, owing to the large quantity of minute blood-vessels which it contains, it presents a bright red hue. The choroid coat, seen through the pupil, has exactly the same aspect; so that the pupil is not readily distinguished. During the day, the vision of these Albinos is very indistinct, and the glare of light is painful to them ; and it is only when twilight comes on, that they can see clearly and without discomfort. 546. The eye is a much more perfect optical instrument than we might be led to suppose, from the cursory view we have hitherto taken of its functions; for by the peculiarities of its construction, certain faults and defects are avoided, to which all ordinary optical instruments are liable. One of these, termed spherical aberration, results from the fact, that rays falling upon the central and outer parts of an ordinary convex lens, whose surfaces form part of a sphere, are not brought to meet in one point,—the focus of the central portion being rather more distant than that of the outer part. This is shown in Fig. 200, where. Fig. 200. l l is the lens, r l, r l, are rays falling upon its circumfer- ence, and r' l', r' l', are rays falling near its centre. The former set of rays meet in/; whilst the latter pass on to f, before they meet in a focus. This may be shown by covering the cen- tral and outer portions of the lens, alternately, with some opaque substance, which shall stop all the
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