340 DR GEORGE WILSON ON THE 



Idly. A good deal of light must undergo irregular reflection and dispersion 

 from the retina and choroid. But so much of this light as passes towards the 

 front of the eye will be arrested in greater part by the abundant pigment of the 

 ciliary processes, and anterior lateral portions of the choroid ; and what scatters 

 laterally will only produce a general excitation of the retina without developing a 

 second complete image of any visible object on the nervous membrane, which is 

 the chief optical evil to be apprehended from intra-ocular reflection. So long 

 as the direct seat of vision is not exposed to strong reflex illumination, and the 

 same entire pencil of rays does not twice depict the image of the same object on 

 different parts of the retina, and thus produce double vision of single things, the 

 general reflection and dispersion of light within the eye cannot do more than di- 

 minish the darkness of the eye-chamber as a whole. In the eye of the casual 

 albino, light is liable to be returned from the back of the greatly contracted iris to 

 the place of most perfect vision, and to disturb it, so that, as far as internal 

 reflection is concerned, he would be better without an iris at all. Moreover, his 

 iris is transparent, and is continually transmitting light from without as well as 

 reflecting it from within. 



But even in the human albino, although unquestionably there must be painful 

 dazzling of the eye from the continual action of light on every point of the retina, 

 still, unless during sudden transitions from very faint to very bright illumination, 

 involving great and rapid change in the area of the pupil, the yellow spot or place 

 of perfect vision being opposite that aperture, will be less exposed than any por- 

 tion of the retina to the impact of light which has undergone intra-ocular reflec- 

 tion ; and as the utmost contraction of the iris does not close the pupil, a pencil 

 of rays reflected from the yellow spot, can never return in toto to repeat upon 

 another portion of the retina the image which it has already produced upon that 

 spot. 



It thus appears that the laws of luminous reflection do not necessitate imper- 

 fect vision, as applied to the fact, that the retina and choroid return much of the 

 light which reaches them, for : — 



1st, In the normal and also in the albino vision of all animals, man included, the 

 amount of direct retinal and choroidal reflection is necessarily coincident with the 

 width or degree of dilatation of the pupil ; the larger the pencil of light entering 

 the pupil, the larger the pencil leaving it, so that in every case the reflected rays 

 are thrown out of the eye and do not disturb vision : further : — 



2nd, In those animals provided with tapeta lucida, such as the cat, the 

 dog, or the ox, which are only partially nocturnal in their habits, the tapetum is 

 so placed that in bright light it is not opposite to the contracted pupil, or is so only 

 to a small degree.* When, however, the choroidal mirror is called into action in 



* John Hunter, in Catal. of Museum of Royal College of Surgeons, London. Vol. iii., p. 165. 



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