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XXI—On the Extent to which the received Theory of Vision requires us to 
regard the Eye as a Camera Obscura. By Grorce Witson, M.D., F.R.S.E., 
Director of the Industrial Museum of Scotland. 
(Read 24 April 1855.) 
In the course of those researches on Colour-Blindness, which, at intervals, I 
have recently been engaged in prosecuting, I have encountered some phenomena 
connected with normal vision, which I am desirous to submit to the considera- 
tion of the Society. Those phenomena I have already in part detailed, in the 
account of the researches referred to,* and I shall not, accordingly, repeat the 
description of them here, to a greater extent than is essential to rendering in- 
telligible the question which I wish to submit for discussion. 
I venture to assume, that without adducing a lengthened series of authorities, 
I may take for granted, that, on the received theory of vision, the eye of man, as 
well as that of most of the lower animals, is regarded as essentially realizing, 
during the performance of its function of sight, the condition of a darkened 
chamber, or camera obscura. In more precise words: the theory in question 
teaches, that those rays of light, which reach the eye from the objects which 
they render visible, and entering at its front traverse all its transparent humours 
and membranes, last of all pierce the retina, and after making that impression 
upon it which is supposed to be the most important physical element of vision, 
are stopped, or absorbed by the dark pigment lining the choroid coat, and suffer 
extinction as visible rays. The dark surface of the choroid is thus held to 
abolish all the light which reaches it, so that none of the luminous rays return 
through the retina, or retrace their course across the chamber of the eye. 
The doctrine thus taught appears, in the present state of our knowledge, to be 
in great part beyond dispute. It may suffice on this point to notice :— 
(1.) That no other use has been assigned, or readily suggests itself, for the 
existence of a dark lining to the deepest membrane of the eye on which light 
falls, than the one referred to. 
(2.) That, as theory indicates, and the experience of our artificial camerze ob- 
scuree teaches, the darker all their internal walls are, the more marked and 
sharply defined is the picture which light produces upon the screen at the back 
of the camera. 
(3.) That, apart from the sharpness of definition secured by the contrast be- 
tween the darkness of the ground, and the brightness of the picture in all ca- 
* Researches on Colour-Blindness. Sutherland and Knox, Edinburgh, 1855. 
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