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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 George Wilson, M.D., F.R.S.E., 

 Director of the Industrial Museum of Scotland. 



(Read 2d 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 ravs. 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 camerae ob- 

 scurse 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. 

 VOL. XXI. PART II. 4 T 



