co 
ee 
ba f 
EYE AS A CAMERA OBSCURA. 
Postscript. 
: From my friend, Professor Goopsir, who recently (June 27th) delivered a 
lecture of great interest and originality, on the Retina, to the Anatomical Students 
of the University of Edinburgh, I learn that, as he stated to his hearers, his micro- 
scopical observations on the structure and development of the eye had led him 
to the conclusion, that only the rays of light which are returned from behind 
through the retina produce a luminous sensation, and that the objective percep- 
tion of light commences physically towards the choroidal, not the hyaloid extremity 
of the optically sensific constituents of the retina. 
According to Brucke (as mentioned in the text, Note, p. 334), the bacillar layer 
acts as a mirror, reflecting light forwards, and luminous sensation begins in a 
layer of grey nervous substance, situated nearer the front of the retina,—an opinion 
combated by KoLuiKer. 
According to Mr Goonsir, the objective perception of light begins somewhere 
near the junction of the rod or cone with the Miillerian fibre (see Note, p. 332). 
On this view, the entire arrangement of rod or cone, with its Miillerian filament, 
is not a nerve-structure, as Kouuiker holds, but a peculiar organ referable to 
the same category as the tactile corpuscles and Pacinian bodies, and so con- 
structed as to oppose the extremity of the nerve, which is contained in it, to the 
ray of light passing backwards from the choroid along the axis of the rod or cone, 
so that the ray shall impinge upon its extremity in the line of its axis, this being, 
_ according to Mr Goonstr’s hypothesis, the only direction in which a Juminous ray 
ean optically affect a nervous filament. 
; I have argued, in the preceding paper, for such returned light being accessory 
to vision, but according to this view it is the only light by which it is exercised. 
If this doctrine (however modified in details) be established, the reflection of light 
from the choroid will prove to be essential to the functions of seeing, and the necessity 
_ for the living eye being a Camera Lucida will be based upon deeper grounds of proof 
than I have attempted to offer. 
August 10, 1855. 
VOL. XXI PART III. SA 
