EYE AS A CAMERA OBSCURA. 347 



Postscript. 



From my friend, Professor Goodsir, who recently (Jnne 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 Kolliker. 



According to Mr Goodsir, the objective perception of light begins somewhere 

 near the junction of the rod or cone with the Mullerian fibre (see Note, p. 332). 

 On this view, the entire arrangement of rod or cone, with its Mullerian filament, 

 is not a nerve-structure, as Kolliker 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 Goodsir's hypothesis, the only direction in which a luminous ray 

 can 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. 5 A 



