374 



attention to the effect which it must have as a coloured medium on 

 the light which reaches it,* and on the actinic rays which traverse it. 

 I wish now to carry these views a step further, in connection with the 

 reflection of light from the choroid through the retina, which was dis- 

 cussed before the Society last session, in a paper " On the Eye as a 

 Camera Obscura,^* and which, before and since, has been largely made 

 the subject of independent inquiry by foreign and British observers. In 

 particular, Professor Goodsir has shown, in a lecture delivered in the 

 University of Edinburgh last June, and since published,! that it 

 is not merely the case that light traverses the retina to the cho- 

 roid, and is then reflected so as to return through the retina, but 

 that it is only the rays thus returned which produce a luminous 

 sensation. The light, therefore, which traverses the yellow spot, 

 and is then reflected forwards on the choroidal extremities of the 

 optically sensific constituents of the retina, must have been deprived, 

 to a greater or less extent, of its actinic rays, before it determines a 

 luminous sensation, unless the portion of the retina under notice 

 differ from all other yellow transparent media known to us, in not 

 arresting the chemical rays. If it be not in this respect excep- 

 tional, then the theory of perfect human vision may be simplified 

 by the exclusion from consideration of the actinic rays ; and one use 

 of the yellow spot, for which it has hitherto baffled physiologists to 

 find a use, may be to extinguish these radiations. I offer this only 

 as a suggestion, the value of which must be determined by testing 

 the chemical power of light after it has traversed the yellow spot, — 

 an experiment which only those few anatomists can try who have 

 the opportunity of examining the human eye soon after death. 



I will only, therefore, remark further, in reference to the absorp- 

 tion of the actinic rays by the yellow spot (with which this paper is 

 chiefly concerned), that the views of those who have described visual 

 impressions on the retina, as phenomena of the same kind as pho- 

 tographic impressions on surfaces charged with salts of silver, or 

 other actinolytes, must fall to the ground if the actinic rays of light 

 are stopped before reaching the optically sensific constituents of the 

 retina. The similar opinion, also, that ** spectral vision," and other 

 abnormal peculiarities of sight, are phenomena of the same kind as 



* Researches on Colour Blindness, p. 83. 



t Edinburgh Medical Journal, October 1855. 



