274 Answer to Oljections against 



not be accounted for on any legitimate principles but by ad- 

 mitting the truth of niv therry. 



Now, sir, lest the imperfect view L. S. has given of the prin- 

 ciples of my theory, and tlie confidence v\'ith which his " single 

 objection" is urged, should prejudice the readers of your valuable 

 Magazine, I mean to show that this redoubtable objection is of 

 no avail ; and that the claims of the retiva to the chief function 

 of vision are not only very doubtful, but that those of the optic 

 nerve are more consistent with the laws of optics, and better 

 supported by phnsiwrnena. 



J. Q. R. does not seem to be aware of the difficulties that 

 liave presented themselves to men eminent for science, in their 

 investigations into the seat of vision, or he scarcely would have 

 spoken so confidently respecting the stniclure and use of the 

 retina. I have before noticed (Philosophical Magazine, vol. xlviii. 

 p. 118) the celebrated controversy between Marriotte and Pec- 

 quet res})ecting the retina and choroid membrane. De la Hire, 

 though he took part with Pecquet, arguirg in favour of the re- 

 tina, supposes that our defect of vision, where the optic nerve 

 enters, is produced by the want of the choroides : accordingly, 

 he conceives that the retina does not receive the impressions im- 

 mediately from the rays, but that they penetrate it, are stopped by 

 the opacity of the choroides, by which this membrane is agitated, 

 and these agitations being communicated to the retina, are 

 transmitted to the nerve ; and thus vision is effected. 



When Dr. Priestley was composing his History of Vision, Mr. 

 Michell suggested a number of additional arguments in favour 

 of the choroides as the immediate organ of vision, which had 

 escaped its former advocates. These, though very ingenious, I 

 shall not enumerate. " I must own," says Dr. Priestley, " that 

 after having retained my prejudice in favour of the retina, not- 

 v/ithstanding all that was advanced by Marriotte and the ad- 

 vocates for his opinion among his countrymen, the arguments of 

 Mr. Michell, in favour of the choroides, have more weight with 

 ine than I was at first either able to perceive, or willing to ac- 

 knowledge, in those of the French philosophers." However, the 

 learned author, towards the conclusion of his Historv, in an 

 additional section relating to the seat of vision, gives up his 

 opinion of the choroides as the projier organ, and inclines to the 

 hypothesis of De la Hire. 



But these are not the only remarkable features in the physio- 

 logy of the eye. Even those who contend for the retina, as an 

 expansion of the optic nerve and the principal organ of vision, 

 express themselves either doubtfully, or are at open variance with 

 each other, respecting the structure and requisite sensibility of 

 this membrane. 



Haller, 



