On Mr. Horn's Theory of Vision. 141 



qulries : for, caustic as his applications are, I find them much 

 more likely to inflame than to cure my mind of its tendency to 

 consider his theory as absurd. 

 I am, sir, 



Your inost obedient servant, 

 SkefFhaven, Feb. 10, 1817. W. PatER. 



XXXIV. On Mr. Horn's Theory of Vision, 



By A CoRRESPONDEM'. 



To Mr. Tilloch. 



Sir, — i HE perusal of a paper in your last, from Mr. Horn, 

 upon Vision, induced me by its novelty to refer to his former 

 communication, but without receiving any kind of satisfaction. 

 His theory appeared to me, prima facie, absurd, which 1 think 

 a very little reflection will prove. He asserts that " light acts 

 upon'the optic nerve and excites sensation there, and not upon 

 the retina ; that the chief function of vision is assignable to the 

 base of the nerve, and the optic images are formed by caustic re- 

 flection in the vitreous humour." I really do not understand 

 liis first assertion, that " light acts upon the nerve and not upon 

 the retina," &c. if the retina b, as I have always understood it 

 to he, a delicate membrane formed solely by the expansion of 

 the optic nerve. 



How is it possible that light, which is its proper stimulant, 

 should excite sensation in the trunk, and not in the extremities 

 of the nerve ? Reasoning from analogy, we should infer the con- 

 trary to be the case. The other nerves of the body receive sen- 

 sation only at their extremities. The de|)th in which their trunks 

 lie Imried,' and an every day's occurrence after the loss of a limb, 

 prove that, if the trunk of the nerve, which before the amputa- 

 tion of the limb had ramified upon the toes, be injured, the 

 sensorium refers the pain to its former situation, and the patient 

 does not feel pain at the injured spot, but in that spot where the 

 sensation of the nerve had been usually excited; namely, in his 

 toes. Besides, if the retina does not receive sensation, but is a 

 mere reflecting surface, where is the use of its being composed 

 solely of nerve ? A membrane less liable to injury would have 

 answered the purpose much better. The other proposition is, 

 That " the optic images are formed by caustic reflection in the 

 vitreous humour." I deny that the retina does reflect, or very 

 slightly at most ; the light of a candle is easily distinguished 

 through it, which I imagine would not be the case if it was re- 

 flected. Allowing however, for the sake of argument, that it 



tloes 



