On Mr. Horn's Theory of Vision. 203 



inconsistencies to Mr. A. Horn for which he is net accountable. 

 Mr. Horn does not maintain that the extremities of the optic 

 nerve are insensible to the impressions of light, while its trunk 

 is endued with a power of transmitting such impressions to the 

 brain. After examination, having satisfied himself that " the 

 retina is nothing else than an expansion of the scepta, or mem- 

 braneous substance that pervades the optic nerve," he divests it 

 of its long usurped sensihility, and limits the extremities of the 

 nerve to the circular portion that perforates the cavity of the 

 eye. The retina he considers a concave mirror, to which the 

 choroides serves as a coating. The inverted pictures of exter- 

 nal objects painted on the retina are by its powers as a specu- 

 lum reflected to the anterior concavity of the eye, where (althougii 

 there be nothing much resembling a mirror) a second reflection 

 transfers them to nearly " the middle of the vitreous humour, in 

 their natural order and position. These images make due im- 

 pressions vpon the opposite iase of the nerve, which are trans- 

 mitted by it to the brain: thus the sensation is produced and 

 vision perfected." 



This mode of vision Mr. Horn regards as a real and incontro- 

 vertible discovery. Its originality will scarcely be questioned : 

 but as the existence of his images in the vitreous humour de- 

 pends on two reflections in the cavity of the eye, both destitute 

 alike of proof and probability, others may be inchned to regard 

 it as nothing more than a simple conjecture. Even considered 

 as a conjecture, a single objection* seems sufficient for its destruc- 

 tion. Mr. Horn says that his images in the vitreous humour 

 make due impressions on the opposite base of the " (optic) 

 nerve," but how these impressions are made he has left entirely 

 unex|)lained. 



This omission it may be feared the saffacity of few of his 

 readers will be able to supply. That the circular base of the 

 optic nerve has the facultv of perceiving images situated at a 

 distance, near the middle of the vitreous humour, or that it can 

 be duly impressed by them in such a situation, is a proposition 

 little likely to procure assent without proof. Yet unless some 

 feuch faculty be shown to exist, it will be difficult to explain hoiv 

 these '* due impressions" are efifected ; since the pencils of rays 

 constituting these hypothetical images in the vitreous humour, 

 would in their further progress diverge and diffuse themselves 



* To both Mr. Pater and J. Q. II. :i similar ohjertion omirrcrl. Neither 

 of these gtiitleintu, however, had seen " The Sent of V ision," ike. and 

 neither happened to divine exactly IMr. IJorn's nio'Je of raisintj; iviuges in 

 the vitreous hiirnoiir. lii consequence of tliis, llie objection is by both 

 ijrj;cd in a way that Mr, Horn will probably contend does not affect his 

 discovery, 



over 



