Mr. Horn's Theory of Vision. 275 



Haller, rejecting the hypothesis that goes to establish the 

 choroides as the proper in.strumei)t of vision, speaks of a certain 

 fibrous membrane//? the retina, distinct from its pulpy substance, 

 and supposes that on these fibres the images of the objects are 

 painted. 



_ " We have reason," says Dr. Reid, " to believe that the rays of 

 light make some impression upon the retina; but we are not 

 conscious of this impression ; nor have aiiatomists or philosophers 

 been able to discover t!ie nature and efi'ects of it ; — whether it 

 produces a vibration in the nerve, or motion of some subtile 

 fluid in the nerve, or something different from either, to which 

 we cannot give a name." 



px. Priestley observes, among desiderata at the end of his 

 History of Vision, that " something may possibly be added more 

 decisive for or against the retina being the place where the 

 pencils of rays terminate, — or, in other words, being the seat of 

 vision,— X.\\m\ has as yet been advanced by the advocates for 

 either of the opinions." 



Besides the suspicious circumstances already mentioned in the 

 character of the retina, there is another po'int v/hich is not a 

 little embarrassing to the advocates for it:^ being the seat of vi- 

 sion. Altiiougii tills membrane is expanded over the whole con- 

 cavity of the eye as far as the ligamentum ciliare, they acknow- 

 ledge that it - is not all equally sensible. The ingenious Dr. 

 Porterfield, in his treatise on tl;e eye, observes, that this is evi- 

 dent; — if, for instance, we are disposed to view the first letter of 

 any long word accurately, the other letters, especially those to- 

 wards the en(l,will not appear clear and distinct. The cause of this, 

 he says, does not so nuich arise from the pencils of rays coming 

 from those letters falling obliquely upon the retina, but chiefly 

 because of a certain degree of hardness, callosity, or insensibility, 

 ill all parts of the retina, excepting towards the axis of the eye, 

 directly oppo;-ite the pupil. 



P>ut this opinion is contradicted in a more recent hypothesis 

 advanced by Mr. Walker in his Archives of Science. This au- 

 thor supposes thiit the images of the external objects are firsC 

 formed at the posterior portion of the retina, and by reflection 

 from thence the impressions are made upon its anterior part. 

 " Consistently," he says, " with the theory just delivered, I should 

 conclude, that we have from this circunrstance a decided proof 

 th^X. th^ posterior part of the retina is utterly insensible ; since 

 at the entrance of the nerve, wh-jrc it exists in the greatest 

 quantity, it can be demonstrated to be so j and that \ ision is 

 wanting at this spot precisely, because where tlie nerve enters 

 there is no choroides to reficct the ray-j to t!)e scrtsil/c anterior 

 portion." 



S2 *Ut 



