182 Experimental Philosophy. [Lecture 13. 



position, while the image on the retina is inverted. 

 In truth, we know nothing of the connexion 

 which exists between the thinking faculty and 

 the organs of sensation. It may, however, suf- 

 fice to answer the present question, if we say 

 that the mind certainly does not look upon the 

 image which is painted on the optic nerve. That 

 nerve is sensible of the impression, from the rays 

 of light being reflected upon it, as the organs of 

 touch feel the impression of any external object, 

 by coming in contact with it. Nor is there any 

 reason why the mind should not perceive as ac- 

 curately the position of bodies, if the rays reflected 

 from the upper parts of those bodies are made to 

 touch the lower parts of the eye, as if they had 

 been directed to the upper parts. Suffice it, that 

 such a correspondence is established between the 

 parts of the eye to which the rays are converged, 

 and the different parts of the object, that we do 

 not find that persons blind from infancy, who 

 have been restored to sight by the operation of 

 couching, have been led into the smallest mistake 

 as to this point*. 



To very perfect sight the three humours of 

 the eye appear necessary. Yet by a very bold 

 experiment (for such it undoubtedly was at first), 

 it is found that we can see tolerably well, even 

 though one of them should be taken away, par- 



* For an elaborate disquisition on this subject, the 

 reader may consult the Rev. A. Horn's Essay on Vision. 



