in connexion with Vision. 247 



made upon the optic nerve ; and the same will be the case, it 

 is natural to conclude, if we seek to discover its exact place in 

 the external world. When we desire to find out the ' direction' 

 of a body, our true purpose is to ascertain what is its position 

 in relation to other bodies or to some fixed standard. It is 

 sufficiently obvious that the object itself, if it were altogether 

 detached from surrounding bodies, could never present to the 

 mind this idea. To entertain the conception implied by the 

 term ' direction,' it is required that other objects be presented 

 to the sight besides that which is under our particular view ; 

 and to institute the proper comparison, or to form the neces- 

 sary calculation, it is as indispensable that the sensations of 

 these neighbouring objects should be communicated to the 

 mind as that the sensation of the object itself should be so 

 presented. However varied the direction of the rays proceed- 

 ing from a single object may be when compared with one 

 another, no reasoning upon them can ever acquaint us with 

 the relative position of that body to another, whose rays are 

 not given. Dr. Brewster has forgotten to supply any second 

 body, or to represent its rays, although to establish a ' line of 

 visible direction' the existence of some standard of comparison 

 is necessarily implied. 



When an object is presented before the eye, the surface of 

 the retina is not occupied with it alone, but the whole sur- 

 rounding scene, everything which is near it and can be included 

 within the field of vision, is represented upon the nerve at the 

 same time. Now, these are so many impressions by which the 

 mind can judge of the relative position of objects ; and so long 

 as these find admission into the eye, data shall not be wanting 

 for instituting the necessary comparison. If a diagram be 

 drawn, representing the surrounding objects and their images 

 falling upon the retina, it will be easy to understand how the 

 mind acquires the knowledge of the direction of each of them. 

 We may make the diagram correspond with the experiment of 

 Dr. Brewster, as it will serve as well as any other to demon- 

 strate what is required. 



The card D E being placed before the eye, may prevent 

 objects situated in that direction from casting their images 

 upon the retina ; but, according to the terms of the experi- 



Voi.. II. Nov. 1831. S 



