274 ON SIB DAVID BBEWSTEB's SUPPOSED 



direction peapendicular to the retinal surface. " The celebrated 

 D'Alembert," Sir David himself writes, in an article published in the 

 Philosophical Magazine for May, 1844, " maintains that the action of 

 " ligbt upon the retina is conformable to the laws of mechanics ; 

 " and he adds that it is difficult to conceive bow an object could be 

 " seen in any other direction than that of a line perpendicular 

 " to the curvature of the retina at the point of excitement.' — 

 The opinion here expressed was abandoned by D'Alembert in 

 consequence of conclusions to which he was led from the errone- 

 ous data with which he was furnished as to the structure of 

 the eye ; but, as the consideration which seemed to him to give 

 an a priori likelihood to a law of visible direction identical with that 

 which Sir David Brewster supposes himself to have experimentally 

 established, may perhaps be thought by some to possess a measure 

 of weight, I would observe that neither D'Alembert' s conjecture, nor 

 the inference which he drew from it, is in the least degree warrant- 

 able. On the one hand, it is by no means to be admitted that the 

 action of light upon the living nerve, where the objective and sub- 

 jective meet together, must, as a matter of course, take place accord- 

 ing to the ordinary mechanical laws that prevail within a strictly 

 objective sphere. And, on the other hand, even were that allowed, it 

 would furnish no presumption in favour of the idea that we see objects 

 in a direction pependicular to the surface of the retina at the point 

 of excitement. For who does not perceive that the question as to 

 the direction to which the mind refers the stimulus that produces 

 vision remains entirely undetermined, whatever be the conclusion we 

 adopt as to the direction in which the retina is impressed ? 



Not only has Sir David Brewster failed in proving his law of visible 

 direction, but it may without difficulty be shewn that the mind does 

 not instinctively refer its visual affections to a remote stimulus lying 

 in any determinate direction whatever from the point of the retina 

 excited, so that no definite Law of Visible Direction exists. This 

 view, and also the ground on which it rests, w r ere hinted at in a pre- 

 vious part of the paper ; but it may be proper to bring it out more 

 fully. It is based on the elementary metaphysical distinction between 

 immediate and mediate knowledge • immediate knowledge being 

 realised, when a thing is known in itself ; and mediate, when a thing is 

 known inferentially, through means of something else. Now, w r hen 

 the mind refers an affection ot which it is immediately cognizant, to a 

 remote stimulus, the judgment of the mind assigning a perpendicular 

 direction or position to the stimulus, is mediate. No immediate, 



