LAW OF TISIBLE DIEECTIOTT, 273 



at least is certain, that it is subjective in such, a sense that it has no 

 existence in absolute space, apart from the mind. 



This explanation being made, we are now able to estimate aright 

 Sir David Brewster's reasoning. Suppose rays from an object X T 

 to fall upon the retinal surface y x ; the rays from X being brought 

 to a focus at x : and those from y being brought to a focus at y. — 

 Sir David argues, that, according to his law, an impulse on the retina 

 at x gives vision in a direction perpendicular to the retina at x ; 

 and that an impulse on the retina at y gives vision in a direction 

 perpendicular to the retina at y ; and that, therefore, the phenome- 

 non of an erect object is produced, though the picture on the retina 

 is an inverted one. But " the phenomenon of an erect object," it 

 must be kept in view, is not any thing having existence in space apart 

 from the mind, and standing in an erect posture. It is a subjective 

 (I do not say, purely subjective) representation. Now I presume 

 that Sir David Brewster does not wish us to believe that this subjec- 

 tive representation itself is a corollary from the law of visible direc- 

 tion. He cannot mean more than that the mind's instinctive and 

 determinate reference of the affections of which it is conscious to an 

 erect exterior stimulus, is a corollary from the law of visible direc- 

 tion. And undoubtedly this reference is a demonstrable corollary 

 from the law. But is it not plain, that, to assume that there is such 

 a reference, instinctive and determinate, involved in, or connected 

 with, the phenomenon of an erect object, is to assume the very thing 

 about which there is any controversy ? For what is it which those 

 demand, who ask proof of the law r of visible direction ? They de- 

 mand proof of the assertion, that the mind instinctively refers its vis- 

 ual affections to a remote stimulus lying in any determinate direction 

 whatever from the point of the retina excited. 



Should the above criticisms be well founded, they are applicable 

 to the whole of Sir David Brewster's reasoning; so that it is unne- 

 cessary to examine the details of other experiments to which he 

 appeals. Our conclusion, therefore, is, that both his direct and his 

 indirect proofs are entirely destitute of weight. The solejact which 

 he has established, is, that the subjective affections to which rays imping- 

 ing on the retina give rise, are the same, ivhatever be the obliquity at 

 which the rays strike the retina. 



It is a curious circumstance that Sir David Brewster was antici- 

 pated in his Law of Visible Direction by a conjecture of D' Alembert, 

 founded upon the idea that the stimulus proximately affecting the 

 retina, acts, conformably to ordinary mechanical principles, in a 



