LA.W OF VISIBLE DIEECTION. 269 



position in binocular vision is professedly derived from that of visible 

 direction in monocular vision, it follows that if the latter be destitute 

 of evidence, the former must be given up likewise. 



Sir David Brewster has no where formally explained what he means 

 by visible direction ; at least he has not done this in those papers 

 in the Philosophical Magazine, which are expressly devoted to the 

 proof and illustration of his Law ; in consequence of which, the real 

 import of the Law is involved in considerable doubt. But probably 

 Sir David would accept the following as a true statement of what he 

 holds, viz.: that the mind, being mysteriously united with the retina 

 as part of the living organism of the body, is immediately cognizant 

 of the affections excited in the retina ; and that it refers the affections 

 of which it is thus cognizant to a stimulus situated in the direction 

 of a normal to the retinal surface. A writer in the Athenaeum for 

 February 7th, of the present year, thus states what he supposes to be 

 Sir David's theory : " The mind, residing as it were in every point of 

 " the retina, refers the impression made upon it to a direction coin- 

 " ciding with the last porlion of the ray that conveys the impression.' 

 This is undoubtedly a mistake. Instead of : refers the impression to* 

 a direction coinciding with the last portion of the ray that conveys the 

 the impression, the statement should lave at least been : refers the 

 impression to a direction perpendicular to the retina at the point where 

 the refracted ray falls upon its surface. With this alteration, the 

 sentence quoted would substantially agree with what I have expressed. 

 Now it is important to observe at the outset, that, even if it be true 

 that the mind " residing as it were in every point of the retina," or, 

 to use a less objectionable mode of expression, mysteriously united 

 with the retina as part of the living organism of the body, is imme- 

 diately cognizant of the retinal affections, this is a metaphysical truth, 

 which does not admit of being experimentally demonstrated. It 

 must be established by its proper evidence : and this is of itself 

 enough to shew that Sir David Brewster, in fancying that he has 

 experimentally proved his law of visible direction, must be labouring 

 under some delusion. From the nature of the case, physical experi- 

 ments are inadequate to establish a law whose necessary basis is a 

 metaphysical principle. 



Passing this, however, let us proceed to examine Sir David Brew- 

 ster's experiments The following is perhaps the most beautiful and 

 plausible of the direct experiments on which he relies in support of 

 his Law: "Having expanded the pupil by belladonna, look directly 

 " at a point in the axis of the eye. Its image will be formed by a 



