268 ON SIB DAYID BBEWSTEB's SUPPOSED 



In Nothoceras, the bent edges of the Septa (the goulot of the French 

 palaeontologists) protecting the Siphuncle, instead of being deflected 

 backwards as in Nautilus, Cyrtoceras, &c, are deflected forwards, or 

 towards the opening of the shell, as in the Ammonites. 



ON SIE DAVID BEEWSTEE'S SUPPOSED LAW OF 

 VISIBLE DIRECTION. 



BY THE BEV. GEOBGE PAXTON YOUNG, M. A., 



PEOEESSOB OB LOGIC AND METAPHYSICS, KNOX'S COLLEGE, TORONTO. 



Read before the Canadian Institute, March 7th, 1857. 



Sir David Brewster claims to have proved experimentally, that, in 

 monocular vision, whatever be the direction in which a ray strikes 

 the retina, it gives the sensation of vision in a direction perpendicular 

 to the retina at the point of excitement. This is his Law of Visible 

 Direction. A careful examination of the eye has shewn that the 

 retina and the cornea have a common centre (which may, therefore, 

 be conveniently termed the centre of the eye), and that a normal 

 to the retina at the point where the picture of a small visible object 

 is formed, almost exactly coincides (at least in pencils of moderate 

 inclination to the axis of the eye) with the line joiniDg the centre of 

 the eye and the object ; so that according to the Lawof Visible Direc- 

 tion, a small object is seen in the direction (nearly) of a line drawn from 

 it to the centre of the eye. From this law of visible direction in 

 monocular vision, has been derived a corresponding Law of Visible 

 Position in binocular vision ; which is, that a small object seen with 

 both eyes, appears at the point where the lines of visible direction for 

 the two eyes meet ; the meeting of these lines being a condition 

 indispensable in order that the object may be seen single. 



These laws, while admitted by some philosophers of high authority, 

 have been ealled in question by others, though I have never seen 

 any thing like a satisfactory refutation of the arguments advanced by 

 Sir David Brewster in support of his theory. I agree with those 

 who deny that Sir David's reasoning is valid ; and I propose in the 

 present communication to shew that the experiments on which he 

 relies are quite inconclusive ; in doing which, it will be sufficient to 

 discuss the case of monocular vision ; for, since the law of visible 



