LAW OE VISIBLE DIRECTION. 271 



the same subjective affections, Sir David Brewster has committed 

 the glaring impropriety of assuming that an object in the axis of 

 vision is seen in the direction of the axis. For how does he aigue ? 

 " Each ray gives vision in the same direction, the visible point retain- 

 " ing its position." Let this sentence pass, dubious as it is ; and 

 what next ? " It follows, that, on the part of the retina in the axis 

 " of vision, all rays, however obliquely incident, give the same 

 " visible direction, perpendicular to the surface of the membrane" 

 Indeed ! How does this follow ? Grant that the rays in question all 

 give the same visible direction (though the only thing proved, is, that 

 they give rise to the same subjective affection) ; how does the idea of 

 a direction 'perpendicular to the surface of the membrane creep in ? 

 The cone of light through which vision is produced, contains a line 

 of rays, no doubt, which fall perpendicularly upon the eye, and pass to 

 the retina without refraction ; and it may be fancied that these at least 

 " give visible direction" in the axis of vision. But how can such a 

 thing be proved ? How does it appear, that, when rays come to the 

 eye along the axis of vision, the mind determinately refers the sub- 

 jective affections occasioned by such rays to a remote stimulus, 

 situated somewhere in the axis ? Let E represent the eye, and an 

 object towards which the axis of the eye is turned. It may perhaps 

 be said, that, if you ask the observer, he will tell you. that he refers, 

 and cannot help referring, his sensation to a stimulus in the line E O. 

 But he means nothing more by this, than that he is unable, while his 

 eye is turned towards 0, to alter the character of the perception 

 realized. That nothing more than this ean be intended, and that there 

 is not, in truth, any intuitive or instinctive reference to the direction 

 E 0, is rendered eertain by a consideration which shall afterwards be 

 more fully brought out, viz : that the object O is not an object of 

 intuitive knowledge at all. Distant objects can only be known 

 mediately or infer entially. And if the object O be not immediately 

 known even as existing, it follows, a fortiori, that the direction E is 

 not immediately known ; so that an instinctive, intuitive or immedi- 

 ate reference of a visual impression to the direction E 0, is an 

 absurdity. 



This may suffice, as regards direct demonstration. JEx uno disce 

 omnes. No direct demonstration can possibly indicate any thing else 

 than the similarity or dis-sim ilarity (as the case may be) of the sub- 

 jective affections produced by rays impinging upon particular parts of 

 the retina. Let us proceed to consider next whether Sir David 

 receives any more effectual support for his doctrine from the indirect 



