562 Sir David Brewster on the Law of Visible Position 



common principle in optics, that objects are seen in the direc- 

 tion of the ray 'which they send to the eye*." 



It is almost impossible to believe that D'Alembert is serious 

 in maintaining these doctrines. The major proposition of his 

 syllogism is absolutely incorrect. It is not true that we see 

 the star E nearer than it is. The eye does not see distances 

 directly : the mind only estimates them, and, according to its 

 means of judging, it forms a right or a wrong opinion. The 

 second proposition is equally incorrect. We do not see the 

 star along the lines As, Be. We see it along the lines A E, 

 B E, at the very place where it is, and whether we consider it 

 nearer or more remote than it is, — whether we think that it 

 touches our eye, or exists at the remotest verge of space, — 

 the position of the optical axis of each eye remains as before, 

 and our vision of the star is not affected by the truth or false- 

 hood of our judgement. 



2. On the Law of Visible Direction in Binocular Vision. 



In admitting the correctness of the law of visible direction 

 in monocular vision, which I have endeavoured to establish in 

 the preceding section, Professor Wheatstone justly remarks, 

 "that the result of any attempt to explain the single appear- 

 ance of objects to both eyes, or, in other words, the laiv of vi- 

 sible direction for binocular vision, ought to contain nothing 

 inconsistent with the law of visible direction for monocular 

 vision f-" Properly speaking, however, there is no such thing 

 as a law of visible direction in binocular vision, because there 

 is no such thing as a centre of visible direction, or a line of 

 visible direction in binocular vision. When we see an object 

 distinctly with both eyes, it is actually seen in two directions, 

 and the point where these directions intersect each other de- 

 termines the visible place of the object. But if we follow Mr. 

 Wheatstone in considering such a law as equivalent to the 

 law which regulates "the single appearance of objects to both 

 eyes," we can readily deduce it as a corollary from the law 

 in monocular vision. A visible point is seen single with two 

 eyes only when it is at the intersection of its lines of visible 

 direction as given by each eye separately. It is obvious that 

 this law does not harmonize with the doctrine of correspond- 

 ing points, or with the binocular circle of the German phy- 

 siologists. It is, however, rigorously true ; for no philosopher 

 can adopt the monstrous opinion that the functions and laws 

 of vision which belong to each eye, acting separately, are sub- 

 verted when they act in concert. Hence it is obvious that 



* Ojiusculcs Mathematiqucs, torn. i. mem. ix. § iv. p. 273-4. 

 f Philosophical Transactions, 1838, p. 388. 



