354 SIR DAVID BREWSTER ON THE LAW OF VISIBLE POSITION 



rection for monocular vision." * 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 tmo direc- 

 tions, and the point where these directions intersect each other determines 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 ob- 

 jects to both eyes," we can readily deduce it as a corollary from the law in mono- 

 cular 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 corresponding 

 points, or with the binocular circle of the German physiologists. 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 

 subverted when they act in concert. Hence it is obvious that the single vision of 

 points with two eyes, or with tmo hundred eyes, is the necessary consequence of the 

 convergency of the tmo, or the two hundred, lines of visible direction to the same 

 point in absolute space ; and although we think that objects appear single with 

 both eyes, yet it is only the points to which the optic axes and the lines of visible 

 direction converge that are actually seen single, and the unity of the perception 

 is obtained by the rapid survey which the eye takes of every part of the object. 



The phenomenon of an erect object from an inverted picture on the retina, 

 which has so unnecessarily perplexed metaphysicians and physiologists, is a demon- 

 strable corollary from the law of visible direction for points. The only difficulty 

 which I have ever experienced in studying this subject, has been to discover where 

 any difficulty lay. An able writer, however, in a recent number of Blackmood' s 

 Magazine,^ in discussing the BERKLEY AN theory of vision, has started a difficulty 

 of a very novel kind, and has called upon me personally to solve it. Were this 

 the proper place for such a discussion, I should willingly enter upon it ; but I 

 must content myself with stating, that the doctrine which the very ingenious 

 author calls the ordinary optical doctrine, was never maintained by any optical 

 writer whatever, and that the doctrine which he substitutes in its place is that 

 which all optical writers implicitly adopt, though they have thought it too ele- 

 mentary to require illustration. A visible point which throws out tmo separate 

 particles of light, an upper and an under, will be inverted on the retina, but a 

 smaller visible point, which throws out only one particle of light, cannot be in- 

 verted, because inversion implies a change in the relative position of tmo visible 

 points. 



* Phil. Trans. 1838, p. 388. t June 1842, vol. li. p. 830. 



