in Single and Bi?iocular Vision. 363 



the single vision of points with two eyes, or with two hundred 

 eyes, is the necessary consequence of the convergency of the 

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

 point in absolute space ; and although we think that objects ap- 

 pear single with both eyes, yet it is only the points to which the 

 optic axes and the lines of visible direction converge that are ac- 

 tually 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 metaphy- 

 sicians and physiologists, is a demonstrable 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 Blackwood's Magazine*, in discussing the 

 Berkleyan 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 elementary to require illus- 

 tration. A visible point which throws out two separate par- 

 ticles 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 inverted, because inversion implies 

 a change in the relative position of two visible points. 



3. On the Vision of Objects of Three Dimensions. 



(1.) By Monocular Vision. — If we look with one eye at a 

 solid body, for example a six-sided pyramid with its apex di- 

 rected to the eye, and uniformly illuminated, we recognise at 

 a single glance that it is not a drawing of the pyramid. When 

 the eye adjusts itself to distinct vision of its apex, all the more 

 distant parts are seen indistinctly, but the eye quickly surveys 

 the whole, adjusting itself to distinct vision of its base and of 

 its edges, and by these successive efforts, at one time contract- 

 ing the pupil and the eyebrows to see the near parts^ and ex- 

 panding them to see the more remote ones, it obtains a know- 

 ledge of the relative distance of its different parts. The vision 

 of the pyramid thus obtained is nearly perfect. There is no 

 inequality of illumination produced by the act of single vision ; 

 and there is no flickering in the outlines of the figure. The 

 * June 1842, vol. li. p. 830. 



