XXIV. Ow /i<2 Zw <2/" Visible Position in Single and Binocular Vision, and on 

 the representation of Solid Figures by the union of dissimilar Plane Pictures 

 on the Retina. By SIR DAVID BREWSTER, K. H. D. C. L., F. R. S., and 

 V. P. R. S. Edin. 



(Read 23d Jan. and 6th Feb. 1843.) 



IN the course of an examination of Bishop BERKELEY'S " New Theory of Vision," 

 the foundation of the Ideal Philosophy, I have found it necessary to repeat many 

 old experiments, and to make many new ones, in reference to the functions of the 

 eye as an optical instrument. I had imagined that many points in the physio- 

 logy of vision were irrevocably fixed, and placed beyond the reach of controversy ; 

 but though this supposition may still be true in the estimation of that very limited 

 class of philosophers who have really studied the subject, yet it is mortifying to 

 find that the laws of vision, as established by experiment and observation, are as 

 little understood as they were in the days of LOCKE and BERKELEY. Metaphysi- 

 cians and physiologists have combined their efforts in substituting unfounded spe- 

 culation for physical truth ; and even substantial discoveries have been prema- 

 turely placed in opposition to opinions of which they are the necessary result. 



In prosecuting this subject, my attention has been particularly fixed upon 

 the interesting paper of my distinguished friend Professor WHEATSTONE, " On 

 some remarkable and hitherto unobserved phenomena of binocular vision." * It is 

 impossible to over-estimate the importance of this paper, or to admire too highly 

 the value and beauty of the leading discovery which it describes, namely, the 

 perception of an object of three dimensions by the union of the two dissimilar 

 pictures formed on the retinse : but, in seeking an explanation of this curious 

 phenomenon, and in applying it to explain phenomena previously known, Mr 

 WHEATSTONE has adduced experimental results, and drawn conclusions which 

 stand in direct opposition to what was best established in our previous knowledge. 

 Before entering, however, upon this branch of the subject, I must first explain the 

 law of visible direction, and the phenomena of ocular parallaxes. 



1. On the Law of Visible Direction in Monocular Vision. 



Several philosophers had hazarded the opinion, that every external visible 

 point is seen in the direction of a line passing from its picture on the retina 

 through the centre of the eye considered as a sphere ; while others maintained 

 that every such point was seen in the direction of the refracted ray by which its 

 image was formed. 



* Phil. Trans. 1838, p. 371. 

 VOL. XV. PAR? III. 5 B 



