in Single and Binocular Vision. 357 



larly fixed upon the interesting paper of my distinguished 

 friend Professor Wheatstone, On some remarkable and hi- 

 therto unobserved Phaenomena 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 disco- 

 very which it describes, namely, the perception of an object of 

 three dimensions by the union of the two dissimilar pictures 

 formed on the retinae : — but, in seeking an explanation of this 

 curious phenomenon, and in applying it to explain phaeno- 

 mena previously known, Mr. Wheatstone has adduced expe- 

 rimental results, and drawn conclusions which stand in direct 

 opposition to what was best established in our previous know- 

 ledge. Before entering, however, upon this branch of the 

 subject, I must first explain the law of visible direction, and 

 the phaenomena 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. 



The celebrated D'Alembert, in his Doutes sur differents 

 questions d'Optique, maintains that the action of light upon the 

 retina is conformable to the laws of mechanics; and he adds, 

 that it is difficult to conceive how an object could be seen in 

 any other direction than that of a line perpendicular to the 

 curvature of the retina at the point where it is really excited. 

 He then investigates, mathematically, how the apparent mag- 

 nitudes of objects would be affected, on the two suppositions 

 that the line of visible direction coincides either with the re- 

 fracted ray, or with a line perpendicular to the retina at the 

 point of excitement. On the Jirst of these suppositions, he 

 finds that the apparent magnitude of small objects would be 

 increased about T yth, and on the second supposition, a little 

 more than ^, or y^ 8 /^. This last result is, as D'Alembert 

 justly remarks, so contrary to experience, that we cannot sup- 

 pose vision to be thus performed, however natural the suppo- 

 sition may appear. " In the direction of what line, then," he 

 adds, " do we perceive objects, or visible points, which are not 

 placed in the optical axis? This is a point which it appears 

 very difficult to determine exactly and rigorously. As expe- 

 rience, however, proves that objects of small extent, which are 



* Philosophical Transactions, 1838, p. 371. [Noticed in Phil. Mag. 

 S. 3. vol. xiii. p. 461.] 



