148 TEAK-BOOK OF FACTS. 



BINOCULAR VISION. 



Professor W. B. Rogers has described to the American Asso- 

 ciation some Experiments and Inferences in regard to BinocularVision. 

 In the theory of binocular vision expounded by Sir David Brewster, 

 and maintained by Briicke, Prevost, and others, it is contended that 

 no part of an object is seen single and distinctly but that to which 

 the optic axes are for the moment directed, and that " the unity of 

 the perception is obtained by the rapid survey which the eye takes 

 of every part of the object." So that, according to this, our percep- 

 tion of an object in its solidity and relief is acquired not by a simple 

 but by a cumulative process, in which the optic axes are converged 

 successively upon every point of the object within view. Like con- 

 ditions must obviously apply to the perception of the binocular 

 resultant formed by the reverse of the twin pictures of a stereoscope. 

 On this theory the conditions of binocular vision of a perspective line 

 would be as follows : — 1. The perception of the perspective Jine in 

 the stereoscope would recpuire the optic axes to be successively directed 

 in such manner as to unite every pair of corresponding points of the 

 two composite lines of the diagram, — or, which amounts to the same, 

 they should be successively converged upon every point of the perspec- 

 tive resukant. 2. In cases of two intersecting lines appearing, instead 

 of this single resultant, those lines should neither of them have a per- 

 spective position. 



In an experimental discussion of the subject some years ago, Pro- 

 fessor Rogers showed that the phenomena of the stereoscopic resultant 

 do not necessarily conform to these conditions ; and that the percep- 

 tion of a perspective resultant line, or of a physical line, in the same 

 attitude, does not require the successive convergence of the axes to 

 every point. The truth of this position is proved by the fact that 

 the resultant obtained by combining two inclined lines with or without 

 a stereoscope, presents a perspective attitude, even when the com- 

 ponent lines, instead of being united into one, are brought together 

 to intersect at a small angle, each of the intersecting lines in this case 

 appearing in relief. Professor Rogers described several experiments, 

 in part oew and in part modified repetitions of those already described 

 by Professors Wheatstone and Dove, which offer decisive proof that 

 such a successive combination of pictures, point by point, however it 

 may enter into the complex process of vision, cannot be considered 

 as an essential condition to the singleness and penpeotiveness of the 

 binocular perception. 



One o ■' by holding » 1 tUI inn t Una io :i ;■' ipeotive 



position ni u convenient distanoe midway between the Byes, sad regarding it for 

 afewsei to produce a lasting impression on the retina Qp turning 



the eyes towards a blank n n, the subjective impression wifl 



projec t and having the same perspei Uve attitude as the original line. 



If then I, the line will appear to sub b of the 



screen, taking an Inolined position corresponding to the optical prc> i otion of the 



i by the and I eve, and therefore corresponding to the 



m of the image formed in that eye. By opening snd dosing the eyes alter- 

 Ireoting both to ths toreen. we are able to see the two oblique 

 ending to these projections, and their binocular resultant corre- 

 sponding to the angina] object, i'or the success of this uud the other fineri. 



