316 • Mr. W. Le Conte Stevens on 



Brewster's remark about the phantom wall, that " it generally 

 advances slowly to its new position," is now easily under- 

 stood. At that time the nature of focal adjustment by action 

 of the ciliary muscle in each eye was not known. When E 

 and E' (fig. 2) are binocularly viewed, since E exceeds 

 E'O', there must he dissociation between the two focal adjust- 

 ments which are generally adapted to the same distance. To 

 this must be added the necessary dissociation between axial 

 and focal adjustments, the former being for an infinite distance, 

 the latter for the distance C, when C and C are binocularly 

 viewed. To untrained eves this unusual muscular action is 

 not always easy, and cannot be secured without effort that 

 may consume several seconds of time. The result is momen- 

 tary confusion ; and many persons are hence unable to decide 

 at first whether the surface appears convex or concave. De- 

 spite these difficulties, the image soon becomes clearly defined 

 when the experiment is performed with axial parallelism. 

 With strong axial convergence, as in Brewster's experiment, 

 the dissociation is far more difficuli 

 muscular tension made necessary. 



6. The Theory of Associated Muscular Action. 

 The theory of visual triangulation seems to have been first 

 put forth in 1(304 by Kepler*, who said that the distance be- 

 tween the two eyes is the base which we employ in measuring 

 the distance of objects, that in so doing we Jenrn to estimate 

 distances with a single eye, and hence that the magnitude of 

 a heavenlj' body as perceived in the eye would serve as a base 

 for distances relatively slight. Brewsterf, in opposition to 

 Berkeley^, elaborated the idea still further, making it the 

 foundation of his theory of the stereoscope, which has kept its 

 place in our textbooks of Physics to the present day. The 

 retention of this theory, in the face of facts to the contrary, is 

 explicable only in consideration of the circumstances attendant 

 upon normal binocular vision, and the great authority attached 

 to Brewster's views on account of his discoveries as a physi- 

 cist. Probably neither he nor his followers undertook to 

 ascertain whether the limit of optic parallelism was ever passed 

 iu the actual use of the stereoscope. In 1861, two Germans, 

 Rollet and Becker§, published a mode of securing binocular 

 fusion of similar images with the visual lines slightly diver- 



* Paralipomena, 1604, pp. 62-G6. 

 t Brewster on the Stereoscope, p. 50. 



% 'An Essay towards a New Theory of Vision': Dublin, 1709. 

 § Wiener Sitzunc/sberichte, May 10, 1861, xliii. ; or Helmholtz, Optique 

 Physiologique, p. 827. 



