Physiological Perspective. 317 



gent, the possibility of this having been already mentioned by 

 Burckhardt *, bat with no reference to the possibility of ste- 

 reoscopy from these similar pictures. In 1862, Brewster's 

 theory was criticised by Professor C. F. Himes, of Pennsyl- 

 vania, who described his experiments in stereoscopy from dis- 

 similar pictures with optic divergence, and called attention to 

 the necessity of modifying the current theory f. Helmholtz 

 related experiments in optic divergence ±, but, unfortunately, 

 did not enter fully into an analysis of the visual results. My 

 own attention was drawn to this subject by the discovery of 

 my power to secure stereoscopy with the unaided eyes by 

 voluntary optic divergence. In a paper read before the New- 

 York Academy of Sciences on the (3th of June 1881, I 

 rejected Brewster's theory ; and the phenomena which he had 

 explained by visual triangulation I referred to the associated 

 action of the muscles of the eyes, applying this to both con- 

 vergence and divergence. It has been very gratifying, there- 

 fore, to find my views confirmed by Brigade-Surgeon Tyler 

 Oughton, A.M.D., in two able articles published in the London 

 'Lancet,' of October 22 and December 31, 1881. These 

 articles were quite independent of my own, each writer having 

 been unknown to the other. In 1877 Mr. Oughton had 

 referred certain phenomena of " erect vision " to the sensation 

 of muscular contraction. My experiments in stereoscopy with 

 perfectly similar pictures, by optic convergence, parallelism, 

 or divergence, with direct or inverse relief at will — experi- 

 ments capable of ready accomplishment by any one with un- 

 trained eyes — still further show that there is no longer any 

 room for the application of geometry to the physiology of 

 vision, beautiful and tempting as may be the theory of visual 

 triangulation. 



It is but due to add that the theory of associated muscular 

 action in relation to the stereoscope was virtually stated by 

 Professor Huxley § in 1868, and in such a way as quite plainly 

 to indicate its applicability to the phenomena of optic diver- 

 gence. That he did not elaborate it in refutation of Brewster's 

 theory was probably due to the fact that the fallacy and wide 

 spread of this theory had not been brought specially to his 



* VerhandJuaq. d. naturforsch. Ges. zu Basel, i. p. 1-15; or Helrnholtz, 

 Opt. Phys. p. 827. 



f American Journal of Photography, September 1, 1862 ; also British 

 Journal of Photography, 1864. 



% Optique P7u/iiologiq>te, pp.616 & 828; also 'Popular Lectures on 

 Scientific Subjects,' p. 307 (transl. 1873). 



§ ' Elementary Physiology,' p. 286 (ilacmillan and Co.). 



