THE 



LONDON, EDINBURGH, asd DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



MA T 1882. 



X XX VI. Physiological Perspective. 



By W. Le Contb Stevens*. 



[Plate Yin. figs. 1-5.] 



1. Brewster's Experiment. 



SIR DAVID BRETTSTER, in his essay « On the Know- 

 ledge of Distance given by Binocular Vision ' ("1844), 

 and in his subsequent volume on the Stereoscope (1856), 

 describes the binocular union, by optic convergence, of simi- 

 lar pictures regularly recurring on large surfaces. The 

 result of viewing, by cross vision, flowers twelve inches 

 apart, on a papered wall at three feet distance, he describes 

 partially as followsf : — " The whole papered wall, with all its 

 flowers, will be seen suspended in the air at the distance of 

 six inches from the observer. At first the observer does not 

 decide upon the distance of the suspended wall from himself. 

 It generally advances slowly to its new position ; and when it 

 has taken its place it has a very singular character. The 

 surface of it seems slightly curved." 



The geometric explanation of the position of the phantom 

 wall, as given by Brewster}, is easily understood. Let R 

 and L (fig. 1) be the optic centres of the right and left 

 eyes respectively, and A, B, C, A! , W , C, equidistant points 

 on the wall. If the right eye is directed to A and the left to 

 A', the intersection of visual lines is at a. The points b and 

 c are determined in like manner ; and if the interocular line 



* Communicated by the Author. 



t Brewster on the "Stereoscope, London, 1856, p. 91. 



t Ibid. p. 95. 

 Phil. Mag. S. 5. Vol. 13. No. 82. May 1882. 2 B 



