T II E 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



MAY 184.7. 



XLVIII. On the Kno'wledge of Distance given hij Binocular 

 Vision. By Sir David Bkewster, K.H., D.C.L., F.R.S.^ 

 and V.P.R.S. Edinb:^ 



[With a Plate.] 



IN analysing Mr. Wheatstone's beautiful discovery, that in 

 binocular vision we see all objects of three dimensions by 

 means of two dissimilar pictures on the retina, I trust I have 

 proved that the dissimilarity of these two pictures is in no 

 respect the cause of our vivid perception of such objects, but, 

 on the contrary, an unavoidable accompaniment of binocular 

 vision, which renders it less perfect than vision with one eyef. 

 On the other hand, it is quite true that, in Mr. Wheatstone's 

 experiment of producing the perception of objects of three 

 dimensions by the apparent coalescence of two dissimilar 

 representations of such objects in flano, the dissimilarity of 

 the pictures is necessary in the exhibition of that beautiful 

 phaenomenon. 



In performing with the eye alone, the various experiments 

 detailed in a former paper J, I was very much struck with the 

 fact, that the apparent solid figure, produced by the union of 

 its dissimilar pictures, never took its right position in absolute 

 space; that is, in place of appearing suspended between the 

 eye and the plane upon which the dissimilar figures were 

 drawn, the base of the solid seemed to rest on that plane, 

 whether its apex was nearer the eye or more remote than its 

 component plane figures. 



With the view of finding the cause of this, I placed tRe 

 component figures on a plate of glass suspended in the air, so 

 as to have no vision of the surface on whicli they rested, and 



* Read at tlic Royal Society of Eclinburgli, April 15, 1844. See their 

 Transactions, vol. xv. p. fJG.'}. 



t 'See this Journal for May and June 1844, vol. x.\iv. pj). .'530, 4.31). 

 X Ibid. 



Pliil. Mag. S. 3. Vol. 30. No. 202. May 1 817. Y 



