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XLII1 On the Knowledge of Distance given by Binocular Vision. By Sir DAVID 

 BBEWSTER, K.H., D.C.L., F.R.S., and V.P.R.S. Edinburgh. 



(Read 15th April 1844.) 



IN analysing Mr WHEATSTONE'S beautiful discovery, that in binocular vision 

 we sefe all objects of three dimensions by means of two dissimilar pictures on the 

 retina, I trust I have satisfied the Society 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 eye. 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 piano, the dissimilarity of the pictures is necessary in the exhi- 

 bition of that beautiful phenomenon. 



In performing, with the eye alone, the various experiments detailed in a 

 former paper, 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 the component figures on 

 a plate of glass suspended in the air, so as to have no vision of the surface on 

 which they rested, and after uniting these figures by binocular vision, and con- 

 cealing the two outstanding single figures, I obtained results which, though not 

 entirely satisfactory, proved that there existed some disturbing cause which pre- 

 vented the united image from placing itself in the binocular centre, or the inter- 

 section of the optical axes. This disturbing cause was simply the influence of 

 other objects in the same field of view, whose distance was known to the 

 observer. 



In order to avoid all such influences, and to study the subject under a 

 more general aspect, it occurred to me that these objects would be gamed by 

 using a numerous series of plane figures, such as those of flowers or geome- 

 trical patterns upon carpets or paper-hangings. These figures being always 

 at equal distances from each other, and almost perfectly equal and similar, the 

 coalescence of any pair of them, by directing the optic axes to a point between 

 the paper-hangings and the eye, is accompanied with the coalescence of every 



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