Prof. Wheatstone on the Physiology of Vision. 243 



A, all objects in the space behind it, included as it were in a 

 shadow ECF cast by a candle at A, are invisible to the eye at A; 

 but when the other eye at B is opened, part of these objects become 

 visible to it ; those only being hid from both eyes that are in- 

 cluded, as it were, in the double shadow CD, cast by two lights 

 at A and B, and terminated in D, the angular space EDG beyond 

 D being always visible to both eyes. And the hidden space CD 

 is so much the shorter, as the object C is smaller and nearer to 

 the eyes. Thus the object C seen with both eyes becomes, as it 

 were, transparent, according to the usual definition of a trans- 

 parent thing; namely, that which hides nothing beyond it. But 

 this cannot happen when an object, whose breadth is bigger than 

 that of the pupil, is viewed by a single eye. The truth of this 

 observation is therefore evident, because a painted figure inter- 

 cepts all the space behind its apparent place, so as to preclude 

 the eyes from the sight of every part of the imaginary ground 

 behind it." 



Had Leonai"do da Vinci taken, instead of a sphere, a less 

 simple figure for the purpose of his illustration, a cube, for in- 

 stance, he would not only have have observed that the object 

 obscured from each eye a different part of the more distant field 

 of view, but the fact would also perhaps have forced itself upon 

 his attention, that the object itself presented a different appear- 

 ance to each eye. He failed to do this, and no subsequent 

 writer within my knowledge has supplied the omission ; that two 

 obviously dissimilar pictures are projected on the two retina? 

 when a single object is viewed, while the optic axes converge, 

 must therefore be regarded as a new fact in the theory of vision. 



§2. 



It being thus established that the mind perceives an object of 

 three dimensions by means of the two dissimilar pictures pro- 

 jected by it on the two retina?, the following question occurs : 

 What would be the visual effect of simultaneously presenting to 

 each eye, instead of the object itself, its projection on a plane 

 surface as it appears to that eye ? To pursue this inquiry, it ia 

 necessary that means should be contrived to make the two pic- 

 tures, which must necessarily occupy different places, fall on 

 similar parts of both retinsc. Under the ordinary circumstances 

 of vision, the object is seen at the concourse of the optic axes, 

 and its images consequently arc projected on similar parts of the 

 two retina; ; but it is also evident that two exactly similar objects 

 may 1)0 made to fall on similar parts of the two retime, if they 

 arc place (I one in tlic direct ion of each optic axis, at equal di- 

 stances before or beyond their intersection. 



Fig. 2 represents the usual situation of an object at the inter- 

 It 2 



