CHAP. XVIT.] SINGLE VISION WITH A DOUBLE ORGAN. 61 



tracts, as he had himself inferred from the phenomenon above 

 mentioned. 



What we have before advanced, however, regarding the un- 

 broken sheet of gray nervous matter in the retina, leads us to 

 attach even more importance to the commissural fibres which appear 

 to connect the two retinae together, though the medium of the 

 chiasma, and independently of parts behind it. We conceive that 

 these commissural fibres -may connect corresponding parts of the 

 retina, much in the same way as corresponding parts of the cere- 

 bral convolutions of the opposite hemispheres are linked together by 

 the corpus callosum or other commissures; and that the unity of 

 action of the double organ may depend, as to its physical cause, on 

 the same principle in both. 



This capacity of forming a single conception from a double im- 

 pression may appear, at first sight, to be given simply to obviate 

 confusion; but Mr. Wheatstone has most ingeniously shown that it 

 confers a new power on the sense, viz. that of appreciating forms 

 projected in relief. 



Such objects, if sufficiently near the eyes for the optic axes to 

 converge in viewing them, are seen from two different directions : 

 they are represented on each retina by a different perspective 

 projection; and the more so, the nearer the object to the ob- 

 server. 



Mr. Wheatstone has shown that the single sensation excited by 

 these two images is that of a third image different from them 

 both, but excitable only by both of them at once, and attended 

 with the notion of solidity, or projection in relief. He has illus- 

 trated this by an instrument which he terms the stereoscope. Some 

 object of three dimensions (as a cube) is represented in two draw- 

 ings as it would be seen at a small distance by each of the two 

 eyes. These drawings are then placed symmetrically in the right 

 and left compartments of a small box, so as to be reflected by 

 sloping mirrors to the eyes of the observer, each view to its cor- 

 responding eye. When he looks at each separately, it seems a 

 mere drawing on a flat surface; but when he regards both views at 

 once, they appear to coalesce, and a solid prominent figure seems to 

 occupy their place. Mr. Wheatstone has also shown that the same 

 effect occurs, although there may exist some disparity between the 

 size of the two images; and that the resultant idea is that of a 

 figure of intermediate size. Now, unless an object is placed directly 

 in front of the eyes, its image is larger in one eye than the other, 

 because it is nearer one eye than the other; and this faculty of 



