392 MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 



It may be supposed, that we see but one point of a field of view distinctly at the 

 same instant, the one namely to which the optic axes are directed, while all other points 

 are seen so indistinctly, that the mind does not recognize them to be either single or 

 double, and that the figure is appreciated by successively directing the point of con- 

 vergence of the optic axes successively to a sufficient number of its points to enable 

 us to judge accurately of its form. 



That there is a degree of indistinctness in those parts of the field of view to which 

 the eyes are not immediately directed, and \^ hich increases with the distance from 

 that point, cannot be doubted, and it is also true that the objects thus obscurely seen 

 are frequently doubled. In ordinary vision, it may be said, this indistinctness and 

 duplicity is not attended to, because the eyes shifting continually from point to point, 

 every part of the object is successively rendered distinct ; and the perception of the 

 object is not the consequence of a single glance, during which only a small part of it is 

 seen distinctly; but is formed from a comparison of all the pictures successively seen 

 while the eyes were changing from one point of the object to another. 



All this is in some degree true ; but were it entirely so, no appearance of relief 

 should present itself when the eyes remain intently fixed on one point of a binocular 

 image in the stereoscope. But on performing the experiment carefully, it will be 

 found, provided the pictures do not extend too far beyond the centres of distinct 

 vision, that the image is still seen single and in relief when this condition is fulfilled. 

 Were the theory of corresponding points true, the appearance should be that of the 

 superposition of the two drawings, to which however it has not the slightest simili- 

 tude. The following experiments are equally decisive against this theory. 



Exp. 1. Draw two lines about two inches long and inclined towards each other, 

 as in fig. 10., on a sheet of paper, and having caused them to coincide by converging 

 the optic axes to a point nearer than the paper, look intently on the upper end of the 

 resultant line, without allowing the eyes to wander from it for a moment. The en- 

 tire line will appear single and in its proper relief, and a pin or a piece of straight 

 wire may without the least difficulty be made to coincide exactly in position with it ; 

 or, if while the optic axes continue to be directed to the upper and nearer end, the 

 point of a pin be made to coincide with the lower and further end or with any inter- 

 mediate point of the resultant line, the coincidence will remain exactly the same 

 when the optic axes are moved and meet there. The eyes sometimes become fatigued, 

 which causes the line to appear double at those parts to which the optic axes are not 

 fixed, but in such case all appearance of relief vanishes. The same experiment may 

 be tried with more complex figures, but the pictures should not extend too far beyond 

 the centres of the retinae. 



Another and a beautiful proof that the appearance of relief in binocular vision is 

 an effect independent of the motions of the eyes, may be obtained by impressing on 

 the retinee ocular spectra of the component figures. For this purpose the drawings 

 should be formed of broad coloured lines on a ground of the complementary colour. 



