266 Prof. Wheatstone on the Physiology of Vision. 



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 similitude. The following experiment is equally- 

 decisive against this theory. 



Draw two lines inclined towards each other, as in Plate VIII. 

 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 entire 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 coin- 

 cide exactly in position with it ; or, if while the optic axes con- 

 tinue 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 intermediate 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 

 retina?. 



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 retina? ocular spec- 

 tra of the component figures. For this purpose the drawings 

 should be formed of broad coloured lines on a ground of the 

 complementary colour, for instance red lines on a green ground, 

 and be viewed either in the stereoscope or in the apparatus, fig. 6, 

 as the ordinary figures are, taking care however to fix the eyes 

 only to a single point of the compound figure; the drawings 

 must be strongly illuminated, and after a sufficient time has 

 elapsed to impress the spectra on the retina?, the eyes must be 

 carefully covered to exclude all external light. A spectrum of 

 the object in relief will then appear before the closed eyes. It 

 is well known, that a spectrum impressed on a single eye and 

 seen in the dark, frequently alternately appears and disappears : 

 these alternations do not correspond in the spectra impressed 

 on the two retina 1 , and hence a curious effect arises ; sometimes 

 the right eye spectrum will be seen alone, sometimes that of 

 the left eye, and at those moments when the two appear to- 

 gether, the binocular spectrum will present itself in bold relief. 

 As in this case the pictures cannot shift their places on the 



