382 MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 



occurred to me while examining figures and engraved plates of crystalline forms ; I 

 mean a sudden and involuntary change in the apparent position of a crystal or solid 

 represented in an engraved figure. What I mean will be more easily understood from 

 the figure annexed (fig. 22.). The rhomboid A X is drawn so that the solid angle A 

 should be seen the nearest to the spectator, and the solid angle X the farthest from 

 him, and that the face A C D B should be the foremost, while the face X D C is be- 

 hind. But in looking repeatedly at the same figure, you will perceive that at times 

 the apparent position of the rhomboid is so changed that the solid angle X will ap- 

 pear the nearest, and the solid angle A the farthest ; and that the face A C D B will 

 recede behind the face X D C, which will come forward, which effect gives to the 

 whole solid a quite contrary apparent inclination." 



Professor Necker attributes this alteration of appearance, not to a mental opera- 

 tion, but to an involuntary change in the adjustment of the eye for obtaining distinct 

 vision. He supposed that whenever the point of distinct vision on the retina is di- 

 rected on the angle A, for instance, this angle seen more distinctly than the others is 

 naturally supposed to be nearer and foremost, while the other angles seen indistinctly 

 are supposed to be farther and behind, and that the reverse takes place when the 

 point of distinct vision is brought to bear on the angle X. 



J That this is not the true explanation, is evident from three circumstances : in the 

 first place, the two points A and X being both at the same distance from the eyes, the 

 same alteration of adjustment which would make one of them indistinct would make 

 the other so ; secondly, the figure will imdergo the same changes whether the focal 

 distance of the eye be adjusted to a point before or beyond the plane in which the figure 

 is drawn ; and thirdly, the change of figure frequently occurs while the eye continues 

 to look at the same angle. The effect seems entirely to depend on our mental con- 

 templation of the figure intended to be represented, or of its converse. By following 

 the lines with the eye with a clear idea of the solid figure we are describing, it may 

 be fixed for any length of time ; but it requires practice to do this or to change the 

 figure at will. As I have before observed, these effects are far more obvious when 

 the figures are regarded with one eye only. 



No illusion of this kind can take place when an object of three dimensions is seen 

 with both eyes while the optic axes make a sensible angle with each other, because 

 the appearance of the two dissimilar images, one to each eye, prevents the possibility 

 of mistake. But if we regard an object at such a distance that its two projections 

 are sensibly identical, and if this projection be capable of a double interpretation, the 

 illusion may occur. Thus a placard on a pole carried in the streets, with one of its 

 sides inclined towards the observer, will, when he is distant from it, frequently ap- 

 pear inclined in a contrary direction. Many analogous instances might be adduced, 

 but this will suffice to call others to mind ; it must however be observed, that when 

 shadows, or other means capable of determining the judgement are present, these 

 fallacies do not arise. 



