Prof. Wheatstone on the Physiology of Vision. 519 



times suddenly, and at other times gradually, the converse oc- 

 cupies its place. The reason of this is, that the relief and distance 

 of objects is not suggested to the mind solely by the binocular 

 pictures and the convergence of the optic axes, but also by other 

 signs, which are perceived by means of each eye singly j among 

 which the most effective are the distributions of light and shade 

 and the perspective forms which we have been accustomed to see 

 accompany these appearances. One idea being therefore sug- 

 gested to the mind by one set of signs, and another totally in- 

 compatible idea by another set, according as the mental atten- 

 tion is directed to the one and abstracted from the other, the 

 normal form or its converse is perceived. Tbis mental attention 

 is involuntary ; no immediate effort of the will can call up one 

 idea while the other continues to present itself, though the trans- 

 ition may be facilitated by intentionally removing some of the 

 signs which suggest the preponderating idea ; thus the converse 

 form being perceived, closing either eye will most frequently 

 cause an instant reversion to the normal form ; and always, if 

 the monocular signs of relief are sufficiently suggestive. 



I know of nothing more wonderful, among the phenomena 

 of perception, than the spontaneous successive occurrence of 

 these two very different ideas in the mind, while all external cir- 

 cumstances remain precisely the same. Thus a small statuary 

 group, an elegant and beautiful object, without any apparent 

 cause becomes converted into another totally dissimilar object 

 uncouth in appearance, and which gives rise to no agreeable 

 emotions in the mind ; yet in both cases all the sensations that 

 intervene between objective reality and ideal conception continue 

 unchanged. 



The effects of the pseudoscope I have already mentioned, may 

 be strictly called conversions of relief, because the illusive appear- 

 ance is in each case the converse impression of the relief of the 

 real object. If, however, the object consists of parts detached 

 from and behind each other, the preceding term is inappropriate 

 to denote the effects which result, but the more general expres- 

 sion conversion or inversion of distance may be employed to de- 

 signate them. I proceed to call attention to a few such effects. 

 Skeleton figures of geometrical solids, as cubes, pyramids, &c, 

 readily sbow their converse. 



Two objects at different distances, being simultaneously re- 

 garded, the most remote will appear the nearest and the nearest 

 the most remote. 



An ivory loot-rule, held immediately before the eyes a little 

 inclined to the horizon with its remote end elevated, appears 

 inclined in the opposite way, its nearer end elevated, and as if 

 the observer were looking at its lower surface. Its form also 



