520 Prof. Wheatstone on the Physiology of Vision. 



undergoes a change. Since the nearest end, the retinal magni- 

 tude of which is the largest, appears farthest from the eyes, and 

 the nearest end, the retinal magnitude of which is greatest, ap- 

 pears near the eyes, the rule will no longer be perceived to be 

 rectangular, but trapezoidal. If the rule be placed horizontally, 

 and it be regarded with the pseudoscope at an angle of 45°, it 

 will appear with the form just described standing vertically. 



Any object placed before the wall of a room will appear behind 

 the wall, and as if an aperture of the proper dimensions had been 

 made in the wall to allow it to be seen ; if the object be illumi- 

 nated by a candle, its shadow will appear as far before the object 

 as in reality it is behind. 



The appearance of a plant is very remarkable ; as the branches 

 which are furthest from the eye are perceived to be the nearest, 

 those parts which are actually obscured by the branches before 

 them, appear broken away and allow the parts apparently behind 

 them to be seen. A flowering shrub before a hedge appears to 

 be transferred behind it ; and a tree standing outside a window 

 may be brought visibly within the room in which the observer is 

 standing. 



I have before observed, that the transition from the normal to 

 the converse perception is often gradual ; I will give one instance 

 of this as an illustration. The object was a page of medallions 

 embossed on card-board, and the raised impressions were pro- 

 tected from injury by a thick piece of mill-board having aper- 

 tures in it made to correspond to each medallion. The page 

 was placed horizontally, illuminated by a candle placed beyond 

 it, and looked at through the pseudoscope at an angle of 45°; 

 for the first moment the page appeared as it would have done 

 without the instrument ; soon after the medallions appeared level 

 with the upper surface, and the shadows on the upper parts of 

 the circular apertures were converted into deep depressions as if 

 cut out with a tool ; they next, from horizontal, became vertical, 

 each standing erect on the horizontal plane, and immediately 

 afterwards the reliefs were all changed into hollows ; finally, the 

 page itself stood vertical, but with that change of form which I 

 indicated in the case of the rule, the upper edge appearing much 

 shorter than the lower edge : the series of changes being now 

 complete, the final form remained constant as long as the object 

 w r as regarded. 



In endeavouring to analyse the phenomena of converse per- 

 ception, it must be borne in mind that the transposition of 

 distances has reference only to distances from the retinae, not to 

 absolute horizontal distances in space. Thus, if a straight ruler 

 be held in the vertical plane perpendicular to the optic base, and 

 also inclined 45° to the horizon so that its upper end shall be 



