THE PEMCEPTION OF SPACE. 257 



objects we conceive clearly at the moment of looking at the 

 figure, we seem to see in all its solidity before us. A little prac- 

 tice will enable us to flap the figures, so to speak, backwards 

 and forwards from one object to the other at will. We need 

 only attend to one of the angles represented, and imagine it 

 either solid or hollow — pulled towards us out of the plane 

 of the paper, or pushed back behind the same — and the 

 whole figure obeys the cue and is instantaneously trans- 

 formed beneath our gaze.* 



The peculiarity of all these cases is the ambiguity of 

 the perception to which the fixed retinal imj)ression gives 

 rise. With our retina excited in exactly the same way, 

 whether by after-image, mask or diagram, we see now this 

 object and now that, as if the retinal image per se had no 

 essential space-import. Surely if form and length were 

 originally retinal sensations, retinal rectangles ought not to 

 become acute or obtuse, and lines ought not to alter their 

 relative lengths as they do. If relief were an optical 

 feeling, it ought not to flap to and fro, with every ojjtical con- 

 dition unchanged. Here, if anywhere, the deniers of space- 

 sensation ought to be able to make their final stand. t 



It must be confessed that their plea is plausible at first 

 sight. But it is one thing to throw out retinal sensibility 

 altogether as a space-yielding function the moment we find 

 an ambiguity in its deliverances, and another thing to 

 examine candidly the conditions which may have brought 

 the ambiguity about. The former way is cheap, wholesale, 

 shallow ; the latter difiicult and complicated, but full of 

 instruction in the end. Let us try it for ourselves. 



In the case of the diagrams 72, 73, 74, 75, 76, the real 

 object, lines meeting or crossing each other on a plane, is 



* Loeb (Pflilger's Archiv, xl. 274) has proved that muscular changes 

 of adaptation in the eye for near and far distance are what determine the 

 form of the relief. 



t The strongest passage in Helmholtz's argument against sensations of 

 space is relative to these fluctuations of seen relief: "Ought one not to 

 conclude that if sensations of relief exist at all, they must be so faint and 

 vague as to have no influence compared with that of past experience? 

 Ought we not to believe that the perception of the third dimension may 

 have arisen without them, since we now see it taking place as well against 

 them as icith them?" (Phj'siol. Optik, p. 817.) 



