254 



PSYCHOLOGY. 



The phenomena of the first group were described on 

 page 232. A. W. Volkmann has studied them with his 

 accustomed clearness and care. * Even an imaginarily 

 inclined wall, in a picture, will, if an after-image be thrown 

 upon it, distort the shape thereof, and make us see a form 

 of which our after-image would be the natural projection 

 •on the retina, were that form laid upon the wall. Thus a 

 .signboard is painted in perspective on a screen, and the 

 eye, after steadily looking at a rectangular cross, is turned 

 to the painted signboard. The after-image ajjpears as an 

 oblique-legged cross upon the signboard. It is the converse 

 phenomenon of a perspective drawing like Fig. 71, in which 



Fig. 



really oblique-legged figures are seen as rectangular crosses. 

 The unstable judgments of relative distance and size 

 were also mentioned on pp. 231-2. Whatever the size may 

 be of the retinal image which an object makes, the object is 

 seen as of its own normal size. A man moving towards us 

 is not sensibly perceived to groiv, for example ; and my 

 finger, of which a single joint may more than conceal him 

 from my view, is nevertheless seen as a much smaller object 

 than the man. As for distances, it is often possible to make 

 the farther part of an object seem near and the nearer part 

 far. A human profile in intaglio, looked at steadily with 

 one eye, or even both, soon appears irresistibly as a bas- 

 relief. The inside of a common pasteboard mask, painted 

 like the outside, and viewed with one eye in a direct light, 

 also looks convex instead of hollow. So strong is the illu- 



* Physiologische Untersucliuugen im Ciebiete der Optik, v. 



