THE PERCEPTION OF SPACE. S51 



wliy lines cutting another line make the latter seem to 

 bend away from them more than is really the case. If, 

 he says, wo draw (Fig. 69) the line pm upon the line «&, 

 and follow the latter with our eye, we shall, on reaching 

 the point m, tend for a moment to slip off ah and to follow 

 mp, without distinctly realizing that we are not still on the 

 main line. This makes us feel as if the remainder mh of 

 the main line were bent a little away from its original direc- 

 tion. The illusion is apparent in the shape of a seeming 



Fig. 69. 



approach of the ends h, b, of the two main lines. This to 

 my mind would be a more satisfactory exi3lanation of this 

 class of illusions than any of those given by previous au- 

 thors, were it not again for what happens in the skin. 



Considering all tJie circumstances, I feel Justijied in dis- 

 carding his entire hatch of illusions as irrelevant to our pres- 

 ent inquiry. Whatever they may prove, they do not prove 

 that our visual percepts of form and movement may not be 

 sensations strictly so called. They much more probably 

 fall into line with the phenomena of irradiation and of 

 color-contrast, and with Yierordt's primitive illusions of 

 movement. They show us, if anything, a realm of sen- 

 sations in which our habitual experience has not yet made 

 traces, and which persist in spite of our better knowledge, 

 ■MTisuggestive of those other space-sensations which we all 

 the time know from extrinsic evidence to constitute the real 

 space-determinations of the diagram. Very likely, if these 

 sensations were as frequent and as practically important as 

 they now are insignificant and rare, we should end by sub- 

 stituting their significates — the real space-values of the 

 diagrams — for them. These latter we should then seem to 



