THE PERCEPTION OF SPACE. 265 



Hering lias tried to explain our exaggeration of small 

 angles in the same way. We have more to do with right 

 angles than with any others : right angles, in fact, have an 

 altogether unique sort of interest for the human mind. 

 Nature almost never begets them, but we think space by 

 means of them and jjut them everywhere. Consequently 

 obtuse and acute ones, liable always to be the images of 

 right ones foreshortened, particularly easily revive right 

 ones in memory. It is hard to look at such figures as 

 a, h, c, in Fig. 79, without seeing them in perspective, as 



Fig. 79. 



approximations, at least, to foreshortened rectangular 

 forms. * 



At the same time the genuine sensational form of the 

 lines before iis can, in all the cases of distortion by sug- 

 gested perspective, be felt correctly by a mind able to ab- 

 stract from the notion of perspective altogether. Individ- 

 uals differ in this abstracting power. Artistic training im- 

 proves it, so that after a little while errors in vertical bi- 

 section, in estimating height relatively to breadth, etc., be- 

 come impossible. In other words, we learn to take the 

 optical sensation before us pure, f 



for foreshorteniug bears only on the relations of the parts of special things 

 spread out before us. Cf. Wundt, Physiol. Psych., 2te Aufl. ii. 96-8; 

 Th. Lipps, Grundtatsachen, etc., p. 535. 



* Hering would partly solve in this way the mystery of Figs. 60, 61, and 

 67. No doubt the explanation partly applies ; but the strange cessation of 

 the illusion when we fix the gaze fails to be accounted for thereby. 



f Helmholtz has sought (Phvsiol. Optik, p. 715) to explain the diverg- 

 ence of the apparent vertical meridians of the two retinae, hy the manner 

 in which an identical line drawn on the ground before us in the median 

 plane will throw its images on the two eyes respectively. The matter is 

 too technical for description here ; tlie unlearned reader may be referred 

 for it to J. Le Conte's Sight in the luteriiat. Scient. Series, p. 198 ff. But, for 

 the benefit of those to whom verbum sat, I cannot help saying that it seems 

 to me that the exactness of the relation of the two meridians — whether div^r- 



