252 



P8YCE0L0O T. 



see directly, and the illusions would disappear like that of 

 the size of a tooth-socket when the tooth has been out a 

 week. 



(h) Another hatch of cases which we may discard is that of 

 double images. A thoroughgoing anti-sensationalist ought 

 to deny all native tendency to see double images when 

 disparate retinal points are stimulated, because, he should 

 say, most people never get them, but see all things single 

 which experience has led them to believe to he single. 

 " Can a doubleness, so easily neutralized by our knowledge, 

 ever be a datum of sensation at all ? " such an anti-sensa- 

 tionalist might ask. 



To which the answ^er is that it is a datum of sensation, 

 but a datum which, like many other data, must first be 

 discriminated. As a rule, no sensible qualities are dis- 

 criminated without a motive.* And those that later we 

 learn to discriminate were originally felt confused. As 

 well pretend that a voice, or an odor, which we have 

 learned to pick out, is no sensation now. One may easily 

 acquire skill in discriminating double images, though, as 

 Hering somewhere says, it is an art of which one cannot 

 become master in one year or in two. For masters like 

 Hering himself, or Le Conte, the ordinary stereoscopic dia- 

 grams are of little use. Instead of combining into one solid 

 appearance, they simply cross each other with their doubled 



/ F tf ' y ^ 



Fia. 70. 



lines. Volkmann has shown a great variety of ways in 

 which the addition of secondary lines, differing in the two 



* Cf . supra, p. 515 ff. 



