THE PERCEPTION OF THINGS. 131 



in all its possible bearings, on the basis of a large mass of 

 empirical material, was begun by Mr. Edmund Gurney and 

 is continued hj other members of the Society for Psy- 

 chical Research ; and the ' Census ' is now being applied 

 to several countries under the auspices of the International 

 Congress of Experimental Psychology. It is to be hoped 

 that out of these combined labors something solid will 

 eventually grow. The facts shade off into the phenomena 

 of motor automatism, trance, etc.; and nothing but a wide 

 comparative study can give really instructive results.* 



The part played by tJie peripheral sense-organ in hallucina- 

 tion is just as obscure as we found it in the case of imagina- 

 tion. The things seen often seem opaque and hide the 

 background upon which they are projected. It does not 

 follow from this, however, that the retina is actually in- 

 volved in the vision. A contrary process going on in the 

 visual centres would prevent the retinal impression made 

 by the outer realities from being felt, and this would in 

 mental terms be equivalent to the hiding of them by the 

 imaginary figure. The negative after-images of mental 

 pictures reported by Meyer and Fere, and the negative after- 

 images of hypnotic hallucinations reported by Binet and 

 others so far constitute the only e\ddence there is for the 

 retina being involved. But until these after-images are 

 explained in some other way we must admit the possibility 

 of a centrifugal current from the optical centres downwards 

 into the peripheral organ of sight, paradoxical as the course 

 of such a current may appear. 



* PERCEPTIO]Sr-TIME.» 



The time which the perceptive process occupies has been 

 inquired into by various experimenters. Some call it per- 

 ception-time, some choice-time, some discrimination-time. 

 The results have been already given in Chapter XIII (vol. 

 I, p. 523 ff.), to which the reader is consequently referred. 



* In Mr. Gurney's work, just cited, a very large number of veridical 

 cases are critically discussed. 



