44 "AT" RAYS 



darker. To remove all possibility of illusion, 

 I arranged permanently a box closed by a cover 

 and wrapped in black paper ; in this completely 

 enclosed box the brick was placed, and, in this 

 manner, the dark background on which the slip 

 stood out remained rigorously invariable, but 

 the observed effect remained the same. The 

 experiment can be varied in different ways. 

 For instance, the laboratory shutters being 

 almost closed, and the dial of the clock fixed to 

 a wall which was just sufficiently lighted for the 

 dial, at a distance of 4 metres, to be just per- 

 ceived as a grey patch with no defined contour, 

 if the observer, without changing his place, 

 directs towards his eyes the " N " rays emitted 

 by a previously exposed brick or pebble, he sees 

 the dial whiten ; he can trace distinctly its 

 circular contour, and even succeed in seeing 

 the hands. When the " N " rays are sup- 

 pressed, the dial again grows dark. Neither 

 the production nor the cessation of the 

 phenomenon are instantaneous. 



As in these experiments the luminous object 

 is placed very far away from the source of " N " 

 rays, and as, on the other hand, in order that 



