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 perceived 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 suppressed, 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
