Page:"N" Rays (Garcin).djvu/109



(1) If one proposes only to ascertain the production of "N" rays in given circumstances, a phosphorescent screen, made as follows, may be used with advantage: some powdered calcium sulphide is mixed with collodion, diluted with ether, so as to form a very thin paste; then, with a water-colour brush, drops of this paste are painted on blackened cardboard, so as to produce stains several millimetres in diameter, close to each other. The screen then presents the aspect of a spotted fabric. If, after being exposed to light, it is examined in a dark room, and in perfect silence, some of the spots will appear less luminous than the others. Usually, some will not seem to be sharply separated from their neighbours, but will form a sort of confused nebula less visible than the rest. Now, if one speaks aloud