Page:"N" Rays (Garcin).djvu/47

placed, containing a phosphorescent substance, say calcium sulphide, which has been previously exposed for a short time to solar rays. If, now, on the path of the solar rays, which are supposed to reach the tube through the wood, a sheet of lead, or the hand simply, is interposed, even at a great distance from the tube, the phosphorescent glow is seen to diminish; when the obstacle is removed, the glow reappears. The extreme simplicity of this experiment will incite many persons, I hope, to repeat it. The only precaution one need take is to operate with a feeble preliminary phosphorescence (note 9). It is advantageous to arrange permanently a sheet of black paper, so that the interposition of the screen does not change the background on which the tube stands out. The variations in glow are especially easy to catch near the contours of the luminous patch formed by the phosphorescent body on the dark background; when the "N" rays are intercepted, these contours lose their sharpness, regaining the same when the screen is removed. However, these variations in glow do not appear to be instantaneous. Interposing