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Rh tangent plane. Hence we are in a position to distinguish between the effects produced on phosphorescence by heat on the one hand, and by "N" rays, sound-waves, magnetic and electric fields on the other.

The following is another case in which the effects are different. Take a rectangular cardboard screen, 5 cms. high and 12 cms. long, for instance, coated very uniformly with calcium sulphide, and rendered very feebly phosphorescent. If the temperature of a portion of the screen is raised, this part becomes more luminous than the rest. If, instead of this, we let fall on one half of the screen a pencil of "N" rays, proceeding, for example, from a Nernst lamp, we find no sensible increase in its glow; but if in front of this half-screen a small opaque object is placed, for instance, a small key or a bit of metal foil, cut off by daylight, this is seen to come out very strongly on the luminous background, while if it is placed on the half not receiving the "N" rays, its outline is vague and indeterminate, and seems even to disappear at times. By shifting slowly the object on the screen, its