(millionths millimeter) is
, or four thousand times as much. Since not a third of the incident energy is in wave-lengths shorter than
, a surface of sodium or lithium which is sensitive up to
should require, even if all tills energy were in one wave-length, which it is not, at least 12,000 seconds or 4 hours of illumination by a candle 3 m. away before any of its atoms could have received, all told, enough energy to discharge an electron. Yet the electron is observed to shoot out the instant the light is turned on. It is true that Lord Rayleigh has shown[183] that an atom may conceivably absorb wave-energy from a region of the order of magnitude of the square of a wave-length of the incident light rather than of the order of its own cross-section. This in no way weakens, however, the cogency of the type of argument just presented, for it is only necessary to apply the same sort of analysis to the case of
-rays, the wave-length of which is sometimes as low as a hundredth of an atomic diameter (
