RECOLLECTIONS AND REFLECTIONS
the nature of light ought to be the same as that of these rays. Rontgen rays ionise a gas through which they pass, and if the wave front of a beam of these rays were continuous no molecule in the path of the beam could escape from being struck by the rays and all would be equally affected by it. We should expect either that no molecules would be dissociated or that a large proportion would be. This, however, is not the case. In the strongest beam of Rontgen we can produce, only an infinitesimal fraction of the molecules struck by the beam are ionised. I pointed out in the Silliman Lectures that I gave at Yale University in 1903 (Electricity and Matter, Scribner) that this indicated that the front of the beam could not be continuous, but must be more like a series of bright spots on a dark background, i.e. that the energy must be concentrated in separate bundles. This is a small part of what was afterwards known as the Quantum Theory of Light, the other part of that theory being Planck's Law that the energy in each bundle is equal to hv.
The quantum of light must, like a corpuscle on the Corpuscular Theory of Light, travel through space without change, and yet it must, like the train of waves of the Undulatory Theory, produce interference phenomena. I suggested in a letter published in Nature, February 8, 1936, that the quantum is a train of waves, but that the waves are of a somewhat different character from those in the ordinary theory. The waves I considered were waves where the lines of electric force were circles, all these circles had their centres on a straight line and their planes at right angles to it. I showed that a train of waves of this kind would travel out in the direction of its axis without suffering any change, thus satisfying the first condition for the quantum, while, since the 410