using a special type of photographic plate, I have, however, been able to photograph radiations produced by electrons whose speed corresponded to only 20 volts, and by increasing the speed of the electrons, to get harder and harder radiations, until at last they were as hard as the kind hitherto studied. The softest radiations obtained in this way could not get through a film of collodion, though this was no thicker than a soap bubble; they are probably identical with those forms of ultra-violet light which are called, after their discoverer, Schumann rays; with these soft rays we may hope to fill up the interval between visible light and the hardest Röntgen rays. These soft Röntgen rays are, I am convinced, likely to prove of great service in investigating the question of the structure of the atom; they promise to enable us to determine the number of different groups or rings present in the atom, and to determine the number of electrons in each ring. Thus, for example, if we can measure the absorption of an element for the whole gamut of Röntgen rays, starting from those characteristic of a heavy element and going down to Schumann rays, then whenever the rays pass through a type corresponding to one given out by the element, there will be a sudden jump in the absorption; by counting the number of these jumps we could get the number of rings of electrons in the atom. Or if we measured the emission of Röntgen rays caused by the impact against the element of cathode rays of different velocities, there would be similar jumps every time the velocity of the cathode rays reached the value which could stimulate a Röntgen ray characteristic of the element.
We could determine the number of electrons in each ring by an extension of the method used to determine the total number of electrons in the atom. When