Röntgen rays harder than the hardest 'characteristic' radiation of an atom are scattered by the atom every electron does its full share of the work, so that the scattering measures the total number of electrons in the atom; if now we take Röntgen rays which, while softer than the hardest characteristic, are harder than any of the other types of radiation given out by the atom, they will not be scattered appreciably by the electrons in the inner ring, but they will be by all the other electrons; thus the scattering of these rays will give us the number of electrons not in the inner ring. We already know the total number of electrons in the atom; the difference of these numbers will be the number in the inner ring. Then if we measure the scattering of Röntgen rays softer than the next hardest characteristic, but harder than any of the others, we can determine the number of electrons outside the two inner rings; this, since we know the total number of electrons and the number in the first ring, will give us the number in the second ring. Thus, by measuring the scattering of softer and softer Röntgen rays, we can determine one after another the numbers of electrons in the rings.
The outer ring of all is the one which gives vibrations slow enough to come within the range of the visible spectrum; we might expect, therefore, if we measured the scattering of light well up in the ultra-violet, to be able to determine the number of electrons in the outer ring, which is in many connexions by far the most important of all. The scattering of light is very closely connected with the refractive index, so that if we know the refractive indices for light going well up in the ultra-violet we could also deduce the number of electrons in this ring. Drude some time ago, and more recently Erfle and Mr. and Mrs. Cuthbertson, have investigated the number of electrons