It was explained long ago that these lines are due to electrons which revolve rapidly round the nuclei of the atoms. Their sudden changes of velocity give rise to a wave (like those caused in water when you drop a pebble into it) in the surrounding medium, and this is one of the characteristic luminous waves of the atom. It reveals itself in one of the lines of the spectrum. The Danish physicist Bohr has recently developed this theory in detail, and has shown that it accurately explains the various spectral lines of the different chemical elements. These, I may note, differ from each other in the number and arrangement of the electrons which revolve within their atoms. Now Sommerfeld has argued as follows. The electrons which gravitate near the centre of an atom must have a higher velocity than those which revolve in its outer part; just as the smaller planets, Mercury and Venus, revolve round the sun far more rapidly than the larger planets, Jupiter and Saturn. It follows if Lorentz and Einstein are right that the mass of the interior electrons of the atoms must be greater than that of the exterior electrons: appreciably greater, as the former revolve with enormous velocities. We can calculate that, in those conditions, each line in the spectrum of a chemical element must in reality consist of a number of fine lines joined together. This is precisely what Paschen afterwards (1916) found. He discovered that the structure of the fine lines is strictly such as Sommerfeld had predicted. It was an astonishing confirmation of an hypothesis: a proof of the soundness of the new mechanics. But that is not all. We know that the X-rays are vibrations analogous to light, the same in origin, but consisting of much shorter waves, or waves with a far higher frequency. Hence, while light comes from the external electrons of the miniature solar system which we call an atom, the X-rays come from the most rapid electrons—those nearest to the centre. It follows that the special structure of the fine lines, due to the variation of the mass of the electron with its velocity, must be much more marked in the case of the X-rays than in the case of the spectral lines of light. This, again, was confirmed by experiment. The figures expressing the observed facts correspond exactly with the calculations of the new mechanics, as regards the predicted variation of mass with velocity. It is therefore settled that the phenomena which take place in the microcosm of each atom are subject to the laws of the new mechanics, not the old, and that, in particular, masses in motion vary as the new mechanics demands. Experience, “sole source of truth,” has given its verdict.