excited in the proper way, say by the impact of cathode rays possessing a suitable amount of energy, and that the lines which are actually observed in the spectrum of an element may be determined by the energy which can be given to the electrons, which are sucked into the atom by the attraction the atom exerts upon them, rather than by the inability of the atom to vibrate in other periods. We may compare an atom to an orchestra with a complete set of instruments; the notes given out will depend upon the players as well as upon the instruments, and the absence of certain notes may be due to the absence of the appropriate players, and not to that of the appropriate instrument.
On this view almost any vibration could be excited if the atom were bombarded with cathode rays of suitable energy, and the vibrations in the visible spectrum are to be regarded as excited by the impact of cathode rays in much the same way as Röntgen rays are excited in a discharge tube, the difference being merely that the cathode particles which excite the Röntgen rays have much more energy than those required to excite the rays in the visible spectrum,—that in fact, in the way it is produced, as well as in its physical nature, visible light is a special type of Röntgen ray.
We can produce a system which is still simpler than the ordinary hydrogen atom, for we can extract the electron from the atom and get merely the positive charge left: these positively charged hydrogen atoms exist in large numbers in the positive rays. The hydrogen atom, minus its electron, is the simplest atom we can conceive; it is much simpler than the normal hydrogen atom, with its electron intact, and essentially different from it. The investigation of its properties is a matter of very great interest. The comparison of the spectrum