This theoretical inference has been confirmed with singular aptness by the discovery of spectroscopic binaries. Pairs circling in orbits too narrow for visual discernment are the natural complement of pairs just divisible with the telescope; the first class represent the unseen, early stages of the second. The two together form an unbroken sequence of stellar systems, for spectroscopic binaries include couples fully separated, and still separating, as well as others barely divided, and revolving almost in contact. Nay, they include specimens, we are led to believe, of globes conjoined into the apioidal figure theoretically investigated by Darwin and Poincaré, which may be regarded as preparatory to the development by fission of two mutually revolving stars from one primitive rotating mass. Some of these supposed dumb-bell systems are variable in light; and if the eclipse-rationale of their obscurations be confirmed by the spectroscope, there is no gainsaying the inference that each flickering object is composed of two stars actually contiguous, if not confluent. Now, compound stars are by no means of exceptional occurrence. Their relative abundance has been found to augment rapidly with every advance in our knowledge of the heavens. From the measures of stellar radial velocity lately carried out at the Yerkes Observatory by Professors Frost and Adams, it appears that the proportion of binary to single stars considerably exceeds Professor Campbell's earlier estimate. If those giving helium-spectra are alone considered, there are most probably as many of one kind as of the other. But why the distinction? it may be asked. The answer is not far to seek. Helium stars are the most primitive, and form the closest and most readily apparent systems. The companions of more fully developed stars would be likely to give less striking spectroscopic signs of their presence. A physically double star must always remain such. There is no law of divorce by which it can put away its companion, although their relations must alter with time. But their alteration tends continually to enhance the difficulty of their detection. For as the members of a pair are pushed asunder by tidal friction their velocity slackens, and the tell-tale swing of their spectral lines diminishes in amplitude, and finally, by its minuteness, evades observation.