The spectrum of Nova Persei, in an instrument of sufficient power, appeared a truly magnificent object. Like other stellar spectra, it displayed the long line of light marked with the hues of the rainbow, but it was unlike the spectra of ordinary stars in respect of the enormous enhancements of the brightness at various parts of this spectrum. For instance, at one end of the long coloured band a brilliant ruby line glowed with a lustre that would at once attract attention, and demonstrated that the object under view must be something totally different from ordinary stars. This superb feature is one of the lines of hydrogen. The presence of that line showed that m the source from which the light came there must have been a remarkable outbreak of incandescent hydrogen gas. At various points along the spectrum there were other beautiful bright lines which, in each case, must have been due to glowing gas. Here we have the evidence of the spectrum telling us in unmistakable language that there were features in this star wholly unlike the features found in any ordinary star. It is impossible to dissociate these facts from the history of the star. Much of what we have said with regard to the spectrum of Nova Persei might be repeated with regard to the spectrum of the other temporary stars which, from time to time, have burst forth. In each case the spectrum characteristic of an ordinary star is present, but superadded to it are bright lines which indicate that some great convulsion has taken place, a convulsion by which vast volumes of gas have been rendered incandescent. In Fig. 54 we show the spectrum of Nova Persei on five dates, from February 27th to March 28th, 1901. These photographs were taken by Mr. Ferdinand Ellerman with the great telescope of the Yerkes Observatory. They show in the clearest manner the bright lines indicating the incandescent gases. We have pointed out the high probability that among the millions and millions of bodies in the universe it may now and then happen that a collision takes place. Have we not also explained how the heat generated in virtue of such a collision might be sufficient, and, indeed, much more than sufficient, to raise the masses of the two colliding bodies to a state of vivid incandescence? A collision affords the simplest explanation of the sudden outbreak of the star, and also accounts for the remarkable spectrum which the star exhibits. ------------------------------------------------------------------------ CHAPTER XIX. CONCLUDING CHAPTER. Comprehensiveness of the Nebular Theory—Illustration—Huxley and the Origin of Species—Rudimentary Organs—The Apteryx—Its Evanescent Wings—The Skeleton—An Historical Explanation—Application of the Same Method to the Nebular Theory—The Internal Heat of the Earth—The Lady Psyche.