ASTRONOMY The view expressed in the ninth edition of this work, that Jupiter shines partly by its own light, has not been confirmed by later researches. The complete disappearance of the satellites, even in the most powerful telescopes, while in the shadow of Jupiter, shows that they cannot receive sufficient light from that planet to be visible, and the uniform darkness of the shadow of the satellite when seen on the planet, shows that the latter cannot be self-luminous. It is also to be remarked that, were it only moderately self-luminous, the colour of the light which it sends to us would be red, such light being that first emitted from a heated body when its temperature is raised. While it is quite possible that a small quantity of such light may reach us from the planet, no facts to prove it have yet been adduced. In April 1898, the probable discovery of a ninth satellite of Saturn by W. H. Pickering was announced from the Harvard College Observatory. The systeman new object was detected by means of four photographs of the region in the immediate neighbourhood of Saturn, taken on 16th, 17 th, and 18 th August 1898. A minute star was found on each of the plates, occupying a position which was vacant on the other plates. The object was of the fifteenth magnitude, and therefore so faint as to be invisible to the eye by any but the most powerful telescopes. Its position relative to Saturn could not be readily determined, because the long exposure necessary to bring out so faint an object resulted in the image of Saturn being completely “ burned ” out on the plates; in fact, the planet and rings appeared only as a large, dark, faintly-defined blotch of light, several times larger than the planet itself. Assuming that the object in question was a satellite of Saturn, it must be at some distance outside of all the other satellites, and its time of revolution must be several months. As the object does not seem to have been certainly detected at the following opppsition, some doubt may yet exist as to its nature. The planet is now in a region rich in stars, and as so faint an object is easily lost among the hundreds which surround it, its detection is difficult. The visible surface of Saturn bears a certain resemblance to that of Jupiter, but the markings are so much fainter that it is difficult, if not impossible, to locate and identify them with precision. There have been two notable exceptions to this rule. Sir William Herschel once saw a spot which lasted several weeks and enabled him to fix the time of rotation of the planet on its axis at 10 h. 16 m. The second case occurred in 1876. On 7th December of that year a bright white spot suddenly appeared in the equatorial region. It continued more than a month, gradually extending its dimensions in an east and west direction, so as to assume the form of a long streak, and at the same time growing fainter. From observations of the brighter portion, Professor A. Hall fixed the time of rotation at 10 h. 14 m. The question whether any markings ordinarily exist on the planet strong enough to be certainly located is an open one among the best observers. Cautious students of the subject will probably give most weight to the negative side. (See Monthly Notices R. A. S., for observations and discussions, by Williams and Barnard.) The most noteworthy addition to our knowledge of the rings of Saturn has been made by Keeler. That these appendages cannot be rings of continuous matter, 1 S e ier rings* itl solid or liquid, has long been well established by theory, which showed that the equilibrium of such an object would necessarily be unstable. The alternative hypothesis that the rings are a cloud of minute satellites, or perhaps mere particles, too small to be individually visible, but so numerous as to look in our telescope like a continuous mass, was exhaustively investigated by J. Clerk Maxwell in his Adams prize essay published about 1860; but no direct evidence bearing on the question was obtainable until the spectro-
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scope was brought into requisition. By the aid of this instrument in its present refined form, the motion of a body to or from the earth can be made evident by the change produced by the motion in the wave length of the spectral rays. The method is equally applicable whether the body is self-luminous or shines by the reflected light of the sun. By photographing the spectrum of Saturn and its rings when the image was thrown upon the narrow slit of a spectrograph, Keeler found that the lines across the spectrum were bent and broken in such a way as to show that the inner part of the rings revolved with greater velocity than the outer part, the motion of each part being that .which would correspond to the motion of a satellite revolving at the same distance. The extreme thinness of the rings was demonstrated in a more striking manner than ever before by Barnard and others in 1892, during one of those rare times at which the plane of the ring passes between the earth and the sun. In such a case the sun shines on one surface of the ring, while only the opposite surface is turned towards us. Considering it as a cloud of satellites, each of the latter should in such a case be fully illuminated by the sun, except so far as it might be in the shadow of those outside of it. The outer satellites of all would necessarily be fully illuminated and also be visible to us if we had sufficient telescopic power, because there would be nothing to hide their light. Now, it is a singular fact that, in these circumstances, the ring was completely invisible, even with the 36-inch telescope of the Lick Observatory. This shows that the entire ring must be so thin that its edge is completely invisible, even in the full light of the sun, at the distance which separates us from the planet. On the other hand, the objects composing it must be completely opaque, as is shown not only by their disappearance in the circumstances we have mentioned, but by the darkness of the shadow which they cast upon the planet when the sun illuminates them obliquely. The cloud of satellites is so dense that a ray of light cannot penetrate the mass. An interesting question still open is the nature of the so-called divisions of the rings. Are these divisions real or are they simply apparent, arising from a darker colour in the matter which composes them 1 In the case of the sharpest and best-known division, to which the name of Cassini has been given from its first observer, there would seem to be little doubt that the division is real. But there is some doubt in the case of the other divisions. It is now well established that the dusky or crape ring, which is on the inside of the brighter one, is really in the nature of an inner border of the bright ring, the one shading off imperceptibly into the other. While many excellent observers have sometimes thought they saw a complete separation between the bright and the crape rings, no such phenomenon has been seen in the great telescopes of our times, and it is almost certain that the dark colour of the crape ring arises merely from its tenuity and transparency. From Barnard’s observation of the passage of Japetus through the shadow of Saturn and its rings it appears that the transparency gradually diminishes from the centre of this ring to its line of junction with the bright ring. If there should ever be a transit of Saturn centrally past a bright star, many questions as to the constitution of the rings might be settled by noting the times at which the star was seen through the divisions of the ring. The great distance of Uranus and Neptune, and the faintness of their illumination by the sun, have as yet prevented our reaching any well-defined views uranjaa as to their physical constitution. Micrometric and Nep= measures seem to show that the globe of Uranus tunian is slightly elliptical, like that of the other outer systemsplanets; this would indicate that the planet has a some-
