ASTRO N o M Y former, .from a preliminary investigation, reached the conclusion that the comet was probably identical with that of Lexell, which had been thrown into some new and unknown orbit by the action of Jupiter more than a century ago. Poor’s inquiry, however, which was made on more complete data, seemed to show that this view is untenable and that the comet had never before been seen. As to the origin of comets, the most important development has been the establishment of the general fact that they belong to the solar system, and are not stray wanderers through space, as was formerly thought possible. Were the latter the case, they would not partake of the motion of the solar system through space. Consequently, as the sun pursued its journey more comets would be found coming from the direction of its motion than from the opposite direction. Those which did come from the former direction would in general move in decidedly hyperbolic orbits. Now we may regard it as established that no comet moves in such an orbit. It is true that some of the computed orbits have a hyperbolic character, this being shown by the eccentricity coming out slightly greater than unity; but the excess, even in these exceptional cases, is always so small that we can only regard it as the result of accidental errors of observation on such ill-defined bodies. The fact being accepted that these bodies, before their observed entry into our system, were accompanying the sun on its journey through space, we are led to the following very probable conclusions:—All comets are to be regarded as having been in the beginning outlying nebulous masses, belonging to the same mass of matter as that which formed the solar system, and moving with it, but mostly at such great distances from the sun that their periods of revolution would be measured by tens or even hundreds of thousands of years. Were it not for the effect of planetary attraction long periods like these would be the general rule, though not necessarily universal. But at every return to perihelion a comet Avill be to some extent either accelerated or retarded in its movements by the action of Jupiter or any other planet in the neighbourhood of which it may pass. Commonly the action will be so slight as to have little influence on the orbit and the time of revolution. But should the comet chance to pass the orbit of Jupiter just in front of the planet, its motion would be retarded and the orbit would be changed into one of shorter period. Should it pass behind the planet, its motion would be accelerated and its period lengthened. In such cases the orbit might be changed to a hyperbola, and then the comet would never return. It follows that there is a tendency towards a gradual but constant diminution in the total number of comets. If we call A the amount by which the eccentricity of a cometary orbit is less than unity, A will be an extremely minute fraction in the case of the original orbits. If we call ± oe the change which the eccentricity 1 — A undergoes by the action of the planets during the passage of the comet through our system, it will leave that system with the eccentricity 1 — A + Se. The possibilities are even whether Se shall be positive or negative. If negative, the eccentricity will be diminished and the period shortened. If positive, the minuteness of A may result in the eccentricity 1 - A + Se being greater than unity, and then the comet will become for ever a wanderer through the stellar spaces. Another discovery is that of the possibility of groups of comets moving in nearly the same orbit. It was formerly thought that only by a very improbable chance would two of these bodies be found describing the same orbit at a long interval of time, and, if an instance was noticed in which such appeared to be the case, it was supposed that what was observed on the second occasion
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was a return of the former comet. In February 1880 a brilliant comet which was suddenly observed in the southern hemisphere was found to be moving in an orbit so nearly identical with that of the great comet of 1843 that no doubt of a relationship could be felt. Yet so slight could have been the deviation from a parabolic orbit in either case that the hypothesis of identity of the two bodies did not seem tenable. The question of possible identity was set at rest by the appearance of a third comet in the autumn of 1882 again describing nearly the same orbit. It now seems likely that these three bodies were parts of one original nebulous mass, situated far beyond the confines of our system, which gradually separated from each other in the course of successive revolutions round the sun. The question whether the motions of Encke’s comet are affected by a resisting medium is not yet conclusively settled. The careful and exhaustive researches of von Asten and Backlund seem to show that resistance is met with during some revolutions and not during others. This would scarcely be the case with a resisting medium, as this agency would always be present if it existed at all. Quite possibly the effect may be due to imperfections in the computation of the perturbations. There is perhaps no celestial object on which observations are so much wanted as on the zodiacal light. Up to the present time the conclusions which can be definitely stated are somewhat general in their nature. The sun is surrounded by an exceedingly rare cloud of matter of very indefinite outline, lenticular in form, extending out somewhat beyond the orbit of the earth. This matter shines only by reflected sunlight, a conclusion which, probable enough in itself, is strengthened by the spectroscopic studies of A. W. Wright, who found the spectrum to be continuous. In northern latitudes, this object can best be seen in the evenings of February and March and in the mornings of September and October. Observations made in the evening seem to show that the central axis of the apparent light is one or two degrees north of the plane of the ecliptic, and this would point to a slight inclination of the median plane of the whole mass to that of the ecliptic. But it is impossible to reach any definite conclusion as to the position of that plane until the apparent axis of the light among the stars has been carefully delineated night after night through an entire year by an observer in a very clear atmosphere within the tropics. It needs only a glance at the figure of the apparent light in the neighbourhood of the horizon when it can first be seen, about the close of twilight, to show that its breadth at the horizon is very considerable, probably 40 degrees. Continuing the outline around the sun it seems probable that its visible border is nowhere less than 25 or 30 degrees from the sun. It follows that an observer stationed at a high elevation, in a very clear atmosphere, in such a position that the sun should be about 20 degrees below his horizon at midnight, should see this portion of the light as a faint glow in the northern horizon. The existence of the mysterious counter glow, or Gegenschein, as it is commonly called, is now fully established. It is a patch of light in the ecliptic opposite the sun, so faint and indistinct in outline that it can be seen only under the most favourable conditions, including absence of moonlight and a considerable elevation above the horizon. It is invisible when projected on the Milky Way. Barnard has observed it to be larger and more diffused in the autumn than in the spring. No wellmarked deviation from the ecliptic or from the point directly opposite the sun in longitude has been detected. The phenomenon has sometimes been supposed to be
