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 ASTRONOMY evidehce of connexion is nearly conclusive. If it does not, we assume that they are simply two isolated stars. In the article Astronomy (Ency. Brit. vol. ii. p. 820) it was shown that five stars of Ursa Major have a common proper motion, all moving nearly in the same direction and with equal velocity. But though no other case like this has yet been discovered, there are several cases of pairs of stars near each other betraying a relationship by their proper motion, which would never have been suspected on any other ground. The two most remarkable cases of this sort are—(1) a pair of stars in B,. A. 15 hours 4 minutes Dec. —15° 54', five minutes apart, and (2) A Ophiuchi, which has a star of the same proper motion following it about 13'. It is not unlikely that pairs like these form binary systems with very eccentric orbits and long periods of revolution. It does not now appear that we can draw any sharp line as regards numbers of stars between the binary systems which we have been considering and clusters clusters whose components are too numerous to be counted. Triple or quadruple stars have long been familiar ; of the latter 9 Orionis in the great nebula of Orion affords a typical example. In this case two other stars much more minute belong to the system. Sometimes it is possible to make an actual count of the stars in a cluster, as is the case with Praesepe, in Cancer, which appears to the naked eye as a nebula, and which the telescope shows to be composed of a definite number of comparatively bright stars. We have also such agglomerations as the great cluster of Hercules, where a complete resolution into stars is just possible with the best telescopic power yet reached. The application of photography to these objects has thrown much light on their general character. Many of them have been photographed at the Harvard Observatory, but even there it has not always been possible to separate all the individual stars in the densest clusters. Perhaps the most remarkable feature brought out has been the great number of variable stars in some of the clusters. The following statement is extracted from a Harvard circular:— Number Stars Cluster, of variables. examined. 125 3000 w Centauri 132 900 Messier 3 185 900 Messier 5 In all 509 variable stars were found among 19,050 examined in twenty-three clusters, a number which exceeds that known to exist in the rest of the sky. Perhaps the most interesting questions suggested by these cluster systems is that of their stability. On any probable hypothesis that we can make as to the constitution of the individual stars, their mutual attraction must be such as would bring them into a single mass in the course of a few thousand years. Why, then, have they not condensed into such a mass? Perhaps the most plausible answer that can now be made is that the individual stars are all moving in irregular orbits under their mutual attraction. If such be the case, the motion would be made manifest by the comparison of photographs taken at intervals of twenty, fifty, or 100 years. The questions which have arisen in our time respecting the constitution of various nebulae, the changes which they undergo, their individual forms, and their reNehulx. jmay ations tQ the gtars seen among them, are so numerous that no discussion of them is possible within our present limits. The greatest addition to our means of knowledge has been made, as in many other cases, by the application of photography. Here the works of Dr Isaac Roberts, F.R.S., and Dr A. A. Common, F.R.S., are worthy of special study; their results are for the most part to be

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found in the publications of the Royal Astronomical Society. One fact brought out quite recently is the superiority of the reflecting telescope over the refractor in this connexion, owing to the absence of chromatic aberration which, even in the secondary character it assumes in the best refracting telescopes, is very appreciable in the long exposures necessary. The great advantage of photography is due to the fact that, in a clear dark sky, it is possible to obtain images of objects which escape vision. This is the case not only with the smaller stars, but apparently in a still higher degree with the nebulae. Among the most remarkable objects brought out by photography is a great nebulous mass winding through a considerable portion of the constellation Orion, first discovered by W. H. Pickering, and afterwards confirmed by Barnard. Among other remarkable nebulosities of this sort is one in the region of p Ophiuchi, photographed by Barnard in 1894; while a third is the singular nebulosity surrounding the Pleiades, carefully studied by Barnard (Monthly Notices B. A. S. January 1900). Quite recently a surprising number of new nebulae have been photographed by the Crossley reflector of the Lick Observatory. Professor Keeler concludes that, judging from the number of these objects brought out with this instrument, there must be as many as 100,000 in the heavens, an average of between two and three to every square degree. It would, therefore, scarcely be an overstatement to say that the heavens are to be regarded as filled with nebulae. The spectrum of these objects (see AWy. Brit. vol. ii. p. 821) seems to show that they are gaseous in constitution, as it consists almost wholly of a very small number of bright lines. An apparent exception is afforded by the great nebula of Andromeda, which gives a continuous spectrum ; but in interpreting such a spectrum it must be noted that a gaseous mass gives a bright line spectrum only when it is transparent through and through. If the mass is of such size that a ray of light cannot pass through it the spectrum will, in general, be continuous, like that of a solid body. If the superficial portions are cooler and more tenuous than the interior, the continuous spectrum will be crossed by dark lines, as in the case of an incandescent body surrounded by a cool atmosphere. We conclude, therefore, that the continuous character of the spectrum of the Andromeda nebulae does not prove it to be composed of solid particles. The transparency of the nebulae in general, which would be evident even without spectroscopic analysis, shows how extremely tenuous these masses are. We cannot seriously doubt that as a general rule their distances from us are at least as great as those of the fixed stars, possibly much greater. In dimensions the most compact of them must far exceed our whole solar system; probably the great majority, even if we omit the diffused masses to which we can assign no definite outline, may exceed the diameter of the orbit of Hep tune a hundredfold. Yet we have reason to believe that a ray of light would not suffer any sensible absorption, except selectively, in passing through them. Constitution of the Stars and Structure of the Heavens. The view that the sun and stars are not bodies fitted to exist for ever in their present form is strengthened by all recent researches on their constitution. They are now regarded as bodies of intensely hot matter ^l^lutioa in the gaseous state, continually losing energy by the radiation of their heat into space, and therefore contracting in volume. This view was first developed by J. Homer Lane in a paper published about 1865 in the American Journal of Science; but his results were merely embodied in mathematical formulae, and not stated with such distinctness as to command attention at the time. S. 1.-95