254 



KNOWLEDGE. 



[November 1, 1895. 



and stars, but also from that striking relationship discovered 

 by Sir William Herschel in his searches for nebulse ; for, 

 as has been so often quoted, after sweeping through a barren 

 region, he would tell his assistant — " Prepare to write, for 

 neiiulffi are coming, "as he found himself already on nebulous 

 ground. The two examples of nebulfe which Dr. Eoberts 

 furnishes in the present number of Knowledge, stand in just 

 such a typical position, near the debateable line between 

 a region rich in stars, and one comparatively barren 

 of them. Such a relation, repeated over and over again 

 in diiJerent parts of the heaven, furnishes the most 

 irrefragable proof, both that nebulse are situated at the 

 same general distances from us as stars, and that the 

 condensations of stars are not apparent only, but real. 



We may be sure, then, that two million times the 

 distance of the sun is the least distance which we can 

 ascribe to them on the average, and that many must be 

 much farther still ; yet even at this minimum remoteness, 

 a nebula, whose vaporous arms stretch over S° of sky, 

 must span within its touch an extent of space greater 

 than that which separates us from a Centauri. 



This one fact is sufficient to show how utterly different 

 is the state of things prevailing in the region of the nebula3 

 from that which prevails in the vicinity of our own solar 

 system. So far as we know, the hu-uhi of the Centaur is 

 our nearest neighbour in the heavens, and we know of 

 nothing existing in the mighty void which intervenes 

 between us. A few comets of medium period form, as it 

 were, a feeble fringe to the solar system, of which, other- 

 wise, the orbit of Neptune would mark the boundary. 

 Possibly some of our comets and meteors come to us from 

 the further side of the gulf ; but on the whole, the solar 

 system remains a solitary island system, separated from 

 any kindred structure by nine thousand times its own 

 semi-diameter. 



" There sinks the nehulous star we call the sun, 

 If that hypothesis of theirs be sound." 



And that the Princess's epithet of "nebulous" justly 

 belongs to the sun, when its origin and past history is 

 considered, is generally conceded. But no contrast could 

 be more striking than that between the intense concen- 

 tration, the almost absolute simplicity, which we see in 

 the solar system, so completely and so widely removed as 

 it is from all neighbouring systems, and the vast far- 

 reaching diffusion of matter seen in such nebulse as those 

 of the Ssorpion of Orion or the Pleiades. 



An amusing jeu d'csptit published a few years ago claimed 

 to have discovered the mathematics of politics, and to find 

 the geometrical expression of the various forms of govern- 

 ment in the conic sections. Pieversiug this analogy, we 

 may look upon the solar system as a despotism of the 

 strictest kind, a pure autocracy, with the entire power 

 vested in one supreme unchallenged head. The sun 

 exceeds in mass the sum of all his planets and dependents 

 seven hundred and forty-five times. Were they all anni- 

 hilated the solar system would have suffered loss by but 

 one and one-third parts out of a thousand. 



But just as the despotic form of government is not by 

 any means the only one to be found in exercise on the 

 earth, so it is abundantly evident that our solar system is 

 by no means the one universal model on which all systems 

 are constructed. The discovery of such multitudes of 

 double stars, the yet more remarkable cases of the Algol 

 stars, and of the " spectroscopic doubles," show that 

 systems in which the supreme control is divided between 

 two or more bodies are very common. 



Nor can we doubt that systems exist which are organized 

 upon what, following the same analogy, we may describe 

 as a thoroughly democratic basis ; systems in which there 



is no one central sun, nor even two or three great bodies, 

 but in which the general mass is distributed not very 

 unevenly between a great number of comparatively small 

 bodies, none of which are large enough to dominate the 

 rest. 



Such a system on a vast scale may perhaps be exhibited 

 to us by the superb object which Dr. Eoberts has photo- 

 graphed for the October number of Knowledge. The 

 approach to circularity in its outline, and the rapid 

 increase in the condensation of the cluster as the centre 

 is approached, point to its being really, and not merely 

 apparently a globular cluster, and we may take it that its 

 depth in the line of sight does not greatly differ from its 

 length at right angles to it. Its more condensed central 

 and spherical portion has a diameter of about five minutes, 

 which its outlying stars increase to fully twelve minutes of 

 arc. This, with a parallax of one-tenth of a second, would 

 give a diameter to the cluster of two-thirds of a billion miles. 

 This is a little more than one-fortieth of the distance 

 between the Sun and a Centauri. But whereas we have no 

 companion Sun in all that distance, here we find thousands 

 of stars massed together within one cluster. The several 

 stars which make up this cluster must, therefore, be on 

 the average, let us say, about one thousand times closer 

 to each other than are the Sun and a Centauri. 



It is, then, in a very different condition as regards the 

 nearness of its component members than we have any 

 experience of in our region of space. And the stars which 

 compose it are of an inferior rank to our sun, if the 

 parallax of one-tenth of a second be adopted. At such a 

 distance our sun would shine like a star of 4j magnitude, 

 if Mr. Gore's estimate of its actual magnitude as — 27 be 

 correct, and four suns as large as our own would give as 

 much light as the entire cluster. Either, then, the in- 

 dividual stars of the cluster are mitch inferior in intrinsic 

 brightness to our sun, or they must, on the average, be 

 smaller bodies than our earth. A system of about the same 

 total mass as our own, but with the place of the sun taken 

 by thousands of bodies of the size of Jupiter and of our 

 earth scattered over a region two hundred and fifty times 

 the orbit of Neptune in diameter would not represent 

 badly the great cluster in Hercules. 



The distance we have assumed is, of course, a minimum. 

 The cluster may be, and probably is, much farther from 

 us, and its components, therefore, larger than we have 

 supposed in the same proportion. But the cluster would 

 remain a cluster still — an association of stars, not greatly 

 dissimilar in size, and, relatively to their size, in pretty 

 close contiguity to each other ; and though, in this 

 particular case, we may assign almost any distance we like 

 to the cluster, yet, as Mr. Eanyard has shown in more 

 than one place, we cannot do the same for the Pleiades 

 and for parts of the Milliy Way. In these regions we are 

 obliged to conclude that the faintest stars we see are either 

 of very low intrinsic brightness, or else are of the same 

 order as the Earth or Jupiter in the matter of size. 

 (To be continued.) 



Hcttcrs. 



[The Editor does not hold himself responsible for the opinions or 

 statements of correspondents.] 



Dr. EOBERTS' PHOTOGRAPHS. 

 To the Editor of Knowledge. 



SiK, — Eeferring to Dr. Eoberts' photographs of 13 M., 

 and the means he suggests for comparing those taken 

 at intervals of eight years for the purpose of detecting 



