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SCIENCE. 



LN. S. Vol. XVII. No. 421. 



enlarged, it was found that the system of 

 the fixed stars was made up of bodies so 

 vastly distant and so completely isolated 

 that it was difficult to conceive of them 

 as standing in any definable relation to 

 each other. It is true that they all emitted 

 light, else we could not see them, and the 

 theory of gravitation, if extended to such 

 distances, a fact not then proved, showed 

 that they acted on each other by their 

 mutual gravitation. But this was all. 

 Leaving out light and gravitation, the uni- 

 verse was still, in the time of Hersehel, 

 composed of bodies which, for the most 

 part, coixld not stand in any known rela- 

 tion one to the other. 



"When, forty years ago, the spectroscope 

 was applied to analyze the light coming 

 from the stars, a field was opened not less 

 fruitful than that which the telescope made 

 known to Galileo. The first conclusion 

 reached was that the sun was composed 

 almost entirely of the same elements that 

 existed upon the earth. Yet, as the bodies 

 of our solar system were evidently closely 

 related, this was not remarkable. But 

 very soon the same conclusion was, to a 

 limited extent, extended to the fixed stars 

 in general. Such elements as iron, hydro- 

 gen and calcium were found not to belong 

 merely to our earth, but to form important 

 constituents of the whole universe. "We 

 can conceive of no reason why, out of the 

 infinite number of combinations which 

 might make up a spectrum, there should 

 not be a separate kind of matter for each 

 combination. So far as we know, the ele- 

 ments might merge into each other by in- 

 sensible gradations. It is, therefore, a 

 remarkable and suggestive fact when we 

 find that the elements which make up 

 bodies so widely separate that we can 

 hardly imagine them having anything in 

 common, should be so much the same. 



In recent times what we may regard as 

 a new branch of astronomical science is 



being developed, showing a tendency to- 

 ward unity of structure throughout the 

 whole domain of the stars. This is what 

 we now call the science of stellar statistics. 

 The very conception of such a science might 

 almost appal us by its immensity. The 

 widest statistical field in other branches 

 of research is that occupied by sociology. 

 Every country has its census, in which the 

 individual inhabitants are classified on the 

 largest scale and the combination of these 

 statistics for different countries may be 

 said to include all the interest of the hu- 

 man race within its scope. Yet this field 

 is necessarily confined to the surface of 

 our planet. In the field of stellar statis- 

 tics millions of stars are classified as if 

 each taken individually were of no more 

 Aveight in the scale than a single inhabit- 

 ant of China in the scale of the sociologist. 

 And yet the most insignificant of these 

 suns may, for aught we know, have plan- 

 ets revolving around it, the interests of 

 whose inhabitants cover as wide a range 

 as ours do upon our own globe. 



The statistics of the stars may be said 

 to have commenced with Herschel's gauges 

 of the heavens, which were continued from 

 time to time by various observers, never, 

 however, on the largest scale. The sub- 

 ject was first opened out into an illimit- 

 able field of research through a paper pre- 

 sented by Kapteyn to the Amsterdam 

 Academy of Sciences in 1893. The capital 

 results of this paper were that different 

 regions of space contain different kinds 

 of stars and, more especially, that the stars 

 of the Milky Way belong, in part at least, 

 to a different class from those existing 

 elsewhere. Stars not belonging to the 

 Milky Way are, in large part, of a dis- 

 tinctly different class. Yet, the extent of 

 each of these classes is as great as that of 

 the universe. Throughout the whole of 

 the extent of the latter, we find in one 

 direction a certain class of stars to be pre- 



