436 



SCIENCE 



[N. S. Vol. XLII. No. 1083 



from the accumulation of data, for which 

 we are indebted to generations of astron- 

 omers, and from the gradual increase in 

 power and perfection of our instruments. 

 The first insight into the stars as a whole 

 naturally came from the survey of their 

 numbers and distribution; and Herschel, 

 who constructed the fiirst great telescopes, 

 explored the heavens with untiring sldH and 

 energy, and speculated boldly on his ob- 

 servations, is justly regarded as the founder 

 of sidereal astronomy. In his great paper 

 "On the Construction of the Heavens," 

 Herschel gives the rules by which he was 

 guided, which I should like to quote, as 

 they may well serve as a motto to all who 

 are engaged in the observational sciences: 



But first let me mention that if we would hope to 

 make any progress in an investigation of this deli- 

 cate nature we ought to avoid two opposite ex- 

 tremes of which I can hardly say which is the most 

 dangerous. If we indulge a fanciful imagination 

 and build worlds of our own, we must not wonder 

 at our going wide from the path of truth and na- 

 ture; but these will vanish like the Cartesian vor- 

 tices, that soon gave way when better theories were 

 offered. On the other hand, if we add observation 

 to observation, without attempting to draw not 

 only certain conclusions but also conjectural views 

 from them, we offend against the very end for 

 which only observations ought to be made. I will 

 endeavor to keep a proper medium; but if I should 

 deviate from that I could wish not to fall into the 

 latter error. 



In this spirit he discussed the "star 

 gauges" or counts of stars visible with his 

 great reflector in different parts of the sky, 

 and concluded from them that the stars 

 form a cluster which stretches to an un- 

 known but finite distance, considerably 

 greater in the plane of the Milky Way than 

 in the perpendicular direction. He gave 

 this distance as 497 times that of Sirius. 

 He did not hesitate to advance the theory 

 that some of the nebula were similar clus- 

 ters of stars, of which that in Andromeda, 

 judging from its size, was the nearest. 



Herschel had no means of telling the scale 

 of the sidereal system, though he probably 

 supposed the parallax of Sirius to be of the 

 order of 1". 



Though some of the assumptions made by 

 Herschel are open to criticism, the result at 

 which he arrived is correct in its general 

 outline. I shall attempt to give a brief 

 account of some of the principal methods 

 used to obtain more definite knowledge of 

 the extent and constitution of this "island 

 universe." The stars of which most is 

 known are, in general, those nearest to us. 

 If the distance of a star has been measured, 

 its coordinates, velocity perpendicular to 

 the line of sight, and luminosity are easily 

 found. In the case of a double star the 

 orbit of which is known the mass may also 

 be determined. But only a very small pro- 

 portion of the stars are sufficiently near for 

 the distance to be determinable with any 

 accuracy. Taking the distance correspond- 

 ing to a parallax of 1" or the parsec as 

 unit — i. e., 200,000 times the distance of 

 the earth from the sun — fairly accurate 

 determinations can be made up to a distance 

 of 25 par sees, but only rough ones for 

 greater distances. 



For much greater distances average re- 

 sults are obtainable from proper motions, 

 and the mean distances of particular classes 

 of stars — for instance, stars of a given 

 magnitude or given type of spectrum — can 

 be found with confidence up to a distance 

 of 500 parsecs, and with considerable un- 

 certainty to twice this distance. The den- 

 sity of stars in space as a function of the 

 distance, the percentage of stars within 

 different limits of luminosity, the general 

 trend of the movements of stars and their 

 average velocities can also be found, within 

 the same limits of distance. 



For all distances, provided the star is 

 sufficiently bright, its velocity to or from 

 the earth can be measured. The general 



