PROBLEMS OF ASTROPHYSICS 447 



astronomer of position, the latter uses the results of the former, and 

 vice versa. Let me give two illustrations. Astrophysics desires to 

 know the relative radiating power of matter in different types of 

 stars, the Sirian and solar types, for example. The meridian circle 1 

 and the telescope 2 discovered a companion to Sirius; the micro- 

 meter determined the form and position of the orbits; 3 the heliometer 

 observed the star's distance ; 4 and the photometer 5 measured the 

 quantity of light received from it. Computations determine from 

 these data that Sirius is but two and one half times as massive as 

 our sun, 6 whereas it radiates twenty-one times as much light; 7 from 

 which it follows that a given quantity of matter in Sirius radiates 

 many times as effectively as the same quantity of solar matter, a 

 fact of prime importance in the astrophysical study of all Sirian stars. 

 The parallaxes of the stars are needed by the student of stellar 

 evolution as well as by the student of the structure of the heavens. 



Again, the measurement of radial velocities of the stars has been 

 left almost completely to those observers who are especially inter- 

 ested in astrophysical problems and methods, yet it is the student 

 of astrometry who is eager to use their results. The overlapping of 

 the two departments of astronomy is but the symbol of progress. 



The term astrophysics is of the present generation, but the begin- 

 nings of astrophysical inquiry are somewhat older. Theories of plan- 

 etary evolution by Kant 8 and Laplace; 9 observations of nebula? 

 and star clusters by the elder Herschel, 10 and his wonderfully saga- 

 cious deductions concerning them; various studies of planetary 

 markings and conditions; systematic investigations of the sun-spots, 

 including Schwabe's discovery u of their eleven-year period; -- these 

 constituted the main body of the science in 1859. But the spirit of 

 inquiry as to the nature of the heavenly bodies was latent in many 

 quarters; and Kirchhoff's immortal discovery of the fundamental 

 principles of spectrum analysis 12 opened a gateway which many 

 were eager to enter. The spectroscope became at once, and has 

 remained, the astrophysicist's principal instrument. However, the 

 spectrum is not his only field, nor the spectroscope his only tool. 

 Radiation in all its aspects, and the instruments for determining its 



1 Bessel, Astronomische Nachrichten, nos. 514, 515, 516; and Monthly Notices, 

 Royal Astronomical Society, vi, 139. 



2 Astronomische Nachrichten, LVII, 131. 



8 Zwiers, Proceedings, Amsterdam Academy of Sciences, May 27, 1899. 

 4 Annals of the Cape Observatory, vm, part n. 



6 Annals, Harvard College Observatory, xiv, part i, 152. 



9 Auwers, Astronomische Nachrichten, cxxix, 232. 



7 Clerke, Problems in Astrophysics, 199. 

 ' Kant's Allgemeine Naturgeschichte. 



9 Laplace's Exposition du Systeme du Monde, u, 295. 



10 Voluminous and frequent papers in Philosophical Transactions, from about 

 1780 to 1820. 



1 Astronomische Nachrichten, xxi, 233; and Humboldt's Cosmos, part n, p. 401 . 

 12 Philosophical Magazine, xx, 93. 



