November 6. 19 19] 



NATURE 



tion of the general magnetic field of the sun by 

 observations of Zeeman effects involving displace- 

 ments usually amounting to less than one- 

 thousandth of an Angstrom unit. Stellar spec- 

 troscopes have been improved by the provision 

 of temperature control and other aids to efficiency, 

 so that radial velocities are now measurable in 

 the case of the brighter stars to within a quarter 

 of a kilometre per second. With the exceptional 

 resources of the Mount Wilson observatory, 

 stellar spectra have even been photographed on 

 a scale comparable with that of Rowland's great 

 map of the solar spectrum, providing data for 

 deductions, among other things, on such a deli- 

 cate matter as that of the pressure in the atmo- 

 sphere of a star. 



Not less important has been the development 

 of experimental researches bearing upon the inter- 

 pretation of celestial spectra. The study of en- 

 hanced lines initiated by Lockyer has been especi- 

 ally productive, not onlv in relation to stellar 

 temperatures, but also in leading to a satisfactory 

 explanation of most of the lines which are met 

 with in the spectra of the hotter stars, where we 

 might well have expected that the reproduction 

 of the conditions would be outside the range of 

 our laboratory resources. The application to sun- 

 spots of Zeeman 's discovery of the effect upon 

 spectrum lines of a strong magnetic field, and 

 Ramsay's discovery of terrestrial helium following 

 its previous detection in the sun's chromosphere, 

 are familiar examples of the close bonds which 

 unite astronomy with other sciences to their 

 mutual advantage. 



The spectrum of the sun has naturally been the 

 subject of an immense amount of detailed study, 

 and as the work has progressed it has become 

 less and less probable that there are any sub- 

 stances in the sun which do not also exist on the 

 earth. The spectra of sun-spots and of the 

 chromosphere have also been minutelv recorded, 

 and most of their peculiarities have been satis- 

 factorily accounted for. The bright lines of the 

 coronal spectrum, however, have not yet been 

 matched in any terrestrial source, but the precise 

 knowledge of this spectrum which has been ob- 

 tained during total eclipses has stimulated theo- 

 retical investigations, and some extremely sug- 

 gestive relations have been deduced by Nicholson 

 in his calculations of the spectra of atoms of 

 assumed simple structure. Similar considerations 

 have also been extended to the unidentified lines 

 which occur in nebulae. 



As regards the stars, many of them have been 

 photographed in great detail for minute analysis, 

 and a multitude more for purposes of classifica- 

 tion. Secchi's classification, at first merely- 

 empirical, soon came to be regarded as indicating 

 the actual sequence of forms assumed by a star 

 in the process of cooling, and the same idea is 

 embodied in the Harvard system of classification, 

 which has been most widely adopted by astro- 

 nomers in recent years. Lockyer, however, has 

 based a classification on the supposition that there 

 must be stars which are becoming hotter as well 

 as stars which are cooling down, in accordance 

 with the theory of condensing masses of gas or 

 meteorites, and this view has lately been greatly 

 strengthened by the work of H. N. Russell on 

 the densities of stars. In either case the impres- 

 sive result is that the different types of stars are 

 not to be looked upon as arising from fundamental 

 differences of composition, but as representing 

 successive stages in an orderly evolutionary pro- 

 gression. 



The spectroscopic determination of the veloci- 

 ties of stars in the line-of-sight, irrespective of 

 distance, has united the old and the new astronomy 

 in the great task of deciphering the intricacies of 

 structure of the sidereal universe. Besides con- 

 tributing the velocities and spectral classes of 

 individual stars, the spectroscope has revealed the 

 existence of a large number of close binary 

 systems, and has provided the most trustworthy- 

 means of investigating the sun's motion in space, 

 the effect of which is to be eliminated in deducing 

 the movements of the stars tliemselves. 



An entirely new field for the spectroscope has 

 been opened up by the remarkable discovery by 

 Adams of a method of estimating the absolute 

 brightnesses, and thence the distances, of the 

 stars by mere iiTspection of photographs of their 

 spectra. This novel method is full of promise, 

 and encourages the hope that other equally un- 

 expected applications of the spectroscope may 

 yet be discovered. 



Lack of space forbids even the enunieration of 

 many other remarkable achievements, but suffi- 

 cient may have been said to convey some impres- 

 sion of the enormous extension of the scope of 

 astronomical research which has been brought 

 about by the introduction of the spectroscope. 

 It cannot be doubted that the spectroscope will 

 continue to play a leading part in the advance- 

 ment of our knowledge of the universe of which 

 we form a part. 



X-RAYS IN PHYSICAL SCIENCE. 



By Prof. W. H. Br.«;g, F.R.S. 



T is twenty-four years since Rontgen made the 

 famous discovery which at once excited such 

 imense and widespread interest. Everyone felt 

 he fascination of the photograph which actually 

 howed the bones of a living human hand. 

 NO. 2610, VOL, ro4] 



Surgeons seized on its obvious application to their 

 craft ; students of physical science realised that 

 a new and most powerful means of investigation 

 had been placed in their hands. And at the 

 present day we see that the first expectations have 



