592 



SCIENCE. 



[N. S. Vol. XII. No. 303. 



absorbent we may study with tbe minutest 

 accuracy the action of the elements of ma- 

 terial bodies in all their variety of combi- 

 nations, upon definite and easily recognized 

 rays of light, and we may discover curious 

 analogies between their aflBnities and those 

 which produce the fixed lines in the spectra 

 of the stars. The apparatus, however, 

 which is requisite to carry on such in- 

 quiries with success cannot be procured by 

 individuals, and cannot even be used in 

 ordinary apartments. Lenses of large di- 

 ameter, accurate heliostats, and telescopes 

 of large aperture are absolutely necessary 

 for this purpose ; but with such auxiliaries 

 it would be easy to construct optical com- 

 binations, by which the defective rays in 

 the spectra of all the fixed stars down to 

 the tenth magnitude might be observed, and 

 by which we might study the effects of the 

 very combustion which lights up the suns 

 of other systems." 



Brewster's words are almost prophetic, 

 and it would almost appear as if he un- 

 knowingly held the key to the elucidation 

 of the spectrum lines, for it was not until 

 1859 that Kirchhoff's discovery of the true 

 origin of the dark lines was made. 



Fraunhofer was the first to observe the 

 spectra of the planets and the stars, and to 

 notice the different types of stellar spectra. 

 In 1817 he recorded the spectrum of Venus 

 and Sirius, and later, in 1823, he described 

 the spectrum of Mars ; also Castor, Pollux, 

 Capella, Betelgeux, and Procyon. 



Fraunhofer, Lamont, Donati, Brewster, 

 Stokes, Gladstone, and others carried on 

 their researches at a time when the princi- 

 ples of spectrum analysis were unknown, 

 but immediately upon Kirchhoff's discov- 

 ery great interest was awakened. 



With spectrum analysis thus established, 

 aided as it was later by the greater devel- 

 opment of photography, the new astronomy 

 was firmly established. 



The memorable results arrived at by 



Kirehhoff were no sooner published than 

 they were accepted without dissent. The 

 works of Stokes, Foucault, and Angstrom at 

 that period were all suggestive of the truth, 

 but do not mark an epoch of discovery. 



Astronomical spectroscopy divided itself 

 naturally into two main branches, the one 

 of the sun, the other of the stars, each 

 having its many offshoots. I shall just 

 mention a few points relating to each. The 

 dark lines in the solar spectrum had al- 

 ready been mapped by Fraunhofer, and 

 now it only needed better instruments and 

 the application of laboratory spectra with 

 Kirchhoff's principle to advance this work 

 still further. 



Fraunhofer had already pointed out the 

 way in using gratings, and these were fur- 

 ther improved by Nobert and Kutherfurd. 



Kirchhoff's ' Map of the Solar Spectrum,' 

 published in 1861-62, was the most com- 

 plete up to that time ; but the scale of refer- 

 ence adopted by him was an arbitrary one, 

 so that it was not long before this was im- 

 proved upon. Angstrom published in 1868 

 his ' Map of the Normal Solar Spectrum, ' 

 adopting the natural scale of wave-lengths 

 for reference, and this remained in use until 

 quite recent times. 



The increased accuracy in the ruling of 

 gratings by Eutherfurd materially improved 

 the efi&ciency of the solar spectroscope, but 

 it was not until Professor Eowland's inven- 

 tion of the concave grating that this work 

 gained any decisive impetus. The maps 

 (first published in 1885) and tables (pub- 

 lished in the years 1896-98) of the lines of 

 the solar spectrum are now almost univer- 

 sally accepted and adopted as a standard 

 of reference. These tables alone record 

 about 10,000 lines in the spectrum of the 

 sun, which is in marked contrast to the 

 number 7 recorded by WoUaston at the be- 

 ginning of the century (1802). Good work 

 in the production of maps has also been 

 done in this country by Higgs. 



