662 REPOKT— 1900. 



The memorable results arrived at by Kircbhoff were no sooner publisbed than 

 they were accepted without dissent. The woiiis 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 

 already been mapped by Fraunhofer, and now it only needed better instruments 

 and the application of laboratory spectra with KirchhofTs principle to advance this 

 work still further. 



Fraunhofer had already pointed out the way in using gratings, and these were 

 further improved by Nobert and Rutherfurd. 



Kirchhoff's Map of the Solar Spectrum, published in 1861-62, was the most 

 complete up to that time ; but the scale of reference adopted by him was^an arbi- 

 trary one, so that it was not long before this was improved 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 Rutherfurd materially im- 

 proved the efficiency of the solar spectroscope, but it was not uutil Professor 

 Rowland's invention of the concave grating that this work gained any decisive 

 impetus. The maps (ffrst published in 1885) and tables (publisbed in the years 

 1896-98) of the lines of the solar spectrum are now almost universally accepted 

 and adopted as a standard of reference. These tables alone record about 10,000 

 lines in the spectrum of the sun, wliich is in marked contrast to the number 7 

 recorded by Wollaston at the beginning of the century (1802). Good work in the 

 production of maps has also been done m this country by Iliggs. 



Michelson has also recently invented a new form of spectroscope called the 

 ' Echelon,' ^ in which a grating with a relatively small number of lines is employed, 

 the dispersion necessary for modern work being obtained by using a high order 

 (say the hundredth) into which most of the light has been concentrated. 



Besides lines recorded in the visual and ultra-violet portions of the solar spec- 

 trum, maps have been made of the lines in the infra-red, the most important being 

 that of Langley's, published in 1894, prepared by the use of his ' bolometer.' 

 Good work had, however, been done in this direction previously by Becquerel, 

 Lamansky, and Abney ; the last, indeed, succeeded even in photographing a part 

 of it. 



The recording of the Fraunhofer lines in the solar spectrum is not all, however. 

 The application of the spectroscope to the sun has several epoch-marking events 

 attached to it, notablj' those of proving the solar character of the prominences and 

 corona, the rendering visible of the prominences without the aid of an eclipse by 

 the discovery of Lockyer and Jansseu in 18G8, the photography of the prominences 

 both round the limb and those projected on the solar disc hj the invention of the 

 spectro- heliograph by Hale and Deslandres in 1890. 



Success has not yet favoured the many attempts to photograph the corona 

 without an eclipse by spectroscopic means ; but even now this problem is being 

 attacked by Deslandres with the employment of the calorific rays. 



Spectroscopic work on the sun has led to the discovery of many hundreds of 

 dark lines, the counterparts of which it has not yet been possible to produce on the 

 earth. 



But besides those ruiknown substances which reveal their presence by dark 

 lines, there were two others discovered, which showed themselves only by bright 

 lines, the one in the chromosphere, to which the name of Helium was given, and 

 the other in the corona, to which the name of Coronium was applied. 



The former was, however, identified terrestrially by Ramsay in 1895, though 

 the latter is .still undetermined. The revision of its wave-length, brought about 

 by the observations of the eclipse of 1898, may, however, result in this element 

 being transferred from the unknown to the known in the near future, 



' Ast. Phys. Journ., vol. viii. 1898, p, 37. 



