i86 SCIENCE PROGRESS 



results in photographing stellar spectra. One of the firstfruits 

 of his labours was the detection of the series of rhythmically 

 related lines in the spectra of Sirius and other blue stars. 

 The resemblance of the lines of this series to the other lines 

 previously recognised as hydrogen lines in the more refrangible 

 part of the spectrum led him to surmise that they were pro- 

 duced by hydrogen. 



It was in 1885 that Balmer discovered that the wave-lengths 

 6562'!, 48607, 4340'! and 4101*2 of the four hydrogen lines bore 

 to the wave-length 3645 "6 the ratios denoted by the fractions 

 2. 4, 15 and % or 9, 1^ 25 36 ^^ 4I ^ jf_ 6^ j^^^ ^ 



5 3 21 8' 5' 12' 21' 32' 3-4' 4-4' 5^-4' 6--4' 



wave-lengths of these four lines may be deduced from the 

 expression 



X = 3645-6 x^ 



if the whole numbers 3, 4, 5 and 6 are successively substituted 

 for 711. Balmer learnt that five years previously Huggins had 

 discovered a series of lines in stellar spectra at wave-lengths 

 3887*5, 3834 and 3796. These lines were found to correspond 

 with the wave-lengths deduced from this same series, when m 

 has the values 8, 9, 10. And thus the v/ork of the terrestrial 

 physicist is supplemented by that of the astrophysicist. 

 Huggins later detected the members of the hydrogen series 

 up to the value ;;/ = 32 ; and it may be surmised that this work 

 gave the impulse to the search for the isolation of rhythmical 

 series in the spectra of other elements, a search in which 

 various spectroscopists engaged and which culminated in 

 success in the work of Kayser and Runge, and of Rydberg. 



In referring to this application of photography to the record- 

 ing- of stellar spectra far into the ultra-violet region, we are 

 anticipating other remarkable work and we would now revert 

 to Huggins's solution of the mystery of nebulae which was 

 achieved in 1864. 



Probably few of Herschel's discoveries struck the imagina- 

 tion more than that series of observations in which by 

 the use of increasingly powerful telescopes he was able to 

 " resolve " into groups of separate stars or clusters many of 

 those celestial objects which to the naked eye or in a small 

 telescope appear as faint nebulae. He passed no less than 

 2500 nebulae and clusters in review, and was thus led to 



