1868.] Mr. W. Huggins on the Spectra of the Sun. 



383 



The author has smce, for several years, devoted much time and labour 

 to this investigation, and believes that he has obtained a satisfactory 

 result. 



He refers to Doppler, who first suggested that the relative motion of 

 the luminous object and the observer would cause an alteration of the 

 wave-length of the light ; and to Ballot, Klinkerfues, Sonnche, Fizeau, and 

 Secchi, who have vvritten on the subject. 



The author is permitted to enrich his paper with a statement of the 

 influence of the motions of the heavenly bodies on light, and of some ex- 

 periments made in an analogous direction, which he received in June 1867 

 from Mr. J. C. Maxwell, F.R.S. 



It is shown that if the light of the star is due to the luminous vapour 

 of sodium or any other element which gives rise to vibrations of definite 

 period, or if the light of the star is absorbed by sodium-vapour, so as to 

 be deficient in vibrations of a definite period, then the light, when it 

 reaches the earth, will have an altered period of vibration, which is to the 

 period of sodium as \-\-v is to V, when V is the velocity of light and v is 

 the velocity of approach of the star to the earth. Equal velocities of sepa- 

 ration or approach give equal changes of wave-length. 



§ II. Description of Apparatus. 

 A new spectroscope is described, consisting in part of compound prisms, 

 which gives a dispersive power equal to nearly seven prisms of 60° of 

 dense flint glass. Various methods were employed for the purpose of 

 ensuring perfect accuracy of relative position in the instrument between 

 the star spectrum and the terrestrial spectrum to be compared with it. 

 A new form of apparatus, which appears to be trustworthy in this respect, 

 was contrived. Many of the observations were m.ade with vacuum-tubes 

 or electrodes of metal, placed before the object-glass of the telescope. 



§ III. Observations of Nebulae. 



The author states that he has examined satisfactorily the general cha- 

 racters of the spectra of about seventy nebulae. About one-third of these 

 give a spectrum of bright lines ; all these spectra may be regarded as mo- 

 difications of the typical form, consisting of three bright lines, described 

 in his former papers. 



Some of these nebulae have been reexamined with the large spectro- 

 scope described in this paper, for the purpose of determining whether any 

 of them were possessed of a motion that could be detected by a change of 

 rcfrangibility, and whether the coincidence which had been observed of 

 the first and the third line with a line of hydrogen and a line of nitrogen 

 would be found to hold good when subjected to the test of a spreading 

 out of the spectrum three or four times greater than that under which the 

 former observations were made. The spectrum of the Great Nebula in 

 Orion was very carefully examined by several different methods of com- 

 parison of its spectrum with the spectra of terrestrial substances. 



