262 TEE POPULAR SCIENCE MONTELY. 



troscopy, first inaugurated by his researches, to some other inves- 

 tigations, such as differences which may present themselves in the 

 photographic region in the case of the variable stars, the differ- 

 ence of relative motion of two stars in the line of sight, the sun's 

 rotation from photographic spectra of opposite limbs, and the 

 spectra of the different parts of a sun-spot." The British Associa- 

 tion address of 1891 includes a fine summary of the results to date 

 of observations of this character as they bear upon the evolu- 

 tional order in which in this paper he arranged the stars from 

 their photographic spectra. Substantially the same order had 

 been proposed by Vogel in his classification of the stars in 1874. 



Dr. Huggins presented a paper on his examination of the 

 great nebula in Orion in 1868, and referred in it to earlier observa- 

 tions. The discussion was continued in 1872, and in 1882, when 

 the author threw out the suggestion of a hope that the further 

 knowledge of the spectra of the nebulae afforded us by photog- 

 raphy might lead, by the help of terrestrial experiments, to more 

 definite knowledge as to the state of things existing in those 

 bodies. In communications to the French Academy of Sciences 

 and to the Royal Society in 1889 he considered it probable that 

 nebulae yielding a spectrum of luminous rays, with a very faint 

 continuous spectrum, which is probably formed in part by lumi- 

 nous rays in close proximity, are at or near the beginning of the 

 cycle of their celestial evolution. " They consist probably of gas 

 at a high temperature and very tenuous, where chemical dissocia- 

 tion exists, and the constituents of the mass, doubtless, are ar- 

 ranged in the order of vapor density. As to the conditions which 

 may have been anterior to this state of things the spectroscope is 

 silent. We are free, so far as the spectroscope can inform us, to 

 adopt the hypothesis which other considerations make most prob- 

 able. On Dr. Croll's form of the impact theory of stellar evo- 

 lution, which begins by assuming the existence of stellar masses 

 in motion, and considers all subsequent evolutional stages to be 

 due to the energy of this motion converted into heat by the col- 

 lision of two such bodies, these nebulae would represent the second 

 stage in which these existing solid bodies had been converted into 

 a gas of very high temperature. They would take the same place, 

 if we assume, with Sir William Thomson, the coming together 

 of two or more cool, solid masses by the velocity due to their 

 mutual gravitation alone. I pointed out in 1864 that the gaseous 

 nature of these bodies would afford an explanation of the appear- 

 ance of flat disks without condensation which many of them pre- 

 sent. ... In other gaseous nebulae strong condensations are seen, 

 and a stronger 'continuous' spectrum. The stage of evolution 

 which the nebula in Andromeda represents is no longer a matter 

 of hypothesis. The splendid photograph recently taken by Mr. 



