troscopy, first inaugurated by his researches, to some other investigations, such as differences which may present themselves in the photographic region in the case of the variable stars, the difference 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 Association address of 1891 includes a fine summary of the results to date of observations of this character as they bear upon the evolutional 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 observations. 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 photography 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 luminous 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 dissociation exists, and the constituents of the mass, doubtless, are arranged 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 probable. On Dr. Croll's form of the impact theory of stellar evolution, 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 collision 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 appearance of flat disks without condensation which many of them present. . . . In other gaseous nebulæ 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.
