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Mr. W, Huggins on the Spectrum 



[May 14, 



His observations are in accordance generally with those communicated 

 by Mr. Lockyer to the Royal Society. 



The author describes the examination of a spot on April 15th, 1868. 

 He shows that about three-fourths of the apparent light of the umbra 

 came from that region of the sun, and the remaining fourth from the in- 

 tervening illuminated atmosphere of the earth. He observed an increase 

 of width in most of the dark lines of the solar spectrum. The lines C 

 and F, due to hydrogen, did not appear stronger in the spectrum of the 

 umbra. No new lines were detected, nor were any of those of the normal 

 solar spectrum observed to be wanting in the spectrum of the light from 

 the umbra. No bright lines were seen. 



Some of the conditions of the solar surface are considered which the 

 phenomena observed may be supposed to indicate. 



A cooler state of the heated vapours by which the lines of absorption 

 are produced would diminish the radiation from the gas itself, and so 

 leave more completely uncompensated the absorption by the gas of the 

 light from behind it. Though in this way an apparent increased inten- 

 sity of the dark lines would result, the observations seem to suggest a state 

 of the vapours connected with tension and temperature in which their 

 power of absorption for each line embraces an increased range of wave- 

 length. Some of the conditions under which this state of things may be 

 brought about are discussed. 



The absence of bright lines is not considered as conclusive of the com- 

 plete absence of light in the umbra from luminous gas ; for if there existed 

 in the spot or above it the same vapours in a cooler state, the light would 

 be almost wholly absorbed, and the feebler emanations of the cooler vapour 

 might not do more than render less intense the dark gaps produced by 

 the vapours in the stronger light of all refrangibilities which is evidently 

 present. 



What is the source of the light in the umbra which gives the continuous 

 spectrum? May the dense and intensely heated gases, which probably 

 form the inner substance of the sun, emit, in some cases, lines so greatly 

 expanded as to form, when numerous spectra are superposed, a sensibly 

 continuous spectrum ? Dr. Balfour Stewart has suggested that, as gases 

 possess a power of general absorption of light, a heated mass of gas, if 

 sufficiently dense to be opaque or nearly so, would give a continuous spec- 

 trum as well as the spectrum of bright hues peculiar to it. 



VIII. On the Spectrum of Brorsen's Comet, 1868.'' By Wil- 

 liam Huggins, F.B.S. Beceived May 14, 1868. 



In January 186G I communicated to the Royal Society the result of an 

 examination of a small com.et visible in the beginning of that year*. I 

 Proceedings of the Royal Society, vol. xv. p. 5. 



