Mr. W. Huggins on the Spectra of the Sun fyc. 57 



servations were made with vacuum-tubes or electrodes of metal, placed 

 before the object-glass of the telescope. 



§ III. Observations of Nebula. 



The author states that he has examined satisfactorily the general 

 characters of the spectra of about seventy nebulae. About one-third 

 of these give a spectrum of bright lines ; all these spectra may be re- 

 garded as modifications of the typical form, consisting of three bright 

 lines, described in his former papers. 



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

 troscope described in this paper, for the purpose of determining 

 whether any of them were possessed of a motion that could be de- 

 tected by a change of refrangibility, 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 as great as 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 comparison of its 

 spectrum with the spectra of terrestrial substances. 



The coincidence of the lines with those of hydrogen and nitrogen 

 remained apparently perfect with an apparatus in which a differ- 

 ence in wave-length of 0*0460 millionth of a millimetre would have 

 been detected. These results increase greatly the probability that 

 these lines are emitted by nitrogen and hydrogen. 



It was found that when the intensity of the spectrum of nitrogen 

 was diminished by removing the induction-spark in nitrogen to a 

 greater distance from the slit, the whole spectrum disappeared with 

 the exception of the double line, which agrees in position with the 

 line in the nebulae ; so that, under these circumstances, the spectrum 

 of nitrogen resembled the monochromatic spectra of some nebulae. 

 It is obvious that if the spectrum of hydrogen were greatly reduced 

 in intensity, the strong line in the blue, which corresponds to one of 

 the lines of the nebular spectrum, would remain visible after the line 

 in the red and the lines more refrangible than F had become too 

 feeble to affect the eye. 



It is a question of much interest whether the few lines of the spectra 

 of these nebulae represent the whole of the light emitted by these 

 bodies, or whether these lines are the strongest lines only of their 

 spectra which have succeeded in reaching the earth. Since these 

 nebulae are bodies which have a sensible diameter, and in all proba- 

 bility present a continuous luminous surface, we cannot suppose that 

 any lines have been extinguished by the effect of the distance of the 

 objects from us. If we had reason to believe that the other lines 

 which present themselves in the spectra of nitrogen and hydrogen 

 were quenched on their way to us, we should have to regard their 

 disappearance as an indication of a power of extinction residing in 

 cosmical space, similar to that which was suggested from theoretical 

 considerations by Cheseaux, and was afterwards supported on other 

 grounds by Olbers and the elder Struve. 



