384 



Mr. W. Hug-gins on the Spectra of the Sun. [May 14, 



The coincidence of the lines with those of hydrogen and nitrogen re- 

 mained apparently perfect with an apparatus in which a difference in 

 wave-length of 0'0460 milHonth 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 dis- 

 tance 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 nebulse. It is obvious that if the spec- 

 trum 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 refran- 

 gible 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 nebulge 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 nebulss.are bodies which 

 have a sensible diameter, and in all probability present a continuous lumi- 

 nous 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 cosm^ical space, similar to that which was suggested from theo- 

 retical considerations by Cheseaux, and was afterwards supported on other 

 grounds by Olbers and the elder Struve. 



It is also shown that at the time of the observations this nebula was 

 not receding from us with a velocity greater than 1 miles per second ; 

 for this motion, added to the earth's orbital velocit}^, would have caused 

 a want of coincidence of tLe lines that could have been observed. If the 

 nebula were approaching our system, its velocity might be as much as 

 20 or 25 miles per second, for part of its motion of approach would be 

 masked by the effect of the motion of the earth in the contrary direction. 



§ IV. Observations of Stars. 



A detailed description is given of the comparisons of the line in Sirius 

 corresponding to F, with a line of the hydrogen^ spectrum, and of the 

 various precautions which were taken against error in this difficult and 

 very delicate inquiry. The conclusions arrived at are : — that the sub- 

 stance in Sirius which produces the strong lines in the spectrum of that 

 star is really hydrogen ; further, that the aggregate result of the motions 

 of the star and the earth in space, at the time the observations were made. 



