380 



Dr. W. Huggins on the 



[June 13, 



nebular line was double, as is the case with the corresponding line of 

 nitrogen. 



Subsequently I took some pains to determine this important point by 

 using a spectroscope of greater dispersive power. I found, however, that 

 the light furnished by the telescope of eight inches aperture, to which the 

 spectroscope was attached, was too feeble, even in the case of the brightest 

 nebulae, to give the line with sufficient distinctness when a narrow slit 

 was used. The results of this later examination are given in a paper I had 

 the honour of presenting to the Royal Society in 1868. I there say* : — 

 " I expected that I might discover a duplicity in the line in the nebula cor- 

 responding to the two component lines of the line of nitrogen, but I was not 

 able, after long and careful scrutiny, to see the line double. The line in the 

 nebula was narrower than the double line of nitrogen ; this latter line may 

 have appeared broader in consequence of irradiation, as it was much brighter 

 than the line in the nebula." When the spark was placed before the 

 object-glass of the telescope, the light was so much weakened that one 

 line only was visible in the spectroscope. " This line was the one which 

 agrees in position with the line in the nebula, so that under these circum- 

 stances the spectrum of nitrogen appeared precisely similar to the spectra 

 of those nebulae of which the light is apparently monochromatic. This 

 resemblance was made more complete by the faintness of the line ; from 

 which cause it appeared narrower, and the separate existence of its two 

 components could no longer be detected. When the line was observed 

 simultaneously with that in the nebula, it was found to appear but a very 

 little broader than that line." I also remark : — " The double line in the 

 nitrogen-spectrum does not consist of sharply denned lines, but each com- 

 ponent is nebulous, and remains of a greater width than the image of the 

 slit. The breadth of these lines appears to be connected with the condi- 

 tions of tension and temperature of the gas. Pliickerf states that when an 

 induction-spark of great heating-power is employed, the lines expand so as 

 to unite and form an undivided band. Even when the duplicity exists, the 

 eye ceases to have the power to distinguish the component lines, if the in- 

 tensity of the light be greatly diminished." I state further : — " I incline 

 to the belief that it [the line in the nebula] is not double." 



One of the first investigations which I proposed to myself when, by the 

 kindness of the Royal Society, I had at my command a much more powerful 

 telescope, was the determination of the true character of the bright line in 

 the spectra of the nebulae which is apparently coincident with that of 

 nitrogen. From various circumstances, chiefly connected with the altera- 

 tions and adjustments of new instruments, I was not able to accomplish 

 this task satisfactorily until within the last few months. 



Description of Apparatus. 

 It seems to me desirable to give a description of the spectroscopic 

 * Phil. Trans. 1868, pp. 542, 543. t Phil. Trans. 1865, p. 13. 



