134 Royal Society : — Dr. W. Huggins on the 



the case of the brightest nebulae, to give the line with sufficient di- 

 stinctness 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 corresponding 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 circumstances 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 com- 

 plete 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 appearbut a very little 

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

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

 component 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 conditions of tension and temperature of the gas. Pliickerf 

 states that when an induction-spark of great heating-power is em- 

 ployed, 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 intensity 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 alterations 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 spectro- 

 scopic apparatus with which the observations in this paper were 

 made. In the former paper, to which I have already referred, I gave 

 some reasons* to show that the ordinary method of comparison, by 

 reflecting light into the spectroscope by means of a small prism placed 

 before one half of the slit, is not satisfactory for very delicate ob- 

 servations unless certain precautions are taken. I then describe an 



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



+ Ibid. 1868, pp. 537 ? 538. : 



