202 Dr. and Mrs. Huggins. On the Principal 



" On a Ke-detenhination of the Principal Line in the Spec- 

 trum of the Nebula in Orion, and on the Character of the 

 Line." By WILLIAM HUGGINS, D.C.L., LL.D., F.R.S., and 

 Mrs. HUGGINS. Received March 20, Read June 12, 1890. 



We think it desirable to put on record the results of a re- determina- 

 tion of the position of the principal line in the spectrum of the nebula 

 in Orion, under the more favourable conditions of a higher position of 

 the nebula, and of some improvements in the instrumental arrange- 

 ments.* The spectroscopes have been furnished with new and sensibly 

 perfect object-glasses by Sir Howard Grubb, and a new bright pointer 

 has been fitted to the spectroscopes by Mr. Hilger, which is illuminated 

 by a small incandescent lamp, of which the brightness is controlled by 

 suitable resistances. In all other respects the instrumental arrange- 

 ments have remained unaltered. The same spectroscope, giving a 

 dispersion of about four prisms, which was described in my paper of 

 1872 as Spectroscope B,f and was used in the work on this line con- 

 tained in my paper of 1874, J and also throughout the work of last 

 year, with the exception of one single confirmatory observation with 

 a more powerful spectroscope, was employed in the present investiga- 

 tion, and also the same arrangements for the comparison spectrum 

 from burning magnesium. 



In my earlier spectroscopic work I pointed out that a possible paral- 

 lactic error of the comparison spectrum may easily come in when a 

 small reflecting prism is placed in the usual way before one half of the 

 slit; and also the possibility of errors from the unavoidable flexure 

 of the spectroscope or of its attachments to the telescope. In 1872, I 

 adopted the plan of placing " the spark or vacuum-tube within the 

 telescope at a moderate distance from the slit. For this purpose holes 

 were drilled in the telescope-tube, opposite to each other, at a distance 

 of 2 feet 6 inches within the principal focus. Tubes were fixed by 

 screws over these holes, and in these tubes slide suitable holders for 



* [In a communication last January to the Royal Society, Professor Lockyer 

 stated that he and his assistants had by different methods and with great dispersion 

 compared directly the chief line in the spectrum of the Nebula in Orion with the 

 band of the magnesium-flame spectrum, and that they had found perfect coincidence 

 between the nebular line and the terminal line of the band. Professor Lockytr 

 also stated that they had always seen the line as a fluting. These statements being 

 in direct contradiction to my early observations and to the conclusions of our paper 

 of last year, the necessity was thrown upon us of going over our work again. 

 July 4.] 



t ' Roy. Soc. Proc.,' rol. 20, 1872, p. 382. 



J ' Roy. Soc. Proc.,' vol. 22, 1874, p. 252. 



' Roy. Soc. Proc.,' rol. 46, 1889, pp. 50, 51. 



