210 Dr. and Mrs. Hugging. On the Principal 



the comparison spectrum similar to that described in the first part of 

 this paper, bat in which the small mirror has been replaced by a very 

 small total reflecting prism. The correctness of position of the 

 comparison spectrum was ascertained by repeated comparisons of the 

 bright lines of magnesium at b with the corresponding dark lines in 

 the Sun's light reflected from the sky. 



When in this spectroscope the spectrum of lead is observed 

 together with that of burning magnesium, the lead line is seen to fall 

 well within, and to be separated by a clear space from, the terminal 

 line of the magnesium-flame fluting. 



The principal line of S. 5, like that of the nebula of Orion, appears 

 when the slit is made narrow to be very thin and clearly defined at 

 both edges. The lead line is a thin and defined line ; if, therefore, 

 the nebular line were coincident with the terminal line of the 

 magnesium-flame fluting, it would appear in the spectroscope to be 

 separated by a clear space from the lead line towards the red. As 

 the angular diameter of the nebula is small, the line is much shorter 

 than the lead line not longer than about one-third of the height of 

 the spectrum, and consequently its position relatively to the lead line, 

 even when it falls partly upon it, can be very accurately determined. 



The nebular line was seen as a short thin bright line partly upon, 

 and partly clinging to, the lead line. The nebular line in our 

 instrument certainly fell upon the lead line, but overlapped it a very 

 little, though not so much as by half its breadth, on the less 

 refrangible side. This position agrees precisely with that described 

 in my early observations made nearly twenty years ago, when I 

 employed for the first time lead as a fiducial comparison line.* As I 

 stated in 1874,f "if greater prism power could be brought to bear 

 upon the nebulae, the line in the lead spectrum would be found to be 

 in a small degree more refrangible than the line in the nebula;" and, 

 of course, if sufficient power of dispersion were employed, the 

 nebular line would be seen separated from the lead line towards the 

 red, and not, as in our instrument, partly upon the lead line. 



These observations, both those by myself in 1874 and the recent 

 observations made by both of us independently on four different 

 nights, place the nebular line exactly where it was ound to be by our 

 direct comparisons with burning magnesium in the nebula of 

 Orion (which were confirmed by Professor Liveing), namely, as not 

 coincident with, but as falling well within, the terminal line of 

 the magnesium-flame spectrum. 



It should be stated that on two nights we made comparisons of 

 2. 5 with burning magnesium, both directly, and indirectly by means 

 of the illuminated pointer. The observations completely confirmed 



* ' ttoy. Soc. Proc.,' vol. 22, p. 252. 

 t Hid. 



