

402 On the Chief Line in the Spectrum of ///>> Xcl-ulrc. [Mar. 19 t 



apply to my own observations, which were made with this difficulty 

 in view. If the distance between the line and the edge of the flutii 

 could be measured with a slit-width vanishingly small, the 

 interval would be obtained. With a practicable slit-width, tl 

 position of the centre of the line is unchanged, but the edge of tl 

 fluting is shifted toward the red by half the width of the line, 

 my observations of nebulae, the slit-width used was such as to 

 the bright, sharp load line (and hence, also, the nebular line) just 

 width of the coarse micrometer wire (about 0'4 tenth-metre). Tl 

 bright lines were observed by occulting them with the wire, 

 observations thus referring to their centres, but the magnesiui 

 fluting was observed by bringing its extreme edge and the lower 

 of the micrometer wire into coincidence, the centre of the wire fallii 

 therefore upon the edge of the fluting with infinitely narrow slit 

 Measures of the interval between the lead line and the edge of tl 

 magnesium fluting, made with the fine micrometer wire and 

 narrow a slit as could be used, gave the same value as measi 

 made in the manner just described.* The correction mentioned 

 Mr. Maunder is therefore unnecessary. 



It appears to me, from what has been shown above, that the non- 

 coincidence of the chief nebular line and the magnesium fluting nu 

 be regarded as proved. 



In regard to the character of the line, recent observations at Mount 

 Hamilton have shown nothing which does not confirm the opinion I 

 have already expressed,! that under no circumstances of observation 

 does the line tend to assume the aspect of the remnant of a fluting. 



The observations which have been made at Mount Hamilton 

 demonstrate the incorrectness of the view that the chief nebular line 

 is in any way connected with the magnesium fluting at X 5006'36, for 

 reasons which may be briefly summarised as follows : 



(1). The nebular line is 0'43 tenth-metre more refrangible than 

 lower edge of the magnesium fluting. 



(2). The nebular line has no resemblance to a fluting. 



(3). Flutings and lines of magnesium, which could not fail to 



* I may call attention to the fact that my own value of this interval (1*86 tenth* 

 metres) is 0'04 tenth-metre smaller than the most reliable measures which have yet 

 been published. 



t " A single prism of 60 was Qrst employed, then a compound prism of about 

 three and one-half times the dispersion of the latter, and finally a Rowland grating 

 of 14,438 lines to the inch. With all these different degrees of dispersion, and alia 

 with other spectroscopes employed, the nebular lines appeared to be perfect mono* 

 chromatic images of the slit, widening when the slit was widened and narrowing to 

 excessively fine, sharp lines when it was closed up. The brightest line showed no 

 tendency to assume the aspect of a ' remnant of a fluting ' under any circumstance* 

 of observation." ' Publications of the Astronomical Society of the Pacific," No. 11, 

 p. 266 and 280. 



