1889.] On the Spectrum of the Great Nebula in Orion. 51 



line. These comparisons were repeated and confirmed generally on 

 March 9, March 11, and March 16. On March 9, a single successful 

 comparison was made with a more powerful spectroscope, giving a 

 dispersion equal to nearly eight prisms of 60°. [Comparisons have 

 been made since with the planetary nebula in Hydra. The short 

 line of the nebula was found to fall within the termination of the 

 magnesium band at about the small distance which corresponds to the 

 known position of the two lines. — April 26.] On all these nights 

 the comparisons were repeated independently and fully confirmed by 

 Mrs. Huggins. 



These comparisons can be successfully imitated in the laboratory 

 by directing a spectroscope of sufficient power to the line of lead 

 which the nebular line is sufficiently near, the slit being narrow and 

 the electrodes of lead near each other ; and then causing, with the 

 necessary precautions, the light of burning magnesium to fall also 

 upon the slit. The lead line will be seen to fall within the end of 

 the band, and to form with it a double line. 



It may be mentioned in this place that this line of lead, and the 

 iron line at 4957 at the position of the second nebular line, can be 

 conveniently used in the laboratory in any chemical research on the 

 nature of the nebulae. ~No terrestrial line which does not fall almost 

 exactly at these positions in the spectrum can have any claim to 

 further consideration. 



It might be suggested that the want of coincidence observed 

 between the nebular line and the magnesium band, amounting to 

 A, 0001 '9 nearly, might be due to a motion of translation of the 

 nebula towards the earth. The motion required to produce this 

 shift of position is about sixty-seven miles in a second. 



[The earth's motion at the time of comparison with the magnesium- 

 flame band may be taken at about 17 miles in a second of recession 

 from the nebula. This motion would bring the nebular line nearer 

 the red, and diminish the apparent interval between that line and the 

 termination of the band. If the nebula has a motion of approach, 

 the earth's recession would bring the line back again, to an extent 

 corresponding to about 17 miles in a second, towards its true place. — 

 May 13.] 



I showed in my paper on this subject in 1874,* that in. the case of 

 the Orion nebula and six other gaseous nebulas, namely, 4234, 4373, 

 4390, 4447, 4510, 4964, of Sir J. Herschel's ' General Catalogue of 

 Nebulas,' "in no instance was any change of relative position of the 

 nebular line and the lead line detected." We should have to resort, 

 therefore, to the overwhelmingly improbable supposition that all 



* < Eoy. Soc. Proc.,' vol. 22, p. 253. 



E 2 



