20(5 Dr. and Mrs. Hugging. On the Principal , 



We have since gone farther, and attempted a quantitative estima- 

 tion of the distance of the nebular line within the termination of the 

 band. For this purpose we made use of the minute apparent breadth 

 of the illuminated pointer-tip as a measuring unit. The value of 

 this unit was determined by measuring with it the distance of 6 3 from 

 6 4 in the solar spectrum. 



Independent estimations made by both of us on several occasions 

 agreed in assigning to this distance, after taking into account the 

 minute displacement of the comparison-spectrum by the little mirror 

 towards the blue, 



A wave-length of about X 0001'5. 



Deducting this distance from X 5006'5, the position of the termina- 

 tion of the band, we get for the nebular line 



A position of about X 5005'0. 



At the time of these observations the earth's motion caused the 

 nebular line to be degraded towards the red by about XOOOO'25. If, 

 therefore, the Great Nebula has no motion of its own, this interval 

 must be deducted from the observed position of the nebular line, 



Placing it at about X 5004'75. 



The observations recorded in the paper of 1874* gave the position 

 of the nebular line relatively to the fiducial lead line with an accuracy 

 not less than X 0000'5. This relative position was translated into 

 wave-lengths in our paper on the Nebula (loc. cit., p. 45), showing 

 that the nebular line lies from about X 5004'6 to about Xi>004'8. 



The question whether this nebula has a motion in the line of sight 



could possibly arise from a change of position of the magnesium during its burning. 

 After a detailed account of the experiments, he wrote in my note-book : " I could 

 not detect any shift ; and I came to the conclusion that there is no sensible shift 

 due to moving the burning magnesium. I next compared the position of the lead 

 line near the edge of the MgO band, as seen in the same spectroscope detached 

 from the telescope, with the said edge of the band. Both oould be seen at the 

 same time, and the apparent distance between them was so great that even if there 

 should be some shift of the lines from the method of throwing in the light when 

 the spectroscope is attached to the telescope, I am satisfied that it could not amount 

 to anything comparable with the distance between the lead line and the edge of the 

 MgO fluting. So far as my memory will serve, the distance from the edge of the 

 MgO fluting at which the nebular line appeared when I observed it on February 9 

 was not far short of the distance now observed between the lead line and the edge 

 of the MgO fluting." July 4.] 



* ' Koy. Soc. Proc.,' vol. 22, 1874, p. 254. This paper claims for the determination 

 of the position of this line in the case of seven nebulre an accuracy sufficiently great to 

 show a motion of 25 miles per second. This motion corresponds to about A. 0000'67, 

 but as some of the nebulae were more difficult to observe than the bright nebula in 

 Orion, the accuracy of the determination of the line in this nebula may certainly be 

 taken as not less than the amount given in the text, namely, A OOOO'o. 



