448 The Satellites of the Mercury Lines. 



than the one made in the third. The positions o£ the 

 satellites are as follows : — 



+ •212 Faint : not shown on print, and never observed 

 + '168 [previously. 



+ •130 

 + •091 

 + •029 



Main line. 



-•034 

 -•111 

 -•124 



I Barely resolved. 



-•174 



— •219 ] 



_.non [ Very faint and never observed previously. 



These values agree well with the ones found by Gale and 

 Lemon, though we have not taken the main line at the same 

 point, which results in a slight shift of the one set of 

 readings with respect to the other. I consider the position 

 of the main line which I have taken to be the correct one, 

 for the middle line (triplet) is clearly resolved and the 

 central component is by far the brightest. Gale and Lemon 

 evidently took the left-hand component, but there is scarcely 

 any trace of resolution in their photograph. 



The satellite + *lb'8, which is the narrow bright one to the 

 right in my photographs, was found by von Baeyer (given 

 as +*188) but was not found by Janicki, Galitzin, or 

 Lunelund, which is sufficient evidence that the interference 

 spectroscopes are not very reliable. 



The resolving power of the best ones is only a very little 

 greater than that of the grating with which 1 have worked, 

 and in my opinion they can only be used to advantage for a 

 supplementary study of the groups of satellite lines after 

 the chief features of the group have been ascertained with 

 the grating. 



It is my opinion that the four photographs reproduced 

 with this paper furnish the most reliable information re- 

 garding the structure of these mercury lines which we have 

 at the present time. 



The placing of the large spectrograph in an underground 

 tunnel will improve its performance, I am sure, as I always 

 found some trouble from striae ("■ schlieren ") stirred up by 

 air-draughts down the long tube. It is my hope also that a 

 somewhat larger grating may be available by that time. 



Note added Feb. 10. — Prof. Hale has just sent me a photo- 

 graph of the iodine absorption spectrum made with one of 



