58 



Dr. W. Hug gins and Mrs. Huggins. [May 2, 



matter. Whether they are peculiar to these particular stars and the 

 matter close about them, or whether they will be found everywhere 

 in the nebula, or in certain parts of greater condensation only, can 

 be known only from future photographs. 



The first group shows some general agreements with a strong iron 

 group, but there are also formidable discrepancies. 



The position of the third group suggested the well known cyanogen 

 group, especially as this group, beginning at X3883, is the first to 

 appear under the chemical conditions which might have been 

 conceived to exist under circumstances of condensation.* Under 

 these conditions this group appears alone in a photograph, without 

 the less refrangible group, as was probably the case in the photograph 

 I took of Comet II, 1881. I therefore took a photograph of an oxy- 

 coal-gas flame, the coal-gas having passed through ammonia, and a 

 magnesium-flame spectrum on the same plate for comparison. 



On comparing this photograph with that of the nebula it was seen 

 by eye, and afterwards confirmed by measurement, that the nebula 

 group begins sooner by one strong line than the cyanogen group, and 

 presents besides in the relative strength and grouping of the lines a 

 distinctly different character. The evidence appears to me to be 

 against attributing these lines to cyanogen. 



I took great pains to ascertain if the group of lines which accom- 

 panies the triplet of the magnesium-flame spectrum could be made to 

 agree with the much longer group of lines in the nebula at this part 

 of the spectrum. Again, as in the case of the cyanogen group, the 

 whole aspect of the grouping of lines is quite different. The groups 

 begin and end differently, and the relative strength of different parts 

 of the group is not the same. The great increase of strength which 

 is seen in the middle of the magnesium group is not present at the 

 corresponding part of the nebula group. I do not think therefore 

 there should be much weight given to the near positions of several 

 individual lines of the two groups, which in the case of so close a 

 grouping might well be accidental, especially as the wave-lengths can 

 be but approximate onlv- 



[The strongest lines of the magnesium-flame group are those 

 forming the triplet which appears also in the spark and the arc 

 spectrum. A nebular line is near the middle line of the triplet, but 

 there are no lines corresponding to the other two lines of the triplet. 

 The other lines of the flame group are too faint to be expected to 

 appear, unless the triplet at 3720 — 3730 were strong upon the plate. 

 —May 13.] 



The three pairs of lines in the photograph of 1889, which are 

 * See Liveing and Dewar, ' Koy. Soc. Proc.,' vol. 34, 1883, p. 128. 



