1870.] 



Argo and Orion, and of Jupiter. 



249 



Jupiter and sky diagram). These are the only lines seen with certainty 

 between C and D. 



The identity of 914 and 838 rests partly on measures and partly on 

 spark comparison, where, for the identification of 914, it is seen that this 

 line is near to the air band 807 of Mr. Huggins's chemical scale. 



The line 914 is so easily seen, that having in mind Mr. Huggins's state- 

 ment concerning the difficulty of discerning it at all, originally very imper- 

 fect measures on a bad night and with the apparatus imperfectly adjusted 

 misleading in the same direction, this line was at first mistaken for 882, 

 from which, however, it is separated far beyond the limit of error in a 

 proper state of adjustment of apparatus. 



882 is not seen at all with Jupiter at considerable altitude. On the night 

 of December 29th, however, between the hours of 12.30 and 1, Jupiter 

 being low, 882 was seen almost as conspicuous as 914, which, I may 

 remark, did not seem to have perceptibly increased in darkness by the ad- 

 ditional absorption of the earth's atmosphere. 



On the night of December 14th (both objects being near the meridian) 

 the spectroscope was turned on Jupiter and the moon alternately several 

 times. On Jupiter 914 and 838 were easily visible, the former (as usual) 

 the more conspicuous; on the moon no line could be certainly made out 

 between C and D. Mr. Ellery was present at the time and gave the same 

 verdict. 



So far these observations are merely confirmatory of those made by Mr. 

 Huggins. There is one point, however, not unworthy of consideration, 

 arising from a comparison of the observations in connexion w r ith the condi- 

 tions under which they are made. 



It is probable that Mr. Huggins, with his earlier apparatus, was under 

 more favourable conditions as regards light than, if not the best at my 

 command, at least those under which 914 is now plainly seen. When 

 condensed as much as arrangements allow (about four times), I probably 

 get a somewhat brighter image at the slit than that produced by Mr. Hug- 

 gins's telescope ; but with little or no condensation, and a dispersion of near 

 7° (B to H = 6° 50'), the line in question is still conspicuous. Yet Mr. 

 Huggins speaks of this line as barely distinguishable, or not at all visible 

 with his earlier apparatus. Width of slit, of course, plays a prominent 

 part ; but I cannot be wrong in assuming that, for prospecting purposes, 

 Mr. Huggins tried various widths. Moreover when the slit is gradually 

 cut down, 914 is visible as long as the chief Fraunhofer lines, and is still 

 readily seen when the light is insufficient to show a trace of C or 838 

 near it. 



These considerations, if not entitled to much weight, at least point to a 

 possible variability of the line in question. If this prove to be the case, it 

 will be interesting to note its degree of visibility in connexion with the cha- 

 racter of the surface at the different times of observation. 



I cannot find whether 914 or 838 is involved in the lines proved by M. 



t 2 



