244 Mr. A. Le Sueur's Spectroscopic Observations. [Mar. 3, 



altitude of the planet and the state of the atmosphere at the times of obser- 

 vation ; considering, moreover, that in Mr. Huggins's observation (as he 

 himself remarks) the relative positions of the sun and Jupiter were such 

 as considerably to exaggerate the effect of the earth's atmosphere on the 

 sky spectrum, it is difficult (in the absence of a more crucial observation 

 pointing in a contrary direction) to escape the impression that this line is 

 in no small degree due to Jupiter's own atmosphere. The band specially 

 examined by Mr. Huggins I have not yet succeeded in seeing with any 

 degree of certainty ; but the opportunities have been so few, that the opti- 

 cal conditions for its most favourable development have not been fairly tried. 



On one night the eye-aspect of Jupiter was as follows : — N to P (Plate I. 

 fig. 1) of yellowish colour, with occasional appearance in good denning 

 moments of hair-line structure ; P to Q almost white, slight tinge of blue ; 

 Q to R yellowish, but much darker than N P ; R to S also yellow, slightly 

 brighter than N P, and with no suspicion of fine lines. 



P, Q, R brown, much darker than general surface, the two latter with 

 a red or yellow tinge, the former with a greenish one. [These are merely 

 the impressions without attempt to eliminate effect of contrast.] 



The absolute positions and definite shape of these bands, as given in the 

 diagram, have no special pretensions to minute accuracy ; considerable care 

 was, however, employed, and in any case the sketch in its broad features 

 is sufficiently near to the truth for the special purpose in view. In the 

 spectrum, G, F, E, D, C G , C are laid down from measures on Jupiter. I 

 have called the band between C, D, C G for reference purposes, subject to 

 rectification, although there can, I think, be little doubt of the coincidence. 



A point specially aimed at in these observations was to note any peculi- 

 arity in the appearance of spectral lines of known atmospheric origin ac- 

 cording to the part of the surface viewed. 



With the slit perpendicular to Jupiter's equator and the advantage of a 

 large image, an admirable opportunity is afforded of noting the behaviour 

 of the lines as they cross the different parts of the surface, a spectroscopic 

 picture of the planet, as it were, being presented beautifully to the eye. 



The nebulous line C 6 was specially and narrowly watched, but without 

 any satisfactory evidence being elicited ; as this line crosses the bright 

 band P Q it is perhaps slightly less nebulous ; on the assumption that C G 

 is in great part due to Jupiter's atmosphere, this peculiarity, by no means 

 marked, is yet in the direction to be accounted for by the usual suppositions 

 concerning the natureof J upiter's visible disk. On this assumption, however, 

 one would expect more decisive evidence of change in the line according as 

 it is due to the cloud -band or to the surface ; there is evidence certainly, 

 but so faint that, due regard being had to the possible .disturbing effect 

 of the somewhat greater brightness of cloud band, and to the bias which 

 cannot be totally eliminated from the mind, it does not seem entitled to 

 much weight. 



This almost, if not altogether, complete sameness of the line might perhaps 



