1871.] 



of Uranus and Comet I., 1871. 



489 



By means of the equatorial refractor of 15 inches aperture, by Messrs. 

 Grubb and Son, recently placed in my hands by the Royal Society, I have 

 succeeded in making the observations described in this paper of the re- 

 markable spectrum which is afforded by the light of the planet Uranus. 



It should be stated that the spectrum of Uranus was observed by Father 

 Secchi in 1869*. He says: "le jaune y fait completement defaut. Dans 

 le vert et dans le bleu il y a deux raies tres-larges et tres-noires." He re- 

 presents the band in the blue as more refrangible than F, and the one in the 

 green as near E. 



*7 44 49 SO Si £2 53 S% SS £<S 5? 59 C0 fff GS 03 04 63 CS 



1 1 1 1 1 1 1 1 1 1 1 1 iii 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 i l 1 1 ul 1 1 fi i i lii nli i i 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 rn 1 1 n 



The spectrum of Uranus, as it appears in my instrument, is represented 

 in the accompanying diagram. The narrow spectrum placed above that of 

 Uranus gives the relative positions of the principal solar lines and of the 

 two strongest absorption-bands produced by our atmosphere, namely, the 

 group of lines a little more refrangible than D, and the group which occurs 

 about midway from C to D. The scale placed above gives wave-lengths 

 in millionths of a millimetre. 



The spectrum of Uranus is continuous, without any part being wanting, 

 as far as the feebleness of its light permits it to be traced, which is from 

 about C to about G. 



On account of the small amount of light received from this planet, I 

 was not able to use a slit sufficiently narrow to bring out the Fraunhofer 

 lines. The positions of the bands produced by planetary absorption, 

 which are broad and strong in comparison with the solar lines, were deter- 

 mined by the micrometer and by direct comparison with the spectra of 

 terrestrial substances. 



The spectroscope was furnished with one prism of dense flint-glass, 

 having a refracting- angle of 60°, an observing telescope magnifying 5^ dia- 

 meters, and a collimator of 5 inches focal length. A cylindrical lens was 

 used to increase the breadth of the spectrum. 



The remarkable absorption taking place at Uranus shows itself in six 

 strong lines, which are drawn in the diagram. The least refrangible of 

 these lines occurs in a faint part of the spectrum, and could not be mea- 



* Comptes Rendus, vol. lxviii. p. 761, and 1 Le Soleil/ Paris, 1870, p. 354. 



