Dr. Hugglns on the Spectra of Uranus and Comet L, 1871. 225 



refrangible of these lines occurs in a faint part of the spectrum, and 

 could not be measured. Its position was estimated only, and on 

 this account it is represented in the diagram by a dotted line. The 

 positions of the other lines were obtained by micrometrical measures 

 on different nights. The strongest of the lines is that which has a 

 wave-length of about 544 millionths of a millimetre. The band at 

 572 of the scale is nearly as broad but not so dark; the one a 

 little less refrangible than D is narrower than the others. 



The measures taken of the most refrangible band showed that 

 it was at or very near the position of F in the solar spectrum. The 

 light from a tube containing rarefied hydrogen, rendered luminous 

 by the induction- spark, was then compared directly with that of 

 Uranus. The band in the planet's spectrum appeared to be coinci- 

 dent with the bright line of hydrogen. 



Three of the bands were shown by the micrometer not to differ 

 greatly in position from some of the_ bright lines of the spectrum of 

 air. A direct comparison was made, when the principal bright lines 

 were found to have the positions, relatively to the lines of planetary 

 absorption, which are shown in the diagram. The band which has 

 a wave-length of about 572 millionths of a millimetre is less refran- 

 gible than the double line of nitrogen which occurs near it. The two 

 planetary bands at 595 and 618 of the scale appeared very nearly 

 coincident with bright lines of air. The faintness of the planet's 

 spectrum did not admit of certainty on this point ; I suspected that 

 the planetary lines are in a small degree less refrangible. There 

 is no strong line in the spectrum of Uranus in the position of the 

 strongest of the lines of air, namely the double line of nitrogen. 



As carbonic acid gas might be considered, without much impro- 

 bability, to be a constituent of the atmosphere of Uranus, I took 

 measures with the same spectroscope of the principal groups of 

 bright lines which present themselves when the induction-spark is 

 passed through this gas. The result was to show that the bands of 

 Uranus cannot be ascribed to the absorption of this gas. 



There is no absorption-band at the position of the line of sodium. 

 It will be seen by a reference to the diagram that there are no lines 

 in the spectrum of Uranus at the positions of the principal groups 

 produced by the absorption of the earth's atmosphere. 



Spectrum of Comet I., 1871. 



On April 7 a faint comet was discovered by Dr. Winnecke. I ob- 

 served the comet on April 13 and May 2. On both days the comet 

 was exceedingly faint, and on May 2 it was rendered more difficult 

 to observe by the light of the moon and a faint haze in the atmo- 

 sphere. It presented the appearance of a small faint coma, with an 

 extension in the direction from the sun. 



When observed in the spectroscope, I could detect the light of the 

 coma to consist almost entirely of three bright bands. 



A fair measure was obtained of the centre of the middle band, which 

 was the brightest ; it gives for this band a wave-length of about 510 

 millionths of a millimetre. I was not able to do more than esti- 

 mate roughly the position of the less refrangible band. The result 



Phil. Mac/. S. 4. Vol. 42. No. 279. Sept. 1871. Q 



