490 



On the Spectra of Uranus and Comet I., 1871. [May 25 3 



sured. Its position was estimated only, and on this account it is repre- 

 sented 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 direc^" with that of Uranus. The band in the 

 planet's spectrum appeared to be coincident 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 refrangible 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 faint- 

 ness 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 improbability, 

 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 re- 

 sult was to show that the bands of Uranus cannot be ascribed to the ab- 

 sorption 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 observed 

 the comet on April 13 and May 2. On both days the comet was exceed- 

 ingly faint, and on May 2 it was rendered more difficult to observe by the 

 light of the moon and a faint haze in the atmosphere. 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. 



