1882.] on Comets. 5 



which appear in certain compounds of carbon. Professors Liveing 

 and Dewar had recently shown that these groups indicate a nitrogen 

 compound of carbon, namely, cyanogen. On this view there must be 

 in the cometary matter, besides carbon and hydrogen, the element 

 nitrogen, 



A few days after my photograph was taken, Dr. Draper succeeded 

 in obtaining a photograph of the comet's spectrum, which appears to 

 confirm mine so far as the bright lines, but does not give the 

 Fraunhofer lines. 



About the same time that the observations were made on the comet, 

 Professor Dewar succeeded in confirming his results, by the reversal 

 of the groups, employing either titanic cyanide or boron nitride. 



The positions and characters of these bands, together with those in 

 the visible spectrum, leave no doubt that the substances, carbon, hydro- 

 gen, and nitrogen, and probably oxygen, are present in the cometary 

 matter, and that this light-emitting stuff appears to be essentially 

 of the same chemical nature for all the comets, some twenty, which 

 have been observed up to the present time. Certain minor modi- 

 fications of the common type of spectrum are often present, and 

 show, as was to be expected, that the conditions prevailing in different 

 comets, and indeed in any one comet from day to day, are not rigidly 

 uniform. 



The temperature, the state of tenuity, the more or less copious 

 supply from the nucleus of the gaseous matter, must be subject to con- 

 tinual variation. At times it is probable that the hydrocarbon 

 spectrum is complicated by traces of the spectrum of the oxygen com- 

 pounds of carbon. These and. other possible variations betray them- 

 selves to us in the spectrum, by the length of range of refrangibility 

 through which each group can be traced, by an alteration in the posi- 

 tion of maximum brightness in the groups, by the relative brightness 

 of the groups, by a more or less breaking up of the shaded light of the 

 bands and the visibility or otherwise of bright lines, by a more or 

 less distinctness of the violet group, and, lastly, by the visibility 

 in the brightest comet of last year of a less refrangible band of 

 the hydrocarbon spectrum which occurs between C and D of the 

 spectrum.* 



We must now consider the information about the nature of comets 

 which has come to us from a wholly different source. 



On almost any fine night, after a short watch of the heavens, we 

 shall see the well-known appearances of " shooting stars." At 

 ordinary times, these are small, and appear indifferently in all parts 

 of the heavens, but on certain nights they show themselves in great 

 numbers, and of such brilliancy as to present a spectacle of much 

 magnificence. On such occasions one remarkable feature presents 



* For these reasons measures of these bands should be considered as strictly 

 applicable to the particular comet at the time of observation only, and not 

 necessarily as applicable to other comets. 



