194 Mr. J. N. Lockyer. On the Classification [Jan. 10 T 



and the observations of Gothard between November 1st and 

 November 18fch showed the second (see fig. 15). 



This sequence may not have been apparent in some comets for two 

 reasons. In the first place, a complication is introduced by the 

 unequal displacements of the bands at different times due to motion 

 in the line of sight, which is variable, and is sometimes very great. 

 Many, apparently, faulty observations are probably to be accounted 

 for in this way. 



Again, different observers may not have recorded the spectrum of 

 exactly the same part of the comet, though in general it may be 

 assumed that the brightest part will have been examined. There 

 must be regions of different temperatures in the same comet, and* 

 from what I have shown in this paper, the spectra of different 

 portions will vary considerably. One part of the comet may give hob 

 carbon, whilst another may give cool carbon radiation. The wave- 

 lengths of the bands seen in the two cases would differ, and the results 

 would apparently disagree. In future observations, therefore, it is 

 very important that the exact portion of the comet examined should 

 be stated. 



VIII. MORE DETAILED DISCUSSION OF CERTAIN COMETS, WITH SPECIAL 

 REFERENCE TO APPROACH AND RECESSION FROM PERIHELION. 



Comet Wells. 



Comet Wells was first seen on the 17th of March, 1882, its peri- 

 helion passage occurring on June 10th. During the earlier observa- 

 tions, made by Vogel, Tacchini, and others in April, its spectrum 

 presented no feature of special interest, consisting merely of " faint 

 traces of the customary three bands close to the weak, faint, con- 

 tinuous spectrum of the nucleus."* At Greenwich, on May 20th, 

 Maunder suspected " a dark band near D on the blue side of that 

 line," due niost probably to the absorption of the second manganese 

 fluting at 586, the first being masked by the citron carbon band. 



By May 22nd, when the spectrum was again observed by Vogel, the 

 comet had much increased in brightness, and " the continuous 

 spectrum of the nucleus had increased in intensity and extent, and 

 was not different from the spectrum of a fixed star." 



On May 27th, however, Copeland and Lohse noticed a bright line, 

 so faint as to require some attention to see it, in the less refrangible 

 end of the spectrum, which they identified with the D line by com- 

 parison on the following day. At the same time they observed a 

 bright part at wave-length 558, due, there can be little doubt, to the 

 first manganese fluting at 558. A maximum at 503 may have been 



* Hasselberg, ' Astr. Nachr.,' No. 2441. 



