214 Mr. J. N. Lbckyer. On the Classification [Jan. 10, 



in iron meteorites, but to a far greater extent in stony ones. A 

 photograph of the arc spectrum of the Obernkirchen meteorite 

 shows barely a trace of D, but the spectrum of a mixture of iron and 

 stones shows it fairly bright. 



XI. CONCLUSIONS. 



I must again refer to the vast difference in the way in which the 

 phenomena of distant and near meteoric groups are necessarily 

 presented to us ; and, further, we must bear in mind that in the case 

 of comets, however it may arise, there is an action which drives the 

 vapours produced by impacts outward from the swarm in a direction 

 opposite to that of the sun. 



It must be a very small comet which, when examined spectro- 

 scopically in the usual manner, does not in consequence of the size 

 of the image on the slit enable us to differentiate between the spectra 

 of the nucleus and envelopes. The spectrum of the latter is usually 

 so obvious, and the importance of observing it so great, that the 

 details of the continuous spectrum of the nucleus, however bright it 

 may be, are almost overlooked. 



A moment's consideration, however, will show that if the same 

 comet were so far away that its whole image would be reduced to a 

 point on the slit-plate of the instrument, the differentiation of the 

 spectra would be lost; we should have an integrated spectrum in 

 which the brightest edges of the carbon bands, or some of them, 

 would or would not be seen superposed on a continuous spectrum. 



The conditions of observation of comets and stars being so different, 

 any comparison is really very difficult; but the best way of proceeding 

 is to begin with the spectrum of comets, in which, in most cases, for 

 the reason given, the phenomena are much more easily and accurately 

 recorded. But even in the nucleus of a comet as in a star it is much 

 more easy to be certain of the existence of bright lines than to record 

 their exact positions,* and as a matter of fact bright lines, as we 

 have seen, including in all probability hydrogen, have been recorded, 

 notably in Comet Wells and in the Great Comet of 1882. 



Allowing for these differences in the conditions of observations, 

 the discussion shows that the changes in the spectrum of a meteor- 

 swarm in the solar system are closely related to those which take 

 place in a swarm outside the solar system. 



In both cases, when the number of collisions is just sufficient to 

 render the swarms visible, the spectra are identical, consisting simply 

 of the radiation of the fluting of magnesium at 500. 



* " Observations of Comet III, 1881, June 25. The spectrum of the nucleus is 

 continuous ; that of the coma shows the usual bands. With a narrow slit there are 

 indications of many lines just beyond the verge of distinct visibility." Copeland, 

 ' Copernicus,' vol. 2, p. 226. 



