28 



Mr. J. N. Lockyer. On the Classification [Apr. 12, 



upon it when, in addition to furnishing us with a general grouping, it 

 is used to indicate how the groups should be still further divided, and 

 what specific differences may be expected. 



The presence or absence of carbon will divide this group into two 

 main sub-groups. 



The first will contain those nebulae in which only the spectrum of the 

 meteoric constituents is observed with or without the spectrum of 

 hydrogen added. 



It will also contain those bodies in which the nebula spectrum gets 

 almost masked by a continuous one, such as Comets 1866 and 1867, 

 and the great nebula in Andromeda. 



In the second sub-group will be more condensed swarms still, in 

 which, one by one, new lines are added to the spectra, and carbon 

 makes its appearance ; while probably the last species in this sub- 

 group would be bodies represented by 7 Cassiopeia?. 



Species of Nebulce. 



I have elsewhere referred to the extreme difficulty of spectro- 

 scopic discrimination in the case of the meteor swarms which are just 

 passing from the first stage of condensation, and it may well be that 

 we shall have to wait for many years before a true spectroscopic 

 classification of the various aggregations which I have indicated, can 

 be made. 



It is clear from what has gone before that in each stage of evolu- 

 tion there will be very various surfaces and loci of collision in certain 

 parts of all the swarms, and we have already seen that even in the 

 nebulosities discovered by Sir Wm. Herschel, which represent possibly 

 a very inchoate condition, there are bright portions here and there. 



If the conditions are such in the highly elaborated swarms and in 

 the nebulosities that the number of collisions in any region per cubic 

 million miles is identical, the spectroscope will give us the same 

 result. In the classification of the nebula?, therefore, the spectro- 

 scope must cede to the telescope when the dynamical laws, which 

 must influence the interior movements of meteoric swarms, have been 

 fully worked out. The spectroscope, however, is certainly at one with 

 the telescope in pointing out that the so-called planetary nebula? are 

 among the very earliest forms — those in which the collisions are most 

 restricted in the colliding regions. The colour of these bodies is blue 

 tinged with green ; they do not appear to have that milkiness which 

 generally attaches to nebula?, and the bright nebulous lines are seen 

 in some cases absolutely without any trace of continuous spectrum. 

 In higher stages the continuous spectrum comes in, and in higher 

 stages still possibly also the bands of carbon ; for in many cases 

 Dr. Huggins in his important observations has recorded the weakness 



