1866.] 



of Comet 1, 1866. 



7 



We know, from observation, that the comae and tails of comets are formed 

 from the matter contained in the nucleus. 



The usual order of the phenomena which attend the formation of a tail 

 appears to be that, as the comet approaches the sun, material is thrown 

 off, at intervals, from the nucleus in the direction towards the sun. This 

 material is not at once driven into the tail, but usuali}^ forms in front of 

 the nucleus a dense luminous cloud, into which for a time the bright mat- 

 ter of the nucleus continues to stream. In this way a succession of enve- 

 lopes may be formed, the material of which afterwards is dissipated in a 

 direction opposite to the sun, and forms the tail. Between these envelopes 

 dark spaces are usually seen. 



If the matter of the nucleus is capable of forming by condensation a 

 cloud-like mass, there must be an intermediate state in which the matter 

 ceases to be self-luminous, but yet retains its gaseous state, and reflects but 

 little light. Such a non-luminous and transparent condition of the come- 

 tary matter may possibly be represented by some at least of the dark spaces 

 which, in some comets, separate the cloud-like envelopes from the nucleus 

 and from each other. 



Several of the nebulae which I have examined give a spectrum of one line 

 only, corresponding in refrangibility with the bright line of the nucleus of 

 the comet referred to in this paper. Other nebulae give one and two fainter 

 lines besides this bright line. Whether either or both of these were also 

 present in the spectrum of this comet I was unable to determine. The light 

 of the comet was feeble, and the presence of the continuous spectrum made 

 the detection of these lines more difficult. I suspected the existence of the 

 brighter of these lines. I employed different eyepieces, and also gave 

 breadth to the bright point by the use of the cylindrical lens, but I was 

 not able to obtain satisfactory evidence of more lines than the bright one 

 already described. 



In my paper " On the Spectra of the Nebulae," I showed that this 

 bright line corresponds in refrangibility with the brightest of the lines of 

 nitrogen. This line may perhaps be interpreted as an indication that 

 cometary matter consists chiefly of nitrogen, or of a more elementary sub- 

 stance existing in nitrogen. 



The great varieties of structure which may exist among comets, as well 

 as the remarkable changes which the same comet undergoes at different 

 epochs, will cause all those who are interested in the advance of our know- 

 ledge of the cosmical relations of these bodies, and of the gaseous nebulae, 

 to wait with some impatience the visit of a comet of sufficient splendour 

 to permit a satisfactory prismatic examination of the physical state of 

 cometary matter during the various changes which are dependent upon 

 the perihelion passage of the comet. 



