1868.] On Undevelopable Uniquadric Homographics. 389 



the spectrum of Comet I., 1864*. The positions of the three bands seen 

 by him appear to agree with those which the bright bands of this comet 

 occupy. 



This comet differs remarkably from the two small comets which I ex- 

 amined in the much smaller relative proportion of the light which forms a 

 continuous spectrum. In Brorsen's comet, as it now appears, the bright 

 middle part of the nebulosity seems to have a constitution analogous at 

 least to that of the nucleus, and to be self-luminous ; in the other comets 

 the coma, which surrounded a distinctly marked nucleus, gave a continuous 

 spectrum. The three comets resemble each other in the circumstance 

 that the light of the bright central part was emitted by the cometary 

 matter, while the surrounding nebulosity reflected solar light. 



The telescopic observations of the heads of Donati's comet and of other 

 large comets have shown that the luminous material is not at once driven 

 off into the outer portions of the coma and the tail, but usually forms in 

 front of the nucleus a dense luminous cloud, which for a time seems to be 

 identical in the character of its light with that of the nucleus. It is, I 

 believe, the outer portions only of the coma, which are frequently sepa- 

 rated by dark spaces from the nucleus, and the tail, which the polariscope 

 has shown to shine by reflected light. 



The positions of the bands in this comet would seem to indicate a che- 

 mical constitution different from that of the nebulae, which give a spectrum 

 of bright lines. It will be seen in the diagram that, though the brightest 

 of the bands in the spectrum of the comet differs but little in position from 

 the brightest line of the nebulae, the other bands are found in parts of the 

 spectrum widely removed from those in which the other lines of the 

 nebulae occur. The suggestion presents itself whether the broad, nebu- 

 lous bands may not indicate conditions of temperature and molecular state 

 different from those which occur in the gaseous nebulae. Pliicker has 

 shown that nitrogen and some other substances give totally different spectra, 

 under different conditions of temperature and tension. The spectrum of this 

 comet, however, does not resemble the other spectrum of nitrogen, which 

 Pliicker distinguishes as the spectrum of the first order f. 



IX. Memoir on 'Undevelopable Uniquadric Homographics.'-'' By 

 Martin Gardiner, C.E. Communicated by the Rev. R. Towns- 

 end, F.R.S. Received April 13, 1868. 



(Abstract.) 



In this paper the author's method of investigation is purely geometrical 

 throughout, its arrangement of details is systematic and natural, and it is 

 divided into eight chapters, the first seven of which are preparatory to the 

 consideration of the interesting problem discussed at length in the eighth, 



^ Philosophical Transactions, 1864, p. 158. 



t Ibid. 186.5, p. 9. 



2 N 2 



