244 



Mr. Cayley on Skew Surfaces. 



[May 26, 



magnesium, and probably of iron. The presence of sodium was also indi- 

 cated in Procyon and a Cygni. 



In no single instance bave tbe authors ever observed a star-spectrum in 

 whicb lines were not discernible, if tbe light were suifficiently intense, and 

 the atmosphere favourable. Rigel, for instance, which some authors state 

 to be free from lines, is filled with a multitude of fine lines. 



Photographs of the spectra of Sirius and Capella were taken upon collo- 

 dion ; but though tolerably sharp, the apparatus employed was not suffi- 

 ciently perfect to afford any indication of lines in the photograph. 



In the concluding portion of their paper, the authors apply the facts 

 observed to an explanation of the colours of the stars. They consider that 

 the difference of colour is to be sought in the difference of the constitution 

 of the investing stellar atmospheres, which act by absorbing particular 

 portions of the light emitted by the incandescent solid or liquid photo- 

 sphere, the light of which in each case they suppose to be the same in 

 quality originally, as it seems to be independent of the chemical nature of 

 its constituents, so far as observation of the various solid and liquid ele- 

 mentary bodies, when rendered incandescent by terrestrial means, appears 

 to indicate. 



III. "A Second Memoir on Skew Surfaces, otherwise Scrolls.''^ By 

 A. Cayley, Esq., F.R.S. Received April 29, 1864. 



(Abstract.) 



The principal object of the present memoir is to establish the different 

 kinds of skew surfaces of the fourth order, or Quartic Scrolls ; but, as preli- 

 minary thereto, there are some general researches connected with those in 

 my former memoir " On Skew Surfaces, otherwise Scrolls " (Phil. Trans, 

 vol. 153. 1863, pp. 453, 483), and I also reproduce the theory (which may 

 be considered as a known one) of cubic scrolls ; there are also some con- 

 cluding remarks which relate to the general theory. As regards quartic 

 scrolls, I remark that M. Chasles, in a footnote to his paper, "Description 

 des Courbes de tous les ordres situees sur les surfaces reglees du troisieme 

 et du quatrieme ordres," Comptes Rendus, t. liii. (1861), see p. 888, 

 states, '* les surfaces reglees du quatrieme ordre .... admettent quatorze 

 especes." This does not agree with my results, since I find only eight spe- 

 cies of quartic scrolls ; the developable surface or " torse " is perhaps in- 

 cluded as a " surface reglee ; " but as there is only one species of quartic 

 torse, the deficiency is not to be thus accounted for. My enumeration ap- 

 pears to me complete, but it is possible that there are subforms which M. 

 Chasles has reckoned as distinct species. 



