152 Royal Society : — Prof. Maxwell on a 



The planetary nebula, 4390, 2 6, Tauri Poniatawskii ; 4514, 73 

 H. IV. Cygni ; 4510, 51 H. IV. Sagittarii ; 4628, 1 H. IV. Aquarii ; 

 4964, 18*H. IV., the annular nebula in Lyra 4447, 57 M., and 

 the Dumb-bell in Vulpecula 4532, 27 M., gave spectra identical 

 with the spectrum of 37 H. IV., except that in the case of some of 

 these the strongest only of the three bright lines was seen. 



It is obvious that these nebulas can no longer be regarded as clus- 

 ters of stars. In place of an incandescent solid or liquid body trans- 

 mitting light of all refrangibilities through an atmosphere which in- 

 tercepts by absorption some of them, such as our sun and. the fixed 

 stars appear to be, these nebulae, or at least their photosurfaces, 

 must be regarded as enormous masses of luminous gas or vapour. 



On this supposition the absence of central condensation admits 

 of explanation ; for even if the whole mass of the gas is luminous, 

 the light emitted by the portion of gas beyond the surface visible to 

 us would be in great measure absorbed by the portion of gas through 

 which it would have to pass, and for this reason there would be pre- 

 sented a luminous surface only. The small brilliancy of the nebulee, 

 notwithstanding the considerable angle which in most cases they 

 subtend, is in accordance with the very inferior splendour of glow- 

 ing gas as compared with incandescent solid or liquid matter. 



The extreme simplicity of constitution which the three bright 

 lines suggest, whether or not we regard them as indicating the 

 presence of nitrogen, hydrogen, and a substance unknown, is opposed 

 to the opinion that they are clusters of stars. 



The following nebulae and resolvable clusters gave a" continuous 

 spectrum :— 4294, 92 M. Herculis ; 4244, 50 H. IV. Ilerculis ; 

 116, 31 M., the Great Nebula in Andromeda; 117, 32 M. Andro- 

 meda ; 428, 55, Andromedse ; 826, 26 H. IV. Eridani. 



In the spectrum of 31 M., the nebulas in Andromeda, and in that 

 of the companion nebula, 32 M., the red and part of the orange are 

 wanting. 



December 8. — Dr. William Allen Miller, Treasurer and Vice-Presi- 

 dent, in the Chair. 



"A Dynamical Theory of the Electromagnetic Field." By Pro- 

 fessor J. Clerk Maxwell,' F.H.S. 



The proposed Theory seeks for the origin of electromagnetic effects 

 in the medium surrounding the electric or magnetic bodies, and as- 

 sumes that they act on each other not immediately at a distance, 

 but through the intervention of this medium. 



The existence of the medium is assumed as probable, since the 

 investigations of Optics have led philosophers to believe that in such 

 a medium the propagation of light takes place. 



The properties attributed to the medium in order to explain the 

 propagation of light are — 



1st. That the motion of one part communicates motion to the 

 parts in its neighbourhood. 



2nd. That this communication is not instantaneous but progres- 

 sive, and depends on the elasticity of the medium as compared with 

 its density. 



