1864.] 



of some of the Nebula. 



493 



way between b and F, and was found, by the method of simultaneous 

 observation, to be coincident with the brightest of the hues of nitrogen. 



A Httle more refrangible, a second line is seen. At about three times the 

 distance of the second line, a third, very faint line occurs ; this coincides 

 in position with Fraunhofer's F, and one of the lines of hydrogen. Besides 

 the three bright lines, an exceedingly faint continuous spectrum of the cen- 

 tral bright point was perceived. 



The planetary nebula, 4390, S 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 clusters of 

 stars. In place of an incandescent solid or liquid body transmitting light 

 of all refrangibilities through an atmosphere which intercepts 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 ex- 

 planation ; 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 presented a luminous surface 

 only. The small brilliancy of the nebulae, notwithstanding the considerable 

 angle which in most cases they subtend, is in accordance with the very 

 inferior splendour of glowing 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 spec- 

 trum :— 4294, 92 M. Herculis ; 4244, 50 H. IV. Herculis ; 116, 31 M., 

 the Great Nebula in Andromeda; 117, 32 M. Andromedae ; 428; 55, 

 Andromedse ; 826, 26 H. IV. Eridani. 



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

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



VI. "On the Composition of Sea Water in different Parts of the 

 Ocean." By Dr. George Forchhammer, Professor in the 

 University of Copenhagen. Communicated by the President, 

 Received July 28, 1864. 



This Paper was in part read. 



2 p 



