Mr. A. S. Davis on a Theory of Nebula and Comets. 405 



line, whilst in other parts the light gradually fades away into 

 darkness. We may suppose that in the one case we are looking 

 edgewise at a shell, so that the line of sight is tangential to its 

 surface, whilst in the other case we are looking at a part where 

 the luminous shell gradually thins out. Now in the first case 

 we find that the light increases towards the border, consistently 

 with the supposition that the visible parts are in thin shells. 



This peculiarity is, I apprehend, what Professor Bond and 

 Mr. Lassell refer to when they speak of the scroll-like appear- 

 ance of the nebula in Orion. 



In those nebulae which appear to have assumed a more stable 

 form, as in the elliptic and circular nebulae, the light diminishes 

 from the centre to the circumference. 



These nebulae may be in a later stage of formation ; they may 

 have become more condensed, and a more intimate mixture of 

 the gases may have taken place, and in consequence they may 

 be more or less luminous throughout. Their shape, and the 

 fact that they are generally resolvable in the telescope, point to 

 the conclusion that they have been a long time in existence. 



Before turning to the subject of comets, I may remark that 

 the theory that there are large masses of invisible gas traversing 

 space, affords an explanation of the sudden temporary outbursts 

 in the intensity of certain stars. Such an outburst might occur 

 if a star became enveloped with a mass of gas with which it 

 could chemically combine. 



It also explains the phenomena of periodical stars. Mr. Hug- 

 gins has found that the diminution in the intensity of the light 

 of these stars is due to an increase in the number and size of 

 the absorption-bands in their spectra. 



It has already been suggested that large opaque bodies may 

 be revolving in orbits round such stars and periodically obscu- 

 ring their light. We have only to suppose that large masses of 

 invisible gas instead of opaque bodies revolve about them, to ex- 

 plain the phenomenon in accordance with spectroscopic obser- 

 vation. 



I will now examine what account the present theory gives of 

 comets. 



There are some reasons for supposing that the sun itself is a 

 nebulous star, or that it is enveloped by matter extending to an 

 immense distance beyond its visible photosphere. The pheno- 

 menon of the zodiacal light, the bands of meteoric matter passed 

 through by the earth in its revolution about the sun, the retar- 

 dation experienced by comets, all point to this conclusion. 



We cannot suppose that this envelope consists of solid or 

 liquid matter only, without the presence of gaseous matter; for 

 at no known temperature can liquid or solid matter exist in a 



