258 ANNUAL OF SCIENTIFIC DISCOVERY. 



really consist, not of stars or separate masses of condensed solid mat- 

 ter, but of gaseous or vaporous fluid. He was thence led to those 

 beautiful speculations which, enlarged and illustrated by Laplace, 

 who showed the mathematical possibility of such an action of conden- 

 sation as Sir W. Herschel required, have become widely known as 

 the Nebular hypothesis, and as to the validity of which, so much con- 

 troversy has been excited. We all know, too, that of late years the 

 tendency of telescopic observation, as conducted by the magnificent 

 instruments of Lord Rosse, and corroborated by the splendid achro- 

 matic of Harvard University, has been to reduce the numbers of the 

 irresolvable nebulae ; and in particular that, after the great nebula of 

 Orion had, in parts at least, been resolved, the general impression 

 was that the nebular hypothesis had lost all substantial evidence in its 

 favor, and though it still might be contended that the existing sys- 

 tems had such an origin, yet that examples of matter in the nebulous 

 form were not to be found in the heavens. Still, it must not be for- 

 gotten that Lord Rosse, while resolving many of the nebula?, had dis- 

 covered others which resisted his instrumental powers, and that to 

 many clusters there were fantastic wisps of nebulous light appended, 

 and diffuse patches of light attached, which defied resolution, though 

 they were evidently connected with the objects he pronounced to be 

 starry in construction. 



AmOng the most wonderful of the nebulous bodies are those called 

 by Sir W. Herschel planetary nebulae. These present the appearance 

 of small discs of uniform light, usually circular, and generally blue, 

 or bluish green in color. They are few in number and bright, looking, 

 in the telescope with a low power, very much like stars out of focus. 

 Now it was apparent to Sir W. Herschel that bodies like these, hav- 

 ing no central condensation of light nor stellar nucleus, could not be 

 globular clusters of stars, which necessarily would be brighter in the 

 centre than at the edges. A cylinder form of stars seen endways 

 might certainly present such an appearance, or a flat disc of stars 

 placed at right angles to our line of sight would also look like a plan- 

 etary nebula ; but such forms are too improbable to detain us a mo- 

 ment. He therefore,' came to the conclusion that these were masses 

 of truly nebulous or vapory matter in the primal form, and present- 

 ing various stages of the process of condensation, a few points of 

 light being visible in some, where a little solid or liquid had probably 

 formed. This supposition, with the addition that, at some distance 

 from the surface the matter was sufficiently condensed to prevent 

 light from behind penetrating it, was then sufficient to account for 

 the existence of a uniform planetary light-giving surface. 



Lord Rosse discovered an analogy between the annular nebula? 

 and the planetary forms, increasing the number of the former from 

 two to seven, and showing that a nebula with a hollow center, imper- 

 fectly seen in telescopes of less aperture, might present the appear- 

 ance of a planetary nebula ; but the question remained one of ex- 

 treme difficulty, when .Mr. lluggins, in the autumn of 1864, took up 

 the subject and attempted to bring analysis by the prism to bear upon 

 these remarkable bodies, which seemed a class, sui generis, and of an 

 order entirely different from the sun and fixed stars. 



The apparatus used was essentially the same as we have described ; 



