SPIRAL NEBULM- — PUISEUX. 145 



Under a sky more transparent than that of England the harvest was 

 yet richer and the general summary published in 1864 gave the 

 positions of 5,079 of these objects. Ver^^ few nebulas found later 

 worthy of mtercst escaped the eyes of the Herschels. 



No thought was taken at that time as to what could be the origin 

 of these curious objects. Their vague aspect gave little faith in their 

 permanency. At one time the hope was held that they might rapidly 

 change before our eyes. Laplace, after meditating upon the spheri- 

 cal or flattened figures of the planets, upon the existence of the ring 

 system of Saturn, upon the close coincidence of the planes of the 

 equators and the orbits of the planets, became convinced that the 

 sun and the planets must have once been parts of the same large, 

 ver}" diffuse cloud. We might then expect the history of the solar 

 system to repeat itself among the many other nebulous clouds in the 

 reahns of space. What would be more natural than to see among the 

 nebulae successive stages in this evolution from such clouds, the 

 material of suns and planets of the future. And, accordingly, he 

 devised that celebrated hypothesis which has since been the cause of 

 so many polemics. 



For the convenience of reasoning, Laplace gave to his primitive 

 cloud a figure of revolution, a general rotation about an axis and a 

 density decreasing regularly from the center outward. Upon all 

 these points the great mathematician showed no spirit of intolerance, 

 and would liave vrillingly consented to improvements. But it was 

 much later that objections were raised. 



The assiduous observers of nebulte found that these objects were 

 mostly of a much less simple structure. This was shown first by the 

 principal nebula of Orion, which was selected because of its extent 

 and brightness. Within the same limits where Huygens drew a 

 uniformly bright sin-face, astronomers provided with better telescopes 

 found strong contrasts of light and shade, filaments and entangled 

 jets, indications of physical connection between this cosmic cloud and 

 numerous stars. All these poitits are revealed in the beautiful draw- 

 ings left by J. lierschel, De Vico, W. Bond, Lassell, G. Bond, and 

 Lord Rosse. The divei-gencies, often strildng, may be interpreted 

 through the marvollous ])lates taken by Prof. Ritchey at the Yerkes 

 Observatory and at Mount Wilson. The same features are not shown 

 by the various artists and by the chemical processes. Even photo- 

 graphic plates have their "personalities" as well as artists. How- 

 ever, we have the right to hope that the plates are more impartial in 

 the features which tiiey reproduce. The long exposures employed 

 often destroy the dc^tails (?asily recognized by the eye in the central 

 and brighter parts. But for the reproduction of the faint and more 

 extended portions the superiorit}'' of the plates is unquestioned. 



