BIBLIOGRAPHY OF HERSCIIEL'S WRITINGS. 581 



Herschel, W.: SvNorsis of the Writings of— Contimit'd. 

 A. D. Vol. p. 



regard to each other in a progresnive course, denoting a periodical revolu- 

 tion round each other. 

 Our suu dofs not belouj^ to such a Hystein. 



III. Of more complicated Sidereal Systems, or trchle, quadruple, quintuple, 

 and multijile stars. 

 1802 92 487 Theorem as to the permaneut connection of revolving stars, when the 

 forces acting on any one of them reduced to a direction as coming 

 from the empty centre are in the direct ratio of the distances from 

 that centre. 

 488-494 Hypothetical examples of such multiple stars. 



494 Such combinations as I have mentioned are not the inventions of fancy ; 



they have an actual existence ; and, were it necessary, I could point 

 them out by thousands. 



495 I do not imagine that I have pointed out the actual manner in which 



they are held together, but only the possibilities of such unions. 



495 IV. Of Clustering Stars and the Milky Way. 



Marks of clustering in the milky way. Example of the stars between 

 /i and Z Cygui. 



49G "We may indeed partly ascribe the increase, both of brightness and 

 of apparent comi>ressiou, to a greater depth of the space which con- 

 tains these stars ; but this will equally tend to show their clustering 

 condition, for since the increase of brightness is gradual, the space 

 containing the clustering stars must tend to a spherical form if the 

 gradual increase of brightness is to be explained by the situation of 

 the stars." 

 V. Of groups of Stars. Definition. 



497 VI. Of clusters of Stars. Definition. 



Vll. Of Nebulw. Perhaps they are all to be resolved into the three 

 last-mentioned species. 



498 Power of a telescope to penetrate not only space but time past. 



499 VIII. Of Stars ivith Burs or Stellar Nebula^. 

 IX. Of Milky Nebulosity. 



Probably of two kinds. 1st, apparent, which is formed by distant 

 ["widely-extended"] clustering stars; and, 2d, real, and possibly 

 at no very great distance from us. The nebula of Orion of this lat- 

 ter kind. 



500 X. Of Nebulous Stars. 



501 XI. Of Planetary Nebula: 



Perhaps they are allied to nebulous stars. 

 XII. Of Planetary Nebula; with Centres. 

 503 Catalogue. 



216 to No. 22H 

 769 to No. 907 

 Nc. 748 to No. 978 

 .59 to No. 78 

 45 to No. 52 

 36 to No. 42 

 56 to No. 67 

 79 to No. 88 



528 Plates XVI and XVII, 16 figures of hypothetical orbits of multiple 

 stars. 

 1803 93 214 Observations of the Transit of Mercury over the disk of the Sun; to which 

 is added an investigation of the causes which often prevent the proper 



