SKCT. XXXVH. NEBULA. 377 



and use of this nebulous matter, scattered over the 

 heavens in such a variety of forms, is involved in the 

 greatest obscurity. That it is a self-luminous, phos- 

 phorescent, material substance, in a highly dilated or 

 gaseous state, but gradually subsiding by the mutual 

 gravitation, of its particles into stars and sidereal systems, 

 is the hypothesis most generally received. And indeed 

 this is the hypothesis of La Place with regard to the 

 origin of the solar system, which he conceived to be 

 formed by the successive condensations of a nebula, 

 whose primeval rotation is still maintained in the rota- 

 tion and revolution of the sun and all the bodies of the 

 solar system in the same direction. Even at this day 

 there is presumptive evidence in the structure and in- 

 ternal heat of the earth, of its having been at one period 

 in a gaseous state from intensely high temperature. 

 But the only way that any real knowledge on this mys- 

 terious subject can be obtained is by the determination 

 of the form, place, and present state of each individual 

 nebula ; and a comparison of these with future observa- 

 tions will show generations to come the changes that 

 may now be going on in these supposed rudiments of 

 future systems. With this view, Sir John Herschel 

 began in the year 1825 the arduous and pious task of 

 revising his illustrious father's observations, Avhich he 

 finished a short time before he sailed for the Cape of 

 Good Hope, in order to disclose the mysteries of the 

 southern hemisphere ; indeed, our firmament seems to 

 be exhausted till farther improvements in the telescope 

 shall enable astronomers to penetrate deeper into space. 

 In a truly splendid paper read before the Royal Society 

 on the 21st of November, 1833, he gives the places of 

 2500 nebulae and clusters of stars. Of these 500 are 

 neWj the rest he mentions with peculiar pleasure as 

 having been most accurately determined by his father. 

 This work is the more extraordinary, as from bad 

 weather, fogs, twilight, and moonlight, these shadowy 

 appearances are not visible, on an average, in England, 

 above thirty nights in the year. 



The nebulae have great variety of forms. Vast multi- 

 tudes are so faint as to be with difficulty discerned at all 

 till they have been for some time in the field of the 

 u2 



