THOUGHTS ON NATURAL PHILOSOPHY. 2Q 



In the early period of their discovery, a discovery 

 that followed naturally upon Galileo's first applica- 

 tion of the telescope to astronomical observation in 

 1609, nebulae were regarded as diffusions of a lucid 

 medium shining by its own inherent lustre. In 

 1780, the year that marked the commencement of 

 Sir William Herschel's classical researches upon 

 them, less than 150 were known ; but as the result 

 of these researches, which extended over a period of 

 twenty-one years, their number had been increased 

 to close upon 2,500. By their extended distribution 

 in space, as well as by the detailed structure revealed 

 in many of them by Herschel's observations, the 

 nebulae had acquired a new importance in the 

 system of the Universe. 



From a not altogether satisfactory deduction from 

 the universality of gravitation, an extension of 

 natural law that his own discovery of the mutual 

 revolution of the components of double stars went 

 far to establish, Herschel was led, in the earlier 

 period of his researches, to reject the generally 

 received view regarding the nature of nebulae, and to 

 substitute for it one according to which they were 

 clusters of stars, the component stars being too faint, 

 by reason, it was supposed, of excessive distance, for 

 their individuality to be recognized. While main- 

 taining this view with regard to the constitution of 

 some nebulae, Herschel, however, subsequently re- 

 verted to the former hypothesis to account for many 

 of them, these including the Nebula of Orion, re- 

 garding them as " extensions of a shining fluid of a 

 nature unknown to us." He further framed a first 



