A HISTORY OF SCIENCE 



ing placed on record with a degree of accuracy hither- 

 to unapproachable. Moreover, other millions of stars 

 are brought to light by the negative, which are too dis- 

 tant or dim to be visible with any telescopic powers 

 yet attained a fact which wholly discredits all pre- 

 vious inferences as to the limits of our sidereal system. 

 Hence, notwithstanding the wonderful instrumental 

 advances of the nineteenth century, knowledge of the 

 exact form and extent of our universe seems more un- 

 attainable than it seemed a century ago. 



The Structure of Nebula 



Yet the new instruments, while leaving so much 

 untold, have revealed some vastly important secrets of 

 cosmic structure. In particular, they have set at rest 

 the long-standing doubts as to the real structure and 

 position of the mysterious nebulae those lazy masses, 

 only two or three of them visible to the unaided eye, 

 which the telescope reveals in almost limitless abun- 

 dance, scattered everywhere among the stars, but 

 grouped in particular about the poles of the stellar 

 stream or disk which we call the Milky Way. 



Herschel's later view, which held that some at least 

 of the nebulae are composed of a "shining fluid," in 

 process of condensation to form stars, was generally 

 accepted for almost half a century. But in 1844, when 

 Lord Rosse's great six-foot reflector the largest tele- 

 scope ever yet constructed was turned on the nebulae, 

 it made this hypothesis seem very doubtful. Just as 

 Galileo's first lens had resolved the Milky Way into 

 stars, just as Herschel had resolved nebulae that resist- 

 ed all instruments but his own, so Lord Rosse's even 



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