Biographical Memoir of Sir William HerschcL 7 



to llie tube, and when the instrument is turned toward the hea- 

 vens, this mirror reflects the shining image of the stars. The 

 observer is himself transported along with the tube in any di- 

 rection required, for he is placed in a seat attached to the upper 

 extremity ; the objects which he observes are behind him, and 

 he views their reflected images. 



Herschel discovered, with this telescope, two new satellites of 

 Saturn ; they are both nearer the planet than those made known 

 by Iluygens and Cassini. Never had the heavens been ob- 

 served with so extraordinary an instrument ; and, it may be said, 

 that the greatest phenomena displayed themselves under a no- 

 vel aspect. The nebulosities, those small luminous and irregu- 

 lar clouds which may be remarked among the fixed stars, in va- 

 rious regions of the heavens, appeared almost all to resolve 

 themselves into an innumerable multitude of stars ; others, hi- 

 therto imperceptible, seemed to have acquired a distinct light. 

 On the entrance of Sirius into the field of the telescope, the eye 

 was so violently affected, that stars of less magnitude could not 

 immediately after be perceived ; and it was necessary to wait for 

 twenty minutes before these stars could be observed. 



The instruments, of which he had previously made use, were 

 less advantageous for the observation of some phenomena ; but 

 it was more easy for him to multiply them, and vary their modes 

 of application. No astronomer had yet been able to acquire so 

 complete and so distinct a knowledge of the phenomena of the 

 heavens. For example, the ring of Saturn always ceased to be 

 jxjrceived when its plane was directed toward the earth ; but the 

 feeble light which it reflects in that position was enough for Her- 

 schel, and the ring still remained visible to him. 



An entirely new and very important observation made by him, 

 was that of certain remarkable points on the surface of Saturn's 

 ring. From these points, Herschel concluded, that this satellite, 

 remarkable for its singular form, turns upon itself round an axis 

 perpendicular to its plane; and he measured the duration of tliis 

 rotatory motion, which is about ten hours and a-half. 



Not long before, a great geometrician in France investigated 

 tlie same question, and solved it by mathematical anal3'sis, which 

 is also a very powerful instrument, and the most general of all. 

 M, dc Laplace demonstrated, that the rotation of the ring of Sa- 



