SI 8 Biographical Memoir of Sir William Herschel. 
Catalogue. The object of this investigation was to determine 
the extent and nature of the changes that take place among the 
fixed stars ; and the method of comparing their relative lustres 
employed by our author, is admirably adapted for this purpose. 
The deductions, however, to which these observations will lead, 
must belong to another age, when astronomers of equal activity 
and intelligence shall have marked with the same accuracy the 
comparative lustre of these distant bodies. 
The determination of the variable brightness of the Fifth Satel- 
lite of Saturn, led our author to a series of “ Observations on the 
changeable brightness of the Satellites of Jupiter , and of the 
variation in their apparent Magnitudes , with a determination of 
the Time of their rotatory motion on their axes” This interest- 
ing memoir, which appeared in the Transactions for 1797, con- 
tains many curious results. It proves beyond a doubt, that 
considerable changes take place in the brightness of the satel- 
lites; — that the 1st satellite is white (sometimes to a very intense 
degree) ; the 2d white, bluish, and ash-coloured ; the 3d always 
white ; and the 4th of a dingy orange, or sometimes reddish ; 
— that the 3d satellite is the largest ; the 1st a little larger than 
the 2d, and nearly of the size of the 4th ; and the 2d a little 
smaller than the 1st and 4th, or the smallest of them all. It 
appears also extremely probable, that all the satellites revolve 
about their axes in the same time that they perform their perio- 
dical revolution about the planet. 
Hitherto it had been supposed, that a telescope afforded a 
distinct view of remote objects, merely by presenting a magni- 
fied picture to the eye ; but Dr Herschel was led by an acci- 
dental experiment, to notice a peculiarity in their mode of action, 
which he has designated by the name of the power of pene- 
trating into space. When he viewed in the dusk of the even- 
ing the dial of a distant steeple, he was able to observe the 
hour of the day, though the steeple itself was invisible to his 
naked eye. Hence he concluded, that though magnifying 
power was necessary to see the hours on the dial-plate, yet none 
was required to see the steeple itself ; and therefore that the tele- 
scope had a power of penetrating into space. The use of night 
glasses with large apertures was no doubt suggested by the 
practical knowledge of this principle, and it is not difficult to 
