2,94 Dr . Herschel’s observations of the satellites 
wanting, is fully equal to what may be required to view ex- 
tremely small objects ; but this branch of the properties of 
optical instruments seems not to be generally understood : 
the question how much a telescope magnifies, admits of vari- 
ous answers. To resolve it properly, we ought in all circum- 
stances to consider how far the magnifying power of a tele- 
scope is supported by an adequate quantity of light ; as without 
it, even the highest power and distinctness cannot be effcie?it. 
The question therefore ought to be limited to an inquiry into 
the extent of what may be called the effective magnifying 
power? It will however be found, that even then, the quan- 
tity of this power cannot be positively assigned. For if a card 
containing engraved letters of a certain size be put up at a 
given distance, the effective power of a telescope directed to 
it, will be that wherewith we can read these letters with the 
greatest facility ; but if either the size of the letters, or their 
distance from the telescope, be changed, the quantity of this 
power will no longer remain the same. 
An obvious consequence of this consideration is, that the 
effective power of telescopes has a considerable range of 
extent, and can only be assigned when the object to be viewed 
is given; and that in this determination two circumstances 
are concerned, which require a separate investigation ; and 
this is abundantly confirmed when a ten feet reflector, such 
as has been mentioned, is directed to the Georgian planet; for 
with none of its highest powers can we possibly ascertain 
even the existence of the satellites. 
Since, then, it is absolutely necessary that the power of 
magnifying should be accompanied with a sufficient quantity 
of light, to reach the satellites of this remote planet, it may 
