2 g6 Dr. Herschej/s observations of the satellites 
minutes, the twenty feet telescope may be properly adjusted 
and directed so as to have the planet in the field of view. 
In the next place I have to mention, that it has constantly 
been a rule with me, not to observe with a larger instrument, 
when a smaller would answer the intended purpose. To use 
a manageable apparatus saves not only time and trouble, but 
what is of greater consequence, a smaller instrument may 
comparatively be carried to a more perfect degree of action 
than a larger one ; because a mirror of less weight and dia- 
meter may be composed of a metal which will reflect more 
light than that of a larger one ; it will also accommodate 
itself sooner to a change of temperature ; and when it con- 
tracts tarnish, it may with less trouble be repolished ; to 
which may be added, that having two mirrors for the twenty 
feet always ready, my observations could never be interrupted 
by accidents which often happen to large mirrors, such as 
greatly injure, or even destroy their polish. 
The quantity of light reflected by the mirror of a twenty 
feet telescope of my construction being known, and the satel- 
lites of the Georgian planet being the objects to be viewed, I 
may now examine the combined powers of this instrument, 
and assign the limits to which they may be stretched. It will 
however be proper first, to point out from experience some 
of the advantages that may be taken, if not to increase, at 
least not to obstruct, the penetrating power, by the full effect 
of which the magnifying power is to be supported. 
The first precaution I ought to give is, that in these deli- 
cate observations, no double eye glass should be used, as it 
cannot be prudent to permit the waste of light at four surfaces, 
