344 Dr. Herschel's Observations of the 
to avoid such ambiguities, when the condition of the satellites 
is under investigation. Nor is it possible to throw the size and 
light into one general idea, and take the first coup d’oeil in 
looking at them, to decide about the general impression this 
compound may make. When our attention is forcibly drawn 
by a considerable power to the apparent size of the satellite we 
are looking at, its brightness can no longer be taken in that 
general way, but must be abstracted from size. 
Let us now see what use may be drawn from the observa- 
tions I have given. 
It appears in the first place very obviously, that considerable 
changes take place in the brightness of the satellites. This is 
no more than might be expected. A variegated globe, whether 
terraqueous like the earth, or containing regions of soil of an 
unequal tint, like that side of the moon which is under our 
inspection, cannot, in its rotation, present us with always the 
same quantity of light Reflected from its surface. 
In the next place, the same observations point out what we 
could hardly expect to have met with ; namely, a considerable 
change in the apparent magnitude of the satellites. Each of 
them having been at different times the standard to which 
another was referred, we cannot refuse to admit a change so 
well established, singular as it may appear. 
The first of these inferences proves that the satellites have 
a rotatory motion upon their axes, of the same duration with 
their periodical revolutions about the primary planet. 
The second either shews that the bodies of the satellites are 
not spherical, but of such forms as they have assumed by their 
quick periodical and slow contemporary, rotatory motions, and 
which forms in future may become a subject for mathematical 
