482 Dr. Herschel’s Catalogue 
for reflection, and, I may add, for calculation. They shew, even 
before we come to more complicated combinations, where the 
same will be confirmed, that there is an essential difference 
between the construction of solar and sidereal systems. In 
each solar system, we have a very ponderous attractive centre, 
by which all the planets, satellites, and comets are governed, 
and kept in their orbits. Sidereal systems take a greater scope : 
the stars of which they are composed move round an empty 
centre, to which they are nevertheless as firmly bound as the 
planets to their massy one. It is however not necessary here to 
enlarge on distinctions which will hereafter be strongly sup- 
ported by facts, when clusters of stars come to be considered. 
I shall only add, that in the subordinate bodies of the solar 
system itself, we have already instances, in miniature, as it may 
be called, of the principle whereby the laws of attraction are 
applicable to the solution of the most complicated phenomena 
of the heavens, by means of revolutions round empty centres. 
For, although both the earth and its moon are retained in their 
orbits by the sun, yet their mutual subordinate system is such, 
that they perform secondary monthly revolutions round a centre 
without a body placed in it. The same indeed, though under 
very narrow limits, may be said of the sun and each planet 
itself. 
That no insulated stars, of nearly an equal size and distance, 
can appear double to us, may be proved thus. Let Arcturus and 
Lyra be the stars : these, by the rule of insulation, which we 
must now suppose can only take place when their distance from 
each other is not less than that of Sirius from us, if very accu- 
rately placed, would be seen under an angle of 60 degrees from 
each other. They really are at about 59 0 . Now, in order to 
