49^ Dr. Herschei/s Catalogue 
milky appearances deserve the name of clustering collections, 
as they are certainly brighter about the middle, and fainter 
near their undefined borders. For, in my sweeps of the heavens, 
it has been fully ascertained, that the brightness of the milky- 
way arises only from stars; and that their compression in- 
creases in proportion to the brightness of the milky-way. 
We may indeed partly ascribe the increase, both of brightness 
and of apparent compression, to a greater depth of the space 
which contains these stars; but this will equally tend to shew 
their clustering condition : for, since the increase of brightness 
is gradual, the space containing the clustering stars must tend 
to a spherical form, if the gradual increase of brightness is to 
be explained by the situation of the stars. 
V. Of Groups of Stars. 
From clustering stars there is but a short transition to groups 
of stars; they are, however, sufficiently distinct to deserve a 
separate notice. A group is a collection of closely, and almost 
equally compressed stars, of any figure or outline ; it contains 
no particular condensation that might point out the seat of an 
hypothetical central force; and is sufficiently separated from 
neighbouring stars to shew that it makes a peculiar system of its 
own. It must be remembered, that its being a separate system 
does not exclude it from the action or influence of other systems. 
We are to understand this with the same reserve that has been 
pointed out, when we explained what we called insulated stars. 
The construction of groups of stars is perhaps, of all the ob- 
jects in the heavens, the most difficult to explain ; much less 
can we now enter into a detail of the numerous observations I 
