STAR STREAMS AND STAR SPRAYS. 
401 
argue that, because we have no means whatever of determining 
the actual distances of the orbs in that group, we are not at 
liberty to assume that they form a real clustering aggregation 
of stars. In so doing, we should undoubtedly be losing sight 
of evidence which absolutely demonstrates the clustering nature 
of the Pleiades. We have only to consider the mathematical 
probability that so many orbs would be gathered together 
within a certain portion of the heavens in the Pleiades, when 
the total number of stars between the same limits of magnitude 
is such and such, to see that we have not to do with an acci- 
dental phenomenon due merely to the apparent association of 
stars of many orders of distance in nearly the same direction, 
but with a real aggregation of stars into a definite cluster, sur- 
rounded on all sides by comparatively vacant regions. W e know 
that William Mitchell, more than a hundred years ago, by 
simply considering the six brighter stars of the Pleiades, was 
able to show that the odds are about half a million to one 
against the association of these stars being apparent only.* 
Now it is worthy of notice that, even among stars of the first 
three or four orders of magnitude, signs of aggregation are 
discernible, which appear too marked to be due to mere chance 
distribution. For instance, if we take an equal-surface (iso- 
graphic) chart of the northern heavens, showing all stars down 
to the fourth magnitude inclusive, we are struck by the singular 
vacancy lying where modern astronomers place the constella- 
tion of the Cameleopard. Within an oval space, having Polaris 
and Castor as the ends of its longer diameter, Dubhe and S 
Aurigse as the ends of its shorter diameter, there are but three 
stars (of the fourth magnitude), although this region extends 
* Mitchell’s paper, in Vol. lvii. of the Philosophical Transactions , antici- 
pates in the clearest possible manner one of the general laws of stellar 
distribution which I have lately endeavoured to establish. The following 
passage, in particular, may be quoted in illustration : — “ It has always been 
usual with astronomers to dispose the fixed stars into constellations ; this 
has been done for the sake of remembering and distinguishing them, and 
therefore it has in general been done merely arbitrarily and with this view 
only ; nature herself, however, seems to have distinguished the stars into 
groups. What I mean is, that from the apparent situation of the stars in 
the heavens, there is the highest probability that, either by the original act 
of the Creator, or in consequence of some general law (such, perhaps, as 
gravity), they are collected together in great numbers in some parts of 
space, whilst in others there are either few or none. The argument I make 
use of in order to prove this is of that kind which infers either design or 
some general law, from a general analogy, and the greatness of the odds 
against things having been in the present situation, if it was not owing to 
some such cause.” 
