WHAT FILLS THE STAR-DEPTHS ? 
279 
really as it seems (from the third of the above results), a stream 
of stars of many different orders, with an enormous preponder- 
ance of relatively small stars, is it not abundantly evident that, 
supposing any portion of this stream removed to a greater dis- 
tance, the fainter regions would vanish first, and that the 
brighter regions would appear as small discrete patches of 
nebulous light, or naked-eye nebulae? Now conceiving the 
stream yet further removed, until even these patches become 
telescopic objects, would they not in all respects resemble the 
stellar but irresolvable nebulae ? 
Thus it appears to me that we have not only no grounds for 
believing that the nebulae are external galaxies, but tolerably 
distinct evidence that the stellar nebulae are distant aggrega- 
tions of stars of many orders of magnitude. Such aggregations 
may also themselves present, and doubtless they do present, all 
orders of magnitude, precisely as within the two rich stellar 
regions above referred to we find every conceivable variety of 
aggregation, and precisely also as within the Milky Way we 
find, on the one hand, bright regions as extensive as that which 
lies in the constellation Cygnus, and, on the other hand, regions 
as limited as the double cluster in Perseus. 
Have we, on the other hand, any satisfactory reasons for re- 
garding the nebulae as external galaxies ? Tracing back the 
course of that process of discussion which has led to the nebulae 
being commonly so regarded, can we indicate any one argument 
which may be looked upon as definitely pointing to such a con- 
clusion? I know not of one. I have carefully studied the 
writings of Sir William Herschel, and I venture to assert, 
without fear of valid contradiction, that every single considera- 
tion adduced in favour of the nebulae being external galaxies 
has been founded on the assumption that the sidereal system 
is continuous — that is, on an assumption which Sir William 
Herschel himself was the first to throw doubt upon. 
But the evidence derivable even from Sir William Herschel’s 
own writings goes farther than this. He not only formed 
views respecting the sidereal system diametrically opposed to 
those which he had entertained when his conceptions respect- 
ing the stellar nebulae were put forward, but he arrived, by 
his careful study of the nebulae system, at a conclusion which, 
rightly interpreted, brings the nebulae into close association with 
the sidereal system. For, noticing the aggregation of nebulae 
at the northern region, which lies farthest from the Milky Way, 
he confidently expressed his belief that any sound theory of 
the universe must account for that peculiar relation. In other 
words, no theory of the universe can be regarded as sound 
which treats that relation as accidental. So that Sir William 
Herschel himself regarded the nebulae system in a light which 
