320 The Construction of the Heavens. (July, 
stars in a state of rest. For though our sun, and all the 
Stars we see, may truly be said to be in the plane of the 
Milky Way, yet I am now convinced by a long inspection and 
continued examination of it that the Milky Way itself con- 
sists of stars very differently scattered from those which are 
immediately about us. ae - “Qna very slight ex- 
amination it will appear that this immense starry aggrega- 
tion is by no means uniform. The stars of which it is 
composed are very unequally scattered, and show evident 
marks of clustering together into many separate allotments.” 
ack ‘The “milky appearances deserve the name of 
clustering collections, as they are certainly much brighter 
about the middle and fainter near their undefined borders. 
For in my sweeps of the heavens it has been fully ascer- 
tained that the brightness of the Milky Way arises only from 
stars; and that their compression increases in proportion to 
the brightness of the Milky Way.” . . . “We may 
indeed “partly ascribe the increase both of brightness and 
compression,” in these clustering regions,” to a greater 
depth of the space which contains these stars; but this will 
equally tend to show 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.” 
It was in this paper that Herschel first put forward the 
hypothesis that many nebule are formed of some substance 
possessing ‘‘the quality of a self-luminous milky luminosity,” 
and “possibly at no great distance from us.” But the 
requirements of space render it advisable that I should 
leave untouched the whole of those considerations which 
Herschel adduced, in this paper and elsewhere, in favour of 
his nebular hypothesis. 
That portion of Herschel’s observing career which we have 
now reached may fairly be regarded as including the most 
important of his researches into the constitution of the 
heavens. His mental powers were now attheir prime. He 
had acquired great experience. His circumstances were such ° 
that he could give his whole attention to scientific work. 
Moreover, he had seen the importance of being solely 
guided by known facts, since the theory which he had 
adopted more than a quarter of a century before (and almost 
in the beginning of his observations on the stars) had had 
to be abandoned. 
Nine years of earnest labour passed before he again spoke 
at any length on the subject of the star-depths. lt will be 
