for ascertaining the distances of clusters of stars, &c, 431 
whether they belonged to the class of nebulae, properly so 
called, or whether they might not consist of an aggregation 
of stars, at too great a distance from us to be distinctly per- 
ceived ; but it is evident that the distance of a cluster of stars 
cannot be ascertained, as long as it remains doubtful whether 
the object consists of stars ; and that, consequently, their first 
perceptibility must be the gaging power by which its profun- 
dity in space is to be ascertained. 
II. A series of observations of clusters of stars, from which the 
order of their profundity in space is determined. 
Observation of the yth cluster of stars in the with class of my 
catalogues of celestial objects * 
“ 1784,, 20 feet telescope, power 157. An excessively faint 
“ cluster of stars, intermixed with resolvable nebulosity, 8 or 
“10 minutes in diameter. The stars are so small that they 
“ cannot be seen without the greatest attention ; 240 verified 
“ it beyond all doubt. I have suspected many such in this 
“ neighbourhood.” 
At the time of this observation, the 20 feet telescope was 
of the Newtonian construction, and its power to penetrate 
into space was 61.18 times that of the eye, which it has been 
shown can see stars of the 12th order rf and since it appears 
from the foregoing observation, that with this power the 
telescope could but just reach the stars of the cluster, we may 
conclude that its profundity in space cannot be less than of 
the 734th order. 
* For these catalogues, see Phil. Trans, for 1786, page 471 ; 3789, page 22 6 , and 
1802, p. 503. 
f Phil. Trans, for 1817, page 317. 
