224 Biographical Memoir of Sir William Herschel. 
This paper was followed, in 1817, by a Observations tending to 
investigate the local arrangement of the Celestial Bodies in 
space , and to determine the extent and condition of the Milky 
Way? which contains much interesting discussion, and in which 
its author concludes, that not only our sun, but all the stars we 
can see with the eye, are deeply immersed in the milky way, 
and form a component part of that immense nebula. 
The last paper which Dr Herschel wrote on this subject, 
and indeed the last which he communicated to the Royal So- 
ciety, was entitled u Astronomical Observations and Experiments 
selected for the purpose of ascertaining the relative distances of 
Clusters of Stars , and of investigating how far the power of 
our Telescopes may be expected to reach into space , when di- 
rected to ambiguous celestial objects From these observations, 
our author concludes, that a star of the first magnitude would 
just come to be visible by the naked eye, if removed to 12 times 
its distance, and by the most powerful telescope hitherto con- 
structed, if removed to 2800 times its distance. Yet such a 
telescope still shews stars in the milky way at the utmost limits 
of their visibility. This extraordinary assemblage of stars is 
therefore equally fathomless by our eyes and by our finest tele- 
scopes. Conceiving, however, that the united lustre of side- 
real systems may reach us from a still greater distance in space, 
Dr Herschel estimates their distance by the aperture of the 
speculum, which just resolves them into stars, and in this way 
he has estimated the distances of 47 clusters. Such clusters 
are again taken as connecting links with such ambiguous objects 
as cannot be resolved by the telescope. Resolvable clusters are 
actually found to put on similar appearances with inferior tele- 
scopes, and hence we may compare their distances with those of 
the former kind, by the same principles as those, with the near- 
est fixed star. When objects of this kind are lost to the sight, 
the utmost limits of human vision seem to be obtained, and our 
author supposes that this must take place at about the 35,000th 
order of distances. 
In consequence of the great reputation which Dr Herschefs 
telescopes possessed, he received numerous applications for in- 
struments, from the sovereigns, as well as from the astronomers 
