468 Sir W. Herschel’s astronomical observations 
As there are but few clusters of stars that can be seen by 
the eye, the observations of their visibility in the finders of 
telescopes, and their appearance in them, are of eminent use 
in ascertaining the distance at which we can expect to see 
celestial objects in large telescopes ; when, therefore, a cluster 
of stars cannot be seen by the eye, its visibility in the finder 
must first of all be reduced to the standard of the eye. I 
have already noticed that the power of my finders to show 
stars, has generally been about four times that of the eye ; 
then, as they would show a star at the distance of the 48th 
order, a celestial object, situated at this distance, would 
require to be brought to one quarter of that distance to 
become visible to the eye. 
The 2d cluster of the connoissance, for instance, was seen 
in a finder with the above mentioned power, and its profun- 
dity having been ascertained to be 243, we may conclude 
that it would be visible to the eye, if it were only of the 
60.75th order ; this being admitted, it will follow that the 20 
feet telescope would still show this cluster of stars as an 
ambiguous object, if it were removed to the 4561st order ; 
and with a space- penetrating power of 191.69, the 40 feet 
telescope, by which it was also observed, would have shown 
this cluster under the semblance of a star that might be dis- 
tinguished from others by having rather a larger diameter, 
if it had been at the distance of the 11645th order. 
In the foregoing instances, I have assigned the extent of 
the power to reach celestial objects, as it is in the same in- 
struments whereby they were observed, but this is not a 
necessary condition ; for when the visibility, and the particu- 
