498 Dr. H'erschel's Catalogue 
us with the appearance of a nebula of any shape, which, like 
the real object of which it is the miniature, will seem to be gra- 
dually brighter in the middle. Groups of stars also may, by 
distance, assume the semblance of nebulous patches ; and real 
clusters of stars, for the same reason, when their composition 
is beyond the reach of our most powerful instruments to resolve 
them, will appear like round nebulae that are gradually much 
brighter in the middle. On this occasion I must remark, that 
with instruments of high space-penetrating powers, such as my 
40-feet telescope, nebulae are the objects that may be perceived 
at the greatest distance. Clustering collections of stars, much 
less than those we have mentioned before, may easily contain 
50000 of them ; and, as that number has been chosen for an 
instance of calculating the distance at which one of the most 
remote objects might be still visible,* I shall take notice of an 
evident consequence attending the result of the computation; 
which is, that a telescope with a power of penetrating into space, 
like my 40-feet one, has also, as it may be called, a power of 
penetrating into time past. To explain this, we must consider 
that, from the known velocity of light, it may be proved, that 
when we look at Sirius, the rays which enter the eye cannot 
have been less than 6 years and 4^- months coming from that 
star to the observer. Hence it follows, that when we see an 
object of the calculated distance at which one of these very 
remote nebulae may still be perceived, the rays of light which 
convey its image to the eye, must have been more than nine- 
teen hundred and ten thousand, that is, almost two millions of 
years on their way ; and that, consequently, so many years ago, 
* See Phil. Trans, for 1800, page 83. N. B. In the same page, line 22, for 5000 
read 50000. 
