x THE STARRY HEAVENS 425 
NEBULA 
From Stars we pass insensibly to Nebule, 
which are so far away that their distance 
is at present quite immeasurable. All that 
we can do is to fix a minimum, and this 
is so great that it is useless to express it 
in miles. Astronomers, therefore, take the 
velocity of light asa unit. It travels at the 
rate of 180,000 miles a second, and even at 
this enormous velocity it must have taken 
hundreds of years to reach us, so that we see 
them not as they now are but as they were 
hundreds of years ago. 
It is no wonder, therefore, that in many of 
these clusters it is impossible to distinguish 
the separate stars of which they are composed. 
As, however, our telescopes are improved, 
more and more clusters are being resolved. 
Photography also comes to our aid, and, as 
already mentioned, by long exposure stars can 
be made visible which are quite imperceptible 
to the eye, even with aid of the most powerful 
telescope. 
Spectrum analysis also seems to show that 
