of 500 new Nebula % and Clusters of Stars. 483 
make these stars appear to us near enough to come under the 
denomination of a double star of the first class, we should re- 
move the earth from them at least 41253 times farther than 
Sirius is from us. But the space-penetrating power of a 7-feet 
reflector, by which my observations on double stars have been 
made, cannot intitle us to see stars at such an immense distance ; 
for, even the 40-feet telescope, as has been shewn,* can only 
reach stars of the 1343d magnitude. It follows, therefore, that 
these stars could not remain visible in a 7-feet reflector, if they 
were so far removed as to make their angular distance less than 
about 24^- minutes ; nor could even the 40-feet telescope, under the 
same circumstances of removal, shew them, unless they were to 
be seen at least 2-J minutes asunder. Moreover, this calculation 
is made on a supposition that the stars of which a double star is 
composed, might be as small as any that can possibly be per- 
ceived ; but if, on the contrary, they should still appear of a 
considerable size, it will then be so much the more evident that 
such stars cannot have any great real distance, and that, con- 
sequently, insulated stars cannot appear double, if they are situ- 
ated at equal distances from us. If, however, their arrangement 
should be such as has been mentioned before, then, one of them 
being far behind the other, an apparent double star may cer- 
tainly be produced ; but here the appearance of proximity 
would be deceptive ; and the object so circumstanced could not 
be classed in the list of binary systems. However, as we must 
grant, that in particular situations stars apparently double may 
be composed of such as are insulated, it cannot be improper to 
consult calculation, in order to see whether it be likely that the 
700 double stars I have given in two catalogues, as well as 
• See Phil. Trans, for 1800, Part I. page 83. 
