[ 261 ] 
been made ; for at this diftance, the didance between 
the earth and the biggeft ftar of this fyftem not fab- 
tending an angle of more than about three minutes 
(that is, about a twelve hundredth part of the radius) 
the bars of this fyftem muft appear lefs luminous than 
they do to ourfelves, in the proportion of the fquare of 
1200 (or 1440000) to 1. And fuppofmg the light 
of Sirius to exceed that of the Ieaft vifible ftar in the 
proportion of about 1 200 to 1 , the brighteft ftar there- 
fore would ftill require to have its light increafed in 
the proportion of 1200 to 1, before it would begin to 
be diftinguifhable. To do this would require a tele- 
fcope, that fhould take in a pencil of rays of a larger 
diameter than the pupil of the eye, in the proportion 
of 35 to 1, that is, a pencil of about a foot diameter, 
exclufive of deductions ; for the pupil of the eye is 
not lefs than a third of an inch in diameter, in a clear 
ftar-light night, when there is no Moon ; but to obtain 
fuch a pencil, we muft not make ufe of a refracting 
telefcope (with two lenfes only) of lefs than 1 5 inches, 
nor a refle&or of lefs than nearly two feet aperture. 
This may ferve to fhew us, that thofe nebulae, in which 
we cannot difting.uifh any ftars, may yet reafonably 
be fuppofed to confift of ftars, though too far diftant 
to be fingly viftble ; fince this would be the cafe with 
our own fyftem, feen from as great a diftance as we 
may well fuppofe thofe nebulas to be from us, if we 
judge of it from the magnitude of the vifible area, 
which they occupy in the heavens. 
Of the twinkling of the fxedfars. 
Having never yet feen any folution of the twink.- 
Lng of the fixed ftars, with which I could reft Tatis- 
fled,. 
