[ 437 3 
not more than one in Sirius himfelf ; though he 
had been placed in the pole b of the ecliptic, and ia 
thole, that appear much lefs luminous, fuch for ex- 
ample as y draconis, which is only of the third magni- 
tude, it cGuld hardly be expected to be fenfible with 
fuch inftruments as have hitherto been ufed in fearch 
of it. 
We have aflumed the magnitude of the fixed 
fhrs, as well as their brigbtnefs, to be equal to thofe 
of the fun ; it is however probable, that there may 
be a very great difference amongft them in both 
thefe refpedts ; and how much foever we may there- 
fore be wide of the truth, in attempting to fix the 
didance of particular fhars from this reafoning, yet 
there is a very great probability, that their mean 
diftances, fettled by this method, will not be muck 
out, fome exceeding and fome falling (hort of it. 
And perhaps the confideration, that a dar muff be a 
thoufand times as great, cceteris paribus , to appear 
equally bright, if it is placed at ten times the didance,, 
may ferve to make it probable, that the limits of the 
errors, which we are likely to commit, in judging 
by fuch a rule, are not fo great as we might other- 
wife imagine them to be. 
With regard to the difference there may be in the* 
native brigntnefs of different ftars, though it is pro- 
bably very confiderable, yet I think we can hardly 
fuppofe, that it is equal to their difference in magni- 
tude, at lead if we except thofe, which are fubjedt to. 
certain changes, and which for that reafon we may 
fuppofe to be luminous in fome parts of their furfaces 
b The latitude of Sirius being only 39 0 33', his parallax-, will be 
a. little lefs than two thirds of the whole parallax. 
