Parallax of the Fixed Stars . p t 
is lefs than half the tenth of a fecond : now this may either 
be taken into conflderation, or fuch low obfervations may be 
avoided, as being by no means neceflary, and but ill fiuting 
the high powers a telefcope proper for this purpofe ought to 
bear. 
The change of polition of the earth’s axis I look upon as an 
unfurmountable obftacle to taking the parallax of ftars by the 
method of zenith diftances : for though refraction is much re- * 
duced in the zenith, this change is there no lefs fenfible than in 
other parts of the heavens ; but as this will always afFeCt our 
two bars exactly alike, we are entirely freed from this embar- 
raffment. 
The aberration of light can have no influence of the leaft 
conflderation upon our two ftars, as a mere infpeCtion of the 
tables will (hew. In a whole degree, its effects, when greateft, 
amount but to four-tenths of a fecond, and confequently in 
$" to no more than ,0005, or the two thoufandth part of a 
fecond. 
Obfervations of the relative diftance of the two ftars that 
make up a double ftar, being thus cleared of every impedi- 
ment, are capable of being continually improved by every de- 
gree of perfection the telefcope may acquire: we can chufe 
ftars that may be viewed fufftciently high to be clear of the 
vapours that fwim near the horizon, and confequently employ 
the greateft powers our inftruments are capable of. From 
experience I can alfo affirm, that the ftars will bear a much 
higher degree of magnifying than other celeftial objeCts. Too 
much has hitherto been taken for granted in optics : every na- 
tural philofopher is ready enough to allow the neceffity of 
making experiments, and tracing out the fteps of nature ; why 
this method Ihould not be more purfued in the art of feeing 
N 2 does 
