I0 4 Mr. herschel on the 
telefcopes may be of confiderable ute, as tney w ill undoubtedly 
give us the relative places or thofe ftars to a much gi eater de- 
gree of accuracy than they can be bad by tranfit inftruments 01 
fectors, and thereby much fooner enable us to difcover any 
apparent change in their fituation occafioned by this new kind 
of fvftematical parallax, if I may be allowed to ufe that expref- 
iion, for lignifvmg the change arilmg tiom the motion of the 
whole folar fyftem. 
I (ball now endeavour to deliver a theory of the annual pa- 
rallax of double ftars, with the method of computing from 
thence what is generally called the parallax of the fixed ftars, 
or of (ingle ftars of the firft magnitude, fuch as are neareft to 
us. It may be oblerved, that the principles upon which I have 
founded the following theory are of fuch a nature, that they 
cannot be ftridtly demonftrated, in confequence of which they 
are only propofed as poftulata, which have fo great a piobabi- 
lity in their favour, that they will hardly be obje&ed to by 
thofe who are in the leaft acquainted with the do&rine of 
chances. 
GENERAL P O S T U L A T A, 
i. Let the ftars be fuppofed, one with another, to be about 
•the fize of the fun L 
a. Let the difference of their apparent magnitudes be owing 
to their different diftances, fo that a ftar of the fecond, third, 
\ 4 * J 
* See Mr. michell’s Inquiry into the probable Parallax and Magnitude of 
the Fixed Stars, Phil. Tranf. vol. LVII. p. 234. 236. 237. 240. and Dr. hal- 
eey on the Number, Order, and Light, of the Fixed Stars, Phil. Tranf. 
vol. XXXI. 
or 
