parallax of the fixed stars. 271 
A good telescope and a good micrometer are all the in- 
struments required. This advantage is really incalculable. 
Instead of confining our attention to one or two principal 
stars, it puts an almost unlimited range of objects in our 
power, by enabling us to employ in this research the largest 
telescopes, and thus easily obtain measures of those stars, 
which, from their faintness, must present insuperable diffi- 
culties with instruments of ordinary apertures. 
In selecting objects for examination by this method, we 
must be chiefly guided by their angles of position and dis- 
tances. Taking such whose distances are below if', (which, 
on the supposition of i° periodical variation of position, cor- 
responds to of a second of relative parallax in stars properly 
situated), the angle of position ought to be such, that the line 
joining the two stars shall point as nearly as may be, to the 
pole of the ecliptic. Ten, twenty, or even thirty degrees of 
deviation either way from this direction, will however not 
materially vitiate the application of this method to stars near 
the ecliptic, while, for such as have considerable latitudes, 
proportionally greater deviations may be allowed, and within 
thirty degrees of the pole of the ecliptic this element is of 
comparatively small moment. 
In general, to ascertain whether any double star is or is 
not favourably situated for the application of this method, we 
must (if we would take up the problem on strict mathematical 
grounds), proceed as follows : 
Let / represent the longitude, and -f X the N. latitude of 
the star, <r its angle of situation ,* or the angle included at the 
* This angle in many astronomical books is called the angle of position, but as 
in speaking of double stars we have always hitherto called the angle made by the 
