parallax of the fixed stars. 279 
If I am not mistaken, then, we have here a method of in- 
vestigating parallax very much more delicate than any which 
has yet been proposed — open to no insuperable objections — 
demanding no laborious reductions — and depending on a 
branch of practical astronomy, which is now happily culti- 
vated with the assiduity it so richly deserves. It may not 
then be too much to hope, that in a few years we shall have 
data for a positive decision, respecting at least a certain 
number of individual stars, and thus be enabled to form a 
much better judgement as to the probable maximum of 
parallax in any. Meanwhile, the number of stars to which 
the method is applicable must not be judged of from the spe- 
cimen above given. The only position micrometer of which 
I have any knowledge from practice is the wire micrometer ; 
but the double image micrometer, if used in a manner, for 
the knowledge of which I am indebted to Captain Kater, 
and which consists in bringing the 1st image of one star 
almost, but not quite in contact with the 2nd image of the 
other, and in the same straight line, presents the singular 
advantage of an attainable exactness proportional to the dis- 
tance of the stars ; and by employing angles of position thus 
measured, stars of the 4th, 5th and 6th classes are rendered 
equally, or more available than those of the 1st, 2nd and 3rd ; 
so that our range of objects becomes, by the use of this instru- 
ment in this manner really unlimited. 
It is possible, that by some an apology may be thought 
necessary for a communication like the present — offered as 
it is to the Royal Society as a mere proposal — not followed 
out into actual practice. A variety of other pursuits, how- 
ever, and circumstances less under my own controul, have 
