of some of the principal fixed Stars, &c„ 425 
at their bases in an axis about 12 inches long, leaving suffi- 
cient space between the limbs for a telescope feet long, 
and an aperture of 2-| inches, to pass between. The two 
circles are firmly united at their extreme borders by a great 
number of bars, which stand perpendicular between them ; the 
whole of which will be readily understood by referring to the 
figure. The square frames, which appear as inscribed in 
the circle, were added to give additional firmness to the whole. 
The circle is divided by fine lines into 5' of a degree ; and 
subdivided into single seconds by two micrometer micro- 
scopes, the principles and properties of which are now too 
well known to require any particular explanation. 
At the time these observations were made, the microscopes 
were firmly fixed opposite to the horizontal diameter: but 
when I considered that, by continuing the observations, the 
error of division would never be diminished, I suggested to 
Mr. Troughton the possibility of giving a circular motion 
to the microscopes, though I confess with very little hope, 
that the thing was really practicable in an instrument pre- 
viously constructed on other principles. Mr. Troughton" 
approved of the idea, and executed it in a very ingenious 
manner. His talents, as an artist, are too well known and 
too highly appreciated, to stand in need of any praise from 
me ; yet I should consider myself as deficient in justice, if I 
did not endeavour to call the attention of the reader to the 
skill and ingenuity, which have been employed not only in 
this very important alteration, but in every contrivance that is 
peculiar to the instrument, which is the object of our present 
consideration. 
These microscopes can now revolve about 6o° from their 
3I 2 
