of the late Dr. Maskelyne. 13 
the care of giving them to the public has been committed 
to Mr. Vince, professor of astronomy and experimental 
philosophy at Cambridge, known by a Treatise on Plane 
and Physical Astronomy, and the Description of Modern 
astronomical Instruments. We shall perhaps find in them 
some further particulars of the prismatic micrometer, in 
some respects similar to those of M. Rochon and P. 
Boscovich. If we credit the Jatter, Dr. Maskelyne is the 
first who invented it; Boscovich claims to be the second. 
It has been found that the same invention has been made 
about the same time by persons who have not had any 
communication with each other. But hitherto M. Rochon 
is the only one who has published observations made with 
this micrometer ; the idea of using a double refraction be- 
longs incontestably to Dr. Maskelyne, and Boscovich him- 
self acknowledges it. Dr. Maskelyne used only common 
glass, and it seems certain that he first thought of making 
the prism move in the inside of the telescope; it remains 
therefore for us to learn what results he has drawn from 
this construction. 
Dr. Maskelyne, who valued the excellent instruments 
which he constantly used, did his utmost to preserve and 
improve them, and made those additions which his ex- 
perience and love of optics suggested to him. He had the 
eye-glass of the transit instrument made moveable, to 
avoid all parallax, by bringing the eye opposite each of the 
five wires that the star successively passes. He found the 
inconvenience of narrow openings, then used in all observa- 
tories, and therefore had those of Greenwich enlarged. 
Notwithstanding all this caution, it has lately been su- 
spected that his quadrant has become less exact by the 
wearing, from the constant friction of the parts for more 
than fifty years, It was likely that the astronomer, who 
always paid the same attention to his observations, and 
besides did not perceive any sign of age in his instrument, 
should not be the first to perceive these trifling alterations. 
Other more modern instruments placed in the hands of 
attentive astronomers occasioned the first idea of it. Not 
but that the small yariations they think they have remarked, 
may be explained in a manner that will clear the Green- 
wich quadrant. MM, Besset and Ottamans had given 
some probable explanation ; but the most certain plan was 
to get new instruments, and this Dr. Maskelyne adopted. 
He ordered a large and fine circle of the celebrated Trough 
ton, which he had not himself the pleasure to place “4 his 
obser 
