2822.| Memoirs of the Astronomical Society, Vol. I. 151 
degrees, the more correct will be the results. Their right ascen- 
sions, on the contrary, must be as near as possible to each other, 
a circumstance which will moreover prevent the possibility of 
any error arising from a variation in the rate of the clock during 
the interval of the observations.” Passing over the mode of 
computing the useful table of declinations with which the paper 
concludes, we shall copy one of the two examples of its use and 
application, and of the mode of operating in such cases: “ On 
July 1, 1819, { placed my transit instrument nearly in the meri- 
dian; and in order to ascertain how much it deviated from the 
true meridian, I observed the two stars y Lyr@ and + Sagittarii. 
The passage of the former was observed at 18".52’.37”,3, and of 
the latter at 18" . 56’. 4”,5 siderial time. The apparent right 
ascensions of those stars on that day were 18" . 52’.9’,8, and 
18" . 55’ .389”,7 respectively ; and their declinations were 32°, 27’ 
N. and 27° . 55S; consequently the operation will stand thus 
ee OT. Oe | 9,0" Se ee ao aya 
We == 19). 08. 00,7 Tate ao.” BS 
apg 99,9 aP e208 .182)6 
whence (d T — d R) = — 2”,7. This value being negative, 
shows that the deviation is to the west: and in order to deter- 
mine the quantity of the deviation, we must take the sum of the 
declinations (or the difference of the polar distances) of the two 
stars, which in this case is equal to 60°. 22’; or for the sake of 
round numbers, equal to 60°; and the declination of N (or the 
northern star) is about 32°. Consequently against the number 
60, and under the column headed 32°, we shall find 1:39; which, 
being multiplied by — 2”,7, will give — 3”,75 for the deviation 
of the instrument in é¢¢me; and this multiplied by 15 will give 
— 56”,3 for the deviation in arc westerly.” 
The importance of micrometers in the practice of astronomical 
observation is so great, that their improvement has constituted 
an object of continual interest to the philosophical artist. From 
this uninterrupted attention, numerous suggestions have arisen ; 
and the Rev. William Pearson, by his extensive investigations, 
contained in the fourth, fifth, and sixth memoirs, has contributed 
im a high degree to the advancement of this valuable appendage 
to the telescope. To detail a method of measuring small angles 
that has for its basis that singular property of several crystallized 
bodies, double refraction, is the purpose of the first of these 
essays, entitled “‘ On the Doubly-Retracting Property of Rock 
Crystal, considered as a Pyinciple of Micrometrical Measure- 
ments when applied to a Telescope.” The ingenious author 
candidly states, that. the Abbé Rochon, about the year 1783, dis- 
covered, and first made known, a method of compounding two 
prisms of rock crystal in such @ manner that any small object 
seen through them appeared double, and the constant angular 
