294 Dr. Herschel’s Third Catalogue of the 
to a plan which I laid down, she began the work about twenty 
months ago, and has lately finished it. 
The index has been made in the following manner. Every 
observation upon the fixed stars contained in the second vo- 
lume of the Historia Ccelestis was examined first, by casting 
up again all the numbers of the screws, in order to detect any 
error that might have been committed in reading off the ze- 
nith-distance by diagonal lines. The result of the computation 
being then corrected by the quantity given at the head of the 
column, and refraction being allowed for, was next compared 
with the column of the correct zenith-distance as a check. 
Every star was now computed by a known preceding or 
following star; and its place according to the result of the 
computation laid down in the Atlas Ccelestis , by means of pro- 
portional compasses. This was necessary, in order to ascertain 
the observed star : for the observations contain but little in- 
formation on the subject; most of the small stars being without 
names, letters, or descriptions. The many errors in the names 
of the constellations affixed to the stars, and in the letters by 
which they are denoted, also demanded a more scrupulous at- 
tention ; so that only their relative situation, examined by cal- 
culation, could ascertain what the stars really were which had 
been observed. 
Every observed star being now ascertained, its number in the 
British catalogue was added in the margin at the end of the 
line of the observation ; and a book with all the constellations 
and number of the stars of the same catalogue, with large blank 
spaces to each of them, being provided, an entry of the page 
where Flamsteed’s observation is to be found, was made in 
its proper place. 
