$14 Biographical Memoir of Sir William Herschel. 
Society in 1787, under the title of c< On the appearance of the 
Polar Regions of the Planet Marsf &c. 
With the view of examining the structure and arrangement 
of the starry heavens, Dr Herschel had now completed a $0 
feet Newtonian telescope, with an aperture of 18-^th inches. 
The apparatus on which it was mounted, was contrived so as to 
confine the telescope to a meridional situation, and, by its mo- 
tions, to give the right ascension and declination of a celestial 
object in a coarse way. By this instrument, he examined all the 
clusters of stars and nebulas which Messier and Mechain had 
published in the Connoissance des Terns for 1783 and 1784, 
and he found that they could almost all be resolved into an in- 
finite number of small stars. Upon applying the telescope to 
the part of the Milky Way about the Hand and Club of Orion, 
which his former telescopes had not light enough to analyse, he 
was astonished at the 44 glorious multitude of stars, of all possi- 
ble sizes, that presented themselves to his view,'” and he reckon- 
ed that a belt 15 degrees long by $ broad, contained no less 
than 50,000 stars that could be distinctly numbered. In pur- 
suing these observations, Dr Herschel discovered no fewer than 
466 new nebulae, which he often found arranged in strata, and 
so thickly, that he detected 31 nebulae which passed through 
the field of view in 36 minutes. The most interesting result, 
however, to which these observations led, was the theory of the 
Milky Way, which Dr Herschel considers as an extensive branch- 
ing congeries or nebula of many millions of stars, within which 
our own Solar System is placed, the luminous tracks which con- 
stitute the Galaxy, being the projection of the nebula upon the 
concave surface of the sky, as seen from the solar system which 
it incloses. In order to determine the position of the sun with- 
in the nebula, as well as the form of the nebula itself, Dr Her- 
schel ganged the heavens in various parts of the Milky Way. 
This process consisted in repeatedly counting the number of 
stars in ten fields of view near each other, and taking the mean 
of their numbers for the number of stars in that part of the 
Galaxy. Upon the supposition that the stars are equally scat- 
tered, the above mean enabled him to deduce the length of his 
visual ray, or the distance through which his telescope has pene- 
trated, or, what amounts to the same thing, the distance of the 
