hermetically sealed.

* * * * *

In March 1821, the younger Herschel, in conjunction with Sir James
South, undertook a series of observations on the distances and positions
of three hundred and eighty double and triple stars, by means of two
splendid achromatic telescopes of five and seven focal length. These
were continued during 1822 and 1823, and have proved of great service to
astronomers.

Having pursued with much zeal the study of optics, and experimented
largely and carefully on the double refraction and polarization of
light, he compiled a treatise on the subject for the "Encyclopaedia
Metropolitana" It has been translated into French by M. Quetelet; and
both foreign and English men of science have been accustomed to regard
it as indicating a new point of departure in the important branch of
science to which it is devoted.

Astronomy, however, became for him, as for his father, the great
pursuit of his laborious life; and having constructed telescopes of
singular magnitude and power, he entered upon a study of the Sidereal
World. In 1825 he commenced a careful re-examination of the numerous
nebulae and starry clusters which had been discovered by his father, and
described in the "Philosophical Transactions," fixing their positions
and investigating their aspects. He devoted eight years to this _magnum
opus_, completing it in 1832. The catalogue which he then contributed to
the "Philosophical Transactions" includes 2306 nebulae and
star-clusters, of which 525 were discovered by himself. While engaged in
this difficult task, Herschel discovered between three and four thousand