r years of the seventeenth, or, for that matter, as the later years
of the eighteenth century itself. Several of the prominent astronomers
of the later seventeenth century lived on into the opening years of the
following century, however, and the younger generation soon developed
a coterie of astronomers, among whom Euler, Lagrange, Laplace, and
Herschel, as we shall see, were to accomplish great things in this field
before the century closed.

One of the great seventeenth-century astronomers, who died just before
the close of the century, was Johannes Hevelius (1611-1687), of Dantzig,
who advanced astronomy by his accurate description of the face and
the spots of the moon. But he is remembered also for having retarded
progress by his influence in refusing to use telescopic sights in his
observations, preferring until his death the plain sights long before
discarded by most other astronomers. The advantages of these telescope
sights have been discussed under the article treating of Robert Hooke,
but no such advantages were ever recognized by Hevelius. So great was
Hevelius's reputation as an astronomer that his refusal to recognize the
advantage of the telescope sights caused many astronomers to hesitate
before accepting them as superior to the plain; and even the famous
Halley, of whom we shall speak further in a moment, was sufficiently
in doubt over the matter to pay the aged astronomer a visit to test his
skill in using the old-style sights. Side by side, Hevelius and Halley
made their observations, Hevelius with his old instrument and Halley
with the new. The results showed slightly in the younger man's favor,
but not enough to make it an entirely convincing demonstration. The
explanation of this, however, did not lie in the lack of superiority
of the telescopic instrument, but rather in the marvellous skill of the
aged Hevelius, whose dexterity almost compensated for the defect of his
instrument. What he might have accomplished could he have been induced
to adopt the telescope can only be surmised.

Halley himself was by no means a tyro in matters astronomical at that
time. As the only son of a wealthy soap-boiler living near London, he
had been given a liberal education, and even before leaving college
made such novel scientific observations as that of the change in the
variation of the compass. At nineteen years of age he discovered a new
method of determining the elements of the planetary orbits which was a
di