eated on his mind by the micrometer.
He writes: 'The first thing Mr. Gascoigne showed me was a large
telescope, amplified and adorned with new inventions of his own, whereby
he can take the diameters of the Sun or Moon, or any small angle in the
heavens or upon the earth, most exactly through the glass to a second.'

The micrometer is now regarded as an indispensable appliance in the
observatory; the use of a spider web reticule instead of wire having
improved its efficiency. Gascoigne was one of the earliest astronomers
who recognised the value of the Keplerian telescope for observational
purposes, and Sherburn affirms that he was the first to construct an
instrument of this description having two convex lenses. Whether this be
true or not, it is certain that he applied the micrometer to the
telescope, and was the first to use telescopic sights, by means of which
he was able to fix the optical axis of his telescope, and ascertain by
observation the apparent positions of the heavenly bodies.

Crabtree, in a letter to Gascoigne, says: 'Could I purchase it with
travel, or procure it with gold, I would not be without a telescope for
observing small angles in the heavens; or want the use of your device of
a glass in a cane upon the movable ruler of your sextant, as I remember
for helping to the exact point of the Sun's rays.'

It was not known until the beginning of the eighteenth century that
Gascoigne had invented and used telescopic sights for the purpose of
making accurate astronomical observations. The accidental discovery of
some documents which contained a description of his appliances was the
means by which this became known.

Townley states that Gascoigne had completed a treatise on optics, which
was ready for publication, but that no trace of the manuscript could be
discovered after his death. Having embraced the Royalist cause, William
Gascoigne joined the forces of Charles I., and fell in the battle of
Marston Moor on July 2, 1644.

The early death of this young and remarkably clever man was a severe
blow to the science of astronomy in England.

The invention of logarithms, by Baron Napier, of Merchistoun, was found
to be of inestimable value to astronomers in facilitating and
abbreviating the methods of astronomical calculation.

By the use of logarithms, arithmetical computations which necessitated
laborious application for several months could with ease be completed in
as many days. It was remarked by