And if from thy clear path my foot have strayed.
Truant awhile, 'twas but to turn, with warm

And cheerful haste; while thou dids't not upbraid,

Nor change thy guise, nor veil thy beauteous form.
But welcomedst back my heart with every wonted charm.
During the six following years he worked at pure mathe-
matics, astronomy, experimental optics and chemistry. It
was in these years that he made his principal contributions to



134 TEN BRITISH PHYSICISTS

pure mathematics. Several of the papers which he contributed
to the Royal Society dealt with the calculus of finite differences;
for these he received the Copley medal in 182 1. In astronomy,
he revised the catalogue of double stars made by his father;
this work he did in conjunction with (Sir James) South and
with the help of two refracting telescopes the property of that
scientist. The resulting catalogue, printed in the Philosophical
Transactions, brought its author the gold medal of the recently
instituted Astronomical Society of London; also the Lalande
prize for astronomy (of the Paris Academy) for 1825. Herschel
along with Babbage took an active part in the foundation of the
Royal Astronomical Society; he wrote its inaugural address,
and was its first foreign secretary, while his father was its first
president. In optics he investigated the absorption of light
by colored media and the action of crystals upon polarised
light. In chemistry (1819, when philosophical chemistry was
perhaps at its lowest ebb in England) he rediscovered the hypo-
sulphite salts, and ascertained their leading properties, the
principal of which is dissolving the nitrate of silver — a property
applied by Daguerre twenty years later to fixing photographic
pictures. In 1821 he traveled in Italy and Switzerland with
Babbage.
