for the lunar motion; and it had struck him that this force
might be the very same as the familiar force of gravitation which gave
to bodies their weight: but in attempting a numerical verification of
this idea in the case of the moon he was led by the then received notion
that sixty miles made a degree on the earth's surface into an erroneous
estimate of the size of the moon's orbit. Being thus baffled in
obtaining such verification, he laid the matter aside for a time.

The anecdote of the apple we learn from Voltaire, who had it from
Newton's favourite niece, who with her husband lived and kept house for
him all his later life. It is very like one of those anecdotes which are
easily invented and believed in, and very often turn out on scrutiny to
have no foundation. Fortunately this anecdote is well authenticated, and
moreover is intrinsically probable; I say fortunately, because it is
always painful to have to give up these child-learnt anecdotes, like
Alfred and the cakes and so on. This anecdote of the apple we need not
resign. The tree was blown down in 1820 and part of its wood is
preserved.

I have mentioned Voltaire in connection with Newton's philosophy. This
acute critic at a later stage did a good deal to popularise it
throughout Europe and to overturn that of his own countryman Descartes.
Cambridge rapidly became Newtonian, but Oxford remained Cartesian for
fifty years or more. It is curious what little hold science and
mathematics have ever secured in the older and more ecclesiastical
University. The pride of possessing Newton has however no doubt been the
main stimulus to the special pursuits of Cambridge.

He now began to turn his attention to optics, and, as was usual with
him, his whole mind became absorbed in this subject as if nothing else
had ever occupied him. His cash-book for this time has been discovered,
and the entries show that he is buying prisms and lenses and polishing
powder at the beginning of 1667. He was anxious to improve telescopes by
making more perfect lenses than had ever been used before. Accordingly
he calculated out their proper curves, just as Descartes had also done,
and then proceeded to grind them as near as he could to those figures.
But the images did not please him; they were always blurred and rather
indistinct.

At length, it struck him that perhaps it was not the lenses but the
light which was at fault. Perhaps light was so composed that it _could_
not be focused