a transparent substance, the particles would be sucked in and
transmitted; turning their repelled poles, they would be driven away
or reflected. Thus, by the ascription of poles, the transmission and
reflection of the self-same particle at different times might be
accounted for.

Consider these rings of Newton as seen in pure red light: they are
alternately bright and dark. The film of air corresponding to the
outermost of them is not thicker than an ordinary soap-bubble, and it
becomes thinner on approaching the centre; still Newton, as I have
said, measured the thickness corresponding to every ring, and showed
the difference of thickness between ring and ring. Now, mark the
result. For the sake of convenience, let us call the thickness of the
film of air corresponding to the first dark ring _d_; then Newton
found the distance corresponding to the second dark ring 2 _d_; the
thickness corresponding to the third dark ring 3 _d_; the thickness
corresponding to the tenth dark ring 10 _d_, and so on. Surely there
must be some hidden meaning in this little distance, _d_, which turns
up so constantly? One can imagine the intense interest with which
Newton pondered its meaning. Observe the probable outcome of his
thought. He had endowed his light-particles with poles, but now he is
forced to introduce the notion of _periodic recurrence_. Here his
power of transfer from the sensible to the subsensible would render it
easy for him to suppose the light-particles animated, not only with a
motion of translation, but also with a motion of rotation. Newton's
astronomical knowledge rendered all such conceptions familiar to him.
The earth has such a double motion. In the time occupied in passing
over a million and a half of miles of its orbit--that is, in
twenty-four hours--our planet performs a complete rotation; and in the
time required to pass over the distance _d_, Newton's light-particle