ngs was the greatest optical difficulty that
Newton, ever encountered. He quite appreciated the difficulty. Over
his eagle eye there was no film--no vagueness in his conceptions. At
the very outset his theory was confronted by the question, Why, when a
beam of light is incident on a transparent body, are some of the
light-particles reflected and some transmitted? Is it that there are
two kinds of particles, the one specially fitted for transmission and
the other for reflection? This cannot be the reason; for, if we allow
a beam of light which has been reflected from one piece of glass to
fall upon another, it, as a general rule, is also divided into a
reflected and a transmitted portion. The particles once reflected are
not always reflected, nor are the particles once transmitted always
transmitted. Newton saw all this; he knew he had to explain why it is
that the self-same particle is at one moment reflected and at the next
moment transmitted. It could only he through _some change in the
condition of the particle itself_. The self-same particle, he
affirmed, was affected by 'fits' of easy transmission and reflection.

Sec. 9. _Theory of 'Fits' applied to Newton's Rings_.

If you are willing to follow me in an attempt to reveal the
speculative groundwork of this theory of fits, the intellectual
discipline will, I think, repay you for the necessary effort of
attention. Newton was chary of stating what he considered to be the
cause of the fits, but there can hardly be a doubt that his mind
rested on a physical cause. Nor can there be a doubt that here, as in
all attempts at theorising, he was compelled to fall back upon
experience for the materials of his theory. Let us attempt to restore
his course of thought and observation. A magnet would furnish him with
the notion of attracted and repelled poles; and he who habitually saw
in the visible an image of the invisible would naturally endow his
light-particles with such poles. Turning their attracted poles towards
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