gravitation, and had made himself at home amid the operations of this
universal power. Perhaps his mind at this time was too freshly and too
deeply imbued with these notions to permit of his forming an
unfettered judgment regarding the nature of light. Be that as it may,
Newton saw in Refraction the result of an attractive force exerted on
the light-particles. He carried his conception out with the most
severe consistency. Dropping vertically downwards towards the earth's
surface, the motion of a body is accelerated as it approaches the
earth. Dropping downwards towards a horizontal surface--say from air
on to glass or water--the velocity of the light-particles, when they
came close to the surface, is, according to Newton, also accelerated.
Approaching such a surface obliquely, he supposed the particles, when
close to it, to be drawn down upon it, as a projectile is deflected by
gravity to the surface of the earth. This deflection was, according to
Newton, the refraction seen in our last lecture (fig. 4). Finally, it
was supposed that differences of colour might be due to differences
in the 'bigness' of the particles. This was the physical theory of
light enunciated and defended by Newton; and you will observe that it
simply consists in the transference of conceptions, born in the world
of the senses, to a subsensible world.

But, though the region of physical theory lies thus behind the world
of senses, the verifications of theory occur in that world. Laying the
theoretic conception at the root of matters, we determine by deduction
what are the phenomena which must of necessity grow out of this root.
If the phenomena thus deduced agree with those of the actual world, it
is a presumption in favour of the theory. If, as new classes of
phenomena arise, they also are found to harmonise with theoretic
deduction, the presumption becomes still stronger. If, finally, the
theory confers prophetic vision upon the investigator, enabling him to