you have
seen more or less perfectly illustrated on a billiard-table. As
regards the collision of sensible elastic masses, Newton knew the
angle of incidence to be equal to the angle of reflection, and he also
knew that experiment, as shown in our last lecture (fig. 3), had
established the same law with regard to light. He thus found in his
previous knowledge the material for theoretic images. He had only to
change the magnitude of conceptions already in his mind to arrive at
the Emission Theory of Light. Newton supposed light to consist of
elastic particles of inconceivable minuteness, shot out with
inconceivable rapidity by luminous bodies. Optical reflection
certainly occurred _as if_ light consisted of such particles, and this
was Newton's justification for introducing them.

But this is not all. In another important particular, also, Newton's
conceptions regarding the nature of light were influenced by his
previous knowledge. He had been pondering over the phenomena of
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 w