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Six Lectures on Light - Delivered In The United States In 1872-1873 by John Tyndall
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luminous object, than that something should be sent from the ground to
the hand of the blind man when he is conscious of the shock of his
staff.' The celebrated Robert Hooke at first threw doubt upon this
notion of Descartes, but he afterwards substantially espoused it. The
belief in instantaneous transmission was destroyed by the discovery of
Roemer referred to in our last lecture.


ยง 2. _The Emission Theory of Light_.

The case of Newton still more forcibly illustrates the position, that
in forming physical theories we draw for our materials upon the world
of fact. Before he began to deal with light, he was intimately
acquainted with the laws of elastic collision, which all of 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
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