capriciously with facts, but a
well-ordered and disciplined power, whose sole function is to form
such conceptions as the intellect imperatively demands. Imagination,
thus exercised, never really severs itself from the world of fact.
This is the storehouse from which its materials are derived; and the
magic of its art consists, not in creating things anew, but in so
changing the magnitude, position, grouping, and other relations of
sensible things, as to render them fit for the requirements of the
intellect in the subsensible world.[9]

Descartes imagined space to be filled with something that transmitted
light _instantaneously_. Firstly, because, in his experience, no
measurable interval was known to exist between the appearance of a
flash of light, however distant, and its effect upon consciousness;
and secondly, because, as far as his experience went, no physical
power is conveyed from place to place without a vehicle. But his
imagination helped itself farther by illustrations drawn from the
world of fact. 'When,' he says,' one walks in darkness with staff in
hand, the moment the distant end of the staff strikes an obstacle the
hand feels it. This explains what might otherwise be thought strange,
that the light reaches us instantaneously from the sun. I wish thee to
believe that light in the bodies that we call luminous is nothing more
than a very brisk and violent motion, which, by means of the air and
other transparent media, is conveyed to the eye, exactly as the shock
through the walking-stick reaches the hand of a blind man. This is
instantaneous, and would be so even if the intervening distance were
greater than that between earth and heaven. It is therefore no more
necessary that anything material should reach the eye from the
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.

Sec. 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