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light which reaches the retina but fails to excite vision, is
about 1,500 times the luminous radiation of the candle.1
But a candle on a clear night can readily be seen at a dis-
tance of a mile, its light at this distance being less than
one 20,000,000th of its light at the distance of a foot.
Hence, to make the candle-light a mile off equal in power
to the non-luminous radiation received from the electric
light at a foot distance, its intensity would have to be mul-
tiplied by 1,500 X 20,000,000, or by thirty thousand mill-
ions. Thus the thirty thousand millionth part of the in-
visible radiation from the electric light, received by the
retina at the distance of a foot, would, if slightly changed
in character, be amply sufficient to provoke vision. Nothing
could more forcibly illustrate that special relationship sup-
posed by Melloni and others to subsist between the optic
nerve and the oscillating periods of luminous bodies. The
optic nerve responds, as it were, to the waves with which
it is in consonance, while it refuses to be excited by others
of almost infinitely greater energy, whose periods of recur-
rence are not in unison with its own.

10. Persistence of Rays.

At an early part of this lecture it was aifirmed that
when a platinum wire was gradually raised to a state of
high incandescence, new rays were constantly added,
while the intensity of the old ones was increased. Thus
in Dr. Draper’s experiments the rise of temperature that
generated the orange, yellow, green, and blue rays, aug-
mented the intensity of the red ones. What is true of the
red is true of every other ray of the spectrum, visible and
invisible. We cannot indeed see the augmentation of in-

1 It will be home in mind that the heat which any ray, luminous or
non-luminous, is competent to generate is the true measure of the energy

of the ray.

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