's further attention. At a common
distance of a foot the visible radiation of the electric light
employed in these experiments is 800 times the light of a candle. At
the same distance, the portion of the radiation of the electric light
which reaches the retina, but fails to excite vision, is about 1,500
times the luminous radiation of the candle.' [Footnote: It will be
borne 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.] But a candle on a clear night can readily be seen at a distance
of a mile, its light at this distance being less than 1/20,000,000 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 multiplied by 1,500 x
20,000,000, or by thirty thousand millions. Thus the thirty thousand
millionth part of the invisible 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 supposed 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 recurrence are not in unison with its own.

********************

10. Persistence of Rays.

At an early part of this lecture it was affirmed, 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 augmented the
intensity of the red. 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 intensity in the region beyond the red, but we can
measure it and express it numerically. With this view the following
experiment was performed: A spiral of platinum wire was surrounded by
a small glass globe to protect it from currents of air; through an
orifice in the globe the rays could pass from the spiral and fall
afterwards upon a thermo-electric pile