38 PR A QMENTS OF SCIENCE. 
verged by our mirror to a focus, there can be no doubt as to 
the fate of a retina placed there. Its ruin would be inevi- 
table; and yet 'this would be accomplished by an amount 
of wave-motion but little more than half of that which the 
retina, without exciting consciousness, bears at the focus 
of invisible rays. 
This subject will repay a moment'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.* 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 ^ovoV.innr f ^ ts lig' lfc 
at the distant 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 inten- 
sity would have to be multiplied by 1,500x20,000,000, 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 gener- 
ated the orange, yellow, green, and blue augmented the 
intensity of the red. What is true of the red is true of 
*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. 
