ruin would be inevitable; and yet this would be accomplished by an amount of wave motion but little more than half of that which the retina bears without being conscious of it, 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 is 800 times the light of a candle. At the same distance, that portion of the radiation of the electric light which reaches the retina but fails to excite vision, is about 1500 times the luminous radiation of the candle[1]. 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 one 20,000,000th of its light at the distance of a foot. Hence to make the former 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 1500 x 20,000,000, or by thirty thousand millions. Thus the thirty thousand millionth part of the radiation from the electric light, received unconsciously 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 illus-
- ↑ 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.
