RADIATION. 189 



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 Hays. 



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 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 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. 



