RADIATION 57 



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. 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 io^obw ^ 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 in- 

 tensity 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 suffi- 

 cient 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 con- 

 sonance, 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 



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. 



