38 FRAGMENTS 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 ^/ooV.oinF f ^ s light 

 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 beat wbicb any ray, luminous 

 or non-luminous, is competent to generate is the true measure of the 

 energy of tbe ray. 



