302 



KADIATION. 



going experiment reacli the retina at all ?" The answer is that the rays were in 

 part transmitted to the retina, and in part absorbed by the humors. Experi- 

 ments on the eye of an ox showed that the proportion of obscure rays which 

 reached the retina amounted to 18 per cent, of the total radiation, while the 

 luminous emission from the electric light amounts to no more than 10 per cent, 

 of the same total. Were the purely luminous rays of the electric lamp converged 

 by our mirror to a focus, there can be no doubt as to the fate of a retina placed 

 there. Its 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 1,500 times the lumin- 

 ous 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 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 dis- 

 tance, its intensity would have to be multiplied by 1,500x20,000,000, or by 

 30,000,000,000. 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 illustrate that special relationship supposed by 

 Melloni and others to subsist between the optic nerve and the oscillating periocfs 

 of luminous bodies. Like a musical string, the optic nerve responds to the wave? 

 with which it is in consonance, while it refuses to be excited by others of almost 

 infinitely greater energy, whose periods of recuiTence are not in imison with its 

 own. 



X. — PEESISTENCE 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 generated the orange, yellow, green, and 

 blue rays augmented 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 intensity in the region beyond the red, but we can 

 measure it and express it numerically. With this view the following experiment 

 was performed : A spiral of platinum wire was surrounded by a small glass globe 

 to protect it from currents of air; through an orifice in the globe the rays could 

 pass from the spiral and fall afterwards upon a thermo-electric pile. Placing in 

 front of the orifice an opaque solution of iodine, the platinum was gradually 

 raised from a low dark heat to the fullest incandescence, with the following results: 



Appearance of spiral. 



Energ'y of ob- 

 scure radiation. 



Dark 1 



Dark, but hotter 3 



Dark, but still hotter 5 



Dark, but still hotter 10 



Feeble red 19 



Dull red 25 



Bed 37 



Appearance of spiral. 



Energy of ob- 

 scure radiation. 



Full red 62 



Orange 89 



Bright orange 144 



Yellow 202 



AVhite 276 



Intense white 440 



* It Avill be borue in mind that the heat which any I'ay, luminous or non-luminous, is com- 

 petent to generate is the true measure of the energy of the ray. 



