3-10 Prof. Tvndall on Luminous and Obscure Radiat 



ton. 



of the electric light which reach the optic nerve, but are incom- 

 petent to provoke vision, is 1300 times that of the light of a 

 candle. But the intensity of the candle's light at the distance 



of a mile is less than one twenty-millionth of its intensity at 

 the distance of a foot, hence the energy which renders the can- 

 dle perfectly visible a mile off would have to be multiplied by 

 1300 x 20,000,000, or by twenty-six thousand millions, to bring 

 it up to the intensity of that powerless radiation which the eye 

 receives from the electric light at a foot distance. Nothing, I 

 think, could more forcibly illustrate the special relationship 

 which subsists between the optic nerve and the oscillating 

 periods of luminous bodies. The nerve, like a musical string, 

 responds to the periods with which it is in accordance, while it 

 refuses to be excited by others of vastly greater energy which 

 are not in unison with its own. 



44. By means of the opake solution of iodine. I have already 

 shown that the quantity of luminous heat emitted by a bright 

 red platinum spiral is immeasurably small *. Here are some 

 determinations since made with the same source of heat and a 

 solution of iodine in iodide of ethyle, the strength and thickness 

 of the solution being such as entirely to intercept the luminous 

 rays. 



Radiation from Red-hot Platinum Spiral. 

 Through transparent liquid. Through opake solution. 



437 437 



437 437 



These experiments were made with exceeding care, and all 

 the conditions were favourable to the detection of the slightest 

 difference in the amount of heat reaching the galvanometer • still 

 the quantity of heat transmitted by the opake solution was 

 found to be the same as that transmitted by the transparent one. 

 In other words, the luminous radiation intercepted by the former, 

 though competent to excite vividly the sense of vision, was, when 

 expressed in terms of actual energy, absolutely immeasurable. 



45. And here we have the solution of various difficulties which 

 from time to time have perplexed experimenters. When we see 

 a vivid light incompetent to affect our most delicate thcrmo- 

 scopic apparatus, the idea naturally presents itself that light and 

 heat must be totally different things. The pure light emerging 

 from a combination of water and green glass, even when rendered 

 intense by concentration, has, according to Melloni, no sensible 

 heating powerf- The light of the moon is also a case in point. 

 Concentrated by a polyzonal lens more than a yard in diameter 



* Phil. Trans, vol. cliv. p. 327. 



t Taylor's Scientific Memoirs, vol. i. p. 392. 



