KADIATION. 51 



retina a moment before. The platinum became red-hot. 

 No sensible damage was done to the eye by this experi- 

 ment ; no impression of light was produced ; the optic 

 nerve was not even conscious of heat. 



But the humours of the eye are known to be highly 

 impervious to the invisible calorific rays, and the 

 question therefore arises, 'Did the radiation in the 

 foregoing experiment reach the retina at all ? ' The 

 answer is, that the rays were in part transmitted to the 

 retina, and in part absorbed by the humours. Experi- 

 ments on the eye of an ox showed that the proportion 

 of obscure rays which reached the retina amounted to 

 1 8 per cent, of the total radiation ; while the luminous 

 emission from the electric light amounts to no more 

 than 1 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, 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. 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 

 ^ nan 20,000,000 f its light at the distance of a foot. 



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. 



