RADIATION. 51 



placed in the position occupied by the retina a moment 

 before. The platinum became red-hot. No sensible 

 damage was done to the eye by this experiment; 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 ques- 

 tion therefore arises, * Did the radiation in the forego- 

 ing 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. Experiments 

 on the eye of an ox showed that the proportion of ob- 

 scure rays which reached the retina amounted to 18 per 

 cent, of the total radiation; while the luminous emis- 

 sion from the electric light amounts to no more than 

 10 per cent, of the same total. Were the purely lumin- 

 ous 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 atten- 

 tion. At a common distance of a foot the visible radia- 

 tion of the electric light employed in these experiments 

 is 800 times the light of a candle. At the same dis- 

 tance, 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 



* It will be borne in mind that the heat which any ray, lum- 

 inous or non-luminous, is competent to generate is tho true meas- 

 ure of the energy of the ray. 



