Prof. Tyndall on Luminous and Obscure Radiation. 339 



Melloni found sufficient to quench all rays emanating from ob- 

 scure sources. 



41. There cannot be a doubt that the invisible rays which 

 have shown themselves competent to traverse such a thick- 

 ness of the most powerful adiathermic liquid yet discovered 

 are also able to pass through the humours of the eye. The 

 very careful and interesting experiments of M. Janssen *, prove 

 that the humours of the eye absorb an amount of radiant 

 heat exactly equal to that absorbed by a layer of water of the 

 same thickness^ and in our solution the power of alum is added 

 to that of water. Direct experiments on the vitreous humour 

 of an ox lead me to conclude that one-fifth of the obscure rays 

 emitted by an intense electric light reaches the retina; and inas- 

 much as in every ten equal parts of the radiation from an electric 

 lamp nine consist of obscure rays, it follows that, in the case 

 of the electric light, nearly two-thirds of the whole radiant 

 energy which actually reaches the retina is incompetent to excite 

 vision. With a white-hot platinum spiral as source, the mean 

 of four good experiments gave a transmission of 11*7 per cent, 

 of the obscure heat of the spiral through a layer of distilled 

 water 1*2 inch in thickness. A larger proportion no doubt 

 reaches the retinaf. 



42. Converging the beam from the electric lamp by a glass 

 lens, I placed the opake solution of iodine before my open eye, 

 and brought the eye into the focus of obscure rays; the heat 

 was immediately unbearable. But it seemed to me that the 

 unpleasant effect was mainly due to the action of the obscure 

 rays upon the eyelids and other opake parts round the eye. I 

 therefore cut, in a card, an aperture somewhat larger than the 

 pupil, and allowed the concentrated calorific beam to enter my 

 eye through this aperture. The sense of heat entirely disap- 

 peared. Not only were the rays thus received upon the retina 

 incompetent to excite vision, but the optic nerve seemed un- 

 conscious of their existence even as heat. What the consequences 

 would have been had I permitted the luminous third of the 

 condensed beam to enter my eye, I am not prepared to say, nor 

 should I like to make the experiment. 



43. On a tolerably clear night a candle-flame can be readily 

 seen at the distance of a mile. The intensity of the electric 

 light used by me is 650 times that of a good composite candle, 

 and as the non-luminous radiation from the coal points which 

 reaches the retina is equal to twice the luminous, it follows that 

 at a common distance of a foot, the energy of the invisible rays 



* Annales de Chimie et de Physique, torn. lx. p. 71. 

 t M. Franz has shown that a portion of the sun's obscure rays reach the 

 retina. 



