TIADIATION. 37 



9. Deadness of the Optic Nerve to the Calorific Rays. 



The layer of iodine used in the foregoing experiments 

 intercepted the rays of the noonday sun. No trace of 

 light from the electric lamp was visible in the darkest 

 room, even when a white screen was placed at the focus of 

 the mirror employed to concentrate the light. It was 

 thought, however, that if the retina itself were brought 

 into the focus the sensation of light might be experienced. 

 The danger of this experiment was twofold. If the dark 

 rays were absorbed in a high degree by the humors of the 

 eye the albumen of the humors might coagulate along the 

 line of the rays. If, on the contrary, no such high absorp- 

 tion took place, the rays might reach the retina with a 

 force sufficient to destroy it. To test the likelihood of 

 these results, experiments were made on water and on a 

 solution of alum, and they showed it to be very improbable 

 that in the brief time requisite for an experiment any 

 serious damage could be done. The eye was therefore 

 caused to approach the dark focus, no defense, in the first 

 instance, being provided; but the heat, acting upon the 

 parts surrounding the pupil, could not be borne. An 

 aperture was therefore pierced in a plate of metal, and the 

 eye, placed behind the aperture, was caused to approach 

 the point of convergence of invisible rays. The focus was 

 attained, first by the pupil and afterward by the retina. 

 Removing the eye, but permitting the plate of metal to 

 remain, a sheet of platinum foil was 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 humors of the eye are known to be highly im- 

 pervious to the invisible calorific rays, and the question 

 therefore arises, "Did the radiation in the foregoing ex- 

 periment reach the retina at all?" The answer is, that the 

 rays were in part transmitted to the retina, and in part 

 absorbed by the humors. Experiments on the eye of an ox 

 showed that the proportion of obscure rays which reached 

 the retina amounted to 18 per rent, 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 con- 



