RADIATION. 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 cent, 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- 
