56 FRAGMENTS OF SCIENCE 



caused to approach the dark focus, no defence, 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 posi- 

 tion occupied by the retina a moment before. The plati- 

 num 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, <ind in part 

 absorbed by the humors. Experiments on th* 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 

 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 



