188 FRAGMENTS OF SCIENCE. 



could be done. The eye was therefore caused to approach 

 the dark focus, no defence, in the first instance, being pro- 

 vided ; 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, Avas 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 mo- 

 ment before. The platinum became red hot. No sensible 

 damage was done to the eye by this experiment ; no im- 

 pression 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 experi- 

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

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

 sorbed 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 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 bears, 

 without exciting consciousness, 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 the 

 electric light is 800 times the light of a candle. At the 

 same distance, the portion of the radiation of the electric 



