50 FRAGMENTS OF SCIENCE. 



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



The layer of iodine used in the foregoing experi- 

 ments 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 de- 

 gree by the humours of the eye the albumen of the hu- 

 mours might coagulate along the line of the rays. If, 

 on the contrary, no such high absorption took place, the 

 rays might reach the retina with a force sufficient to de- 

 stroy it. To test the likelihood of these results, experi- 

 ments 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 dam- 

 age could be done. The eye was therefore caused to 

 approach the dark focus, no defence, in the first in- 

 stance, 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 ap- 

 proach the point of convergence of invisible rays. The 

 focus was attained, first by the pupil and afterwards 

 by the retina. Eemoving the eye, but permitting the 

 plate of metal to remain, a sheet of platinum foil was 



carbon is incorporated be colourless. To render the bulb of a 

 thermometer a perfect absorbent, the glass ought in the first in- 

 stance to be green. Soon after the discovery of fluorescence the 

 late Dr. William Allen Miller pointed to the lime-light as un 

 illustration of exalted refrangibility. Direct experiments have 

 since entirely confirmed the view expressed at page 210 of his 

 work on ' Chemistry,' published in 1855. 



