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 

 degree by the humours of the eye the albumen of the 

 humours 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 damage 

 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 

 approach the point of convergence of invisible rays. 

 The focus was attained, first by the pupil and after- 

 wards by the retina. Removing the eye, but per* 

 mitting the plate of metal to remain, a sheet of 

 platinum foil was placed in the position occupied by the 



bulb of a thermometer a perfect absorbent, the glass ought in the 

 first instance to be green. Soon after the discovery of fluorescence 

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

 illustration of exalted refrangibility. Direct experiments have 

 since entirely confirmed the view expressed at page 210 of his work 

 on * Chemistry,' published in 1855. 



