RADIATION 55 



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 ab- 

 sorption 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 improb-^ 

 able that in the brief time requisite for an experiment any 

 serious damage could be done. The eye was therefore 



blue glass, the platinum foil glowed with a pink or purplish light. The effect 

 was not subjective, and considerations of obvious interest are suggested by it. 

 Different kinds of black glass differ notably as to their power of transmitting 

 radiant heat. When thin, some descriptions tint the sun with a greenish hue : 

 others make it appear a glowing red without any trace of green. The latter are 

 far more diathermic than the former. In fact, carbon when perfectly dissolved 

 and incorporated with a good white glass is highly transparent to the calorific 

 rays, and by employing it as an absorbent the phenomena of "calorescence" 

 may be obtained, though in a less striking form than with the iodine. The 

 black glass chosen for thermometers, and intended to absorb completely the 

 solar heat, may entirely fail in this object, if the glass in which the carbon is 

 incorporated be colorless. To render 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. 



