RADIATION. 187 



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



The layer of iodine used in the foregoing experiments 

 intercepted the light 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 improbable that in the 

 brief time requisite for an experiment any serious damage 



specimens of blue glass, the platinum-foil glowed with a pink or purplish 

 light. The effect was not subjective, and considerations of obvious in- 

 terest are suggested by it. Different kinds of black glass differ notably 

 as to their power of transmitting radiant heat. In thin plates some de- 

 scriptions tint the sun with a greenish hue : others make it appear a 

 glowing red without any trace of green. The latter are far more diather- 

 mic than the former. In fact, carbon when perfectly dissolved, and in- 

 corporated with a good white glass, is highly transparent to the calorific 

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

 cence " may be obtained, though in a less striking form than with the 

 iodine. The black glass chosen for thermometers, and intended to ab- 

 sorb 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. 



