RADIATION. 301 



results constitute a kind of complempnt to his discoverips. Professor Stokes 

 named the phenomena which he has discovered and investia^ated, Fluorescence ; 

 for the new phenomena liere described, 1 have proposed the term CaJorcscence. 

 He, ])y the interposition of a proper me(hnm, so loivered the refranijil)ih"ty of the 

 uhra-viidet rays of the spectrum as to render tliem visible ; and here, by the 

 interpositi()n of the plantinnm foil, the refranjifibility of the idtrared rays is so 

 exalted as to render them visible. Looking tln-ong-h a prism at the incandescent 

 image of the carbon points, the light of the imago is decomposed and a com- 

 plete spectrum obtained. The invisii)le rays of the electric light, remolded l)y 

 the atoms of the ])latinum, shine thus visibly forth — •ultra-red rays being con- 

 verted into red, orange, yellow, green, blue, indigo, and ultra-violet ones. Could 

 we, moreover, raise the original source of rays to a sufficiently high temperatui-e, 

 we might not only obtain from the dark rays of such a source a single incandes- 

 cent image, but from the dark rays of this image we might obtain a second one, 

 from th(! dark rays of the second a third, and so on — a series of complet(! images 

 and spectra being thus extracted from the invisible emission of the primitive 

 source. * 



IX. — DEADXESS OF THE OPTIC KERVE TO THE CALORIFIC RATS. 



The layer of iodine used in the foregoing experiments, when placed before 

 the eye, intercepted the light of the noonday sun. Nf) trace of light from the 

 electric lamp was visible, even in the darkest room, when a white screen was 

 placed at the focus of the mirror. It was thought, however, tliat 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 coagu- 

 late along the line of the rays. If, on the contrary, no such high absor])tion 

 tof'k 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 tlie 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 instance 

 being provided; but the heat acting upon the parts surrounding the pu{)il could 

 not l>e borne. An aperture was therefore pierced in a plate of metal, and the 

 eye placed behind the aperture was caused to approach the jwint of convergence 

 of invisible rays. The focus was attained, first by the pupil and afterwards b}'' 

 the retina, liemoving the eye, but permitting the plate of metal to remain, a 

 sheet of platinum foil was placed in the position occupie(i by the retina a moment 

 before. The platinum became red-hot. No sensible damage was done to the 

 eye by this experiment ; no impression of light was })roduced ; the optic nerve 

 was not even conscious of heat. 



But the humors of the eye are known to be highly impervious to the invisible 

 calorific rays, and the question therefore arises, " did the radiation in the fore- 



* On investigating: the calorcscence prodticcJ by rays transmitted throngii plassos of vari- 

 ous colors, it was found that in the case of certain specimens of blue plass thu pla- 

 tinum foil glowed Willi a. piulc or purplish light. The ctTcct was not subjt'ctive, and consid- 

 erations of obvious interest are suggested by it. DilVercnt kinds of black glass ditfer notably 

 as to their power of transmitting radiant heat. In thin jjlates some descriptions tint the sun 

 with a greenish hue; others make it appear a glowing nd without any tract: of gn'cii. Tho 

 latter are by far inore diathermic than the former. In fact, carbon, when |)erlrctly 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 whiidi the car- 

 bon is incorporated be colorless. To render the bulb of f thermometer a perfect absorbent, tho 

 glass with which the carbon is incorporated ought in the lirst instance to be preen. Soon 

 alter the discovery of fluorescence, Dr. W. A. Miller pointed to the lime light as an illus'ra- 

 tiou of exalted refrangibility. Direct experiments have since entirely coutirmed the view 

 expressed at page 21U of bis work ou CItcmistry, published in IriO.'). 



