146 ANNUAL OF SCIENTIFIC DISCOVERY. 



well-known effect of light in radiating from a centre, and rendering bodies 

 visible which are not so of themselves, as long as the emission of rays was 

 continual the general nature of the undulatory view, and the fact that the 

 mathematical theory of these assumed undulations was the same with that 

 of the undulation of sound, and of any undulations occurring in elastic 

 bodies, were referred to as a starting position. Limited to this effect of 

 light, it was observed that the illuminated body was luminous only whilst 

 receiving the rays or undulations. But superaddcd occasionally to this 

 effect is one known as phosphorescence, which is especially evident when the 

 sun is employed as the source of light. Thus, if a calcined oyster-shell, a 

 piece of white paper, or even the hand, be exposed to the sun's rays, and 

 then instantly placed before the eyes in a perfectly dark room, they are seen 

 to be visible after the light has ceased to fall on them. There is a further 

 philosophical difference, which may thus be stated: if a piece of white 

 oyster-shell be placed in the spectrum rays issuing from a prism, the parts 

 will, as to illumination, appear red, or green, or blue, as they come under the 

 red, green, or blue rays ; whereas, if the phosphorescent effect be observed, 

 i. e., that effect remaining after the illuminating rays are gone, the light 

 will either be white, or of a tint not depending upcm the color of the ray 

 producing it, but upon the nature of the substance itself, and the same for 

 all the rays. The ray which comes to the eye in an ordinary case of visi- 

 bility, may be considered as that which, emanating from the luminous body, 

 has impinged upon the substance seen, and has been deflected into a new 

 course, namely, towards the eye; it may be considered as the same ray, 

 both before and after it has met with the visible body. But the light of 

 phosphorescence cannot be so considered, inasmuch as time is introduced; 

 for the body is visible for a time sensibly after it has been illuminated, which 

 time in some cases rises up to minutes, and perhaps hours. This condition 

 connects these phosphorescent bodies with those which phosphoresce by 

 heat, as apatite and fluor-spar; for when these are made to glow intensely 

 by a heat far below redness, it is evident that they have acquired a state 

 which has enabled them for a time to become original sources of light, just 

 as the other phosphorescent bodies have by exposure to light acquired a like 

 state. And then again, there is this further fact, that as the fluor-spar, 

 which has been heated, does not phosphoresce a second time \vhen reheated, 

 still it may be restored to its first state by passing the repeated discharge of 

 the electric spark over it, as Pearsall has shown. Then follows on (in the 

 addition of effect to effect) the phenomena of fluorescence, and the fine con- 

 tributions to our knowledge of this part of light by Stokes. If a fluorescent 

 body, as uranium glass, or a solution of sulphate of quinine, or decoction of 

 horse-chestnut bark, are exposed to diffuse daylight, they are illuminated, 

 not merely abundantly but peculiarly, for they appear to have a glow of 

 their own ; and this glow does not extend to all parts of the bodies, but is 

 limited to the parts where the rays first enter the substances. Some feeble 

 flames, as that of hydrogen, can produce this glow to a considerable degree. 

 If a deep-blue glass be held between the body and the rays of the sun, or of 

 the electric lamp, it seems even to increase the effect; not that it does so in 

 reality, but that it stops very many of the luminous rays, yet let the rays 

 producing this effect pass through. By using the solar or electric spectrum, 

 we learn that the most effectual rays are in most cases not the luminous 

 ones, but are in the dark part of the spectrum; and so the fluorescence ap- 

 pears to be a luminous condition of the substance, produced by dark rays 



