SCIENCE. 



37i 



Now let us attend to reflected light. As we attend to 

 it we shall learn that reflected light does not reveal the 

 reflector but the body emitting it. If bodies are seen by 

 reflected light, they should be more clearly seen in pro- 

 portion as they reflect more perfectly the light falling on 

 them. The facts are exactly contradictory to this. In 

 proportion as any given surface is a good reflector it is to 

 that degree invisible, and when a surface becomes a per- 

 fect reflector it becomes invisible. Can it then be true 

 that bodies are seen by reflected light when it is palpably 

 true that the better they reflect light the less visible they 

 are ? The reflected light makes visible the body emitting 

 it, not the reflecting body, and it results that, in studying 

 the stars, the astronomer uses nearly indifferently a reflect- 

 ing or a refracting telescope. Plainly then, we would say, it 

 is not by reflected light that bodies are visible. This con- 

 clusion cannot be escaped by any conjectures as to the 

 extent and form of the reflecting surface. The minutest 

 surface reflecting the sunlight gives a brilliant, dazzling 

 star, not a revelation of itself. Curved, convex or con- 

 cave, or variously warped surfaces give only images va- 

 riously enlarged or diminished, or variously distorted, of 

 the body emitting the light, and not at all of the surfaces 

 reflecting it. If the microscope be applied to the surface, 

 the facts are still found to be as above stated. No theory 

 of minute reflecting surfaces changes any of these facts, 

 unless it were imagined that a surface might be so small 

 as to decompose the light falling on it, but this result would 

 be destructive of the theory now objected against. Thus 

 it appears from all the facts stated and referred to that the 

 proof is conclusive that, in no case is a body seen as such 

 by the light it reflects. 



If, now, we go on to inquire as to the light by which 

 bodies are seen, we may find some good reasons for be- 

 lieving it to be essentially radiant light, even when pro- 

 ceeding from non-luminous bodies. Note, then, that it 

 is the peculiarity of radiant light that it is emitted in 

 straight lines in every possible direction from every 

 luminous point. The light, hence, by which such a 

 point or body is seen is divergent light, and the office of 

 the optical apparatus is to bring it to a focus on the re- 

 tina. It is not possible for a single point (the minimum 

 of visible surface,) in any reflecting surface to reflect 

 light in every direction ; and for light thus to proceed in 

 every direction from a luminous point is the distinguish- 

 ing characteristic of radiant light. What thus charac- 

 terizes the light of what are called luminous bodies 

 will be found to characterize the light by which all non- 

 luminous bodies are visible. From every point of any 

 such visible body the light proceeds in every possible 

 direction ; vvhence we note that every such point is a 

 point of dispersion or radiation, and not a point of reflec- 

 tion. Here, as we learned in the case of luminous bod- 

 ies, the light by which any ordinary non-luminous body 

 (so-called) is seen is divergent, and the office of the 

 optical apparatus is to bring it to a focus on the retina. 

 This brings before us the perfect similarity of the 

 conditions under which luminous and non-luminous 

 bodies are seen ; and which seem to compel us, hence, 

 to regard the light by which non-luminous bodies are 

 seen as having essentially the same qualities and relations 

 as radiant light. 



If, now, we seek to know how this can be explained, 

 seeing that non-luminous bodies are not original sources 

 of light, I think we may find a nearly perfect analogy 

 in the facts of heat that may afford us much help. We 

 are tolerably familiar with radiant and reflected heat. 

 The heat which a body reflects follows all the laws of 

 reflected light, and has this peculiarity, that it does not 

 change the temperature of the reflecting surface. For 

 the rest, the heat which falls on a body, and, as it is said, 

 is absorbed by it, raises the temperature of the absorbing 

 body, and immediately said body begins to radiate 

 heat, and the heat thus radiated shows all the essential 

 characteristics of radiant heat. What we wish to have 



particularly noted here is, that this heat has been all 

 along said to be radiated, not reflected. By the prin- 

 ciple of the correlation of forces the heat which is said 

 to be absorbed is transformed first into increased mole- 

 cular activity in the absorbing body, and then again 

 transformed into what is emitted as radiant heat ; and 

 this emission is in straight lines in every direction from 

 every point in the surface of the body radiating. All 

 this is plain, and in perfect agreement with the accepted 

 theory of heat. We have now only to apply these facts 

 and principles, by analogy, to light, and we may obtain 

 an equally plain and consistent theory of light as to vis- 

 ible bodies. 



We have already called attention to the fact that the 

 light which a surface reflects does not reveal that sur- 

 face. The light by which any non-luminous body is seen 

 is emitted, let us say, radiated, from every point of its 

 surface. This may now be explained by supposing the 

 light (luminous energy) received by such a body as in 

 some degree or manner absorbed by the superficial par- 

 ticles of the body, and then radiated from every such par- 

 ticle as a centre, analogous to what we believe of heat. 

 The light thus taken in appears to be always 

 decomposed, with numberless variations of results ; so 

 that the light emitted or radiated is always of a different 

 color from that received. This difference of color affords us 

 another contrast between the light by which bodies are 

 seen and reflected light ; this last being always of the same 

 color as the incident light. In making this statement we 

 have in mind the fact that the same surface may both reflect 

 and radiate light ; and that, hence, in each case we must 

 take care not to confound the one with the other in mak- 

 ing our observations. When this caution is observed, 

 the statement above concerning the color of reflected 

 light will not, we think, be called in question. 



The explanation, then, that I would offer is, that the 

 light which falls on non-luminous bodies (so far as it is 

 not reflected) is somehow absorbed by them, decomposed, 

 and then radiated, at least in part, that the body is visible 

 by this radiated light, and not at all by that light which 

 it reflects. In these actions and reactions between the 

 luminous energy falling on a non-luminous body and the 

 body itself, we think it not improbable that there are some 

 correlations of force ; and that these may be essential 

 parts of the change that enables the light radiated to 

 make visible the non-luminous body. 



If the views presented in this paper be allowed, they 

 enable us to place the facts of phosphorescence, and may 

 be of fluorescence, in harmony with the action on light of 

 ordinary non-luminous bodies ; and differing from these 

 chiefly, if not wholly, in degree only. And is it not true 

 that this so-called phosphorescence is possessed in some 

 degree by every visible body ? We do not now speak of 

 cases of slow combustion, like exposed phosphorus, but 

 those continuing to emit light for a time after being cut 

 off from extraneous light, like snow and the diamond. 

 We would look for the explanation of these greater de- 

 grees in phosphorescence in the power of the bodies ex- 

 hibiting it to absorb and decompose light more deeply, 

 and then more tardily radiate the luminous energy, than 

 is true of non-luminous bodies generally. 



It may be proper here to notice the facts of iridescence, 

 with which our theme may have some interesting con- 

 nections. Inasmuch as the facts of iridescence are ex- 

 plained by the interference of the luminous waves, caused 

 by the reflection of light from very thin lamina;, it might 

 be thought the same explanation would apply to decom- 

 position of light by ordinary non-luminous bodies. We 

 think the facts in the two cases so different that the same 

 explanation is not applicable to both. In the first place, 

 the facts of iridescence agree with the usual character- 

 istics of reflected light ; while, on the contrary, we have 

 noted in this paper that the facts in the case of ordinary 

 visible bodies do not so agree. And, in the second place, 

 the results of the decomposition of light in iridescence 



