The Astrophysical journal . uences may be at once deduced fromthis law : I. Suppose that the body C is simply rotated, without being replaced by another body ; the ratio - does not change. Thus we are able to deduce at once, in the special case of opaque bodies, laws defining the change of emission with the incidence. Let us suppose that the body, which is suf^ciently thick to permit none of the radiations considered to pass, is bounded by a plane surface, and that without disturbing the diaphragms which limit the beam, this surface is more or less inclined (Fig. 2) ; the ratio - must remain c
The Astrophysical journal . uences may be at once deduced fromthis law : I. Suppose that the body C is simply rotated, without being replaced by another body ; the ratio - does not change. Thus we are able to deduce at once, in the special case of opaque bodies, laws defining the change of emission with the incidence. Let us suppose that the body, which is suf^ciently thick to permit none of the radiations considered to pass, is bounded by a plane surface, and that without disturbing the diaphragms which limit the beam, this surface is more or less inclined (Fig. 2) ; the ratio - must remain constant. The Fig. 2. a area cut out of the surface by the cylinder X, which limits the beams, varies (in the inverse ratio of the cosine of the angle of incidence) so that the emissive power applies to changing surfaces. If the body is perfectly absorptive, a—\, e remains constant, and the emissive i^ow&x per unit of S7irface varies as the cosine of the angle of incidence ; in other words, Lainberf s law is applicable. 2 54 -i- cor TO A to perfectly absorptive bodies. (Leslie found tliis true for lamp-black.) If the body is opaque, but diffuses the light or reflects itregularly, the absorptive jjovver is i — r, r being the reflecting ordiffusing j)o\ver. The emissive powers of the two surfaces (t,(t\will be connected by the relation : e e , Lamberts law will thus be applicable only if r = r, , ifthe body has a perfectly mat surface, if it diffuses equally, what-ever be the incidence. (Mollers experiments have shown thatthis is really the case.) Finally, if the body is polished and highly reflecting, thereflecting power changing with the incidence, formula (i) thenindicates how the emission varies with the incidence. Uljanin,to whom we owe a recent investigation upon this subject, hasshown that this formula well represents the experimental results,for example, those obtained with highly polished platinum.^ It is seen that Kirchhoffs law, co7iji7ied to the case of a singl
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectspectru, bookyear1895
