The Astrophysical journal . isible orinvisible rays) is determined by the temperature alone; in aword, to phenomena of incandescence. In spite of this restrictionit will be seen that the law is still of considerable importance. In order to avoid serious error I shall recall at the outset, inaccordance with the ideas of Kirchhoff, the precise definitions ofemissive and absorptive power. I shall suppose at first that noaccount is taken of polarization phenomena ; later I will returnto this point. I. Emissive Poiver e. Consider a body C (Fig. i) isolated in the midst of a greatenclosure which nei
The Astrophysical journal . isible orinvisible rays) is determined by the temperature alone; in aword, to phenomena of incandescence. In spite of this restrictionit will be seen that the law is still of considerable importance. In order to avoid serious error I shall recall at the outset, inaccordance with the ideas of Kirchhoff, the precise definitions ofemissive and absorptive power. I shall suppose at first that noaccount is taken of polarization phenomena ; later I will returnto this point. I. Emissive Poiver e. Consider a body C (Fig. i) isolated in the midst of a greatenclosure which neither sends to it norreturns to it radiations of any kind (ablack enclosure at an extremely lowtemperature). Imagine a cylinder X meet-ing the body C, cutting from it an elementof the surface cr. (The right section ofthe cylinder will have determinate but verysmall dimensions, which are much greaterthan the wave-length.) The emissive power of the body C for ^^ the radiation \ is the total intensity of the rays of wave-length. KIRCHHOFFS LA W 2 5 I X, which, starting from the body, arc contained within thecylinder. The number measurint; this emissive j)ower depends upon theunit of intensity adopted, and upon the selection of the narrowradiation interval comprising the radiation X, employed in themeasurements. The emissive power also depends upon thecylinder X, the nature and form of the body, the condition ofthe surfaces — in a word, upon the entire body C, and not upon<T alone. It also depends upon the supposedly transparentmedium which envelops the cylinder (S. de Smolan). 2. Absorptive Pozvcr a. Suppose that iinpolarized rays of the same wave-length Xmove in a direction exactly opposite to that of the aboverays, and strike the body C. Let us measure the intensity ofthe incident rays comprised within the cylinder, and then theintensity of what remains of these rays after they have encoun-tered the body, i. c\, the intensity of the rays transmitted,reflected, or diffused by
Size: 1450px × 1724px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectspectru, bookyear1895
