a 
446 £. L. Nichols—Character and Intensity of the 
Art. LVL—On the Character and Intensity of the Rays emitted 
by Glowing Platinum ; by E. L. Nicwoxs, Ph.D. (Géttingen.) 
In 1860, Kirchhoff* issued his well-known paper on the rela- 
tion between the capacity of bodies for emitting and for 
absorbing rays. That essay made a new epoch in the science 
of Radiation. It offered the first complete proof and the first 
universal expression of a principle which had existed in the 
minds of scientists, more or less dimly, since the days of Euler.t 
Although the results of that treatise have been repeatedly 
confirmed by the experience of investigators in Optical Science 
and in the domain of Radiant Heat, there have been, so far as 
know, in spite of the interesting character of Kirchhoff’s 
Function I,¢ no attempts to measure its values. 
* Kirchhoff, Poggendorff’s Annalen, cix; also, “Untersuchungen iber das 
Sonnenspectrum— Anhang.” 
+ For earlier attempts to hhoff’s Law 
Euler, Opuscula Varii Argumenti, Berol. 1746 (Nova Theoria Lucis et Colorum, 
Cap... Vio... Fu i 
Warme, Halle, 1798. Angstrom, Poggendorff’s Annalen, xciy. Balfour Stewart, 
oceedings of the Royal Society of Edinburgh, 1857-58. 
_} In the above-mentioned treatise Kirchhoff gives for I the following formula: 
W, Wa 
ei 3 
where (fig. 1) w, and w, are the projections of the openings (1) and (2) in the 
screens 8, , upon planes perpendicul he axis of a pencil of rays, hich, 
going out from the black body C, passes through both of these ope: ‘Where 
urther, t stance between the two ings. and e the emissive capacity 
of a blac lack body according irchhoff, and the sa ~ 
applies to the term when used in this paper. is a even when 
infinitesimal thickness absorbs all the rays falling upon it. The following sho 
t 
y the terms emissive capacity, absorptive capacity, ete. 
nlite “Before a body C (fig. 1) let us suppos? 
d to be placed, in 
1 t 
: : of 
which are the openings (1) and (2), 
® Sein al size when compared with the 
{ center. uppose the 
i): a pencil of rays throu 
ok 6 . Of this pencil of é 
sider that portion the wave lengths of which lie between / and 2+dA, and let : 
imagine the same resolved into two components, polarized in the planes 4 and 
| Let the p i i 8, ane _ 
be perpendicular to one another. Let, further, Ed’ be the intensity of the com- 
ponent . Ma Poh one ‘the same thi ane a # 
_ energy (lebendige Kraft) of the ether behind the screen 8, suffers in age? 
time by the acti this component. The quantity E is called the emissi0? 
($2 of Kirchhoff’s treatise.) ey eral 
to be black. For its emissive capacity, which in genet 
” e3 
