On the computation of the constant Csub2 of Planck's equation by an extension of Paschen's method of equal ordinates . d is based directly upon Plancks equation,and gives a relation similar to equation (9) but with certain cor-rection terms added. These may be computed exactly, but the ^Coblentz, loc. cit. See also Jalirb. der Radioaktivitat und Elektronik 8, p. i; 1911. 398 Bulletin of the Bureau of Standards [Vol. 7. No. 3 work may be shortened by an approximation which will be shownto be permissible. This method, which is due to the second of thewriters, is simpler in application; at least
On the computation of the constant Csub2 of Planck's equation by an extension of Paschen's method of equal ordinates . d is based directly upon Plancks equation,and gives a relation similar to equation (9) but with certain cor-rection terms added. These may be computed exactly, but the ^Coblentz, loc. cit. See also Jalirb. der Radioaktivitat und Elektronik 8, p. i; 1911. 398 Bulletin of the Bureau of Standards [Vol. 7. No. 3 work may be shortened by an approximation which will be shownto be permissible. This method, which is due to the second of thewriters, is simpler in application; at least as exact, even in itsapproximate form; and altogether preferable to the first method,which retains a value only for checking purposes. Each of themethods presupposes a knowledge of the approximate value of c^,so that in some cases successive approximations might be necessary. 3. COMPUTATION OF WIENS CURVE FROM AN OBSERVED CURVE We now assume, as is justified by the experimental confirma-tions of the vaHdity of Plancks equation, that if the results ofaccurate observations on / = /(^) at 6 constant are plotted, the. X \ Vo \2 resulting curve may be represented by Plancks equation withappropriate values of c^ and c^. The value of c^ influences thevertical scale but not the form of the ciu-ve, and need not occupyour attention. Let PP in the figure be the observed curve, and let WW bea curve plotted from Wiens equation with the same values ofCi and C2 that must be used to make Plancks equation representthe observed curve. Let the horizontal secant ABDE cut thecurves at the wave lengths X \ and \\ \. Let / be the ordinateof A and / the ordinate of a point C on the Wien curve directlybelow A. The curves are not far apart and if the secant is not too closeto the maxinmm of the inner curve VF, the lines BC and DF DiiungJt^^] Constant C^ of Plancks Equation 399 may be treated as straight. Considering the triangle ABC, wenow have CA_dJ_J~J AB dX y- X V- -X = j-rdr whence dXFrom equations
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