548 FOOTE, MOHLER, AND FAIRCHILD: PYROMETER GLASS 



means of showing this is as follows. Let equation (1) represent 

 the condition for any system whatever, that is, one in which, in 

 general, A =f (0). On dividing (1) by its derivative with respect 



to 0, and substituting - — „ = A (d) we obtain : 



dA __ X QS) - X' (0) 



do e 2 x' (0) 



(6) 



Where X (S) is the effective wave length of the system lamp at 

 temperature S + red glass, and X' (0) is the effective wave 

 length of the system furnace at temperature + absorption 

 glass + red glass. 2 



For the case under consideration X (S) - X' (0) = 0. The 

 values obtained by graphical integration are given in table 1 

 together with similar values for the type of absorption glass 



dA 

 ordinarily used. It is seen that jir=0 within the errors of 



CLu 



computation for the new type of absorption glass, while it is 

 far from zero for the Jena glass F3815. 



TABLE 1 



dA 



Values of — for Jena Glass and New Absorption Glass, Using Red Glass 

 de 



Ocular Jena 2745 



Preliminary work indicates that the absorption glass can be 

 experimentally obtained, to a sufficient degree of approximation, 

 by the combination of black and colored glasses. 



2 Cf. Foote. Loc. cit. 



