barus.] METHODS OF PYROMETRY. 55 
data in the regions of high temperature are known. Beyond this inci- 
dental result I do not attempt to fix the absolute value of any high tem- 
perature datum. To do this it is not only necessary to expend *more 
time than I have now at my disposal, but it is expedient that the ob- 
server be specially equipped for the purpose. In the problems of this 
volume, on the other hand, where the end chiefly in view is the trial of 
available methods rather than the investigation of new and accurate 
results, such equipment is superfluous. In one case the variables of 
the thermal apparatus selected, including of course the limits of varia- 
tion of its constants, are chiefly to be studied ; in the other case addi- 
tional time and pains must be spent in the absolute evaluation of the 
constants themselves. 
(709) 
