310 



is focusetl for distant objects, i. e., when O is drawn near L, the solid angle 

 w would be increased if it were not for the diaphragm D. 



The light which reaches the eye is rendered appx'oximately mono- 

 chromatic by a red glass R placed before the eye-piece bnt for temperatures 

 below 800° C. this is not necessary and above 1,200° C. two glasses are 

 generally used. For the extrapolation of the exi^erimentally determined 

 curve for high temperatures Wien's thirtl law may be used. For these high 

 temperatures beyond the safe limit of the lamp three different methods 

 are used for cutting down the incident radiation a determinate amount ; 

 absorbing glasses, mirrors, and sector discs. 



Since the absorbing power of the absorption glasses is different for 

 different wave lengths, if there is any lack of monoehromatism in the red 

 transmission glasses, which is generally the case, Wien's law will not hold 

 for high temperatures. 



To overcome this difficulty Henning" has combined an H.-K. pyrometer 

 with a Hilger constant deviation spectrometer so that homogeneous light 

 may be used. This instrument has the further advantage that any part 

 of the spectrum may also be employed. Dr. Mendenhall has recently de- 

 vised a spectroscopic eye piece to accomplish the same pxn-pose. 



The H.-K. pyrometer is probably the most sensitive pyrometer now 

 in use. 



Fery Total Radiation Pyrometer. — From a consideration of the Stefan- 

 Boltzmann radiation law we have seen that the energy radiated by a black 

 body is proportional to the fourth power of the absolute temperature, or, 



J = KT* (2) 



From this relation it is evident that a comparatively rough method 

 of determining the energy radiated would yield fairly accurate results of 

 temperature measurements. 



The Fery radiation pyrometer is shown in detail in Fig. 6. Radiation 

 from an incandescent body is focused upon a minute and sensitive thermo- 

 couple C, by means of a lens A'. In order to calibrate the pyrometer 

 directly in terms of the Stenfan-Boltzman law the leriS should be trans- 

 parent for all radiations and this is best effected by using a fluorite lens 

 which for temperatures above 900° C. does not absorb an ajipreciable portion 

 of the incident radiation. F is a rack and pinion for focusing the radiation 

 upon the thermo-junction. The screens C and D protect the junction from 



" Zeitschrift Fiir Instrumentenkunde, Miirz, 1910. 



