310 , 
is focused 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 approximately mono- 
chromatic by a red glass R placed before the eye-piece but for temperatures 
below SO00° C. this is not necessary and above 1,200° C. two glasses are 
generally used. For the extrapolation of the experimentally determined 
curve for high temperatures Wien’s third law may be used. For these high 
temperatures beyond the safe limit of the lamp three different methods 
ure 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 monochromatism 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 purpose. — 
The H.-K. pyrometer is probably the most sensitive pyrometer now 
in use. 
Féry 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, 
ie (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 Féry 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 lens 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 appreciable 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, Mirz, 1910. 
