309 
The intensity of the unknown source in terms of the intensity of the 
comparison lamp taken as unity, becomes 
1 
J = (—)? 
d 
where d denotes the length of one side of the square aperture 8S. Due to 
the lack of monochromatism of the red glass this instrument is not so 
accurate as the Wanner. 
Holborn-Kurlbaum Pyrometer.—In this method the luminous intensity 
of the comparison source is varied until a photometric balance is obtained 
between its image and the image of the incandescent object in question. 
In the H.-K. (Holborn-Kurlbauin) pyrometer shown in Fig. 5 a small elec- 
tric lamp L is placed in the focal plane of the objective O and the same 
is viewed by means of an eye-lens E. In making an observation the pyrom- 
eter is focused upon the object whose temperature is sought, thus bring- 
ing the image of the object in the plane of L. The current through the 
lamp is adjusted by means of a rheostat until the lamp filament disappears 
against the bright background. The value of the current strength can be 
read direct from a milli-ammeter. 
In order to measure temperature with this instrument it must be 
empirically calibrated by means of a black body. A curve may then be 
plotted with current in milli-amperes, I, and temperature, t, in degrees C. 
To determine an unknown temperature, it is only necessary to focus the 
instrument upon the object in question and adjust the current through the 
lamp until the filament disappears against the bright object. The pyro- 
neter then indicates the black body temperature unless black body condi- 
tions are realized, in which case it indicates true temperature, i. e., thermo- 
dynamic temperature. 
The reading of the «mmeter will be independent of the distance of 
pyrometer from cbject so long as the solid angle w, Fig. 5, is constant 
This is accomplished by means of the diaphragm D. When the instrument 
