LABORATORY OPTICAL PYROMETER 
71 
body would emit if it were in the same thermal state, and let be 
the temperature of a black-body which appears as bright through 
the pyrometer as the radiating platinum. Then if H be the energy 
of wavelength L emitted by each, 
and 
Equating the right-hand members, we have 
-CJLT _ —CJLS 
or 
j/r = i/Y + log r . L/C^. 
Since the hole in the platinum is emitting like a black-body, its 
temperature determination will yield the true temperature of the 
platinum. The temperature 5 may be found in the usual manner 
and, by means of the above equation, r may be calculated. The 
number r is called the “emissivity” of the substance.^,^^. It varies 
from substance to substance and with the condition of the surface 
for the same substance, as well as with the temperature and with 
the wavelength of the light used in its determination. When r is 
known and S’ has been observed, T can be calculated. Thus we 
have a method for determining the true temperature of a non- 
black-body. 
THE OPTICAL PYROMETER IN PRACTICE 
The objective lens, B of figure 6, should be an achromatic lens 
of good quality. A rapid rectilinear photographic lens having a 
focal length of about 25 cm, and a maximum aperture of about 
F.8 mounted in a barrel with an iris-diaphragm, will serve very 
well. It should be placed at such a distance from the pyrometer- 
lamp D that the image of the background A formed at D is much 
larger than the filament oi D. 
The telescope F is the eyepiece of the pyrometer. It should be 
of good quality and capable of being focused on objects as near 
as one meter. The magnifying power should be sufficiently large 
so that no difficulty is experienced by the observer in fixing on 
the intersection of the pyrometer-filament and the background 
image. The resolving power of almost any laboratory telescope 
is so great that the objective lens (of the telescope )will need to be 
stopped down with a limiting diaphragm B. When the resolving 
power is too great, dark bands are seen on either side of the py- 
rometer filament; these are caused by the diffraction of the light 
