the measurement of temperature is Innde photometrically by adjusting to 
equality two photometric fields produced by a standard source and the 
body to be measured. The intensity of radiation is varied by cutting down 
the objective aperture, as in Le Chatelier, or by a polarizing device, 
as in the Wanner, or by varying the intensity of the standard itself, as in 
the Holborn. 
Since we are using mono-chromatic light a measure of the luminous 
intensity may be taken as a measure of the radiant energy. ‘The intensity 
of radiation of a source may be defined as the ratio of the total energy 
emitted (including all wave lengths) to the energy falling upon unit sur- 
face. A part of the energy emitted by a heated body, however, may be 
luminous and both the luminous and total energy emitted by a body in- 
creases with temperature, but the total luminous energy is not proportional 
to the total energy emitted. The luminous energy of any particular wave 
length, however, is directly proportional to the total radiant energy 
emitted. Hence in any optical pyrometer when photometric comparison 
is made if mono-chromatic light is used the above radiation laws will 
hold. 
Wanner Pyrometer—tlt has been shown that the luminous intensities 
of two bodies may be taken as a measure of their temperatures, if mono- 
chromatic light is used, and since luminous intensities may be compared 
by the rotation of a Nicol prism we have a convenient means of measuring 
high temperatures. 
In this method comparison is made between a standard lamp and the 
body whose temperature is sought. The standard used is a 6-volt incan- 
descent lamp which is in turn compared with some primary standard as 
an amyl acetate lamp. For this work the primary standard is used merely 
as a check for the more convenient electric lamp and so long as it is re- 
producible so that the comparison lamp can always be brought to the same 
condition, we are not concerned with its intrinsic intensity or temperature. 
Photometric comparison is made of the comparison lamp and the unknown 
source by adjusting to equal brightness two halves of a photometric field 
by means of a polarizing arrangement, monochromatic red light being pro- 
duced by a direct vision prism. 
The intensity of the unknown source in terms of the comparison lamp, 
taken as unity, is 
J = tan 
where @ is the rotation of the Nicol prism. 
