308 
Le Chatelier Pyrometer.—bLe Chatelier’s optical pyrometer compares 
the luminous intensity of the red radiation from the body whose tempera- 
ture is derived with the red radiation from a standard light source. The 
radiation from the body whose temperature is to be measured, traverses 
the diaphragm §S, Fig. 4, and the objective O. A part of the radiation 
grazes the right edge of the mirror M and is brought to a focus at the 
focal plane of the eye-piece A. Light from the central portion of the flame 
of the comparison flame L traverses the objective O’, is reflected from the 
inclined mirror M and is also brought to a focus in the focal plane of the 
eye-piece. Thus two images, one of the source whose temperature is 
sought, and one of the comparison flame, are found side by side, in the focal 
plane of the eye-piece. These two images are simultaneously observed by 
means of the eye-piece A provided with a piece of red glass for rendering 
Via. 4. 
the radiations that enter the eye of the same wave lengths. By adjust- 
ing the size of the aperture in the diaphragm S, these two images can be 
brought to the same luminous intensity. The distance from the objective 
O to the focal plane of the eye-piece can be varied in order to focalize the 
radiation from the luminous source, and the distance can be read directly 
from a scale engraved on the draw tube. The aperture in the diaphragm 
S is square and the length of one side can be read directly from the 
screw head which operates it, 
