308 



Le ChatcJicr Pyrometer. — Le Cliatelier's optical pyrometer compares 

 the luminous intensity ot the red radiation from the body whose tempera- 

 ture is derived witli the red radiation from a standard light source. The 

 radiation from the body wliose temperature is to be measured, traverses 

 the diaphragm S, Fig. 4, and the objective O. A part of the radiation 

 grazes tlie 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 



A 



Fig. 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. 



