250 



Profs. H. Rubens and R. W. Wood on 



The arrangement of our apparatus is shown in fig. 1. 

 The radiations from a Welsbach light A pass through a 



Fi£. 1. 



\C^2 



F 



71 



B 



n 



^ 



circular aperture 15 mm. in diameter, B, in a screen made of 

 two large sheets of tin-plate, and then in succession through 

 the quartz lens L, a second aperture F, and the quartz lens 

 L 2 which focusses them upon the thermocouple of the radio- 

 micrometer M. 



The lenses have a diameter of 7*3 cm. and a thickness of 

 •8 cm. in the middle and '3 cm. at the edge : their focus 

 for light rays is 27'3 cm. The central zones of these two 

 lenses are covered by circular disks of black paper a x and a 2 , 

 25 mm. in diameter. A movable shutter is placed between 

 C and L L to cut off the radiation at will. 



The operation of this arrangement of apparatus is easily 

 understood. The distances of the lenses from the circular 

 apertures are so proportioned, that a sharp image of the 

 circular source B is focussed upon F only for radiations for 

 which the refractive index of the quartz is 2*14, the square 

 root of the dielectric constant for slow oscillations. 



Inasmuch as the refractive index of quartz is between 

 1*55 and 1*43 for the shorter heat and light waves (fine 

 dotted lines), which are able to pass through it, these rays 

 form a divergent cone after passing through the lens L^ 

 This cone is intercepted by the screen E, the circular 

 aperture being shielded by the disk of black paper. As a 

 result, the only rays which can pass through the aperture 

 are the very long heat-waves (coarse dotted lines), which 

 converge upon it, as a result of the high value of the 

 refractive index for these radiations. The second quartz 

 lens, acting in the same way, still further purifies the 

 radiation, and eliminates completely the shorter heat waves, 

 scattered or diffused by the surfaces of the first lens. 



