200 Focal Isolation of Long Heat- Waves. 



The arrangement is shown in fig. 5. The rays are converged 

 by the horizontal quartz lens upon the flat surface S ol the 



Fig\ 5. 



a 



C: 



3mt^_ 



7'" 



substance under investigation, s is a small elliptical mirror 

 (made by silvering a flat spectacle-glass) mounted with wax 

 on a cork attached to the under surface of the lens. Thi* 

 mirror serves not only to reflect the converging pencil of 

 rays to the diaphragm F, but acts at the same time as the 

 central stop of the lens. 



The observations were made by measuring* the reflexion 

 first with a plate of silvered glass at S, and then with the 

 fluid or solid surface under investigation. Care was of 

 course iaken to insure that the reflecting surfaces were level 

 and at the same distance from the lens, as shown by the 

 movable marker U. 



In Table VI. are given the reflecting powers of various 

 substances not only for the 108 /j, waves obtained by focal 

 isolation, but also for the 82 fi residual rays from KBr. In 

 the last two columns are given the dielectric constant and 

 the value of the reflecting power for infinitely long waves 



uy|\ 2 

 1- vk/- 



The calculated and observed values are in close agree- 

 ment for rock-salt, fluorite, and glass, while for sylvite the 

 value observed with the 108 /i waves is much nearer the 



calculated from it by the formula E = 100 



