Measurements in the Extreme Infra-red Spectrum. 763 



where they disappeared. If the rotation were discontinued 

 the interference bands remained practically unaltered ; only 

 after a complete revolution could a marked displacement 

 of the central band be noticed, and it seldom amounted 

 to more than one or two sodium bands *. If, on reversal 

 of the rotation, the plates were brought to their original 

 position no change in the system was apparent which could 

 vitiate results. Furthermore, the wave-lengths under con- 

 sideration are of a magnitude 80 to 170 times greater than 

 that of sodium light, hence the inference that the plates, as 

 well as the motion of the interferometer slide, are sufficiently 

 exact for the long-wave measurements. 



By intense illumination of the quartz plates another in- 

 terference system was visible. This could not affect the 

 measurements, however, as the phenomena remained constant 

 during separation of the quartz plates, i. e. the fringes have 

 their origin in the plates themselves. 



Quartz is particularly suitable as a material for the 

 bounding surfaces of the air-film, since it is sufficiently 

 transparent in thin sheets, and also has a high index of 

 refraction for radiations of greater wave-length. Previous 

 investigations t have shown the index of refraction of quartz, 

 for \ = 56/Lt, to be 2*18, and probably this value approaches 

 2*15 as a limit with increasing wave-length. For n = 2*18 

 the reflectivity (R) is equal to 13*8 per cent. ; hence, ac- 

 cording to the Airy formula, the intensity of a completely 

 homogeneous ray after transmission through a plane-parallel 

 air-layer is 



(100-R) 2 



° (100 - R) 2 + 400 R sin 2 ( 2 ^ 8 \ 



In the present instance the value varied between 



I =I and I min = 0-r>711 . 



This variation in the intensity is evidently never com- 

 pletely obtained for a non-homogeneous radiation. 



* The periodic variations in the visibility of the interference bands, 

 which were investigated by Fizeau and Michelson as well, were also 

 noticed. 



t H. .Rubens and E. Aschkinass, Ann. d. Phys. u. Chem. lxvii. p. 459 

 (1899). 



