Physics. — “An Interference Phenomenon Due to the Introduction 
of Sodium Vapour into one of the Paths of the Fizwav- 
Micug.son [nterferometer-Arrangement’. By G. C. Dissrrz Jr. 
and Prof. P. Zeman. 
(Communicated at the meeting of November 26, 1921). 
When sodium vapour is introduced into one of the paths of light 
of the interference-arrangement used in Fizwau’s experiment, surprising 
shifts of interference fringes may be observed. In order to observe 
these shifts we must arrange that the sodium vapour acts as a 
prism, and that by means of a spectroscope the change of the 
distance of the interference fringes with wavelength can be watched 
by projecting the interference fringes on the slit of the spectroscope. 
The above figure is reproduced from an earlier paper’) on 
FreSNEL’S convection coefficient for light of different colours in water, 
in which use is made of Micnerson and Morrrr’s interferometer. 
In a the ray meets Micuetson’s slightly silvered mirror.. There the 
1) ZEEMAN. These Proc. Vol. XVII p. 445 (June 1914). 
