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XI. Experimental Demonstration of the Constancy of Velocity 

 of the Light emitted by a Moving Source. By Q. Majorana, 

 Professor of Physics at the Polytechnic School of Turin *. 



IN a preceding paper | on the second postulate of the 

 theory of relativity I described an experimental arrange- 

 ment of mine by means of which I was able to demonstrate 

 that light propagates itself with constant velocity, inde- 

 pendently of the conditions of movement or rest of the 

 mirror by which the light is reflected. At the end of the 

 above-mentioned paper I hinted at my intention of studying 

 experimentally the eventual influence of the movement of 

 the source on the velocity of the propagation of the light ; 

 the object of the present note is to communicate the result of 

 these researches. 



As is known, the only studies made with luminous sources 

 in motion are astronomical ones, and those with the canal 

 rays. Particularly with the former it has been possible to 

 deduce the measure of the Doppler effect (and therefore the 

 value of the velocity of displacement) for different sources, 

 such as the fixed stars or planets and the limb of the sun. 

 I am not aware of any attempt to prove the Doppler effect 

 with the artificial movement of a common luminous source; 

 the difficulty of this research consists principally in the 

 necessity for giving a specially high rate of velocity of 

 displacement to the source. 



But even if an arrangement of this kind could be realized, 

 its interest does not lie in the verification of the Doppler 

 effect (change of frequency), upon which no doubt any 

 longer exists, so much as in the control of the value of the 

 velocity of the propagation of the light, also in the case of a 

 moving source. This is the reason why the examination of 

 the latter must not be made either with prisms, as in the 

 arrangement of Belopolski, or with diffraction-gratings, as I 

 have before explained. In making my preparations to set 

 up an apparatus with moving source, I resolved, from the 

 iirst, to examine the latter with the interference method 

 already described by me, which is founded on the use of the 



* Communicated by the Author. 



t Phil. Mag. Feb. 1918, p. 163; on the same argument see also the 

 papers of Michelson, Astrophysical Journal, xlii. p. 19 (1913), and of 

 Fabry and Buisson, C. It. clviii. p. 1438 (1914). These works, of which 

 1 heard only lately, arrive in different ways at the same conclusions. 



Phil. Afaq. S. 6. Vol. 37. No. 217. Jan. 19W. L 



