146 Prof. Q. Majorana : Experimental Demonstration of 



Michelson interferometer, with a great difference in path. 

 Admitting the second postulate of the theory of relativity, or 

 if, in any way, the velocity of propagation of light by earthly 

 sources seems to us unchangeable, supposing such an appa- 

 ratus were realized, a certain number /of fringes would be 

 seen to cross the wire of the eyepiece, when the source 

 passes from rest to velocity v, 



where I is the difference of path of the interfering rays, \ the 

 length of the wave, and c the velocity of light. This is 

 analogous to my affirmation previously made with regard 

 to moving mirrors. Now, the values which it is hoped may 

 be attained in a laboratory for v are rather modest, even with 

 regard to those of astronomical luminous sources ; it is 

 therefore necessary to give to I the highest possible value,, 

 which is only to be obtained by using a source of light with 

 a very long visibility curve. 



This can only be the green line of mercury, which, however, 

 being more complex in its structure than the lines of cadmium, 

 allows the observation of the fringes up to the value of 

 Z = 32cm.*, using excitation of vapours by means of a voltaic 

 arc in vacuum. Besides, this source is particularly suited for 

 the present researches because of its exceptional intensity. 

 I believe, therefore, that they would hardly be repeated, using 

 a different source. 



I established anew plan of experiments, intending to endow 

 with swift rotatory movement some mercurial arcs held by 

 airless glass tubes, and to examine by means of the Michelson 

 interferometer the light emitted by them tangentially to the 

 trajectory line. Now in the attaining of a peripheral velocity 

 of nearly 100 m. per second, this being the necessary velocity 

 for a sure appreciation of a displacement in the fringes, two 

 principal mechanical difficulties are found : the enormous- 

 centrifugal force, and the very great resistance of air. To 

 diminish the first, it is convenient to enlarge as much as 

 possible the diameter of the trajectory and lessen the number 



* It must be noted that Michelson observed fringes up to 1=4:0 cm. 

 But that scientist used Geissler tubes with mercury vapour ; it seems 

 that the excitation with the voltaic arc in vacuum, used by me, changes- 

 the visibilitv curve. 



