Constancy of Velocity of Light emitted by Moving Source. 149 



# = 79*77 m./sec; I = 232 mm., \= 0'546 fi; so that a displace- 

 ment is foreseen of 



232 79-77 

 ?= .;!V 10- 2 = 0-113, and 2/= 0-226. 

 J 546'<5 



Experimentally, I have been able to augment considerably 

 the exactitude o£ observation of the fringes; and this because 

 of the higher luminosity of the phenomena. The circular 

 fringes follow each other as is known, with increasing 

 diameter according to the law : 



I / a 2 \ 



n=s x( 1_ ?> 



in which I and A have the usual meaning, n is the increasing 

 ordinal number of the fringes, beginning at the centre, and 

 a is their radius measured in visual angle from the eye of 

 the observer. So that when a displacement of a fringe is 

 observed, for obtaining a higher precision it is necessary to 

 keep in mind that parabolical law referring to the value 

 of the length of the wave. This is what I have done, 

 studying previously the distribution of the above-mentioned 

 fringes in the field of the telescope. Want of space does not 

 allow me to explain this more at length. I may only say 

 that in the series of observations quoted, I obtained an average 

 of displacement 



2/=0-238. 



As we see, this value is somewhat superior to that expected, 

 about 5 per cent. Until now, although I took the greatest 

 care to keep in mind the precision of the different measures 

 which are necessary to arrive at this result, I do not know 

 if any systematic error is the reason for this small difference. 

 Certainly it appears superior to the probable error of the 

 result ; and that is why I have mentioned it. But in con- 

 sideration of the delicacy of the measures I do not register 

 the value of the displacement of the fringes before admitting 

 the above-mentioned discordancy (however slight). For now 

 we may conclude that, under the conditions of the experiment, 

 and within the limits of exactitude of the observations made, 

 the velocity of light does not change by the movement of the source 

 along the direction of propagation. 



From the researches made by Michelson, Fabry and 

 Buisson, and by myself, it results that the velocity of light 



