446 



secoiKl tei-ni of formula (1). The velocity wliieh we are able to 



obtain ill a eoluniu of water transmitting light is of the oixler of 



magnitude of 5 metres per seooud. We have thus to lind a difference 



3 10« 

 of velocity of 5 metres in -jirr ^"•' ^- ®- ^^ one part in tifty millions. 



This was done by Fizkau ^) in one of the most ingenious experi- 

 ments of the whole domain of physics. Fizkau divided a beam of 

 light issuing from a line of light in the focus of an object-glass 

 into two parallel beams. After traversing two parallel tubes these 

 beams pass through a second lens, in the focus of which a silvered 

 mirror is placed. After reflection the rays ai-e returned to the object 

 glass, interchanging their paths. Each ray thus [)asses through the 

 two tubes. A systeui of interference fringes is foi-med in the focus 

 of the first lens. If water is flowing in ojjposite directions in the 

 two tubes, one of the interfering beams is always travelling with the 

 current and the other against it. When the water is put in motion 

 a shift of the centi-al white band is observed: by reversing the 

 direction of the current the shift is doubled. 



The ingenuity of the arrangement lies in the possibility of securing 

 that the two beams tra\'erse identical ways in opposite directions. 

 Every change due for example to a variation of density or of tem- 

 perature of the moving medium equally influences the two beams 

 and is therefore automatically compensated. 



One can be sure that a shift of the system of interference fringes, 

 observed when reversing the direction of the current must be due 

 to a change of the velocity of propagation of the light. 



The tubes used by Fizeau had a length of about 1,5 metres and 

 an internal diameter of 5,3 m.m., whereas the velocity of the water 

 was estimated at 7 metres. With white light the shift of the central 

 band of the system of interference fringes observed by reversing the 

 direction of flow was found from 19 rather concordant observations 

 equal to 0,46 of the distance of two fringes; the value calculated 

 witfj Fresnel's coefficient is 0,404. 



The result is favouiable to the theory of Fresnel. The amount 

 of the shift is less than would correspond to the full velocity of 



the water and also agrees numerically with a coefficient 1— — , if 

 the uncertainty of the observations is taken into account. 



') H. Fizeau. Sur les hypotheses relatives a l'éther lumineux et sur unè experience 

 qui parait démontrer que le mouvement des corps change la vitesse avec laquelle 

 la lumiore se propage dans leur intérieur. Ann. de Chim. et de Phys. (3) 57 

 385. 1859. 



