570 



Professor Oliver Lodge 



[April 1, 



A quicker stratum bends it oppositely. If a medium is both denser 

 and quicker moving, it is possible for the two bendiags to be equa 

 and opposite, and thus for a ray to go on straight. Parenthetically 

 I may say that this is precisely what happens, on Fresnel's theory, 

 down the axis of a water-filled telescope exposed to the general 

 terrestrial ether drift. 



In a moving medium waves do not advance in their normal direction, 

 they advance slantways. The direction of their advance is properly 

 called a ray. The ray does not coincide with the wave-normal in a 

 moving medium. 



All this is well-shown in Fig. 5. 



Fig. 5. 



Successive Wave Fronts in MoviDg Medium. 



S is a stationary source emitting successive waves, which drift as 

 spheres to the right. The wave which has reached M has its centre 

 at C, and C M is its normal ; but the disturbance, M, has really 

 travelled along S M, which is therefore the ray. It has advanced as 

 a wave from § to P, and has drifted from P to M. Disturbances 

 subsequently emitted are found along the ray, precisely as in Fig. 2. 

 A stationary telescope receiving the light will point straight at S. 

 A mirror, M, intended to reflect the light straight back must be 

 set normal to the ray, not tangential to the wave front. 



The diagram also equally represents the case of a moving source 

 in a stationary medium. The source, starting at C, has moved to S, 

 emitting waves as it went, v^hich waves as emitted spread out as 

 simple spheres from the then position of source as centre. Wave- 

 normal and ray now coincide : S M is not a ray, but only the locus of 



