PKESrDENTlAL ADDRESS SECTION A. 45 



i^reater density of sther in its material). In definite figures 

 this would require the velocity of light in the i)rism to be in- 

 creased by V (1 M owing to the prism's velocity z' in the 



direction of the light. 



Exp. 2. — In 1 85 1, Fizeau tested Fresnel's explanation in 

 the following way. He passed part of a ray of light through one 

 of two parallel tubes of water; then reflected it back through the 

 other tube: another part of the ray was made to follow the 

 opposite course. Interference effects were produced. 

 He now gave the water in the tu'bes a velocity v, so 

 as to be against the one part of the ray and with the other. He 

 found a distinct change in the interference effects ; and hii 

 measurements showed that the effect of the velocity z' on the 

 velocity of the light in water agreed exactly with Fresnel's 



theoretical result, z' ( i -)■ 



This result of Fizeau's, with the discovery of the alteration 

 of light by Bradley, seemed to ])oint definitely to the theory 

 of the c'ether iiot moving: matter must UKne through if and if 

 through uiaffcr, freely. 



Exp. 3. — But in 1887, Michel>en and (with refinements) 

 Morley designed an experiment which should show the velocit}^ 

 of the earth relation to the ?ether to the second order of the small 

 fraction z'/( velocity of light) (Fizeau's would only work to the 

 first order). By means of moving mirrors from which a ray of 

 light is reflected, a velocity of the earth relative to the sether 

 should betray itself in interference elTtects. if it is as great as 

 one-tenth of the earth's velocity in its orbit. The result of the 

 experiment was " no trace of any such effect " ; and repetition of 

 the experiment with modified ajiijarattis and greater refinement 

 in 1005 confirmed this "result. 



Tluis the (unpalatable) fact of no relative motion of aether 

 and earth seemed to be established. Flowever, an alternative 

 explanation was (as usual) f(M-thcoming : an Irish physicist, 

 Fitzgerald, pointed out that the null result might implv (not a 

 stagnant ccther but ) a physical change in the apparatus due to the 

 motion : a contraction in the ratio V (i — v^ /c-)* in the direction 

 of motion would do. The conditions of the experiment showed 

 that this efl:'ect must be the same for all materials. In_ 189c;, 

 H. A. Lorentz. of Leyden (who, with our Larmor and Einstein 

 of Leipzig, is in the forefront of modern discussions of aether 

 and matter") hit independently on Fitzgerald's idea. He (and 

 Larmor subsequently to second order effects) working on the 

 electron theorv. of which Maxwell's electro-magnetic equations 

 are an accepted part, established the following results: (i.) If 

 the equations are satisfied by one system of electric charges at 

 rest in the ^ther, they are enually well satisfied bv a second 

 system of electric changes all moving with velocity v, pro- 



*r = velocity of body, f = velocity of light. 



