732 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
et dans I’etlier envifoniiant proveuait uniquement d’une difference de densite, I’elasticite etant la meme 
de part et d’autre ; mais il est tres possible que les deux milieux diffm’ent en elasticity comme en 
densite. On con 9 oit meme que Telasticite d’un corps solide peut varier avec le sens suivant lequel on 
le considere ; et c’est tres probablemeut ce qui occasionne la double refraction, comme Ta observe le Dr. 
Young. Mais quelle que soit riiypotbese que Ton fasse sur les causes du ralentissement de la marolie de 
lumiere dans les corps transpareuts, on peut toujours, pour resoudre le probleme qui m’etait propose, 
substituer jiar la pensee, au milieu reel du prisme, un fluide elastique en equilibre de tension avec I’etber 
envirounant, et d’une densite telle que la vitesse de la lumiere soit precisement la meme dans ce flaide et 
dans le prisme supposes en repos ; cette egalite devra subsister encore dans les deux milieux entraines par 
le mouvement terrestre: or, telles sout les bases sur lesquelles repose moii calcul.”—(‘Ann. de Chim. et 
de Phys.,’ 1818, t. 9, p. 128 or 286.) 
And Mr. Glazebeook (‘ Phil. Mag.,’ December, 1888) shows that in the interaction of ether 
and matter, a term depending on relative acceleration is sufficient to sustain the results achieved by 
Ekesnel’s hypothesis. In other words, that a virtual density, or loading of the ether by matter, is quite 
enough without true condensation. 
It is, however, still appropriate to speak of the exti-a ethereal density inside matter; meaning 
the coefficient of this acceleration term. 
5. A plausible mode of exhibiting the naturalness of Fresnel’s law is as follows :— 
The constant which determines the speed of electromagnetic waves through any 
medium is /rK ; by the differential equation to wave motion. 
In a dense body the value of this constant is fi'K'. 
Shift a lump of this body fi’om one jtlace to another. Its constant p'K' has been 
shifted in position too, but the ordinary space-value p-K remains behind; so the 
I’esultant shift of the property determining the velocity of light (the effective medium) 
is a fraction (p'K’ — pK)/p'K', of the shift of the body. 
So, if the lump moves with velocity the property of it concerned with the 
velocity of light shifts with velocity (p'K'— pK)/(p'K').w ; that is with speed 
(1 — l/«^)w. 
And, as in all probability the velocity of wave motion relative to its medium is 
unaltered, this may be taken as the extra speed of the light caused by the motion of 
the matter. 
6. It is here assumed that the medium simply carries the wave motion with it as 
air carries sound. It is not customary to doubt that wave motion must be aftected 
by any motion of its medium in that simple manner. But a singular investigation by 
Professor J. J. Thomson (‘Phil. Mag.,’ April, 1880 ) seems to show that on electro¬ 
magnetic principles the speed of ether waves is affected with only half the velocity of 
the medium conveying them. 
This extraordinary result is not at present positively contradicted by the Fizeau 
experiment, even as repeated by Michelson, because the value of 1 — 1 /n~ for water 
is not sufficiently different from ^ to afford a certain criterion ; and water is the only 
substance for which a positive result has as yet been obtained. Certainly the 
negative result obtained for air by both Fizeau and Michelson is in accord with 
Fresnel’s theory and not in accord with J. J. Thomson’s. But a definition of what 
