DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
7.S1 
Ignoring’ this possible finite speed of affection of ether by matter, unless circum¬ 
stances make us revert to it, the question faces us, what is meant by the travelling 
of the modified ether ? 
It is not a question easy to state without some looseness of language, but we 
may ask ; 
(a) Does it mean that the identical stuff inside the matter travels from one place 
to the other ? If so, the free ether which it has displaced must stream back 
round the body in the same way as a material fluid would have to do, 
[b) Or does it mean that no ether travels at all, that the mere presence of the 
matter causes the modification wherever it is, so that it is only the modifi¬ 
cation or affection whicb travels? If so, the ether abandoned by the matter » 
becomes free in situ, while the ether encroached on by the matter becomes 
modified in situ, and there is no question as to its motion. 
On hypothesis (b) the whole ether is fixed and imperturbable by tlie motion of 
matter. The portion enshrouded by matter at any instant has properties differing 
from those of free ether, but the modification is only connected with the matter 
causing it in the same sort of way as a shadow is connected with the object casting it. 
Of the two hypotheses, there can be no question but that the second is the simpler 
and considered as a hypothesis is preferable, but we must enquire whether it is 
competent to sustain the weight of all known facts, 
Fresnel’s Hypothesis. 
4. It is notorious that the hypothesis at present holding the field is not exactly 
either of these, but is some form of the bold and picturesque idea of Fresnel; viz., 
that in addition to the free and undisturbed ether of space existing equally everywhere 
and flowing through the pores of gross matter, there is an extra quantity of 
bound ether fixed to the matter and travelling with it; this additional quantity being 
(1 — of the whole. 
This idea of Fresnel’s seems, at first sight, essentially to involve the condensation of ether by matter, 
so that its density inside bodies is ; for the fixed ether is superposed upon the normal ether of 
space. (Certainly the converse is true ; viz., that extra ethereal density involves Fresnel’s law, as will 
be shortly shown.) 
Now the facts of gravitation, and many electrostatic experiments, suggest that the ether is practically 
incompressible ; hence the notion of any actual increase of densityAnside gross matter is repugnant, 
Fresnel, hoAvever, himself pointed out, in a subsequently written postscript to bis original letter 
to Akago* promulgating his famous hypothesis, that the extra density need not be taken too literally. 
(As this postscript seems rather to have been overlooked it may be worth while to quote it). 
“ Note additionnelle d la lettre de M. Fresnel d M. Arago, inseree dans le dernier Cahier des Annales. 
En calculant la refraction de la lumiere dans un prisme entraine mouvement terrestre, j’ai 
suppose, pour simplifier les raisonneraents, que la difference entre les vitesses de la lumiere dans le prisme 
* ‘ Ann de Chirn. et de Phys.’ (2), vol. 9, p. 56. 
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