[FOURTH SERIES.]
APRIL 1860.
MANY theories have been proposed with aview of accounting for the phænomenon of the aberration of lightaccording to the undulatory theory. In the first instance Fresnel, andmore recently Doppler, Stokes, Challis, and several others havepublished important researches on this subject; though none of thetheories hitherto proposed appear to have received the complete approvalof physicists. Of the several hypotheses which have been necessitated bythe absence of any definite idea of the properties of luminiferousæther, and of its relations to ponderable matter, not one can beconsidered as established; they merely possess different degrees ofprobability.
On the whole these hypotheses may be reduced to the following three,having reference to the state in which the æther ought to be consideredas existing in the interior of a transparent body. Either, first,the æther adheres or is fixed to the molecules of the body, andconsequently shares all the motions of the body; or secondly, theæther is free and independent, and consequently is not carried with thebody in its movements; or, thirdly, only a portion of the ætheris free, the rest being fixed to the molecules of the body and, alone,sharing its movements.
The last hypothesis was proposed by Fresnel, in order at once to satisfythe conditions of the aberration of light and of a celebrated experimentof Arago's, which proved that the motion of the earth does not affectthe value of the refraction suffered by the light of a star on passingthrough a prism. Although these two phænomena may be explained withadmirable precision by means of this hypothesis, still it is far frombeing considered at present as an established truth, and the relationsbetween æther and matter are still considered, by most, as unknown. Themechanical conception of Fresnel has been regarded by some as tooextraordinary to be admitted without direct proofs; others consider thatthe observed phænomena may also be satisfied by one of the otherhypotheses; and others, again, hold that certain consequences of thehypothesis in question are at variance with experiment.
The following considerations led me to attempt an experiment the resultof which promised, I thought, to throw light on the question.
It will be observed that, according to the first hypothesis, thevelocity with which light traverses a body must vary with the motion ofthat body. If the motions of the body and the ray are like-directed, thevelocity of light ought to be increased by the whole velocity of thebody.
If the æther be perfectly free, the velocity of light ought not to bealtered by the motion of the body.
Lastly, if the body when moving only carries with it a portion of theæther, then the velocity of light ought to be increased by a fractionalpart of the velocity of the body and not by the whole velocity, as inthe first case. This consequence is not as evident as the two precedingones, though Fresnel has shown that it is supported by mechanicalconsiderations of a very probable nature.
The question then resolves itself to that