Are we then in presence of two mechanical explanations,
different indeed, but nevertheless both adaptable to all the facts,
and between which it will always be impossible to make a choice? Or,
on the contrary, shall we succeed in realising an _experimentum
crucis_, an experiment at the point where the two theories cross,
which will definitely settle the question?
Professor Wiener thought he could draw from his experiment a firm
conclusion on the point in dispute. He produced stationary waves with
light polarized at an angle of 45 deg.,[22] and established that, when
light is polarized in the plane of incidence, the fringes persist; but
that, on the other hand, they disappear when the light is polarized
perpendicularly to this plane. If it be admitted that a photographic
impression results from the active force of the vibratory movement of
the ether, the question is, in fact, completely elucidated, and the
discrepancy is abolished in Fresnel's favour.
[Footnote 22: That is to say, he reflected the beam of polarized light
by a mirror placed at that angle.
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