95 years before Special Relativity was born, Arago attempted to detect the
absolute motion of the Earth by measuring the deflection of starlight passing
through a prism fixed to the Earth. The null result of this experiment gave
rise to the Fresnel's hypothesis of an ether partly dragged by a moving
substance. In the context of Einstein's Relativity, the sole frame which is
privileged in Arago's experiment is the proper frame of the prism, and the null
result only says that Snell's law is valid in that frame. We revisit the
history of this premature first evidence against the ether theory and calculate
the Fresnel's dragging coefficient by applying the Huygens' construction in the
frame of the prism. We expose the dissimilar treatment received by the ray and
the wave front as an unavoidable consequence of the classical notions of space
and time.Comment: 16 pages. To appear in European Journal of Physic