A very important consequence of the electromagnetic theory foreseen by
Maxwell is that the luminous waves which fall on a surface must
exercise on this surface a pressure equal to the radiant energy which
exists in the unit of volume of the surrounding space. M. Lebedeff a
few years ago allowed a sheaf of rays from an arc lamp to fall on a
deflection radiometer,[26] and thus succeeded in revealing the
existence of this pressure. Its value is sufficient, in the case of
matter of little density and finely divided, to reduce and even change
into repulsion the attractive action exercised on bodies by the sun.
This is a fact formerly conjectured by Faye, and must certainly play a
great part in the deformation of the heads of comets.
[Footnote 26: By this M. Poincare appears to mean a radiometer in
which the vanes are not entirely free to move as in the radiometer of
Crookes but are suspended by one or two threads as in the instrument
devised by Professor Poynting.--ED.]
More recently, MM. Nichols and Hull have undertaken experiments on
this point.
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