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.

They have measured not only the pressure, but also the energy of the radiation by means of a special bolometer.

They have thus arrived at numerical verifications which are entirely in conformity with the calculations of Maxwell.
The existence of these pressures may be otherwise foreseen even apart from the electromagnetic theory, by adding to the theory of undulations the principles of thermodynamics.