"Relativity principles require us to associate mass with the energy of radiation, and it is reasonable to suppose... an exchange of ... [T]he exchange of momentum between free electrons and radiation is very similar to the exchange... when two particles collide. ...[A] beam of light should be considered as an assembly of "units", each or which [using light (\nu), (h), speed of light (c)] possesses energy (W), momentum (p), and mass (m), given byW = h\nu; \; p = \frac{h\nu}{c}; \; m = \frac{h\nu}{c^2}. \quad 17(14)...This general picture was first suggested by Einstein... The units are now called photons... [T]he spreading of light by diffraction cannot be permanently concentrated in a small volume like the energy of a material particle. ...The pressure p, exerted by a parallel beam incident normally on a body which completely absorbs it, is...p = \rho_p, \quad ...17(15) where \rho_p is the energy per unit volume of the incident radiation. ...[C]onsider the radiation pressure of a parallel beam of light, incident on an absorbing body... the light is of frequency \nu and... there are N quanta per unit volume. Then...\rho_p = Nh\nu. \quad ...17(18)[A]ll the quanta in a cylinder of volume c [speed of light multiplied by unit area] cubic centimetres are incident upon unit area of the surface in one second, the pressure...p = NcP, \quad ...17(19)where P is the momentum of one photon. Combining...P = \frac{h\nu}{c} = \frac{h}{\lambda}.[Experimental] results... for isotropic radiation are in agreement..."