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The first page reads: APPARENT WEIGHT OF PHOTONS R. V. Pound and G. A. Rebka, Jr. Lyman Laboratory ofPhysics, Harvard University, Cambridge, Massachusetts (Received March 9, 1960) NUMBER 7 As we proposed a few months ago,' we have now measured the effect, originally hypothesized by Einstein, of gravitational potential on the ap- parent frequency of electromagnetic radiation by using the sharply defined energy of recoil-free y rays emitted and absorbed in solids, as discovered by Mössbauer. We have already reported a detailed study of the shape and width of the line obtained at room temperature for the 14.4-kev, 0.1-microsecond level in Fes. Particular attention was paid to finding the conditions required to obtain a narrow line. We found that the line had a Lorentzian shape with a fractional full-width at half-height of 1.13 x10-12 when the source was carefully prepared according to a prescription developed from experience.