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A general treatment of electro-and magneto-optical phenomena in the presence of resonant absorption is given. The resulting expressions in terms of the real and imaginary parts of a matrix index of refraction are easily utilized in calculations. We consider in detail the effects of these phenomena in Mossbauer experiments with polarized y rays, presenting expressions for absorption as a function of polarization as well as for the polarization of the transmitted beam, which are valid for arbitrarily thick absorbers. Comparisons

The recoil-free emission and resonant absorption 1 of the 14.4-kev nuclear gamma ray of Fe 57 , has been used to determine the quadrupole coupling for the 3/2-excited state of Fe 57 bound in Fe 2 0 3 , and to measure an energy shift of this nuclear gamma ray which is attributed to effects of chemical binding. This effect is corollary to the effects of chemical environment on internal conversion coefficients 2 and on electron capture disintegration rates. 3 These measurements also yield the value of the internal magnetic field at the position of the Fe 57 nucleus when it is bound in antiferromagnetic Fe 2 0 3 .The Ml emission line of Fe 57 bound in ordinary metallic iron is split into six components by the magnetic hyperfine interaction. 4 The resonant absorption of this emission spectrum by Fe 57 bound in Fe 2 O s has been examined, as well as the much simpler absorption pattern which results when the "unsplit" emission line from Fe 57 bound in a stainless steel lattice 5 is used. Because the interpretation of the two sets of measurements agree, only the latter measurements are presented in this Letter. The former measurements, in which a Co 57 source co-plated with iron onto 1-mil copper was used, will only briefly be remarked upon.The ~2-mg/cm 2 Fe 2 0 3 absorber used in these measurements contained Fe 57 enriched to -30%. The source consisted of Co 57 plated onto 0.001-inch stainless steel (25% Cr, 20% Ni). After plating, this source was annealed for one hour at 900°C in a hydrogen atmosphere. All measurements were made with source and absorber at a temperature of 25°C.In order to obtain a Doppler shift of the emission line, a uniform motion was obtained by coupling a pneumatically driven cylinder to another cylinder filled with oil, the ports of which were connected via a needle valve. A wide range of uniform velocities could be selected by adjusting this valve. The direction of source travel was reversed automatically by means of microswitches. Additional microswitches, set to exclude the region of nonuniform motion near the travel limits, were used to provide gate signals for the counters. The distance of travel between the limits of the counting gates was 0.973 cm. Source velocity was determined by counting the cycles from a 1000 cps tuning fork oscillator during the time between the gate limits.The 14.4-kev gamma ray was detected with a Nal(Tl) scintillation counter. The phosphor was 2 mm thick and 1.5 inches in diameter. The counter face was located 5.4 cm above the upper limit of vertical travel of the source. The absorber was placed 4.7 cm from the counter face. A single-channel pulse-height analyzer selected the 14.4-kev gamma ray photopeak. The outputs of this analyzer and the 1000 cps clock were switched between two pairs of scalers so as to record counting rate and velocity separately for both directions of source motion. Figure 1 shows the counting rate (in arbitrary units) as a function of source velocity relative to the Fe 2 0 3 absorber for the stainless steel source. Absorption of the "unsplit...

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