Exciton absorption occurs, for weak exciton lines, at an energy which is the energy of an exciton having a wave vector equal to that of the light in the medium. These excitons have a finite wave vector, and therefore, a finite velocity. In a uniform magnetic field, the Lorentz force on the electron and hole due to the center-of-mass velocity produces a magnetic perturbation in addition to those ordinarily considered. The measurement of such a perturbation measures the velocity of an exciton of known wave vector, and therefore determines the total exciton mass. In addition, the measurement of this effect which depends on the exciton velocity provides a positive distinction between exciton absorption lines and absorption lines due to impurities. It is shown that this perturbation can be measured by the measurement of the Stark effect on excitons in the presence of a uniform magnetic field. The exciton mass for the $n=2\mathrm{}$ states of excitons formed from the top valence band in CdS was measured by this technique, and found to be 0.92±0.18 in reasonable agreement with the mass calculated from independent experiments. The Stark effect in the absence of a magnetic field was also studied to ensure an understanding of the effect in the presence of a magnetic field. The stark effect in a magnetic field sometimes exhibits peculiar behavior which was attributed to an extraneous Hall field. This interpretation gives an estimate of ${\omega }_c{\tau }_r\approx 2$ for electrons in "good" CdS crystals at 1.6°K and at 31 000 gauss.

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