Thin sections of the ionic ferromagnet CrBr3 are transparent to visible light. Below the Curie point, in the demagnetized condition, these sections possess a fine‐grained domain structure in which the characteristic distance is comparable to the wavelength of visible light. We find that near 5000 Å the optical properties of this material are closely associated with the magnetic properties, and the modulation in magnetization direction which constitutes the domain structure has an associated modulation in the optical properties. This modulation constitutes a grating which gives rise to readily observed diffraction rings. Such a large fraction of the green light is diffracted that meaningful absorption measurements can only be made if the domains are removed by saturation. Spacings deduced from the size of the diffraction rings may be compared with domain theory, as well as used in the detailed interpretation of nuclear magnetic resonance results. The study of these rings constitutes a new technique for the investigation of domain structure.

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