The eigenelectromagnetic modes characterizing a uniaxial and dichroic crystal exhibiting an isotropic point for refractive indices and submitted to a natural (optical activity) or artificial perturbation are theoretically investigated. It is demonstrated that these modes are two elliptical and linearly independent vibrations whose respective axes are not perpendicular. Expressions of elliptical birefringence and elliptical dichroism are derived, and it is demonstrated that, for low perturbations, the linear dichroism of the unperturbed crystal cannot be neglected. The experimental study is performed at room temperature with a crystal of CdS submitted to a uniaxial pressure parallel to the  direction. The radiation transmitted by the crystal in the filter configuration and the transmission spectrum in light polarized parallel and perpendicular to the optical axis are analyzed, with the linear dichroism of the unperturbed sample taken into account. Finally, the value of the elasto-optic coefficient p44 at the isotropic point is reexamined.
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