To the Editor:
In reviewing the literature on corneal birefringence, I was intrigued by the observations of Cogan1 on Haidinger's brushes, particularly the opposite rotation of the brushes when cellophane was interposed. Subsequent work on the phenomena by Stanworth and Naylor2 provides beautiful corroboration of the dichroic macular theory of Helmholtz.3 This theory explains the entoptic observation of Haidinger's brushes as the result of dichroic elements, having planes of polarization oriented with axial symmetry, within the macular region. Cogan's observations, too, fit this theory.A dichroic filter of this sort can be modeled readily using a Polaroid plane polarizing filter. The Polaroid filter itself consists of a multitude of uniformly oriented dichroic elements. The polarizer is cut into many narrow isosceles triangles or sectors, with the short base of each triangle parallel to the polarizing axis. These sectors are positioned side by side such that the sharp
Post D. BIREFRINGENCE OF THE CORNEA. Arch Ophthalmol. 1965;74(1):144–145. doi:10.1001/archopht.1965.00970040146037
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