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To the Editor.—
A 56-year-old physician, Donald R. Thomas, was able to see the opacities of his cataracts when he illuminated his eye at a distance of 15 to 20 cm from the oculars of a microscope while under high power. Several ophthalmologists did not explain this phenomenon.Its explanation is to be found in a rule of optics that states, "a ray of light that passes through the center of an optical system does not undergo refraction; i.e., the light ray does not change course but emerges in the same direction it entered." A central (axial) ray of illuminating light, as in Dr Thomas' case, qualifies for this effect.In this case, the illumination is essentially an axial ray of light that cannot be refracted (bent) by the optical system of the eye. Thus, any opacity within the pupil of the eye will simply cast its image as a
Delp JR. Visualization of One's Own Cataract. JAMA. 1983;249(9):1152. doi:10.1001/jama.1983.03330330034028