Ocular haloes and coronas

Abstract

The author describes in detail three ocular diffraction phenomena and discusses the physics of their formation based on observations made on his own eyes. (a) It is confirmed that the coloured rings sometimes called the lenticular halo form a circular spectrum produced by the radial fibres of the crystalline lens, which become sufficiently uniform in size and regular in arrangement near to the periphery of the lens to act as an optical grating. The angular radius of the yellow ring (λ= 0.00057 mm.) is found to be 3.1° and the radii of the other coloured rings to be in linear proportion to their wavelength. The optical measurements give 8,μ as the width of the fibres which are optically active to within 1 or 1.5 mm. of the centre of the lens. Methods of differentiating the lenticular halo from other coloured rings are discussed. (b) Observations of the corona first described by Descartes in 1637 and usually seen within a few minutes of waking were made each morning during 5 months. This corona consists of a bright white central aureole surrounded by three broad coloured rings: the first red, the second green, and the third red; traces of further red and green rings could sometimes be seen. The central radii of the first three rings are 4.1°, 6.0°, and 8.6° respectively; from this it is deduced that the corona is caused by particles with diameter of 8,μ. Experiments show that these particles are present on the anterior surface of the cornea; they cannot be removed by blinking or rubbing but are immediately dissolved in a bath of distilled water or saline solution. The nature of the particles is discussed without any conclusion being reached. The name "Descartes corona" is suggested for these rings. (c) The glow seen around bright lights is called the ciliary corona because it is frequently seen to be composed of bright fibre-like rays of light. It is shown that this corona is also a diffraction effect, the rays becoming visible when the source of light subtends an angle of at least 19′, i.e., when the distance of the light from the observer is greater than 180 times its diameter. The diffraction is due to particles of uniform size (diameter less than 10 μ) somewhere in the body of the eye. The nature of these particles and their exact location in the eye are as yet undetermined.