Birefringence of Viscacelle

Abstract

DR. N. H. HARTSHORNE, of University College, Swansea, writes: “With reference to my letter on the birefringence of ‘Viscacelle’ in the issue of NATURE of February 16 (p. 269), my attention has been directed to a note on the birefringence of ‘Cellophane’ contributed by Mr. Arthur M. Grundy to Watson's Microscope Record in 1931 (Sept., p. 22). The name ‘Cellophane’ is commonly, though mistakenly, used as a general term for artificial cellulose sheet, and as Mr. Grundy did not state the source of his material, I am uncertain as to its exact nature. It was, however, doubtless very similar to, if not identical with ‘Viscacelle’. He noted the uniformity of its optical character, and the fact that specimens of different thicknesses gave appropriately different polarisation colours, but he gave no data for the relative retardations associated with different thicknesses. He expressed the view that the optical uniformity of the material ruled out stress as a cause of its double refraction. As regards ‘Viscacelle’, I do not think that the double refraction is due in any degree to stress in the finished material, but, as I indicated in my letter, the stress involved in the spinning process (that is while the material is being drawn from the spinning slot into the coagulating bath) almost certainly causes an orientation of the cellulose molecules, and this accounts for the observed direction of ‘slow’ vibration. I regret that I did not see Mr. Grundy's note earlier, and I gladly acknowledge his prior discovery of the double refraction of artificial cellulose sheet.”