Methods of Improving Visibility

Abstract

THE observations of Prof. C. V. Raman (NATURE, October 20, p. 242) on a method of improving the visibility of distant objects by the elimination of reflected light by means of a Nicol's prism placed in the eye-piece of a telescope are certainly interesting, but at the same time they are thoroughly well known, and the idea of increasing the visibility by the elimination of polarised light has received a great deal of attention lately. During the war a considerable amount of experimental work was carried out by the Admiralty in connection with fog-penetration and the beamless searchlight, both of which dealt with polarisation phenomena. It was, however, found advisable to substitute for the Nicol's prism as used by Prof. C. V. Raman a few plates of plain glass placed obliquelv at a suitable angle across the axis of the particular instrument, since it is extremely difficult to make really large Nicol's prisms, in addition to which the actual absorption by Nicol's prisms is rather excessive. The matter was also taken uo by the late Sir William Crookes, and formed the basis for lenses cut from quartz crvstals at right angles to the principal axis, thus utilising the rotary polarising effect of this material, I have lately been using tourmaline for obtaining a similar effect, and I have found that a very thin plate of tourmaline cemented as a semi-lens on to an ordinary pair of spectacles is best for this purpose.