On the colours of the striæ in mica, and the radiation from laminar diffracting boundaries

The phenomenon which forms the subject of this paper was briefly described in a preliminary communication in ‘Nature,' made jointly by Prof. C. V. Raman and the author of this paper, accompanied by a short but suggestive note by Lord Rayleigh. It is observed when a clear sheet of transparent mica is placed in front of a good achromatic lens, and examined by the well-known Foucault “knife-edge” test, otherwise referred to also as the Topler “Schlieren” method. The arrangement is shown in fig. 1. White light from a slit, S (illuminated by a half-watt incandescent lamp of 100 C.P.), is incident upon the lens, L, which forms an achromatic image of the slit at some distance from the lens. A knife-edge, K, is put in the focal plane, and so placed that the light coming to a focus in the ordinary way is completely cut off. The telescope, T, with its objective just behind the plane of the knife-edge, is pointed towards the lens, and is focussed on the surface of the mica (M. M.) placed immediately in front of the lens.