Prisms and Plates for Showing Dichromatism

Abstract

DICHROMATISM, or the change of colour of an absorbing medium with increasing thickness, is usually shown with plates of coloured glass. It is not always easy to obtain the right kind of glass, and only a few of the aniline dyes are suitable for the purpose. The medium should transmit two distinct regions of the spectrum, the absorption coefficient for one being greater than for the other. I have found that it is better to give the medium the form of a prism, for then the transmitted colours are separated, and the more rapid fading of one as the eye is moved from the refracting edge to the base can be followed. A number of years ago I found a small amount of an unlabeled dye which transmitted a red band and a green band, that is, it had a strong absorption band in the yellow and the blue. Thin layers of this dye were bright green, thick layers were blood red. I have never been able to find the dye again, though I have examined a large number of dyes, but I have found that a mixture of commercial “brilliant green” with a little naphthol yellow has identical optical properties. Brilliant green alone in thin layers is blue rather than green, and though it shows dichromatism, the change from blue to red is not nearly so striking as a change from green to red. The prisms can be made in the following way.