A Curious Optical Effect

Abstract

A FORTNIGHT ago, while standing with my back to the sun, which for a few minutes happened to be shining brightly, and with my face within a few inches of some darkly painted boards, which were covered with minute sparkling particles, presumably from an adjacent coke-grinding machine, I noticed that on approaching my face a little closer, the particles became iridescent and apparently magnified to a size of about one-eighth of an inch. On closing one eye and looking closer, concentric circles appeared, with a cross × over them, and in some cases there was a smaller circle just touching the inner margin of the larger one; in others the small circle seemed to be nearer the centre of the larger one. On a subsequent examination, when the sun was not so bright, the concentric circles seemed to be wavy and indistinct, as was also the case with the cross. The whole thing reminded me of illustrations I have seen of effects produced by tourmalines under certain conditions.