White Light Interference Fringes with a Thick Glass Plate in One Path

The object of this paper is to describe and account for certain interference phenomena which are observed with white light, when a thick plate of glass, or other dispersive medium, is placed in the path of one of the beams in the Michelson interferometer. The mirrors are adjusted in sodium light for circular fringes, and the paths made equal. The glass plate is then inserted in one of the paths, and that path is shortened, or the other path lengthened, by moving the mirror, till the sharpest sodium maximum is reached. The path containing the glass plate will be shorter than the other by about one-half the thickness of the plate. If white light is now substituted, the fringes should be in the field. It is better before throwing on the white light, to re-adjust the mirrors for perfect parallelism of the two beams, which may be known by the almost startling distinctness of the sodium fringes in this position. It will be easier if the first trials are made with a plate not more than 5 mm. thick