XLIV. New contributions to interferometry.—Part VIII. Interferometric examination of small local irregularities in mica

Abstract

Abstract Two local irregularities in thin mica slips are studied by multiple beam interference methods using (a) monochromatic Fizeau fringes (b) new white-light fringes called “fringes of equal chromatic order.” The Fizeau fringe technique formerly employed has been developed so that high magnification (80 times) can be employed. The features studied extend over less than 1 millimetre. The fringes of equal chromatic order prove to be a powerful weapon for the interpretation of the features observed. One irregularity consists of a small surface hillock adjacent to a complex occlusion pocket. The hillock is interpreted as being a growth nucleus, and is shown to be built up in discontinuous steps. The hillock is about 2500 A.U. high and 34 steps have been measured in it. Of these, 56 per cent, are less than 3 1/2 “molecules” and 85 per cent. less than 7 1/2 “molecules” in height. The second irregularity appears as a hillock flanked by two valleys. This is considered to be due to the inclusion of a small transparent foreign solid body during the laying down of the mica. The observed valleys are optical, arising from vacuoles associated with the foreign body. Two closely related possible mechanisms for the lay down of the mica sheets over the foreign body are proposed. The definition obtained with the fringes representing this second feature supersedes all earlier observations.