The question here proposed has been considered by Roiti and by Zecher. My experiments were made in ignorance of the work of these observers, and the results would scarcely be worth recording were it not that the examination seems to have been pushed further than hitherto. It may be well to say at once that the result is negative.

The interference fringes were produced by the method of Michelson as used in his important investigation respecting “The Influence of Motion of the Medium upon the Velocity of Light.” The incident ray ab meets a half-silvered surface at b, by which part of the light is reflected and part is transmitted. The reflected ray follows the course abcdefbg, being in all twice reflected in b. The transmitted ray takes the course abfedcbg, being twice transmitted at b. These rays having pursued identical paths are in a condition to form the centre of a system of fringes, however long and far apart may be the courses cd, ef.

There is here nothing to distinguish the ray ab from a neighbouring parallel ray. The incident plane wave-front perpendicular to ab gives rise eventually to two coincident wave-fronts perpendicular to bg. With a wave incident in another direction the case is different. The two emergent wavefronts remain, indeed, necessarily parallel, both having experienced an even number of reflexions (four and six).