470 ON METHODS TOR DETECTING SMALL OPTICAL RETARDATIONS [415 To examine the sudden transition from one uniform retardation to another, I used a piece of plate glass which had been etched in alternate strips with hydrofluoric acid to a depth of about |X*. When this was set up in front of the first aperture with strips vertical, the division-lines shone out brightly, when the intervening areas were uniformly dark or nearly so. No marked difference was seen between the alternate division-lines corresponding to opposite signs of p. Perhaps this could hardly be expected. The whole relative retardation, reckoned as a distance, is %\, and is thus intermediate between the values specified in Table IV. It would be of interest to make a similar experiment with a shallower etching. [1919. For further developments reference may be made to Banerji, Phil. Mag. .Vol. xxxvil. p. 112, 1919.] * Compare Nature, Vol. ixiv. p. 385 (1901); Scientific Papers, Vol. iv. pp. 546, 547.erely to integrate (36) as it stands. For although the denominators become zero when f = 0 or £ := cr, the four fractions themselves always remain finite. The line of transition between the two halves of the field is not so marked as when there was an actual discontinuity in the retardation itself.