the actual experiment, however, they are all superposed. At the middle point, where the two paths are equal, all theFIG. 29 colors will be superposed, the result being a white central band. At no other point will this be true, and the result will be a series of 1 colored fringes symmetrically disposed about the central white fringe, the succession of colors being exactly the same as in the case of thin films (cf. Plate II).

The breadth of the fringes is determined by the smallness of the angle under which the two pencils meet. This is shown in Fig. 30. In the right-hand figure the angle between the pencils is smaller than in the other, while the breadth of the fringes is correspondingly greater in the former than in the latter. The exactFIG. 30 relation is readily obtained. We have only to note that ac is the wave length l (very nearly) and bc is (very nearly) the width b of a fringe; whence, if c is the very minute angle