monochromatic light will appear when these two images have been made to coincide with the help of the adjusting screws ss. The fringes in white light appear only when the lengths of the two paths AD and AC are the same. TheFIG. 40 width and the position of the fringes in the field of view can be varied by slightly moving the adjusting screws. We shall have occasion to discuss this particular form of interferometer in a subsequent lecture.

1. The objection to the wave theory of light, that light moves in straight lines while sound waves can bend around an obstacle, is shown to be groundless, since we have seen that if the sound waves are sufficiently short they cast a sound shadow, while by devices which take into account the