4. Make a Fox's wedge. Split off as large a sheet of mica as can be obtained.^l Its thickness must be uniform, and as nearly as possible about 1/8 wave-length. This thickness will, when placed on the stage D, cause a dark field to become grey, but will give no colour. On the lower stage it should, of course, act as a 1/4-wave plate. (It is not absolutely necessary that it should be 1/8-wave.) Place it on the polariscope, and mark the direction in which it produces the most effect. Cut it into rectangles parallel to this direction, each rectangle being about 1/2 inch in one direction, and in the other direction 1/16, 1/8, 3/16, 1/4 inch, and soon. These rectangles are now to be mounted upon a microscope slide, one upon another to form a flight of steps (Fig. 372), with Canada balsam dissolved in benzol or xylol. Leave them until the balsam is set; and then cover them with an ordinary thin glass, with more balsam. Be careful not to displace them in so doing.

^l To split mica, insert a thin needle, perpendicular to an edge, about in the middle of one side, and then work it each way to split off the film so produced. By watching the Newton's rings which are seen where the films are separating, it is easy to see if the film is splitting property. Any discontinuity forming in these rings, indicates a change in the thickness of the film. Before cutting the film, it must be examined with a polariscope, and if the thickness is not uniform all over, a scratch must be made surrounding the part which is of one thickness, and the rectangles must all be cut from this piece, so that each step in the wedge may increase by an equal amount. The wedge of Experiment 356 (iii.) is supposed to have sixteen films. The scratches will all disappear when the films are mounted in Canada balsam.