. Carnegie Institution of Washington publication. 112 DISPLACEMENT INTERFEROMETRY BY plane of symmetry parallel to the edge of the prism. In the interest of variety, moreover, it is preferable not to employ strictly accurate surfaces, so that the prisms with which grandfather used to decorate his gas-fixtures will, as a rule, suffice admirably. In figure no, P is such a prism (truncated) on a plate of obsidian Q, the long edges being normal to a white window curtain at L near by, illuminated with sunlight or daylight; or any light toward the front over- head is good. The rays that are wanted,
. Carnegie Institution of Washington publication. 112 DISPLACEMENT INTERFEROMETRY BY plane of symmetry parallel to the edge of the prism. In the interest of variety, moreover, it is preferable not to employ strictly accurate surfaces, so that the prisms with which grandfather used to decorate his gas-fixtures will, as a rule, suffice admirably. In figure no, P is such a prism (truncated) on a plate of obsidian Q, the long edges being normal to a white window curtain at L near by, illuminated with sunlight or daylight; or any light toward the front over- head is good. The rays that are wanted, s, will enter symmetrically at a mean angle of about 30° to the vertical and after reflection at the base of the prism and the plate reach in the eye in the direction E. The rays totally reflected, t, come from a greater angle to the vertical and are not wanted. --s. The limit of total reflection here (also easily recognized) is usually a sharp parabolic or cuspidal apex. The light seen through either face enters by the opposed face. On looking down from a steeper angle and with properly selected faces, brilliant groups of complete confocal ellipses (major axes to over 2 inches), or of confocal hyperbolae may be seen in each of the roof- faces. To find advantageous face combinations, the three faces of each prism should be examined in succession, and it is well to rub P on Q to improve the contact. On moving the eye fore and aft or using different pressures, any type of ellipse with white or colored disk may be produced at pleasure. It is usually preferable to use a shorter plate Q than is given in the figure, about one-half the length of the prism. When well produced the ellipses may also be seen by side-light, with different patterns in the two roof-faces. The type of interference figure clearly depends on micrometric differences of the faces in contact. The ellipses are Newton's rings modified by the color dispersion of the glass. The hyperbolae, however, are about equa
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