Test pieces of calcite were prepared in order to measure directly the refraction of the ordinary and extraordinary rays in all crystallographic directions. Two half-cylinders were made from transparent calcite crystals; one being a semi-circle in cross section, and the other ahalf-ellipse. The dimensionsof the ellipse were directlyproportional to the values of the velocities of the extraordinary ray—parallel and normal to crystallographic c axis. The minor axis of the ellipse was made parallel toand the major axis normal tocrystallographic c. The semi-circular specimen was cut with the crystallographic c parallel to the diameter. Forboth specimens observations were made on a goniometer with transmitted light. The ordinary ray emerged, refracted only at the surface of incidence of the semicircular cylinder, but the extraordinary wave normal was also refracted when leaving the calcite-air surface. The extraordinary ray was observed to follow a course in air after deflection at the surface of the calcite half-cylinder which agrees with the course to be expected according to computations. With the elliptical half-cylinder the extraordinary ray appeared to emerge from the calcite with almost no deflection from the computed course of the wave normal, and the ordinary ray, of course, was deflected.

In order to check the accuracy of the values obtained, observations were made on an optically true circular half-cylinder of identical size made by Bausch and Lomb Optical Company. This half-cylinder produced results essentially identical with thoseof the handmade specimen.

The test pieces were difficult to make, but the results demonstrate clearly the behavior of the ordinary and extraordinary wave fronts inanisotropic media.

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