Page:Collected Physical Papers.djvu/84

64 The following are for the D ray at a temperature of 18°:—

If we reject the first two values as somewhat indefinite and take the last two, we find a difference of more than 1 part in a thousand, Gladstone and Dale's result being the lower.

With C ray at the temperature 18°:—

Here also the same difference is observed. Thus the minimum difference between the best observations is about 1 part in a thousand.

My determinations were made with ordinary distilled water, condensed by passing steam through a coil of copper immersed in cold water. The experiments were carried out with the two following apparatus:—
 * I. A large circle, roughly graduated into degrees, with a diameter of 45 cm.; the glass trough used is 25 cm. in diameter.
 * II. The circle was obtained from an old theodolite. The Vernier reads to 1′. The glass cylinder used has a diameter of 14·4 cm.

It must be remarked here that the cylinders at my disposal, though circular in section, were slightly conical in shape. I tried to obviate this defect by using a very small portion of the height of the cylinder for light to pass through, the length of the slit being reduced to about 5 mm.

Just before total reflection, a series of images of the slit are seen reflected by the air-film. When white light