Page:Collected Physical Papers.djvu/88

68 Fraunhofer's lines are present. As the cylinder is rotated, a shadow gradually moves along the spectrum, beginning with the most refrangible end. The shadow is made to coincide with the different absorption lines, and the differences between these readings and the "D" reading give the absolute critical angles for the different rays. In this way, the values of the indices for different rays are found, and hence the dispersive power. A greater degree of accuracy is obtained by repeating the difference.

The incandescent filament of a glow lamp may also be used as a source of light. The micrometer in the eye-piece of the spectroscope should have been previously calibrated.

The following determination was made to find the difference between D and F lines. This was repeated ten times, which gave a total difference of 2° 5′. The critical angle for the F ray is therefore less than the angle for D ray by 12·5′. Hence taking the mean critical angle for D to be 48° 39·5′.

Indices for solids.—The method for the determination of the indices for solids is precisely the same as that for liquids. The solids are cut in the form of two small semi-cylinders, a process which is not so difficult as the cutting of a prism.