Page:Proceedings of the Royal Society of London Vol 69.djvu/171

Rh diamond. In this way a scale of wave-lengths was secured. For the determination of refractive indices we require, however, the angular deviation of the different rays. This can be determined if we know the focal length of the lens forming the image of the spectrum on the slit of the spectrometer, and the actual deviations measured in millimetres of the images of the first slit formed by light of various wave-length at this point. To secure a record of this a glass plate, ruled with half- millimetre lines, was placed over the slit of the spectrometer, and the position of the lines on the glass plate in the focus of the large lens recorded with the writing diamond. This scale enabled the recorded deviations to be reduced to the actual deviations as they existed on the slit of the instrument.

The dispersion curve obtained in this manner, with the half-milli- metre marks, is reproduced on the same scale as the original in fig. 4, the prismatic spectrum having been converted into a normal spectrum. It will be seen that for all wave-lengths shorter than those of the D lines, the refractive index is less than one, while for the rest of the spectrum it is greater than one. This is the only case that I know of in which we have a medium, beautifully transparent even in consider- able thicknesses, in which light travels faster than in a vacuum.

Fig. 4.

To determine the refractive indices we require the angle of the sodium prism, and here we encounter a grave difficulty, for it appears to be quite impossible to determine this directly. As I halve said before, we are dealing with the optical equivalent of a prism, namely, a non-homogeneous medium, in which the vertical wave-front is retarded or accelerated in a progressively increasing amount as we pass from its upper to its lower edge. The equivalent prism appears from experiment to be bounded by concave instead of plane surfaces, in other words the effective angle is greater near the bottom of the tube than at the top. This can be shown by screening off different levels.

The lower part of the tube acting alone gives a much greater deviation than the upper. In practice I have found it best to screen