Page:Proceedings of the Royal Society of London Vol 69.djvu/168

160 after traversing which the parallel rays passed lengthwise through the dispersion tube. A second lens brought them to a focus on the slit of a spectroscope, when the dispersion was to be studied by the method of crossed prisms, or in the focus of an eye-piece when the anomalous spectrum was to be viewed subjectively (fig. 2).

The first experiments were made by the method of crossed prisms, the spectrometer being furnished with a Rowland plane grating, which showed the sodium lines widely separated. It was at once apparent that far better results could be obtained with the dispersion tubes than had ever been observed with prismatic flames. The curved branches of the diffraction spectrum on each side of the D lines were perfectly sharp and steady, and the dispersion could be traced to a considerable distance up and down the spectrum. On the slit of the spectrometer appeared, instead of the white image of the horizontal slit, a most beautiful anomalous spectrum, of great brilliancy and purity. The spectrometer was at once removed and an eye-piece put in its place, when a most superb spectrum revealed itself. The general appearance is shown in the coloured plate accompanying this paper, though it is quite impossible to represent by means of pigments the sparkling brilliancy of the colours. (See Plate 1.)

Before discussing this spectrum in detail it will be better to take up the results of the experiments made by the method of crossed prisms. On first heating the tube the curvature of the spectrum between the D lines as well as on each side is observed, the appearance being identical with that figured by Becquerel, but in a few seconds the vapour becomes so dense that total absorption of all the light between the lines occurs. Julius expresses the opinion that this disappearance of the light between the lines is only a result of the strong dispersion, that is, it is not absorbed but turned off to one side so that it does not enter the instrument. It appears to me, however, that this is not the case, for I have observed the same effect under conditions where lateral deflection seems quite out of the question. As I shall show later on the breadth of the absorption band is sometimes twenty times the width of the spectrum comprised within the D lines.

The oppositely-curved branches adjacent to the region of absorption grow out rapidly as the tube grows hotter, the ends finally passing out of the field of the instrument. A beautiful fluted absorption appears in the red and the greenish-blue, which finally blots out a