If we calculate the refractive indices from the data given by the curve traced with the diamond point, assuming the equivalent angle of the stratified vapour to be 130, we get for the wave-lengths adjacent to the absorption band the values //. = T0024 and //, = 0'9969.
The value given by Becquerel (1 -0009) is intermediate between the values obtained by the two methods, but I feel sure that his flame must have been non-homogeneous, and his numerical results more or less inaccurate in consequence.
For the exhibition of the actual spectrum produced by a prism of sodium vapour a long dispersion tube with a battery of four of five prisms gives the best results. A small Bunsen burner should be used for each of the fragments of sodium, which should be at least 6 or 8 cm. apart. (This arrangement is shown in fig. 2, p. 160.)
The coloured drawing of the spectrum (Plate 1), to which I have already alluded, was made when the spectrum was obtained in this manner. A single prism gives a very pretty anomalous spectrum, but the magnificent effect produced by the battery makes the slight amount of extra trouble well worth while. If the electric arc is employed as the source of light extreme violet will be found to occupy the position of the un deviated image of the slit. Then comes the blue, sometimes in contact with the violet and sometimes slightly separated by a fine dark line, owing to the fact that the violet light comes from the fluted carbon band of the arc, which is separated from the bhie by a comparatively dark region. Then comes a wide gap corresponding to light absorbed by the sodium vapour in the blue- green region (the channelled spectrum), and above this a beautiful flare of colour ranging from blue-green through grass-green to yellow. The red and orange portion of the spectrum is on the other side of or below the undeviated image, forming another brilliant flare of colour. It is separated from the violet by a wide dark band, due to the absorption in the vicinity of the D lines. If the density of the vapour is increased by heating the tube to a higher temperature the red flare extends lower down, grows fainter, and finally fades away owing to the presence of the fluted absorption bands in the red. The green and blue persist, however, becoming more widely separated, but finally the green disappears almost entirely. It is best to arrange the gas- cock so that the height of the flames can be controlled without leaving the eye-piece, for it is surprising how slight a change is necessary to completely alter the general appearance of the spectrum.
The glass tube should not be allowed to cool until the experiment is at an end, otherwise it will immediately fly to pieces as soon as the flame is again applied to it.
While glass answers very well if the experiment is to be of short duration, sheet iron is much more satisfactory. Suitable tubes can be made by any tinsmith. They should be made of thin sheet iron, and
