length of which could be changed by turning the prism, which was furnished with a mirror in the manner described by Wadsworth.
On heating the tube the black vapour was seen to spread out and fill the prismatic clear space between the iron tubes, while the lateral deviation of the image of the slit, as observed in the telescope, indicated that, to a certain extent at least, the desired prismatic form had been secured. In the extreme red the deviation was very slight, but as the spectrum was advanced across the slit by slowly turning the prism the image in the telescope moved off to one side, the deviation in this direction reaching its maximum value just before the wave-length reached that of the sodium lines. At this point the image jumped abruptly to the other side just as we should expect it to do on crossing the D lines in the spectrum, and from now on the image slowly crawled back to its undeviated position. The focal length of the telescope was 460 mm., and the maximum deviation of the rays adjacent to the D lines on the red side, as measured by an eye-piece filar micro- meter, was but 1 mm., while the deviation in the opposite direction of the rays on the other side of the D lines was 1-2 mm. The angle of the prism was 130, from which data we get the following values for the refractive index of the vapour for these two wave-lengths (relative to hydrogen) /x = 1-0005 and /* = 0-9994.
Similar results were obtained with the device shown in fig. 6, when two elliptical pieces of perforated sheet iron were used for moulding the vapour. The images formed in this case were blurred by diffraction.
FIG. 6.
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I place very little value however on these figures, for I am of the opinion that the effective angle of the sodium prism is much less than the angle between the ends of the tubes, it seeming probable that the vapour bulges out into the tubes, especially near the centre. That this is to a certain extent the case is indicated by the fact that the image of the slit is not very sharp, though this may well be caused by the varying density of the vapour. I have not yet despaired of getting a prism bounded by plates of glass, about which there can be raised no question, though the problem is a difficult one, and observations will have to be made with great rapidity.
The deviations obtained by this method are very much less than those obtained with the dispersion tubes, indicating either that the equivalent angle is very large in the latter case (it may be nearly 180 for all we know) or that the angle of the prism formed by the iron tubes was less than the estimated value.
