286 



NA TURE 



[January 23, 1902 



spectrum on each side of the D lines were perfectly sharp and 

 steady, and the dispersion could be traced a considerable 

 distance up and down the spectrum. On the slit of the spectro- 

 meter 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 

 itseli (Fig. I). 



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. 

 The oppositely curved branches adjacent to the region of 

 absorption extend rapidly up and down as the tube grows 

 hotter, the ends finally pa.ssing out of the field of the instru- 

 ment. A beautiful fluted absorption appears in the red and the 

 greenish-blue, which finally blots out a region in the blue 

 almost entirely. Meanwhile the curvature of the spectrum 



increases in a most remarkable manner, and the entire red end 

 is lifted high above the green-blue end. As the density cf the 

 vapour increases the red gradually fades away, leaving only the 

 yellow and green and the remote blue and violet, the curvature 

 increasing all the while. 



I-igs. 4 and 5 are photographs of the continuous spectrum 

 after dispersion by means of a prism of sodium vapour. They 

 give a very good idea of what is seen with the apparatus 

 arranged as shown in Fig. 3. The sodium lines were impressed 

 on the plate by holding a sodium flame in front of the slit of 

 NO. 1682, VOL. 65] 



the spectrometer after the exposure was over. In Fig. 5 the 

 arrows indicate the points to which the curved branches 

 could be traced in the original negative. Kye observations 

 enable them to be traced much farther, for they are very feeble 

 at the tips, and the light is not very actinic. 



For the exhibition of the actual spectrum produced by a 

 prism of sodium vapour a long dispersion tube with a battery of 

 four or 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. A coloured draw- 

 ing of the spectrum, which has already been mentioned, was made 

 when the spectrum was obtained in this manner. A single 

 prism gives a very pretty anomalous spectrum, but the 

 magnificent efi'ect 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, the extreme violet will be 

 found to occupy the position of the undevialed image of the slit 

 (Fig. i). Thencomesthe 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 blue 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, or 

 below the undeviated image, forming another brilliant fiare 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 alinost 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 when 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 the turned-over seam should be 

 hammered until a tight joint is formed. These tubes can be 

 heated and cooled any number of times and can be kept in 

 operation for an hour or two, at the end of which time the 

 sodium is generally used up, a moss-like deposit of oxide 

 gradually filling up the tube. The tubes can be used over and 

 over again without deterioration, and are most satisfactory in 

 every respect. Their only fault lies in their conductivity, the 

 sealing-wax softening and the glass plates falling off ; but this can 

 be prevented by wrapping a strip of cloth around each end and 

 wetting it from time to time. One tube was made with water 

 jackets at each end, but it seems to have no especial ad- 

 vantage, and is more complicated. I'orcelain tubes are quite 

 satisfactory, but the iron is to be preferred on the whole. 



By employing a tube of about 5 cm. diameter the anomalous 

 spectrum can be projected, but the appearance is so very 

 inferior to that of the phenomenon when seen subjectively 

 that the method is not recommended. 



Royal Astronomical Society, January 10. — Dr. J. W. L. 

 Glaisher, F. R.S., president, in the chair. — In a paper on 

 periodic orbits Mr. E. T. Whittaker communicated two theorems 

 relating to the periodic solutions of the difterential equations of 

 dynamics and astronomy. The first theorem furnished a criterion 

 for the discovery of periodic orbits ; if a certain function of 

 position be negative for all points of a closed curve and positive 

 for all points of a curve enclosing this, then a periodic orbit 

 exists in the ring-shaped space between the curves. The second 

 theorem was concerned with an integral, the value of which when 

 integrated over the region bounded by a periodic orbit is equal 

 to the number of centres of force enclosed by the orbit. — Prof. 

 H. M. Turner read a paper by Major Burrard, of the Indian 

 Survey, on the attraction of the Himalaya Mountains upon 

 the plumb-line in India. A chart was exhibited, showing a 

 supposed underground source of attraction running across central 

 India. — Lantern slides were shown of photographs taken by 



