January 23, 1902] 



NATURE 



285 



before mentioned. By means of prisms of sodium vapour it is 

 possible to form a complete anomalous spectrum, in which all 

 the colours between the extreme red and violet are present, 

 with the exception of a very narrow range at the D lines. It has 

 been found that the refractive index is one for the extreme 

 violet, greater than one for all colours on the red side of the D 

 lines, and less than one for all colours on the other side, the 

 maximum and minimum values occurring, of course, close to 

 absorptionbands (the D lines). 



In the spectrum produced by a prism of sodium vapour the 

 violet occupies the position of the undeviated image of the slit, 

 the red and orange flaring off to one side, and the blue, green 

 and yellow to the other, as is shown in Fig. I.' 



Inasmuch as sodium vapour appears to be the substance best 

 adapted to class demonstration of anomalous dispersion, it is worth 

 while lodescribe in some detail the apparatus by which the remark- 

 able optical properties of the vapour can be exhibited, referring 



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the reader to the original paper for a more complete discussion. 

 (The paper is to appear also in the Philosophical Magazint. ) 



The first experiments were made with a large prism of cast 

 iron, furnished with windows of mica or thin plate glass, in 

 which the metal was heated in an atmosphere of hydrogen. 

 Very beautiful results were at once obtained, but certain pecu- 

 liarities of the vapour's action showed that the refraction was 

 due chiefly to the action of a non-homogeneous medium, the 

 planes of constant density being hori- 

 zontal. Great trouble was caused by the 

 windows, which soon became covered with 

 a white deposit, which cut off most of 

 the light. It being apparent, however, 

 that the oblique faces played but a very 

 small part, the effect being due almost 

 wholly to the variable density of the 

 vapour, it seemed best to make the most 

 of this circumstance and dispense with the 

 trouble entirely by removing the glass plates to such a distance 

 from the heated vapour that no deposit took place. The ar- 

 rangement finally adopted was simply a tube of glass 

 about 30 cm. long, provided with plate-glass ends cemented 

 1 This --pectrum is illustrated by .1 coloured plate in the original paper. 



NO. 1682, VOL. 65] 



on with sealing-wax. Hydrogen dried by passage over 

 calcium chloride was conducted into and out of the tube 

 by meahs of two fine glass tubes, arranged as shown in 

 Fig. 2. 



The diameter of the tubes should not be more than 2 mm. , and 

 they should lie close against the sides of the large tube in order 

 not to cut off any of the light. The most suitable diameter for 

 the large tube is 2 cm. The ends of the tube are first warmed 

 and thickly coated with sealing-wax. One of the glass straws 

 is placed in position, and a small piece of plate glass, pre- 

 viously warmed, applied to the end of the tube, any crevices 

 around the straw being closed with wax. The leading-in tube 

 is next placed in position and a piece of freshly cut sodium 

 (about 5 mm. on a side) inserted. The other window is then 

 cemented to the tube and the current of hydrogen started as 

 soon as possible. Some experience is necessary to regulate 

 properly the hydrogen stream during the experiment. When 

 the tube is first heated much white smoke forms. If a stream 

 corresponding to about one bubble per second is allowed to flow, 

 the smoke will usually clear up in a few minutes and give little 

 trouble. The tube should be heated by means of a Bunsen. 

 burner turned down low, the tip of the flame playing against 

 the bottom of the tube. If a sodium flame is placed behind the 

 tube the formation of the vapour can be watched, for it appears 

 almost jet black against the flame, though quite colourless 

 in white light. The behaviour of the vapour is somewhat 

 peculiar. It grows out from the sodium globule as a dark 

 atmosphere with a sharply defined surface, which clings to 

 the globule with great tenacity. It resembles at fiKt a 

 thick growth of mould more than anything else, and a 

 sudden gust of hydrogen scarcely moves it at all. A wire 

 pushed up through it drags a certain amount above the free 

 j surface, in much the same manner as a stick pushed up through 

 ' the surface of thick molasses would do. If the tube be inverted 

 the black cloud clings to the upper surface, behaving, on the 

 whole, like a very viscous mass. It is even possible to dip some 

 of it up on a wire. 



These peculiar physical properties of the metallic vapour have 



■ as yet only been studied in a very superficial manner, and they 



I are mentioned now only because it appears that there is 



some connection between them and the 



optical behaviour of the medium. 



The opinion is expressed that the appa- 

 rent viscosity is an illusion ; that the sharply 

 defined surface is merely the boundary at 

 which either condensation or chemical 

 action (the hydrogen was not absolutely 

 pure) is taking place. The process of 

 dipping the vapour up on the wire might 

 be explained by condensation on the wire 

 followed by vaporisation. A more care- 

 ful study of the physical behaviour of the 

 vapour will be made some time in the 

 future. 



The apparatus employed in the study 

 of the dispersion of the vapour was essen- 

 tially identical with that used by Becquerel. 

 The light of an arc lamp was focussed on the horizontal slit of 

 a collimator, after traversing which the 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 prisins, or in the focus of an. 

 eye-piece when the anomalous spectrum was to be viewed 

 subjectively (Fig. 3). 



The first experiments were made by the method of crossed 



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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 



