WOOD. — ANOMALOUS DISPERSION OF SODIUM VAPOR. 369 



matter in the near future by a modification of the method of striae. The 

 channelled bands affect the magnetic rotatory dispersion in a powerful 

 degree, and this subject is under investigation at the present time. The 

 magnetic dispersion is many times greater than the ordinary dispersion, 

 as"has been found by examining the vacuum dispersion tubes in a power- 

 ful magnetic field with polarized light, and the whole subject will be 

 fully taken up in a subsequent paper. 



The absolute values of the refractive index for different densities have 

 been determined with the interferometer for monochromatic light of dif- 

 ferent wave-lengths. Relative values (dispersion) have been obtained in 

 the same way, as well as by the method of crossed prisms. 



Determination of the Dispersion with the Interferometer. 



The absolute determinations being the more difficult, it was decided to 

 begin with an investigation of the dispersion, in order to get as much 

 experience as possible with the behavior of the vapor under various 

 conditions. This was a wise decision, as was subsequently found, for a 

 number of very elusive sources of error were found, which were 

 eliminated in the final work. 



The general plan of the apparatus is show in Plate I. The inter- 

 ferometer was the form of Michelson instrument designed by Brace. 

 The base of the instrument consisted of a heavy tube of brass 12 cms. in 

 diameter and a little over a meter long. The arrangement of the mirrors 

 is shown in the diagram, all but one being of speculum metal. Light 

 from an arc lamp was focussed on the slit of the monochromatic illumi- 

 nator. The beam issuing from the slit of this instrument comprised a 

 range of wave-lengths about equal to the range between the D lines- 

 Near the slit was mounted a helium tube, which furnished a light of 

 constant wave-length, and the observations consisted in counting the 

 fringe shifts for the two kinds of light, when a given amount of sodium 

 vapor was introduced into the path of one of the interfering beams. A 

 small mirror was mounted in front of the interferometer, which deflected 

 a portion of the emergent light into a telescope, the remainder of the 

 beam entering a second telescope. By a little adjustment it was possible 

 to get the fringes formed by helium light distinct in one telescope, and 

 those formed by the light from the monochromatic illuminator distinct in 

 the other. The helium tube standing a little to one side of the slit, very 

 little overlapping of the two systems occurred. 



The sodium was contained in a seamless steel tube, covered with 



VOL. XL. — 24 



