WOOD. — ANOMALOUS DISPERSION OF SODIUM VAI'OR. 395 



The Ql'kstion ov thk Sklective Reflection of 

 Sor)iu.M Vatou. 



The vapor of sodium shoiikl exhibit strong selective reflection in the 

 vicinity of tlie D lines, just as tiie aniline dyes have metallic reflecting 

 power for light of certain wave-lengths. For obvious reasons the vapor 

 must be very dense if the phenomenon is to be detected experimentally. 

 It must moreover terminate abruptly in a flat surface. This can only be 

 accompanied by confining it in closed vessels made of some transparent 

 material, since in a vacuum tube we have a gradual transition from dense 

 to rare vapor at the free surface. I feel justified in speaking of the free 

 surface of a gas in a vacuum tube in this particular case. A more precise 

 detiuitiou of the apparent surface may be an isothermal surface on one 

 side of which we have sodium vapor and on the other sodium fog. From 

 such a surface we should hardly expect to get any trace of reflection, 

 which requires that the transition from dense to rare be sudden. If we 

 confine the vapor in glass bulbs we at once encounter many difficulties. 

 The glass is at once attacked and discolored, and we have the reflection 

 from the glass surfaces. An attempt was however made to observe the 

 phenomenon in the following way. A small amount of the metal was 

 sealed up in a hard glass bulb, highly exhausted. The bulb was mounted 

 in a small air bath of sheet iron which could be rapidly raised to a red 

 heat. 



The reflection of the filament of a Nernst lamp by the inner and outer 

 surfaces of the bulb was observed through a large direct vision prism. 

 The two minute points of light were seen drawn out into spectra, and it 

 was hoped that any trace of selective reflection in the vicinity of the D 

 lines would manifest itself by a change in the relative intensities of the 

 two spectra at the point in question. The bulb was heated very rapidly, 

 but no conclusive observation made. The glass discolored so rapidly that 

 the densest vapor could not be studied. In one case it was thought that 

 a slight brightening in the yellow occurred, and the subject will be further 

 investigated. Possibly by employing light polarized in such a plane as 

 to be refused reflection by glass surfaces we may be able to get the desired 

 results. The phenomenon is doubtless connected intimately with the 

 fluorescence of the vapor. In studying this phenomenon with very dense 

 vapors I was forced to the couclusion that the illumination of the vapor 

 with light of the wave-length of the D lines did not provoke the fluores- 

 cence. Wiedemann and Sclimidt observed a bright band in the fluorescence 

 spectrum in the case of vapor confined in glass bulbs which appeared to 



