222 THE CHEMISTRY OF THE SUN. [CHAP. 



line, when a small image of the arc, carefully focussed for the 

 yellow light, is thrown upon the slit and considerable dispersion 

 is employed. 



If the arc is observed before the introduction of the sodium 

 between the poles, with the poles slightly separated, the continuous 

 spectrum of each pole will be bounded by a sharp line, and in 

 the included region the exquisite flutings of the carbon vapour 

 will be seen together with the lines due to any metallic sub- 

 stances present. The metallic lines will be thickest near one 

 pole, and will overlap its continuous spectrum, while the carbon 

 flutings will overlap the other. The D lines in the arc should 

 occupy the centre of the tield of view. 



If now a piece of metallic sodium be placed on the lower 

 pole, the whole of the light will be blotted out, if the field of 

 view be small. Gradually the two ends of the spectrum of the 

 arc will begin to appear on either side of the field, the sharp 

 boundary lines to which reference has been made having 

 disappeared, as the poles are no longer incandescent. 



The absorption in its retreat to the central region will next 

 take the appearance of a truncated cone, its base resting on that 

 side of the arc formerly occupied by the carbon flutings. The 

 intense blackness gradually changes into a misty veil through 

 which, as it were, the D lines gradually make their appearance 

 as enormous truncated cones with their bases turned in the 

 opposite direction to that occupied by the original absorption. 



The more refrangible line is twice as thick as the other, and 

 is often contorted while the other is rigid. Gradually, as the 

 quantity of sodium vapour is reduced, the poles regain their 

 original incandescence, and the one to which the carbon bands 

 attach themselves will become more vividly incandescent than 

 the other. Then begins a new set of phenomena the absorption 

 of the light of either pole. Generally on the more incandescent 

 pole the absorption widens for a space, then narrows, and finally 

 puts on a trumpet appearance and is lost. 



