430 Mr. J. N. Lockyer. On some Phenomena [Mar. 20, 



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, as if 

 the molecules to which the absorption is due were then, owing to 

 the reduction of temperature, being reconstructed, thus increasing the 

 quantity of available absorbing material of this particular kind. 



Very often on the opposite pole the line is seen merely as a bright 

 one, or again the absorption is reduced to its smallest proportions. 



Having thus stated the phenomena with regard to the D line, it will 

 be convenient to make some general statements supported by the 

 various photographs which I now submit to the notice of the Society. 



I. We have first a general absorption of the light of the arc over 

 the region to be eventually occupied by the bright line. 



II. Next the disappearance of this indefinite absorption and the 

 formation of a truncated absorption of a symmetrical bright and 

 wider line. 



III. Next the parallelism of the boundaries of the bright and dark 

 lines in the centre of the arc itself. 



IV. Next the various absorption phenomena on the two poles. 



V. Finally the extinction of the absorption line in the arc. 



The other lines in the sodium spectrum are also good representa- 

 tives of cases in which the absorption leads to different appearances, 

 or in which absorption phenomena are entirely wanting. For instance, 

 the double green line of sodium shows scarcely any trace of absorp- 

 tion when the lines are visible ; but before the lines are produced out 

 of a general brightness which fills the whole field the absorption is 

 visible as line absorption, the less refrangible member being thicker 

 and darker than the other, exactly the opposite to what holds with the 

 D lines. 



I have observed no absorption in the case of the blue line, but the 

 radiation phenomena are extremely curious taken in connexion with 

 the other lines. While the D lines put on the appearance of black 

 truncated cones, while the green lines widen at their bases towards 

 the red and not at all towards the more refrangible side, the blue 

 lines are only widely developed in the centre of the arc, and are least 

 developed in that portion of it where the phenomena of the other lines 

 are seen in their strongest intensity ; thereby affording a striking in- 

 stance of the irregular absorption and radiation of the molecules of 

 the same element in the same sectional plane of the arc. 



The red double line of sodium I have never seen reversed nor irre- 

 gularly widened. 



When a Siemens' lamp is employed, the absorption phenomena of 

 the flame referred to in another communication merit a most careful 



