( 187 ) 



R, and cause the flame to emit a beautiful, clear and constant sodium 

 light, the intensity of which can he controlled and regulated by means 

 of an aini)è)'emeter and a variable resistance. 



In Fig, 2, a and h are shown two dif- 

 ferent wa3-s in which the light travels 

 through this long sodium flame. L repre- 

 sents the crater of an electric arc of 20 

 ampères. The lens A throvs^s an image of 

 the crater on the slit ^S\, which, in its turn, 

 is depicted by the lens B on the slit S^ of 

 a grating spectroscope. About half of the 

 conical beam of light which leaves A is 

 intercepted by the screen P, and the part, 

 which the slit S^ allows to pass, falls almost 

 entirely on the screen Q, wliich has been 

 shifted so close to the optical axis of both 

 lenses, that only a narrow streak of light 

 can reach the slit S^, through the middle of 

 B. The large gas burner stands on a hori- 

 zontal slide, which is movable up and down 

 and round a vertical axis ; thus, by means 

 of screws, it can easily be put in any 

 position required. 



When the axis of the flame (which we 

 assume to be in its most luminous part, 

 i.e. a little above the blue-green core) coin- 

 cides with the optical axis of the system 

 of lenses, both the D-lines will be seen 

 symmetrically widened in the spectroscope. 

 If not perfect, the symmetry will easily be 

 corrected by slightly shifting the screens 

 P and Q. 



Picture 1 Fig. 3 (see Plate) refers to the 

 case when the flame N is not burning; the narrow absorption lines 

 are due to traces of sodium surrounding the carbon points. When 

 the flame is burning, a very weak current passiug through the sodium 

 solution will produce the effect shown in 2. The photographs 3, 4 

 and 5 were obtained with currents of about 1, 3 and 6 amperes, 

 the flame always being in the symmetrical position. 



We will now examine the case represented by Fig. 2, a. Here 

 the axis of the flame has been shifted 3 m.M. towards the right. 

 The narrow beam of light which reaches aS,, only penetrates that 



Fig. 2 a. Fig. 2 h. 



