SOME ADDITIONS TO THE ARC SPECTRA OF THE 

 ALKALI METALS. 



By F. a. Saunders. 



Presented by Charles R. Cross. Received September 8, 1904. 



Lenard * discovered a uew series and several other lines in the arc 

 spectrum of sodium. He removed the slit of a spectroscope and in its 

 place focussed a real image of the arc. "With suitable dispersion he ob- 

 served tliat different parts of the arc gave different - lines," and the new 

 ones which he discovered were emitted by the hottest vapor, near the 

 positive pole. Konen and Hagenbach f succeeded in photographing 

 many of these lines and finding others in the lithium spectrum which 

 are ap[)arently emitted under similar circumstances. The writer, also, 

 hoped to obtain photographs of such lines, using the usual slit, if his 

 spectrograph were designed to give very bright spectra free from astigma- 

 tism, and if an image of the proper part of the arc were cast on the slit. 

 The attempt was made with all the elements of the lithium family and 

 the results, which were partially successful, are given below. 



The essential feature of the apparatus used was a Rowland concave 

 grating of about 10 cm. width and 305 cm. radius, ruled with lines of 

 somewhat unusual length on a parabolic surface. The ruled surface had 

 an area of 5 by 8 cm. The grating was mounted with the slit close to 

 it on a solid iron casting bolted to a brick wall. An arm, supported 

 from this casting and constructed of heavy gas-pipe, carried the camera 

 at its end and could be turned about a point immediately below the 

 grating. The table carrying the grating turned with this arm. Tiie 

 light from the slit fell on a parabolic mirror, simihir to the one on 

 which the grating was ruled, placed at such a distance that the rellected 

 light formed a parallel beam. This then fell upon the grating and the 

 spectra were formed about 150 cm. from the grating along a curve 

 which was nearly a circle of 75 cm. radius. The incident and reflected 

 beams at the mirror made an angle of 3 degrees with one another, while 

 the angle between the axis of the grating and the beam incident on it 



* Ann. der Pliys., 11, G3G (1903). t Pliys. Zeitsclir., 4, 592, 801 (1903). 



