LYMAN. — FALSE SPECTRA FROM THE ROWLAND GRATING. 2-17 



distinctly visible, — A 2 , th, C 2 , A 3 , b 2 , and C s . A 2 resembles group 

 Ai, C 2 group Cx; C 3 resembles group C 4 , A 3 group A 4 . It is to be 

 noted that while A 2 and C 2 lie on the right or ultra-violet side of A x 

 and Ci, C 3 and A 3 lie on the left or red side of C 4 and A 4 . In addition 

 to the simple groups of lines already mentioned, the whole spectrum, 

 from one end to the other between A x and C 4 , is filled with a number 

 of fine, faint lines. These lines are not regularly arranged so as to pro- 

 duce fluting, but rather present the appearance of a reproduction of 

 groups of lines. 



MAGNESIUM. 



ALUMINUM. 



Group. 



I 



Bi 

 Ci 



Do 



E 2 

 B 3 

 C s 



Apparent 

 wave length. 



2376 n 



2075/x 



1890 m 



1300 m 



1184 m 



1023 m 



933 m 



Displacement. 



38.8 mm. 



36.1 mm. 



36.7 mm. 



34.9 mm. 



35.3 mm. 



36.0 mm. 



36.6 mm. 



If we examine the displacements produced in these groups when the 

 light is allowed to pass through the quartz prism before falling upon the 

 grating, the following facts become evident : A steady increase in dis- 

 placement is observed in the successive lines going from the strong group 

 at 2790 (C) towards the ultra-violet until we get in the neighborhood of 

 group A t . At this point the amount of displacement suddenly returns 

 to about the value it had at 2790 fx. Between groups Ax and D 2 the dis- 



