LYMAN. — FALSE SPECTRA FROM THE ROWLAND GRATING. 247 



distinctly visible, — Ag, bi, C2, A3, K, and C3. Ag resembles group 

 An C2 group Ci ; C3 resembles group C4, A3 group A4. It is to be 

 noted that while A2 and C2 lie on the right or ultra-violet side of A^ 

 and Cx, C3 and A3 lie on the left or red side of C4 and A4. In addition 

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

 from one end to the other between Aj and C4, 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. 



If we examine the displacements produced in these groups when the 

 light is allowed to pass through the quartz prism before falling u[)on the 

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

 placement is observed in the successive lines going from the strono^ oroup 

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

 group Ai. At this point the amount of displacement suddenly returns 

 to about the value it had at 2790//,. Between groups Ai and D3 the dis- 



