INDEX TO PLATES. 



Plate I, Figure 1. Tin arc in hydrogen, 1st order. 



Plate I, Figure 2. Upper part, copper arc in hydrogen, 1st order. Lower part, 

 copper arc in air. 



Plate I, Figure 3. Tin arc in hydrogen, 1st order. All lines are second order 

 except H a at 6563. 



Plate I, Figure 4. Copper arc in hydrogen, 2d order, showing hydrogen-copper 

 fluting and the H y line. 



Plate II, Fig ore 1. Aluminium arc in ammonia, 2d order, showing hydrogen- 

 aluminium fluting. 



Plate II, Figure 2. Middle, magnesium arc in hydrogen, showing hydrogen- 

 magnesium fluting at 5210, 1st order. Outside, magnesium arc in air. 



Plate II, Figure 3. Upper part, zinc arc in hydrogen, 1st order, showing hydro- 

 gen-zinc lines. Lower part, zinc arc in air. 



Plate II, Figure 4. Sodium (and copper) arc in hydrogen, 1st order, showing 

 hydrogen-sodium lines. 



