82 The Spectroscope in Mint Assaying. _[January, 
constituents, viz., oxygen, nitrogen, &c.; these, of course, 
write their signatures in the spectroscope, and it is neces- 
sary to eliminate the numerous bright air lines which thus 
appear in all the spectra. Some of the lines of different 
metals appear in close proximity, and might readily be mis- 
interpreted. Thus a bright blue line of bismuth is almost 
identical in position with one of zinc. A green line of iron 
is nearly coincident with a bright gold line. The difficulty 
which presented itself in the exact comparison of these 
proximate lines was overcome by using a pure metal as one 
electrode and another pure metal as the other electrode. 
The effect thereby produced was very curious. With pure 
gold and pure copper as the electrodes, the gold lines 
extend across only one half the field of the spectrum, and 
the copper lines extend only across the other half, the 
medial termini of both sets of lines being perfeCtly sharp 
and bright. By this means a double spectrum of copper 
and gold is obtained, or, rather, a section of a complete gold 
spectrum and a section of a complete copper spe¢trum are 
visible in immediate juxtaposition, thereby enabling a most 
accurate comparison of lines, which in reality are not 
identical in position, but which by the previous method were 
apparently so. 
By a slight modification of the experiment, substituting 
pure copper as one electrode and an alloy of silver and gold 
as the other, the proximate lines of these three metals are 
presented mapped, as it were, on a natural scale. (Fig. 1.) 
Further modifications of this principle suggested them- 
selves, and were tried with indications of valuable results. - 
By using as one ele¢trode an alloy of gold and copper of 
comparative fineness, and a baser alloy of the same metals 
as the other electrode, a result not before observed presented 
itself. The lines of both copper and gold crossed the entire 
field of vision, but in the section representing the fine alloy, 
