122 THE ART OF PROJECTING.
Lithium, and Copper are especially good for this work
and give satisfactory spectra.
When this monochromatic light frotai the stick of
glass or the saturated solution of sodium chloride is
made to appear, it will be a good time to give atten¬
tion to its effects upon other colors. Observe the
faces of individuals, the colors of flowers, of ribbons,
of pictures. It is a good plan to have prepared a set
of strips of bright-colored papers, or ribbons, or the
Newton’s disk, for exhibition in monochromatic light.
REVERSED LINE.
The dark sodium line is the only one that is ever
projected, owing to the great difficulty there is in
making the vapors of other substances sufficiently
dense to absorb the powerful rays from the electric arc
or of the lime light. With either, a pure spectrum
must first be projected, and the slit should be nicely
focussed, as described. — Then having provided a
gas jet with Bunsen burner, or an alcohol lamp in
front of the slit, hold in it a small iron spoon con¬
taining a lump of metallic sodium as large as a pea.
It will take fire and burn with a yellow blaze and a
white vapor, through which the light from the lantern
must pass. If this vapor is dense enough it will stop
rays from the other light that have the same refrangi-
bility ; and as its own luminousness is not very great,
it will leave a black line upon the screen in the place
where the sodium line would appear if the light came
from it.
It will be best to have a screen a foot square with a
hole through it, to set in front of the sodium flame to
prevent its light from falling upon the large screen and
injuring the effect.