THE SPECTRUM 
3° 9 
into which slides a metal tube, enlarging at the front end into 
a long cone r, and at the back end bearing a plate with a slit 
s. The cone r is made to extend nearly to the lens which 
focusses the slit ; the latter being made thus, to slide out, 
because it will need cleaning pretty frequently. To the door 
at the back is attached, by a projecting piece, a metal ring, in 
which can be supported capsules, b, for the burning sub¬ 
stances. 
In using this combustion-lantern, as I venture to term it, 
it is arranged in front of 
the ordinary lantern, in the 
optic axis, with the door 
open as in the second 
figure, so that the light 
from the optical lantern 
can be passed through the 
slit s, and enable the latter 
to be focussed on the 
screen, with its spectrum 
all in focus as described 
in § 175. The optical 
lantern may then be turned 
off or removed. The pow¬ 
der used is then placed 
in the capsule b, in a little 
heap, a piece of cotton wick 
about an inch long is stuck 
perpendicularly into it so 
as to be half-buried in the 
powder; the wick is lighted, and the door shut; Prof. Wein- 
hold advises soaking the wick in lead chromate. As soon as 
the flame reaches the powder it flames up, and gives excellent 
line-spectra, though for a short time only. 
Prof. Weinhold recommends the following mixtures as 
effective. For sodium lines: 3 parts sodium nitrate, 1 part