on Spectrum Analysis. 367 
of acid, to maintain the evolution of hydrogen, keeps the 
flame burning brilliantly for some time ; this method might, 
therefore, be useful for spectroscopic observations for com- 
parison. 
ii. Continuous Spectrum showing Bright Bands. 
If the slit is narrow, the orange and green bands of calcium 
may often be seen on the spectrum obtained with the lime- 
light. If the chlorides of the alkalis or alkaline earths are 
previously melted upon the surface of the lime-cylinder, 
the bands of these compounds become very bright. Four 
different compounds might be melted upon different portions, 
and the spectra of each shown in turn by turning the cylinder. 
The lime-cylinder should be a “ hardlime;”’ the softer cylinders 
are too porous. 
il. Reversed Spectra. 
The important fact that the light emitted by incandescent 
sodium vapour is opaque to light from the same source, is 
easily shown in a striking manner upon the screen by placing 
the apparatus used for showing the yellow band of sodium in 
§i.in the centre of an ordinary optical lantern ; a bright 
‘disk of monochromatic sodium-light may be obtained on the 
screen from 4—5 feet in diameter. A small Bunsen burner is 
then placed as near as possible to the condensing lens, with 
the top of the tube projecting $ inch above the edge of the 
lens, which shows the shadow of the tube at the top of the 
disk upon the screen. When a platinum wire containing a 
quantity of sodium chloride is introduced into the flame of 
the Bunsen burner, the dark appearance of the cooler flame is 
clearly seen on the screen. 
Reversal of D line. 
Mr. Lewis Wright states that this can be done with the 
limelight by burning sodium in a Bunsen burner placed as 
close to the slit as possible. I have not been able to obtain 
very satisfactory results by this method ; the flame is too hot. 
I have been more successful with the following methods :— 
The sodium is burnt in a small spoon placed between the lens 
and prism, just below the focus of the rays from the lens, 
which must be made as small as possible, so that all the rays 
are made to pass through the flame where the sodium vapour 
is densest, and none of the light from the sodium vapour is 
_ focused on the screen, and a much smaller flame may be used. 
Or the spoon may be placed a little in front of or beyond the 
focus in the path of the rays ; a shadow of the spoon is seen 
2C 2 
