342 
rorULAR SCIENCE REVIEW. ■ 
soiiiul produces silence, the waves interfering and neutralising ’ 
each other. But the disappearance of light in these black lines 
is not due to this cause. In the case of these black lines it 
arises from the circumstance that a body which is emitting 
light consisting of vibrations of a definite degree of fre- 
(jiiency, can absorb those portions of the light of other bodies 
which possess a corresponding rate of vibration, and can then 
radiate it forth anew in all directions ; much in the same way 
as a tuning-fork produces a resonance when held opposite the 
mouth of a box holding a column of air of such length as to 
vibrate in unison with itself, though it produces no such reso- 
nance when held opposite a box of different length which does 
not vibrate in harmony with it. The air in the resounding box 
first absorbs and then gives forth the vibrations of the fork with 
which it corresponds. 
In the case of sodium, for instance, the vapour of this metal 
absorbs the light of that particular portion of the spectrum of 
the body behind it, which corresponds with it in its rate of- 
vibration, and it allows all the rest of the light behind to pass 
on unaffected. 
If the sodium vapour is at a considerably lower temperature 
than the body behind, the absorbed rays will elevate the tem- 
perature of the metallic vapour somewhat, and will cause the 
sodium to give out a light which is a little greater than that 
line to the sodium alone ; but it is considerably less than that 
which v/ould be produced by the continuous spectrum of the 
body behind it, and the result is that when the combined image 
of the two spectra is thrown upon the screen we obtain what 
appejirs to us as a black line : but it really is .a line of low illu- 
minating power, which, being contrasted with the intense light 
of the spectrum on either side, produces upon our eyes the 
impression of a black line. 
If the sodium be raised in temperature until it acquires the 
same degree as that of the body behind it, the light which falls 
upon the sodium flame will still be absorbed as before; but now, 
as the intensity of the sodium light is ecpial to that of the spec- 
trum behind it, no sensible effect will be produced upon the 
screen. But if, on the other liand, the sodium flame be still 
liotter than the body behind it, it will be more intensely lumi- 
nous, and instead of a black line we shall have a bright line 
crossing the spectrum at this point. 
The vapour of sodium, according to its temperature, may 
tlierefore give rise to three different effects. 1. It may produce 
a black line, when the temperature of the sodium is low. 2. It 
may j)roduce no sensible effect, in which case the temperature 
and the light of tlic sodium arc cf[ual to those of the body 
behind it. 3. It may produce a bright line, but in this case the 
