164 On the Production of Light by Chemical Action. 
shell. These, without the prism, would all have pursued the 
same normal path, and produced a commixed effect on the eye ; 
but with it are separated transversely, and each becomes fer- 
ceptible. 
It might be supposed that, in the familiar instance of an oil 
lamp, if we put any check on the supply of air, and thereby 
check the intensity of combustion, we ought to have the flame 
emitting rays of light the refrangibility of which becomes less 
and less, and which, from being quite white, should pass through 
various shades of orange, and end with a dull red. But the 
compound nature of the burning vapor interferes with that result. 
For, when a certain point is gained, the hydrogen for the most 
part alone burus, the carbon being set free as smoke, and such a 
flame cannot support itself in strict accordance with the princi- 
ple given. ails 
We must then search for other conditions under which carbon 
is found, and which are free from this difficulty. Two at once 
present themselves; they are carbonic oxyd and cyanogen gas. 
In the former the carbon is already united with half the quantity 
In place of the burning coal of the former experiments, I sub- 
stituted a jet pipe through which the various gases might be made 
to pass, and the rays emitted by their flames enter the telescope 
after passing through the slit and prism. In this arrangement 
the slit should be horizontal, and not vertical. So far from being 
immaterial which of the two positions is selected, very great ad- 
vantages arise from the former. If the slit be vertical, the prism, 
it is true, will separate the constituent colors from one another, 
but it fails to show their relative position. If it be horizontal, the 
relative positions of the different colors can be demonstrated ; 
it can be proved that a horizontal section of a flame is in reality, as 
has been already remarked, a colored ring, the red being the iuner- 
most color, and the violet outside. For if this is the order in 
which the colors occur, the red ring must necessarily have a less 
diameter than the green, and the green than the violet ; and when 
the prism, set also in a horizontal position, separates those colors 
from each other, the sides of the resulting spectrum ought not to 
be parallel but inclined to each other, the breadth being least in 
the red, and increasing as we pass to the violet end. This in- 
creasing breadth proves that the constituent colored shells of the 
flame envelop each other, the violet being outermost and there- 
fore br This valuable indication would be wholly lost if 
the slit was vertical. . , i mids ot 
