ACCORDING TO THE UNDULATORY THEORY. 489 
sorption proceeds toward the red end of the spectrum, new black stripes 
arise before them, till at last, at a certain intensity of the colour of the 
iodie gas, the whole spectrum is absorbed, with the exception of asmall 
piece of the red, which is now entirely filled with black stripes. This 
beautiful phenomenon of absorption is explained with the greatest ease 
and exactness by supposing two different causes of retardation. We 
need only presuppose that the one retardation is about equal to the 
length of the wave of red light, and the other about 150 times greater. 
The part of the curve of the intensity which represents the first re- 
tardation has the form A B (fig.3); but that part of it which repre- 
sents the second has the form C D (in the same figure). The results 
of both may therefore be expressed by a curve of the form of E F. 
With increasing intensity of the colour of the iodic gas, we must sup- 
pose that 7 and r’ are increased, and that this increase can of course 
have an influence, not on the station of the maxima and minima, but 
merely on the absolute magnitude of the intensity. The greater r and 
r' become, the less must also the intensity become. And as we must 
imagine that the light, in order to be visible, must have a certain inten- 
sity, and as we can also express these limits of perception of the light 
by a line, it is evident that the increase of r and 7’ must force down 
the curves of the intensity toward this line. 
If we now place fig. 2 on fig. 3, and conceive the line A B in 
fig. 2 to represent the limit of perceptibility for the eye, we shall 
render the effeet of the increase in the value of 7 and 7’ evident by 
sliding fig. 2 gradually higher on fig. 3. If fig. 2 lies on the line 
a 6, we see some stripes appear in the blue; if we move it higher 
towards a! b', we see that the blue end is absorbed, and the stripes now 
make their appearance in the green; if we move it still higher to a! b", 
we find the whole spectrum absorbed, with the exception of a piece of 
red, which is now filled with black stripes. This is precisely the pro- 
cess in the spectrum of the iodic gas. ‘The phenomena of absorption in 
bromic gas are explained just in the same manner. In order to ex- 
plain the spectra arising from the light which has traversed nitric acid 
gas, or euchlorine, we must suppose several causes of retardation. We 
must not be astonished at this difference, as the two last gases belong 
to the compound, while the two first belong to those which we consider 
as simple. It appears to me quite natural to assume that the ele- 
mentary constituent parts of a compound body may each of itself cause 
different retardations; and if we consider nitric acid gas as a compound 
of nitric acid and nitrogen, instead of considering it as a binary com- 
pound of azote and oxygen, we then easily conceive how very possible 
it is that a great number of retarding causes may be contained therein, 
each of which arises in the same manner as in the single gases. 
Without endeavouring to explain the presupposed causes of retarda- 
