4:94: BARON VON WREPE ON THE ABSORPTION OF LIGIT 
gas, but also that which arises through absorption in oxalate of chrome 
and potash. In these experiments the magnitudes ry and 7’ can be 
varied up to a certain degree, and consequently the breadth of the 
black stripes can be changed in proportion to the breadth of the bright 
ones. For this purpose it is only necessary to let the light fall on the 
surface of the cylinder, under different angles of incidence. For it is 
evident that the proportion between the light reflected on the first and 
that reflected on the second must be greater the smaller the angle of 
incidence is; and consequently that the black stripes must at smaller 
angles of incidence be less broad than at greater. If we wish to produce 
absorptions with a small difference between the intensities of the maxima 
and minima, it is only necessary to let the light pass through a plate 
of mica instead of being reflected on it. In this experiment the propor- 
tion can be considerably varied by changing the angle of incidence. 
The most complex of all the phenomena of absorption is undoubt- 
edly the solar spectrum, with its numerous irregularly placed stronger 
or fainter black stripes. If we suppose with Herschel that these 
stripes arise from absorption in the atmospheres of the sun and earth, 
it becomes easy to explain them according to the principles already 
laid down. Although I have not yet made any experiments with a 
view to ascertain whether and in what measure the different pres- 
sures of the gases act on the position of the arising absorptions, yet 
I think it highly probable that they have a very considerable influ- 
ence on it. In sucha ease it is evident that the light in its passage 
through both the atmospheres, the density of which varies with the di- 
stance from their respective bodies, must suffer an indefinite number 
of unequal retardations, each of which will produce a certain series of 
maxima and minima. The cause of the number and also of the irre- 
gular position of the black stripes is consequently easily to be conceived. 
But the (at least apparently) vast difference which takes place between 
the intensities of the maxima and minima requires a particular explana- 
tion, which I will now endeavour to make. We have hitherto considered 
only two reflecting surfaces ; it is however evident that, according to 
the hypothesis with which I have set out, we must suppose a series 
of such surfaces which will be greater in proportion to the greater thick- 
ness of the absorbing medium. If we call, as before, a the original in- 
tensity of the light, 7 the loss in each reflection, and m the number of 
reflecting surfaces, we easily perceive that the intensities of the system 
of waves of light must be as we find them given in the following table 
(fig-10), in which A, B,, A, B,, A; B;, &e. represent the reflecting 
surfaces. 
When the thickness of the absorbing medium is rather considerabie, 
m must be a great number; and consequently 7 must be very small, 
because otherwise no considerable portion of light could traverse all 
