480 BARON VON WREDE ON THE ABSORPTION OF LIGHT 
been, in comparison with this, diminished by one portion equal to the 
double distance between the reflecting surfaces. 
If now we confine ourselves to the consideration of the two first sy- 
stems, it is clear that their results must depend on the relation between 
the length of the wave of light and the amount of retardation which 
has taken place in the one system in relation to the other; so that when 
this amount equals 4, 3, 3, $... of the length of the waves of light, 
the intensity of the results must be equal to the difference between the 
intensity of both the systems; and when it is equal to an entire mul- 
tiple of the length of the wave of light, the intensity of the results 
must be equal to the sum of the intensities of the systems. If we now 
admit that waves of light, of all lengths, from the limit @ (the long- 
est) to that of ( (the shortest), traverse a medium which causes a 
delay ¢ in one part of this light, it is evident that the intensity of all 
kinds of light the half length of whose wave » amounts to { ) f, 2 
i Fae mop moet etc. must be a minimum; that, on the con- 
trary, those kinds of light the half length of whose wave is So ‘ &. 
& ; ft $ etc., must attain their maximum of intensity. 
ro a! hen 8 
When these species of light are separated by means of a prism, each one 
whose intensity is a minimum must appear as absorbed in relation to 
the others situated between them, and the whole spectrum must be 
analogous to that which a light which has traversed iodic or bromic 
gas presents. 
Before I enter further into the comparison between the spectra which, 
according to the theory, must originate in consequence of such a simple 
retardation, and those which, as experience shows, are produced by ab- 
sorbing media, I will try to determine what the consequences are of the 
hypothesis just laid down, namely that of an indefinite continued re- 
flection. Ifa represents the original intensity of the light, and 7 the part 
which is lost at every reflection, the intensities of each of the originating 
systems of waves of light attain the value shown in Plate VI. fig.9.* As 
for the rest, the figure requires no other explication than that 1 determine 
the reflecting surfaces with the lines A B and CD, and call 6 the di- 
stance between these surfaces. 
Fresnel has shown, in his excellent paper on the diffraction of light t, 
that the velocity of undulation. which a particle of zther receives 
* Plate VI. will be given, with the rest of this Memoir, in Part IV. 
+ Poggendorff’s Annalen der Physik und Chemie, vol. xxx. p. 100. 
