480 BARON VON WREDE ON THE ABSORPTION OF LTGHT 



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 ^, f, f, f... 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 a (the long- 

 est) to that of /3 (the shortest), traverse a medium which causes a 

 delay (p 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 -?*, S., r.^ 



"r. , -i-,... i — , L. etc. must be a minimum; that, on the con- 



9 11 2m — I 2wj + 1 



trary, those kinds of light the half length of whose wave is -^, -^, -i-, 



Si, -?— , i etc., must attain their maximum of intensity. 



8 2m 2m +2 ^ 



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. If a represents the original intensity of the light, and r 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 C D, and call b the di- 

 stance between these surfaces. 



Fresnel has shown, in his excellent paper on the diffraction of light f, 

 that the velocity of undulation u which a particle of aether receives 



• Plate VI. v.'ill be given, with the rest of this Memoir, in Part IV. 

 t Poggendorff's Annulen der Physik tind Chemie, vol. xxx. p. 100. 



