47'i 



Article XXV. 



Attempt to explain the Absorption of Light according to the 

 Undulatory Theory ; by Baron Fabian von Wrede. 



From PoggendorfF's Annalen der Physik and Chemie, vol. xxxiii. No. 23. p. 353, 

 Nos. 24 and 25.* 



X HE mathematical accuracy with which most optical phaenomena are 

 explained according to the theory of undulations, and the simplicity of the 

 first principles of this theory, as well as the analogy which it presupposes 

 between both the means with which Nature has endowed the living being 

 for the purpose of enabling it to perceive and distinguish those external 

 objects by which it is surrounded, render this theory in itself highly pro- 

 bable, although we have not yet been fortunate enough to bring under 

 its general laws all the simple phenomena. Though we do not consider 

 ourselves authorized to judge of the relative value of this theory, yet 

 everything appears to pronounce in its favour, if we compare it with the 

 theory of emission ; as this, with all its complex and not very probable 

 auxiliary means, accounts for a limited number only of optical phaeno- 

 mena, whilst it leaves without explanation many others which the theory 

 of undulations not only entirely explains but had even anticipated before 

 they were observed. 



The chief objection made to the theory of undulations is that it leaves 

 unexplained dispersion and absorption. With respect to dispersion, 

 Fresnel has it is true not explained it, but he has shown at least that it 

 does not stand in opposition to the undulatory theory. For he has proved 

 that the velocity with which light is transmitted through an elastic me- 

 dium, can only be independent of the length of the waves when the op- 

 posite spheres of action of the molecules of the elastic medium are so 

 small that we may consider them as null in comparison with the length of 

 the waves. If this is not the case, the velocity of transmission for the 

 shorter waves of light must be smaller than for the longer waves, 

 and this therefore the theory of undulations must take for granted, ac- 

 cording to the observations made on the dispersion of light. 



I shall now proceed to the special object of this paper, The Absorption 

 of Light. Brewster, who more than any other person has followed up 

 with attention the phaenomena connected with the absorption of light, 



* Translated by Mr. W. Francis. 

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