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XLVIII. Optical Researches. By A. J. Angstrom*. 



[With a Plate.] 



1. r | ^HE dispersion, absorption, and diffusion of light, are 

 -i- effects, the complete solution of which, it may be 

 assumed, is still very distant from us. Thus the explanation 

 given by Cauchy, in conformity with the notions of Fresnel, 

 that the sphere of attraction of the molecules is comparable in 

 magnitude with the length of undulation, has not yet been 

 applied to doubly refracting bodies; and so far as Cauchy's 

 theory has reference to isophanous bodies it also needs comple- 

 tion, inasmuch as the action of the medium itself is not taken 

 into account. Further, as regards the phenomena of absorp- 

 tion, there is certainly no lack of explanations, but the difficulty 

 is to apply them in each particular case ; and, as several causes 

 work together, to appropriate to each its due proportion in the 

 production of the phenomena to be explained. The grounds 

 more particularly referred to in the explanation of the absorption 

 and diffusion of light, are the following : — 



a. An unsymmetrical distribution of the molecules of the 

 medium, the consequence of which is, that in the differential 

 equations for the motion, differential coefficients of an uneven 

 degree are introduced. As far as I know, this cause was first 

 assigned by Cauchy in a letter to Libri. 



b. The principle adduced by Euler in his Theoria Lucis et 

 Caloris, according to which the colour of a body is produced by 

 the resonance of the oscillations, which can be assumed by the 

 particles themselves. 



c. The interference of light. On this principle Newton is 

 known to have based his celebrated theory of colours, which 

 was further expanded by Biot in the spirit of the theory of 

 emission. M. von Wrede was the first to apply it in the sense 

 of the theory of undulation, by showing that the phenomena, in 

 the case of absorption by gases, &c, might be referred with ease 

 to the principle of interference. 



Now with regard to the first of the three grounds of explana- 

 tion, it alone is by no means sufficient to explain the phenomena 

 of colours. That in general the opacity of bodies might be 

 referred to it as a cause is easy to discern ; but it does not 

 explain why a certain colour, to the exclusion of all others, is 

 produced. 



The principle of Euler also explains, not so much the colour 

 which a body actually exhibits, as that which it is unable to assume, 

 because most of the oscillatory motions which bodies assume in 



* Prom PoggendorfTs Annalen, vol. xciv. j>. 111. 



