254 



PROFESSOR POWELL ON THE DISPERSION OF LIGHT. 



Conclusion. 



Upon comparing the numbers above given as resulting from theory and from ob- 

 servation, and bearing in mind that the assumptions of the constants on which the 

 calculation depends are but tentative and approximative, and open to farther cor- 

 rection, it will, I think, be allowed that the coincidences are quite sufficient to permit 

 us to regard the formula as a very close representation of the law of nature. 



We are thus, I think, justified in concluding, that for all the substances examined 

 by Fraunhofer, viz. for four kinds of flint glass, three of crown glass, for water, 

 solution of potash, and oil of turpentine, the refractive indices observed for each of 

 the seven definite rays are related to the lengths of waves for the same rays, a^ nearly 

 as possible according to the formula above deduced from M. Cauchy's theory. 



Thus, then, for all the media as yet accurately examined, the theory of undulations 

 (as modified by that distinguished analyst,) supplies at once both the law and the ex- 

 planation of the phenomena of dispersion. 



Oxford, 

 February 17, 1835. 



