in the Theory of the Dispersion of Light. 267 



ployed in my later calculations ; but which essentially involve 

 the fault of requiring too large assumptions from observa- 

 tions, and these too, according to the remarks of Prof. Lloyd, 

 leading to conclusions inconsistent with truth : this he has 

 pointed out in a letter to myself; the details of his investiga- 

 tion have not yet been published ; but there is a short abs- 

 tract of them in the Proceedings of the Royal Irish Academy, 

 No. 1 . p. ] 0, and some mention of the subject was, I believe, 

 made at the British Association 1838, in the Physical Section. 

 I have reason to hope that the paper will before long be 

 published, meanwhile it may not be irrelevant to offer one 

 observation more on the subject. 



Since such a formula as the above is unquestionably very 

 nearly accordant with the truth, through, at least, a consider- 

 able range of media, it certainly becomes important to ex- 

 amine, whether any theoretical conditions are conceivable, 

 which may warrant the adoption of it, as at least a good ap- 

 proximation ; and the question obviously reduces itself to 

 this : whether in taking the sum of an indefinite number of 

 terms which are combinations of H^ and A x^ , H,, and A x^^ , 

 H^^^ and A x^^^ , &c. (the form of which is seen in my paper, 

 Phil. Trans. 1838, part ii.), we may not discover some ground 

 for justifying the adoption of a single term containing some 

 mean value of A x combined with a constant coefficient H, 

 such, that we shall have accurately or nearly, 



TT A jr. 



( sin' {^=-=^)^ " 



1^)^ 



I 



+ <H » 



(^')"| 



r^^') 



ri^^i\ 



\ 



L + &C. 



= H^ 





fTT ^X 



How far the dynamical conditions of a system of molecules 

 such as that supposed by M. Cauchy and the other eminent 

 mathematicians who have treated on the subject, may be found 

 susceptible of leading to any such deductions (in which the 

 process would be perhaps somewhat analogous to finding the 

 expression for the centre of gravity of a system of particles,) 



