Mr. Earnshaw on Dispersion, in reply to Prof. Powell. 217 



produced by nacrite, by decomposed films of glass and by 

 polarizing plates, are distinctly referable. Here also we have 

 the probable cause of certain remarkable phenomena of di- 

 chroism in doubly refracting bodies, in which rays of the same 

 refrangibility, but of different colours, pass into the ordinary 

 and extraordinary pencils. 

 Allerly, May 5th, 1837. 



XXXVIII. On the Theory of the Dispersion of Light; in 

 reply to Prof. Powell's Note. By S. Earnshaw, M.A., 

 Cambridge*. 



T^HE object which I had in view in writing the letter printed 

 -* in your Magazine for April, was to show that the " op- 

 probrium of all theories, — the dispersion of light," — has not 

 yet been removed from the undulatory theory. I endeavoured 

 to accomplish this object by showing two things ; — 1st, that a 

 certain formula, derived directly from theory, which was said 

 to have supplied " both the laws and the explanation of the 

 phenomena of dispersion," is insufficient for that purpose ; 

 and 2ndly, that the method of calculation employed in com- 

 piling the tables given in Professor Powell's book is a method 

 of interpolation only, and therefore from its very nature inca- 

 pable of verifying a physical theory of dispersion. It is not 

 necessary to repeat the arguments by which I endeavoured 

 to establish these two points. In answer to the former, the 

 Professor distinctly states that he has "long since discarded " 

 the formula animadverted upon ; and therefore I suppose that, 

 as far as that formula is concerned, I may consider the object 

 of my letter accomplished. In answer to the remaining parts 

 of my letter, the Professor, if I rightly understand his note, 

 puts forward three arguments : — 



1st. That Sir W. R. Hamilton has taken the trouble of 

 simplifying the mode of calculation, a circumstance which im- 

 plies his approval of the general principle. 



2ndly. That that " pre-eminently gifted mathematician 

 M. Cauchy " has considered his own investigations a suffi- 

 cient basis for calculation ; and, 



3rdly, That the method of calculation used in computing 

 the tables " is surely, at all events, a direct deduction from 

 theory." 



Now I will not accuse Professor Powell of bringing forward 

 the first two of these with the intention of carrying the dis- 

 puted point by the force of great names; but if such had been 

 his intention, they are, as it seems to me, better suited for 

 * Communicated by the Author. 



