200 



the principles of the undulatory theory, as proposed by M. Cauchy 

 about 1830, by numerical comparison with the indices observed, 

 more especially in cases of the most highly dispersive media then 

 examined. 



The general result then arrived at was, that while the theory 

 applied perfectly through an extensive range of media of low and 

 moderate dispersive power, it did not apply well to those of higher ; 

 and to the highest in the scale (which of course formed the true 

 test of the theory) it did not apply within any allowable limits of 

 accuracy. Since that time little has been done towards prosecuting 

 the subject. 



In the experimental part of the inquiry, about 1849, I had ob- 

 served the indices for a few new media*; but these were not high in 

 the scale ; yet though perhaps thus of little importance, I have now 

 thought it as well to go through the calculation for them : the 

 results are of the same general character as just described. 



Soon after, finding that my friend, the Rev. T. P. Dale, F.R.A.S., 

 was desirous to carry on some researches of this kind, I placed 

 at his disposal the apparatus with which I had determined all my 

 in dices f. 



In 1850 that gentleman communicated to the Royal Astronomical 

 Society a short general account of his observations | relative to some 

 substances not very high in the scale. 



In 1858, Mr. Dale, in conjunction with Dr. J. H.Gladstone, F.R.S., 

 presented to the Royal Society a valuable series of determinations, 

 evincing highly interesting results relative to the change of refractive 

 power in various substances under different temperatures. 



None of these media being high in the scale, they have little 

 bearing on the main object of my inquiries. In two cases (viz. water 

 and alcohol) the indices agree so closely with mine, that it was not 

 worth while to recalculate them. In two other cases I have carried 

 out the numerical comparison, which affords a good agreement with 

 the theory. 



Very recently the same gentlemen have, however, published some 



* See British Association Reports, 1850, Sect. Proc. p. 14. 

 f Described and figured, British Association Reports, 1839. 

 % Notices, vol. xi. p. 47. Phil. Trans. 1858. 



