Prof. Powell on the Dispersion of Light. 477 



desirable—such, for instance, as bridle rods, or other means known 

 to practical mechanics, and, he was sure, well known to Mr. Osier. 

 Mr. Osier replied, that many methods of supporting this part of the 

 apparatus had been tried and laid aside, as not answering ; among 

 the rest, bridle rods. 



Professor Powell then made a communication ■ On the Dispersion 

 of Light.' The object of this communication is to state the progress 

 of the inquiry into the subject of dispersion since the last meeting of 

 the Association. On that occasion, the author laid before the Phy- 

 sical Section the results of his observations for determining the re- 

 fractive indices of the standard rays for 28 media. These have been 

 since published, with some preliminary remarks, as one of the series 

 of memoirs of the Oxford Ashmolean Society. They are to be con- 

 sidered only as first approximations, and it would be very desirable 

 to have many of them carefully repeated, as well as to extend the 

 inquiry to other bodies. The author regrets that he has been unable, 

 from particular circumstances, to carry on these researches during the 

 past summer, but intends to take the first opportunity ot resuming 

 them. In particular, he was kindly favoured by Mr. Brooke with a 

 specimen of some crystals of chromate of lead for examination, and 

 accordingly put them into the hands of Mr. Dollond, who warmly 

 entered into his views, and has used every endeavour to give them a 

 prismatic form, but, unfortunately hitherto without success. It is 

 only by such co-operation of those engaged in different departments 

 of science that inquiries like the present can be successfully carried 

 on, and the author is anxious to obtain specimens of any transparent 

 media, which are capable of prismatic examination, and especially 

 such as are of high dispersive power. 



Meanwhile, he has been engaged in the comparison of observation 

 and theory, especially among the more highly-dispersive bodies which 

 he has examined. He has performed the calculations by the method 

 of Sir W. R. Hamilton, and has found that for those media whose 

 dispersion is not very great, the coincidences are sufficiently close j 

 but, on proceeding to the more highly-dispersive bodies, especially 

 oil of cassia, the discrepancies increase, and moreover preserve a 

 certain regularity of character, which shows that they are not mere 

 errors of observation : this would seem to warrant the expectation, 

 that a further development of the formula might still give success- 

 ful results. These investigations have been communicated to the 

 Royal Society, and will appear in the next volume of its Trans- 

 actions. 



Since the period of this communication, however, the able and 

 profound memoir of Mr. Kelland has appeared in the Cambridge 

 Transactions*. This gentleman's theory is, in some measure a 

 simplification of Cauchy's; the resulting formula, for the dispersion, 

 though substantially the same, as developed in a different form, and 

 readily capable of being applied to numerical computation. In some 

 correspondence with Mr. Kelland, that gentleman favoured the au- 



* See Lond. and Edinb. Phil. Mag., vol. x., p. 336. 



