British Association for the Advancement of Science. 293 



on the list of those from which invitations had been received 

 on former occasions ; and its situation being also far re- 

 moved from the districts which the Association has hitherto 

 traversed, it was determined to hold the ensuing meeting in 

 that city in August next. The highly interesting and impor- 

 tant country which forms the South-west of England will be 

 conveniently embraced by this meeting, and the zeal which 

 public bodies no less than individuals have shown to facilitate 

 and encourage the arrangements for it, concurs with the high 

 reputation of the men of science connected with Bristol, to 

 hold out the confident expectation of a successful result. 



— — 



Notices of Lectures delivered at the Evening Meetings of the 

 Association. 



Professor Powell gave a lecture on the phaenomena of prismatic 

 dispersion, in relation to the undulatory theory of light. 



After giving a general view of the phaenomena, and a particular de- 

 scription of the black lines in the spectrum whose position is taken as 

 a measure of the refractive and dispersive powers of substances, Pro- 

 fessor Powell proceeded to state the results of some recent labours 

 undertaken by himself in order to ascertain whether the undulatory 

 theory of light, which is admitted to explain almost every fact in 

 optical science except dispersion, could be applied to explain that 

 also*. By reducing to calculation a formula suggested to the author 

 by Professor Airy, as arising out of the researches of M. Cauchy, 

 and expressing a relation between the refractive index of a ray and 

 the length of the wave, a very close agreement was found be- 

 tween the numbers which result from the formula and those observed 

 by Fraunhofer for ten different media, viz. four kinds of flint glass, 

 three of crown glass, water, oil of turpentine, and solution of pot- 

 ash. Professor Powell is engaged in the arduous labour of testing 

 the applicability of M. Cauchy's modification of the undulatory 

 theory to the explanation of the phaenomena of prismatic disper- 

 sion, by individual examples ; and he states, that as far as the calcu- 

 lations have been executed, it appears that even the extreme case of 

 that highly dispersive substance oil of cassia is comprehended with 

 at least considerable accuracy by the theory. It appears, then, that 

 one of the greatest of the remaining objections to the reception of 

 the undulatory theory is at least partially removed. 



The Rev. W. Whewell stated the progress which had been made 

 during the past year in observations of the tides, not only round the 

 coasts of Great Britain and Ireland, but also under the direction of 

 the Governments of Sweden, Denmark, Russia, Spain, France, Hol- 



* Professor Powell's abstract of M. Cauchy's view of the undulatory 

 theory will be found in Lond. and Edinb. Phil. Mag., vol. iv. p. 16 et seq. ; 

 and a notice of the results of his calculations relative to the refractive in- 

 dices observed by Fraunhofer, in vol. vi. p. 374. — Edit. 



