LECTURES DELIVERED. 
5 
derstanding of the spirit in which it is to be pursued and 
the methods by which it is to be advanced. 
Nor does there seem to be any reason to fear that the want 
of a locality for such assemblages will be found to place an 
impediment in their way. At the late meeting there were 
deputies present from five of the chief commercial towns in 
England to invite the Association and to offer suitable ac¬ 
commodation in their respective towns. Bristol stood first 
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 EVEN¬ 
ING MEETINGS OF THE ASSOCIATION. 
Professor Powell gave a lecture on the phenomena of prismatic 
dispersion, in relation to the undulatory theory of light. 
After giving a general view of the phenomena, 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 phenomena 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 
