98 Mr. Rose’s Sketch of the Geology of West Norfolk. 
at Prague, under the title of “Nouveaux Exercices,” &c. They 
will contain the continuation of the theory of dispersion, and 
the development in a form adapted to calculation. The di- 
stinguished author also has recently produced a memoir on 
interpolation, by a new method, which in conclusion he 
briefly applies (but without sufficient explanation) to the cal- 
culation of the refractive indices, in one instance of flint glass 
from Fraunhofer. 
One thing, however, is clear, viz. that from the close accord- 
ance between all the results which I have calculated (by the 
approximate formula) and those of observation, viz. the ten 
sets of indices obtained by Fraunhofer, and since that, ten 
other sets determined by M. Rudberg (very recently com- 
municated to the Royal Society*), it is sufficiently evident that 
at least for all these cases the approximate supposition is as 
near the truth as, perhaps, will be thought sufficient, when 
all circumstances are considered. 
It is, however, still quite conceivable that the differences, 
minute as they are, may be accounted for by a more accurate 
prosecution of the analysis. Again, it remains to be seen 
whether in other cases, especially those of more highly di- 
spersive media, the same method will still apply, or whether 
we must have recourse to a more complex investigation, which 
shall yet include, as a simplified case, the formula which holds 
good for media of low dispersive power. 
= + 
1X. A Sketch of the Geology of West Norfolk. By C. B. Rose, 
Fellow of the Royal Medical and Chirurgical Society of Lon- 
don. 
(Continued from vol. vii. p. 376, and concluded. } 
Diluvium.— CiLsy, sand, or gravel of varying thickness, and 
frequently alternating beds of these substances, 
are found immediately incumbent on the chalk, and obscure in 
many places its outcrop, as they also do that of the gawlz, lower 
greensand, and clays of Marshland+. These irregular beds, 
alternating with each other, without any order of superposition, 
have received the name of diluvium ; but itis so difficult to de- 
termine what has been deposited by diluvial agency, in other 
* So long ago as 1827 we received and inserted in Phil. Mag. and 
Annals, N.S., vol. ii. p. 401, a paper on the undulatory theory of disper- 
sion from M. Rudberg. Has this been lost sight of in the recent investi- 
gations of the subject ? Some of the calculated numerical results obtained 
by M. Rudberg, we observe, are identical with those obtained by Professor 
Powell, as given in Phil. Trans, 1835, pp. 252, 254.—Epir. 
+ The diluvium in Marshland is covered by a considerable thickness of 
alluvial deposits. 
