STO Mr Herschel 07i Achromatic Object-Glasses. 
Thus, we have in the crown lens, 
Focal length, - — 4330 
Radius of one surface, 6.6788 
Index of Refraction, = 1.519 
From which data it is easy to compute, by rules familiar to eveiy 
optician, the radius of the other surface, which will come out 
S.3868. 
Again, in the Flint lens, we have. 
Focal length, - = 7.635 
Radius of one surface, = 13.7229 
Index of Refraction, nr 1.589 
whence we find 3.3871 for the radius of the other surface. 
The four radii are thus obtained for a focal length of 10 
inches ; and to obtain them for 30 inches, we have only to mul- 
tiply them by 3, and we obtain finally, in the case proposed, 
Radius of 1st Surface, of 2d, of 3d, of 4th, 
20.0364 Inch, 10.1604 Inch. 10.1613 Inch. 41.1687 Inch. 
So that here the radii outlie two adjacent surfaces scarcely differ 
more than inch, and they may of course be ce- 
mented together, should it be thought desirable. I am, 
Yours, &c. 
J. F. W. Herschex.. 
Art. XXV . — Proceedings the Royal Society of Edinburgh. 
(Continued from p. 162.) 
17. 1821.— A Paper by Dr Brewster was read, con- 
taining an Account qf a new and extraordinary structure in 
the Faroe Apophyllite*'' ' 
Gn the same evening, a letter from Professor Moll of Utrecht, 
to Dr Brewster, was read, containing an account of some new 
electro-magnetic experiments. This letter is printed in the pre- 
sent number, p. 220. 
1822, Jan. 7. — At this meeting the following Members were 
elected: 
FOREIGN MEMBERS. 
. M. Ampere, Paris. 
M. Van Swinden, Professor of Natural Philosophy, Amsterdam. 
M. Shumacher, Professor of Astronomy, Copenhagen, 
