596 Royal Society. 
thrown forwards, as it must be in order to make it impinge upon 
the focus of the blue rays of the terrestrial eye-glass, then there will 
be produced a great over-correction for the astronomical eye-glass ; 
and vice versa. Hence it appears that the application of Huyghen- 
ian eye-pieces to refracting telescopes, is incompatible with the 
conditions of achromatism, throughout the entire range of magni- 
fying power ; and that in reflecting telescopes they unavoidably in- 
troduce dispersion, because they are not in themselves achromatic. 
These defects the author proposes wholly to obviate, by substituting 
for the Huyghenian eye-pieces, single achromatic lenses of corre- 
sponding magnifying power; consisting of the well-known combina- 
tion of the crown-lens, and its correcting flint-lens, having their ad- 
jacent surfaces cemented together ; thus avoiding internal reflections, 
and enabling them to act as a single lens. The achromatic eye-pieces 
which he uses were made by Messrs. Tulley and Ross, and are of 
the description usually termed single cemented triples. 
A paper was also read, entitled, “ Meteorological Observations 
made between October, 1837, and April, 1839, at Alten in Fin- 
marken.” By Mr. S. H. Thomas, chief mining agent at the Alten 
Copper Works. Presented to the Royal Society by John R. Crowe, 
Esq., Her Britannic Majesty’s Consul at Finmarken. Communicated 
by Major Edward Sabine, R.A., V.P.R.S. 
This memoir consists of tables of daily observations of the baro- 
meter and thermometer, taken at 9 a.m., 2 p.m., and 9 p.m., with re- 
marks on the state of the weather, at Kaafjord, in latitude 69° 58^ 2>" 
north, and longitude 23° 4<3' 10" east of Paris. 
Jan. 16. — A paper was read, entitled, “ On Nobili’s Plate of Co- 
lours ; in a Letter from J. P. Gassiot, Esq., addressed to J. W. 
Lubbock, Esq., V.P. and Treasurer R.S.” Communicated by J. 
W. Lubbock, Esq. 
The effect produced by the late Signor Nobili, of inducing 
colours on a steel plate, excited the curiosity of the author, and led 
him to the invention of the following method of producing similar 
effects. Two of Professor Daniell’s large constant cells were exci- 
ted with the usual solutions of sulphate of copper and sulphuric acid. 
A highly-polished steel plate was placed in a porcelain soup-plate, 
and a Altered solution of acetate of lead poured upon it. A piece 
of card-board, out of which the required figures had been previously 
cut with a sharp knife, was then placed upon the steel-plate. Over 
the card, and resting on it, there was fixed a ring of wood, a quarter 
of an inch thick, and the inner circumference of which was of the 
same size as the figure. A convex copper-plate was made so that 
its outer edge might rest on the inner part of the wooden ring ; and 
its centre placed near, but not in actual contact with the card-board. 
Connexion was then made by the positive electrode of the battery 
with the steel-plate ; the negative being placed in the centre of the 
copper convex plate. The figure was generally obtained in from 
15 to 35 seconds. If a concave, instead of a convex plate be used, 
tlie same colours are obtained as in the former experiment, but in 
an inverse order. 
“ Geographical position of the principal points of the Triangula- 
