the Voltaic Spark before the circuit is completed. 595 
Electrometer, and by which an appreciable space of one five-thou- 
sandth of an inch could be measured with great accuracy. He de- 
scribes this instrument ; and relates several experiments which he 
made with a view to test the correctness of its action. He first 
prepared 160, and then 320 series of the constant battery, in half- 
pint porcelain cells, excited with solutions of sulphate of copper and 
muriate of soda; but although the effects, after the contact had 
been completed, were exceedingly brilliant, not the slightest spark 
could be obtained. He was equally unsuccessful with a water bat- 
tery of 150 series, each series being placed in a quart glass vessel ; 
and also with a water battery belonging to Professor Daniell, con- 
sisting of 1020 series ; but when a Leyden battery of nine jars was 
introduced into the circuit of the latter, sparks passed to the extent, 
in one instance, of six five-thousandths of an inch. 
The author mentions his having been present at the experiment 
of Professor Daniell, on the 16th of February, 1839, when that gen- 
tleman had 70 series of his large constant battery in action ; and 
having been witness of the powerful effects obtained by this appa- 
ratus, he was induced to prepare 100 series of precisely the same 
dimensions, and similarly excited : but although this powerful appa- 
ratus was used under every advantage, and the other effects pro- 
duced were in every respect in accordance with the extent of the 
elements employed, still no spark could be obtained until the circuit 
was completed ; even a single fold of a silk handkerchief, or a piece 
of dry tissue paper, was sufficient to insulate the power of a battery, 
which, after the circuit had been once completed, fused titanium, 
and heated 16 feet 4 inches of No. 20 platinum wire. 
The author then describes a series of experiments made with in- 
duced currents. Twelve hundred and twenty iron wires, each insu- 
lated by resin, were bent into the form of a horse-shoe. A primary 
wire of 115 feet and a secondary of 2268 feet, were wound round 
the iron wires. With this arrangement he obtained a direct spark 
(through the secondary current), sufficient to pierce paper, to charge 
a Leyden jar, &c. Several forms of apparatus employed by the 
author are next described, and also a series of 10,000 of Zamboni’s 
piles. With this arrangement he charged a Leyden battery to a 
considerable degree of intensity, and obtained direct sparks of three- 
fiftieths of an inch in length. He ultimately succeeded in obtaining 
chemical decompositions of a solution of iodide of potassium, 
the iodine appearing at the end composed of the black oxide of 
manganese. 
The Society then adjourned over the Christmas Vacation, to 
meet again on the 9th of January, 1840. 
Jan. 9, 1840. — A paper was read, entitled, “ On the construction 
and use of Single Achromatic Eye-Pieces, and their superiority to 
the double eye-piece of Huyghens.” By the Rev. J. B. Reade, 
M.A., F.R.S. 
The author observes, that experience has shown it to be impracti- 
cable to make a telescope even approach to achromatism, by employ- 
ing the same object-glass with an astronomical, as with a terrestrial 
eye-piece ; for if the focus of the blue rays from the object-glass be 
