minute parts of a Second in Astronomical Observations. 325 
This ingenious invention may be applied to all kinds of tele- 
scopes, and will, we think, be found of great utility for va- 
rious scientific purposes. 
There is one objection, however, of a practical nature, which 
we think it of consequence to mention, and which we fear is 
not susceptible of being removed. It has been shewn by Dr 
Brewster that when the eye is eagerly directed to the contem- 
plation of one object, the retina is thrown into such a state, that 
circumjacent objects, seen by indirect vision, occasionally vanish 
and reappear, so that if the degree of attention with which the 
astronomer is obliged to watch the motion of the star, shall be 
found capable of throwing portions of the retina into a state of 
partial insensibility, he may lose sight of the disk at the very 
instant when it might be of the utmost importance to observe it. 
A singular instance of this illusion will be found in a subse- 
quent article, on the phosphorescence of minerals. 
Art. XVII.' — Remarks on Captain Kater'^s Paper^ containing 
Experiments for determining the Length of the Second's Pen- 
dulum in the Latitude of London, By Mr W illi am W atts. 
Communicated by the Author. 
Long before the Bill for the Equalization of 'Weights and 
Measures was thrown out of the House of Lords in 1816, the 
attention of certain individuals who had promoted the scheme 
of equalization, had been directed to the determination of the 
length of the simple equivalent pendulum in vacuo, vibrating se- 
conds in the latitude of London. On the 30th September 1815, in 
a letter addressed to Davies Gilbert, Esq, M. P. I stated it as my 
opinion, that the length of the second’s pendulum, in the lati- 
tude of London, had never been determined with that degree 
of exactness which is so desirable, and even necessary, in the 
investigation of many physical problems ; such, for example, as 
the figure of the earth. But it was the length of the second’s 
* A notice of these experiments, which was read at the Royal Society of 
Edinburgh, on the 19th January 1818, will appear in an early Number of this 
J ournal. 
