190 Proceedings of the Asti'ommcal Society of London. 
April 10. — A paper was read On the construction and use 
of a new micrometrical eye-piece of a telescope by the Rev. 
W. Pearson, LL. D. F. R. S. Member of the Astronomical So- 
ciety of London. 
The object of this communication is to explain the practical 
construction of the new prismatic micrometer, and the astrono- 
mical eye-piece which Dr Pearson had invented, and to give to 
the Society the results of some observations he had made with 
that instrument. In order to determine the power of the tele- 
scope in any given position of the lenses composing the eye- 
piece, it is necessary to be acquainted with the focal distances of 
the several glasses of which it consists. The mode by which 
Dr Pearson ascertained the focal distance of any small lens was 
as follows : He placed it as the eye-glass to an object-glass, 
whose focal distance was accurately known ; thus forming an as- 
tronomical telescope of the simplest kind, having measured, by 
means of a dynameter, the magnifying power of this instrument, 
the focal distance of the eye-lens is equal to the focal distance 
of the object-glass divided by the magnifying power. At the , 
conclusion of this paper are given several tables of measure- 
ments of several planets and double stars effected by an instru- 
ment of the kind above described. 
May 12. — A letter was read from the Rev. G. Peacock, 
A. M. F. R. S. Fellow of Trinity College, Cambridge, to 
C. Babbage, Esq. F. R. S. L. k E. giving an account of the 
means taken for establishing an observatory at Cambridge. It 
is proposed to provide two sets of instruments, one of which 
will be employed solely by the observer and his assistants. It is 
understood that these are to have every perfection which the pre- 
sent state of the art can give them ; and in mentioning the name 
of Mr Troughton as their maker, all other recommendation will 
be unnecessary. The other set of instruments are to be of less 
costly construction, and are intended for the instruction of those 
members of the University who may be desirous of acquiring a 
knowledge of practical astronomy. The University have given 
L. 5000 as the commencement of a subscription, which it is 
hoped will amount to L. 15,000, — the sum which is estimated 
as the probable expence of the establishment. 
