20 
general similitude to a peculiarly shaped fibula found in some 
ancient burial-places in France. 
The President read the following paper on the construc- 
tion of the Cassegrain Telescope :— 
_ & Tt is probably known to the Academy that an applica- 
tion has been made to Government, under the auspices of the 
Royal Society and the British Association, for the establish- 
ment of a large reflector in some convenient part of the British 
dominions, and its employment on a complete survey of the 
southern nebulae. In the course of the discussion which pre- 
ceded this application, the construction of the telescope was 
an object of some importance, and I suggested that it might 
be desirable to try the Cassegrain : this was thought deserving 
of attention, and, in the hope that it may be acted on, I offer 
some rules for determining the dimensions of its parts, which 
will not be unimportant in so gigantic an experiment. ‘They 
were investigated by me many years ago, when directing the 
arrangements of that which Mr. Grubb constructed for the 
Armagh Observatory. It is 15 inches aperture, and its per- 
formance is such as to justify an expectation that this form 
may be made of much greater magnitude. 
‘«* The Cassegrain has been little used ; in fact, I know but 
of two, besides that referred to, which have been made of 
larger aperture than 6 inches: one of 18 inches by Lord Rosse, 
who, however, uses it as subsidiary to his larger telescopes, 
and has not given special attention to its improvement. The 
other was made by the elder Tulley about 1800: it was 15 
inches aperture, and 7 feet focus; but it appears to have been 
indifferent ; for, according to the notes of William Walker (a 
competent judge) it ‘showed Rigel like a shilling, and the 
companion was not seen at all.’ At that time, however, the 
proper method of supporting the great speculum was not 
known. 
