THE MOST POWERFUL TELESCOPE IN EXISTENCE. 
389 
a rival in England, and was only beaten perhaps by the great 
refractor of the Washington Observatory. With this instrument 
was made a number of observations of the faint satellites of 
Saturn and Uranus, which rendered the Ealing Observatory a 
familiar name to all astronomers. When, in 1877, the astro- 
nomical world was electrified by the announcement of Professor 
Asaph Hall’s discovery of the two satellites of Mars, it was to 
Ealing that astronomers looked for systematic observations of 
these faint objects, and it was from Ealing Observatory that came 
the only systematic series of measures of these objects which 
has been furnished by England. Astronomers may congratulate 
themselves, therefore, upon this new telescope being in good 
hands, and in an observatory where it will not be allowed to 
rust in idleness like so many of the finest instruments in 
England. 
Satisfied from the performance of his eighteen-inch New- 
tonian reflector that it would be possible to successfully construct 
much larger instruments of this kind, it seems to have been 
about two years ago that Mr. Common first seriously thought of 
constructing a very large reflecting telescope with a silver-on- 
glass speculum. It was obvious that this would be a serious 
undertaking, and one which would require much thought and 
ingenuity to carry it out successfully. Many difficulties would 
require to be boldly faced and successfully overcome. The 
expense alone would have been sufficient to deter most men. 
Experience, skill, courage, perseverance, money ; all would 
be required if success was to be won. 
It was decided to first undertake the manufacture of a tele- 
scope with an aperture of 37 J inches and a focal length of about 
eighteen or twenty feet. This was a much shorter focus than 
had usually been thought essential for an instrument of this 
large aperture. Generally instruments of this kind are made with 
a focal length of from nine to ten times their diameter. This 
would correspond to about thirty feet focus for a speculum of 
the given size. The fine performance of his eighteen-inch tele- 
scope had convinced Mr. Common that it was not necessary to 
give a greater focal length than fifteen or sixteen feet. But 
there were two conflicting interests to be reconciled. The 
shorter the instrument the easier it would be to mount, and the 
easier to observe with ; but, on the other hand, the longer the 
focus the better it would be for taking photographs of the 
heavenly bodies, and this last was one of the main uses that the 
new telescope was intended for. With the view of best recon- 
ciling these two views the instrument was designed with a focus 
of some twenty feet. 
The very first step to be taken was to undertake the manu- 
facture of the glass speculum, and here at the outset an 
