ormer, and reflectors given to the latter. All the difficulty in
the execution of these optical instruments lies in the making of the
object-glass, whether they be made of glass or metallic mirrors.

Still at the epoch when the Gun Club made its great experiment these
instruments were singularly perfected and gave magnificent results. The
time was far distant when Galileo observed the stars with his poor
glass, which magnified seven times at the most. Since the 16th century
optical instruments had widened and lengthened in considerable
proportions, and they allowed the stellar spaces to be gauged to a depth
unknown before. Amongst the refracting instruments at work at that
period were the glass of the Poulkowa Observatory in Russia, the
object-glass of which measured 15 inches in width, that of the French
optician Lerebours, furnished with an object-glass equally large, and
lastly that of the Cambridge Observatory, furnished with an object-glass
19 inches in diameter.

Amongst telescopes, two were known of remarkable power and gigantic
dimensions. The first, constructed by Herschel, was 36 feet in length,
and had an object-glass of 4 feet 6 inches; it magnified 6,000 times;
the second, raised in Ireland, at Birrcastle, in Parsonstown Park,
belonged to Lord Rosse; the length of its tube was 48 feet and the width
of its mirror 6 feet; it magnified 6,400 times, and it had required an
immense erection of masonry on which to place the apparatus necessary
for working the instrument, which weighed 12-1/2 tons.

But it will be seen that notwithstanding these colossal dimensions the
magnifying power obtained did not exceed 6,000 times in round numbers;
now that power would only bring the moon within 39 miles, and would only
allow objects 60 feet in diameter to be perceived unless these objects
were very elongated.

Now in space they had to deal with a projectile 9 feet wide and 15 long,
so the moon had to be brought within five miles at least, and for that a
magnifying power of 48,000 times was necessary.

Such was the problem propounded to the Cambridge Observatory. They were
not to be stopped by financial difficulties, so there only remained
material difficulties.

First of all they had to choose between telescopes and field-glasses.
The latter had some advantages. With equal object-glasses they have a
greater magnifying power, because the luminous rays that traverse the
glasses lose less by absorption than the reflectio