n on the metallic
mirror of telescopes; but the thickness that can be given to glass is
limited, for too thick it does not allow the luminous rays to pass.
Besides, the construction of these vast glasses is excessively
difficult, and demands a considerable time, measured by years.

Therefore, although images are better given by glasses, an inappreciable
advantage when the question is to observe the moon, the light of which
is simply reflected they decided to employ the telescope, which is
prompter in execution and is capable of a greater magnifying power; only
as the luminous rays lose much of their intensity by traversing the
atmosphere, the Gun Club resolved to set up the instrument on one of the
highest mountains of the Union, which would diminish the depth of the
aerial strata.

In telescopes it has been seen that the glass placed at the observer's
eye produces the magnifying power, and the object-glass which bears this
power the best is the one that has the largest diameter and the greatest
focal distance. In order to magnify 48,000 times it must be much larger
than those of Herschel and Lord Rosse. There lay the difficulty, for the
casting of these mirrors is a very delicate operation.

Happily, some years before a _savant_ of the _Institut de France_, Leon
Foucault, had just invented means by which the polishing of
object-glasses became very prompt and easy by replacing the metallic
mirror by taking a piece of glass the size required and plating it.

It was to be fixed according to the method invented by Herschel for
telescopes. In the great instrument of the astronomer at Slough, the
image of objects reflected by the mirror inclined at the bottom of the
tube was formed at the other extremity where the eyeglass was placed.
Thus the observer, instead of being placed at the lower end of the tube,
was hoisted to the upper end, and there with his eyeglass he looked down
into the enormous cylinder. This combination had the advantage of doing
away with the little mirror destined to send back the image to the
ocular glass, which thus only reflected once instead of twice; therefore
there were fewer luminous rays extinguished, the image was less feeble,
and more light was obtained, a precious advantage in the observation
that was to be made.

This being resolved upon, the work was begun. According to the
calculations of the Cambridge Observatory staff, the tube of the new
reflector was to be 280 feet long and its mir