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 Masses of troops marching backwards and forwards would also be plainly visible, and we may assert with something like absolute certainty that there are neither towns nor villages in the moon, nor any buildings as large as St. Paul's of London or the colossal railway stations of that metropolis.

This telescope, as we have said before, is the largest hitherto constructed, and cost its noble constructor more than 25,000l. It must also be recollected that it was not a mere scientific toy belonging to an amateur philosopher, but a real working instrument in the possession of a true man of science, who did work with it that will render his name famous while civilization lasts. The present Lord Rosse seems worthy in every way of his father's great name, and has already enriched astronomical science with numerous valuable observations.

We shall finish this chapter by a description of the Newtonian telescope constructed by M. Léon Foucault. The mirror, instead of being made of speculum metal, which is an alloy of tin and copper, is made of glass from the famous manufactory of St. Gobain. The first rough grinding having been finished, it passed into the workshops of M. Secrétan, the optician to the Paris Observatory, to receive its final polish and finishing touches from the hand of M. Foucault himself, the most careful optical tests being applied to it before the commencement of each operation.

The glass mirror having reached the degree of perfection desired, was then silvered on its concave surface by being plunged into a bath of nitrate of silver, dissolved in water, and mixed with certain proportions of gum galbanum, nitrate of ammonia, and oil of cloves. Half an hour in this bath was sufficient for the deposition of a film of silver of sufficient thickness to bear polishing. When finished, the mirror was found to