Of as little importance as the above passage of Alexander, is another of Isidore, often quoted in support of the antiquity of glass mirrors. On the first view it appears to be a testimony of great weight; but when closely examined it becomes reduced to very little. “Nothing,” says he, “is so fit for mirrors as glass[187].” Abat and others, who have considered these words as decisive, make less hesitation to ascribe to the sixth century, in which Isidore lived, a knowledge of mirrors covered on the back with tin and quicksilver, as the same writer, in another place, observes, that quicksilver can be kept in no vessel but one of glass[188]. It is very true that a glass filled with that metal will form a very good mirror; but I am of opinion that this may have been long known, before people thought of making an amalgam of tin and quicksilver in order to cover the backs of mirrors. The first passage, which is properly the one of any consequence, loses its force when we see that it is taken from Pliny and copied incorrectly. The latter says, that one can give to glass every kind of shape and colour, and that no substance is more ductile, or fitter to be moulded into any form[189]. Isidore, as is usual, says the same thing, and in the same words, except, that instead of _sequacior_ he substitutes _speculis aptior_; so that the mention of a mirror is altogether unexpected, and so little suited to what goes before and what follows, that one must believe that this alteration, occasioned perhaps by the similitude of the words, or by an abbreviation, was not made by Isidore, but by some transcriber. But even if we believe that Isidore himself spoke of glass being used at that period for mirrors, we are not able to comprehend, from what he says, how glass mirrors were made in the sixth century. I have met with no information respecting this subject in the whole period between the age of Isidore and the eleventh century. About the year 1100, at least as is supposed not without probability, Alhazen the Arabian wrote his well-known treatise on Optics, in which I conjectured that I should find mention made of glass mirrors; but I searched that work in vain, though I must confess I did not read it through entirely. Where he begins his catoptrical lessons, he however often speaks of iron mirrors, by which we may understand mirrors of the best steel. In explaining a certain phænomenon, he says, that the cause of it cannot be in the darkness of the iron mirror, because if a mirror of silver be used, the same effects will be produced. Would he not on this occasion have introduced glass mirrors, had he been as well-acquainted with them as with those already mentioned? At first, he never speaks of mirrors without adding of iron, of silver; but he mentions them afterwards without any epithet of the kind.