I Had the honour last winter to lay before the Royal Society a few observations upon some of the curiosities found at Herculaneum, _&c._[37]. Among other articles, I just mentioned a piece of a plate of white glass; and now beg leave to inquire into the uses, to which such plates might be applied in the early age, to which this fragment undoubtedly belongs. And here a person, who forms his ideas of ancient customs by what he sees practised in later times, may be ready to offer several conjectures; in some of which he will, probably, be mistaken; as in others he may be justified by the genuine evidences of antiquity. And, first, It is obvious to imagine, that such plates might serve for _specula_, or looking-glasses. And, indeed, that _specula_ were anciently made, not only of metals, and some stones, as the[38] phengites, _&c._ but also of glass, may, I think, be collected from Pliny, who, having mentioned the city of Sidon as formerly famous for glass-houses, adds immediately afterwards, _Siquidem etiam specula excogitaverat_[39]. But then it is to be observed, that before the application of quicksilver in the constructing of these glasses (which, I presume, is of no great antiquity), the reflection of images by such _specula_ must have been effected by their being besmeared _behind_, or tinged _thro’_ with some dark colour, especially black, which would obstruct the refraction of the rays of light[40]. Upon these hypotheses (supposing the tincture to be given after fusion) the _lamina_ before us may be allowed to be capable of answering the purpose here assigned. It may further be suggested, that plates of this kind might be intended to be wrought into lens’s, or convex glasses, either for burning, or magnifying objects placed in their focus. But this designation cannot be supported by proper vouchers from antiquity. On the contrary, we are informed, that the ancients used either _specula_[41] of metal, or balls[42] of glass for the former of these purposes; as it is well known, that glass was not applied to the latter, in optical uses, till the beginning of the XIIIth century[43].