Comparisons of light grasp derived from small, fresh, carefully silvered surfaces are sometimes given which lead to illusory results, and from such experiments Foucault claimed superiority for the silvered speculum over the object-glass.
The Speculum Naturale fills a bulky folio volume of 848 closely printed double-columned pages.
Rutherfurd introduced into common use the reflection grating, finding that speculum metal was less trying than glass to the diamond point, upon the permanence of which so much depends.
No further practical advance appears to have been made in the design or construction of the instrument till the year 1723, when John Hadley (best known as the inventor of the sextant) presented to the Royal Society a reflecting telescope of the Newtonian construction, with a metallic speculum of 6-in.
Francois and Speculum perfectionis, and Lempp's Frere Elie de Cortone.
Rogeri Baconis Anglici de Arte Chymiae Scripta (Frankfort, 1603) - a collection of small tracts containing Excerpta de Libro Avicennae de Anima, Breve Breviarium, Verbum Abbreviatum, 3 Secretum Secretorum, Tractatus Trium Verborum, and Speculum Secretorum; (5) Perspectiva (1614), which is the fifth part of the Opus Majus; (6) Specula Mathematica, which is the fourth part of the same; (7) Opus Majus ad Clementem IV ., edited by S.
Among them the Vinland sagas, also a Norwegian work of the 13th century, called Speculum regale (The King's Mirror), and some papal letters, give interesting glimpses of the life of this colony.
Silvered mirrors have also some advantage in light grasp over those of speculum metal, though, aperture for aperture, the former are inferior to the modern object-glass.
Chiefly owing to the brittleness of this material, Lord Rosse's first larger specula were composed of a number of thin plates of speculum metal (sixteen for a 3-foot mirror) soldered on the back of a strong but light framework made of a peculiar kind of brass (2.75 of copper to 1 of zinc), which has the same expansion as his speculum metal.
With few exceptions all the known events of Defoe's life are connected with authorship. In the older catalogues of his works two pamphlets, Speculum Crapegownorum, a satire on the clergy, and A Treatise against the Turks, are attributed to him before the accession of James II., but there seems to be no publication of his which is certainly genuine before The Character of Dr Annesley (1697).