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But the refraction of the rays of light are chiefly re-markable when the mediums through which they pass areseparated by a spherical surface.
That portion of optics which treats of refraction and itslaws, is called Dioptrics. Glasses used for assisting thesight in viewing distant objects, are called Dioptricalglasses.
In nothing have the moderns excelled the ancients morethan in their discoveries concerning light, which is yetvastly imperfect, and in the invention of optical instruments.
Concave glasses for collecting the rays of light into apoiut, or convex ones for making them diverge, are calledlenses; from their resemblance to the seed of the lentil,(lens), a kind of pulse.
Of lenses, there are five kinds: 1st. a plano-convex, i. e.plain on one side, and convex on the other; 2d. a doubleconvex, convex on both sides ; 3d. a plano-concave; 4th. adouble concave ; and 5th. a miniscus, or concavo-convex; i.e.concave on one side, and convex on the other like a watch-glass.
An oblong glass, with at least three phun sides, boundedby parallel lines, having both ends similar and parallel, iscalled a prism.
In passing through glasses plain on both sides, the direc-tion of the rays is not changed. In passing through a convexlens, the rays converge towards each other, in proportionto the convexity of the glass ; and in a concave lens the con-trary ; oblique rays, however, more than direct rays. Con-vex lenses also become burning glasses; with these theancients are presumed to have been well acquainted; Aristoph.in Nub. ii. 1.140. P/in. xxxvi. 67. xxxvii. 10. Lanctantde iri Dei. e. 10.; and are thought by means of them tohave lighted their sacred fire. Archimedes also is saidto have constructed burning-glasses, or specula, (see asubsequent article,) which destroyed the Roman fleet beforeSyracuse , at the distance of a bow-shot. And Zonoras saysthe same thing was done at Constantinople, by one Proclus ,under Anastasius .
When the rays of light passing through a single or doublelens, are brought into their smallest compass, the pointthereby formed is the focus of the lens.
By placing a convex lens in a small opening through the
subject to corruption; called also Tritogenia, or thrice begotten, because thissuited with the ancient division of the year, imo spring, summer, and winter.They also named her Glaucopis , as some believed from the colour of the eye, grey.But this author says, it is most probable, because that was the colour the air assumed.