The microscope and its revelations . scription is called a plano-convex lens; and it wiH hereafterbe shown to possess properties, which render it very useful inthe construction of microscopes. But if, instead of passingthrough a plane surface, the rays re-enter the air through asecond convex surface, turned in the opposite direction, as ina double-convex lens, they wiU be made to converge still will be readily comprehended, when it is borne in mindthat the contrary direction of the second surface, and the con-trary direction of its refraction (this being/rom the densermedium, instead
The microscope and its revelations . scription is called a plano-convex lens; and it wiH hereafterbe shown to possess properties, which render it very useful inthe construction of microscopes. But if, instead of passingthrough a plane surface, the rays re-enter the air through asecond convex surface, turned in the opposite direction, as ina double-convex lens, they wiU be made to converge still will be readily comprehended, when it is borne in mindthat the contrary direction of the second surface, and the con-trary direction of its refraction (this being/rom the densermedium, instead of i7ito it), antagonize each other; so that thesecond convex smface exerts an influence on the course of therays passing through it, which is almost exactly equivalent to i REFRACTION Br CONVEX LENSES. 41 that of the first. Hence the focus of a donble-coBYex lenswill be at just half the distance, or (as commonly expressed)wdll be at half the length, of the focus of s. plam-coBNQX lenshaving the same curvature on one side (Eig. 2).. Parallel rays, falling on a double-convex lens, brought to a focusin its centre; conversely, rays diverging from that poiut, renderedparallel. 4. The distance of the focus-from the lens will depend, notmerely upon its degree of curvature, but also upon therefracting power of the substance of which it may be formed;since, the lower the index of refraction, the less will theoblique rays be deflected towards the axial ray, and the moreremote will be their point of meeting; and conversely, thegreater the refractive index, the more wiU the oblique rays bedeflected towards the axial ray, and the nearer will be theirpoint of convergence. A lens made of any substance whoseindex of refraction is 15, will bring parallel rays to a focus atthe distance of its diameter of curvature, after they havepassed through one convex surface (Eig. 1), and at the distanceof its radius of curvature, after they have passed through twoconvex surfaces (Ei^. 2); and as this rat
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