Wilson's quarter century in photography : a collection of hints on practical photography which form a complete text-book of the art . are fitted to onegauge. The varying diameters of photographic objectives will ever, of course,prevent the adoption of a universal flange for all purposes; but what can andought to be done is the adoption throughout the world of a series of flanges, asfew as possible, of recognized and standard sizes. When two achromatic lenses are to be mounted, they are first of all placed ina trial mount so adapted as to permit of an approximation or separation of the polygon
Wilson's quarter century in photography : a collection of hints on practical photography which form a complete text-book of the art . are fitted to onegauge. The varying diameters of photographic objectives will ever, of course,prevent the adoption of a universal flange for all purposes; but what can andought to be done is the adoption throughout the world of a series of flanges, asfew as possible, of recognized and standard sizes. When two achromatic lenses are to be mounted, they are first of all placed ina trial mount so adapted as to permit of an approximation or separation of the polygon of an infinite number of sides, and we will do well to take the lens as an infinitenumber of prisms, more so, as the infinitely small portion of the lens struck by the raymay be taken for a tangent plane. Thus a converging lens may be considered as prismsunited at their bases, and a diverging lens of prisms united at their apices. As we alreadyknow that prisms refract parallel rays toward the base, it is easily seen why converginglenses refract the rays B B (Fig. 16) to /, and that diverging lenses diverge the raysto/. Fig. The distance of the focus from the lens depends, 1st, upon the curvature; 2d, upon therefracting power of the material; and 3d, upon the thickness of the lens. Not to make the matter unnecessarily complicated, we will take the supposition thatour lenses have an extremely small, or no thickness at all. For cominon glass of anindex of refraction of , calculation shows that a plano-convex lens has a focal lengthof the diameter of the sphere of which the lens is a part. A double-convex lens of equalradii has its focus half that distance, or equal to the radius of the surfaces. If the double-convex lens of equal radii, say of 10 inches, is made of the following substances, thethickness neglected, the foci would be: ABOUT LENSES. 41 lenses. The test object is a watch dial placed at the extreme eud of the testing-room, and the image of this dial is examined
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