. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. other formof projecting apparatus. We can bend rays of light easily,by sending them at an angle through the surface of someother medium of greater (or less) density than the air ; andthe greater the angle at which the ray strikes the densermedium, the more it is bent; the amount of refraction atlifferent angles being connected by a simple law which need not be discussed only need to remem-ber, that on entering thedenser medium the rayis bent in towards theperpendicular, and inleaving
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. other formof projecting apparatus. We can bend rays of light easily,by sending them at an angle through the surface of someother medium of greater (or less) density than the air ; andthe greater the angle at which the ray strikes the densermedium, the more it is bent; the amount of refraction atlifferent angles being connected by a simple law which need not be discussed only need to remem-ber, that on entering thedenser medium the rayis bent in towards theperpendicular, and inleaving the denser me-dium away from it. If,then, we have a pieceof glass with inclinedfaces, called a prism(fig. 2), to whose faces the dotted lines n i and n e are perpen-diculars or normals,1 the ray si will be bent towards thenormal, to the path i e, and on leaving the prism will besimilarly bent away from b n to e r. Thus it is permanentlybent in, or refracted, towards the thick side of the prism. 1 All angles in optics are reckoned from the perpendiculars or normals,not from the surfaces Fio. » ON PROJECTION 5 A convex or focussing lens must act in the same way,because it is a piece (or combination of pieces) of glassformed so as to be thickest in the middle, the faces graduallybecoming more and more inclined till they go off to an is a circular prism, of gradually-increasing inclinationtowards its edges. If we place such a lens in a beam ofparallel rays, as from the sun, it is easy to see what musthappen. The centre ray, striking the glass perpendicularly,proceeds straight on unrefracted. The next outer rays,meeting the glass at a small angle, are a little bent in towardsthe centre or thickest part. Then rays farther out from the centre are more bent in, because they strike the surfaces at a greater angle ; and so the whole beam of rays meet practically in one point, f (fig. 3), which is the focus for parallel rays, or principal focus. This is the well-kno
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Keywords: ., bookauthorwrightle, bookcentury1900, bookdecade1900, bookyear1906
