Light; a course of experimental optics, chiefly with the lantern . n the screenwith the loose lens, a blackened glass slide with one or moreperpendicular lines, or other figures, scratched in whitethrough the varnish. Now hold across the slide a strip ofplate glass \ inch thick, so as only to cover a portion of thefigure. When this is held flat to the slide there is of course S8 LIGHT. [CHAP. no perceptible refraction; but when the plate glass is heldobliquely, so that one end is farther from the slide than theother, the lines as far as covered are perceptibly deflected orbroken. (Fig. 42.) 35
Light; a course of experimental optics, chiefly with the lantern . n the screenwith the loose lens, a blackened glass slide with one or moreperpendicular lines, or other figures, scratched in whitethrough the varnish. Now hold across the slide a strip ofplate glass \ inch thick, so as only to cover a portion of thefigure. When this is held flat to the slide there is of course S8 LIGHT. [CHAP. no perceptible refraction; but when the plate glass is heldobliquely, so that one end is farther from the slide than theother, the lines as far as covered are perceptibly deflected orbroken. (Fig. 42.) 35. Prisms.—But we further perceive that if the twosurfaces of the glass are inclined to each other, the ray mustbe permanently deflected into a new direction. Any refract-ing body with faces so inclined is called a prism. Fig, 43shows a section of such a prism, and it is clear that a ray,s I, infringing on the first surface at the angle sin with thenormal N,will be refracted in the path 1 e towards the per-pendicular. But en is the normal to the second surface,. Fig. 42.—Deflected Image. and on emerging the ray must be refracted from that, in thedirection e r, widely different from that of the incident is equally clear that the deviation must depend not onlyon the density, but on the angle of the refracting surfaces ofthe prism. It also, however, depends on the position ofthe prism (§ 36). All this is easily demonstrated; but to avoid much colourphenomena, which must be studied separately, it is best totake a water-prism of rather a small angle. Such a prism isreadily constructed by shaping a smooth wedge of beech III.] PRISMS. 59 about ^ inches square, one inch thick at one side, and taperingto an edge at the other. Bore centrally through, from faceto face, a circular hole 2 inches diameter; paint the inside ofthe hole with sealing-wax varnish; and then, heating twoclean pieces of plate glass 3 inches square, cenient them withhot sealing-wax or shellac on the flat s
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Keywords: ., bookcentury1800, bookdecade1880, bookidcu3192403121, bookyear1882
