Light; a course of experimental optics, chiefly with the lantern . revent scratching. Carefully clean them, andthen carefully slide or grind them with moderate pressuresmoothly together. We very soon see beautiful fringes ofgorgeous colour. When satisfactory, pinch one lower corner 166 LIGHT. [chap. of the double plate in the clip, and the three others withloose wooden spring letter-clips. Focus as before : all willbe reproduced on the screen, and as we further pinch any-where, even with the finger and thumb, changes and move-ments of the colours will demonstrate that the particulsircolour who
Light; a course of experimental optics, chiefly with the lantern . revent scratching. Carefully clean them, andthen carefully slide or grind them with moderate pressuresmoothly together. We very soon see beautiful fringes ofgorgeous colour. When satisfactory, pinch one lower corner 166 LIGHT. [chap. of the double plate in the clip, and the three others withloose wooden spring letter-clips. Focus as before : all willbe reproduced on the screen, and as we further pinch any-where, even with the finger and thumb, changes and move-ments of the colours will demonstrate that the particulsircolour wholly depends on the thickness of the film. Fig. 98.—Newtons Rings. 99. Thickness of the Film.—Newtons Rings.— We want to know, if possible, however, what that thickness is,and the last experiment probably suggested to Newton hisfamous rings. He placed a convex lens of very slightconvexity in contact with a flat glass,.as in Fig. 98, againstwhich it was pressed by screws. A simpler method sometimes,employed is to cut two circular flat glasses (they must be a. Fig. 99.—Newtons Rings with Flat Glasses. J-inch thick, or, at least, one must be so), and having care-fully cleaned them, place a ring of gold-leaf between themat their circumference. Mounted as in Fig. 99, pressurefrom the centre screw at the back produces, as in the othercase, Newtons rings, which, in either case, are presentedto the lantern and focused on the screen precisely in the IX.] NEWTONS RINGS. 167 manner of the soap-film (Fig. 97). This method is withinthe power of many who like to construct their ownapparatus. It is clear that, knowing either the curve of the lens, orthe thickness of the gold-leaf, it is very easy to calculate
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