Light visible and invisible : a series of lectures delivered at the Royal Institution of Great Britain, at Christmas, 1896 (1897)
Thompson, Silvanus Phillips, 1851-1916, Royal Institution of Great Britain · New York : Macmillan · leaf 58
Fig. 24. the path a lens made of glass thicker in the middlethan at the edges. At once it is observed—thanks tothe dust in the air—to make these waves converge to afocus at F (Fig. 25). This is again a real focus. A lensthat is thus thicker in the middle than at the edges iscalled a convex lens.… … Fig. 25. would be the opposite. Since the thin middle retardsthe mid parts of the wave-front less than the thick glass
Light · X-rays
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