The penetration depth of magnetic fields in superconducting tin films was measured. The films, 400-2000 A thick, were evaporated in vacuo and had the form of hollow cylinders A magnetic field of 60 c/s was externally applied parallel to the axis of the cylinder and the magnetic field which penetrated through the films into the interior of the hollow body was detected. It is found that there are departures from the law A(X) = A(0) [1-(T/Xc)4]-1'2, which are in agreement with Lewis extension of

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... e Casimir-Gorter two fluid model and with the theory of Bardeen, Cooper, and Schrieffer. The absolute value of the London penetration depth at T = 0°K is found to be A(0) = 700 4_; 100 A for the thickest films and increases with decreasing film thickness.