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The severe limitations imposed upon nearly all astronomical observations by "seeing"-the effects resulting from passage of light rays through the turbulent atmosphere of the earth-are familiar to every observer. With a small instrument the effect may appear largely as a continual shifting and scintillation of the image of a star, but with a large telescope poor seeing usually manifests itself mainly as an enlargement and blurring of the image. The reason for this difference becomes apparent when one makes a knife-edge test with a large telescope on a bright star. The turbulent elements of the atmosphere appear as shifting dark and bright areas on the image of the objective. In general

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