nt objects which subtend only a small anglewith the axis will be sharply in focus on the screen, but theothers will be blurred, and the screen must be moved closer tothe lens to bring them into sharp focus. 41. Astigmatism.—This is a fault which shows itself par-ticularly for pencils of light which… … Figure 53 make this plain. The ellipse L represents a lens seen in per-spective. For convenience in explanation, suppose an opaquepiece of paper pasted over the face of the lens, with a squarehole, so that the beam that comes through is limited to whatpasses through this square aperture. Only the…
Light
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