Brewster, David, Sir, 1781-1868, Bache, A. D. (Alexander Dallas), 1806-1867 · Philadelphia : Lea and Blanchard · leaf 297
hole in its centre. Forvery remote objects the curve of the speculum should be aparabola. For nearer ones it should be an ellipse in whosefarther focus is the object, and in whose nearer focus is theimage; and in both these cases the speculum would be freefrom spherical aberration. But, as these… … -wijHNi^ large speculum is placed a small concave one, C D, whichcan be moved nearer to and farther from the large speculumby means of the screw W at the side of the tube. This spec-ulum should have its curvature elliptical, though it is gene-rally made spherical. An eye-piece consisting of two…
Optics
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