Davies, Charles, 1798-1876 · New York, Cincinnati [etc.] American book company · leaf 193
d perpendicular to the optical axisin front of the object-glass, at a distance from it equal to itsprincipal focal distance. When the wires are thus adjusted to include a certain space,as two feet for instance, upon a rod placed at a distance of100 feet from the specified point on the optical axis,… … Fig. 105. SEC. III.] BANGING OUT LINES. 187 thus obtained, the principal focal distance of the object-glass,plus the distance of the object-glass from the middle of thehorizontal axis, the distance of the rod from the station can beascertained without the use of a chain. The focal distance of the…
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