How? or, Spare hours made profitable for boys and girls (1887)
Holbrook, Kennedy · New York, Worthington co. · leaf 154
and allowed to fall over thetriangular side-openings, so as to still further preventoutside light from reaching the ground glass, is a greatimprovement. In the diagram, the dotted lines show the course of thelight from the object in view, through the lens (where therays cross each other) to the… … vex spectacle-glass, and the board marked b a partition,serving as a screen upon which the image is thrown. Inthis form the lens may be fixed in the end of the box ifdesired, which is much easier than adjusting it in a slid-ing tube. The focal distance of an ordinary spectacle-glass averages about…
Handicraft · Amusements
Internet Archive · Library of Congress · Public Domain Mark 1.0. Free to use for anything.
