This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1869 Excerpt: ...which are of equal focal length, and separated by an interval equal to two-thirds of the common focal length, as in Eamsden's positive eye-piece: lastly comes the diaphragm, d, placed at a distance from m, the second lens of the magnifier, equal to the focal length of this magnifier, which is one-fourth the focal ...
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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1869 Excerpt: ...which are of equal focal length, and separated by an interval equal to two-thirds of the common focal length, as in Eamsden's positive eye-piece: lastly comes the diaphragm, d, placed at a distance from m, the second lens of the magnifier, equal to the focal length of this magnifier, which is one-fourth the focal length of m or p. The best forms of the two lenses are, for the first, a planoconvex, and, for the second, a convex meniscus, the radii of whose surfaces are as 1 to 15; and the advantage aimed at in this construction is to render the image flat, and consequently capable of coinciding with the plane screen upon which it is to be received. A similar purpose is the object of the construction of Eamsden's eye-piece, viz., to obtain, as it is there called, a flat field. The object being placed a little further from p than the focal length of the magnifier, the pencils of rays from each point of the object, after passing through the two lenses, become slightly convergent, and, at a distance from the diaphragm depending upon the distance of the object from the lens p, the magnified image is formed inverted with respect to the object THE CAMERA OBSCURA. This instrument consists of a plane reflector, upon which pencils of light from the various points of a landscape are re ceived and reflected, so as to pass first through a diaphragm, and then through a plano-convex lens, after which the rays of the pencils become convergent, and form an image upon a screen in a darkened chamber placed to receive it. The diaphragm and lens are placed in a tube, which is passed through a hole in the chamber just large enough to receive it, so that no extraneous light may be admitted. The distance of the lens from the diaphragm is determined upon the condition that the image...
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