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Lens Story: 9 of 28
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·Back Cover
Universal photomicrographic apparatus
(Horizontal position)
astronomical and microscopic lenses, thereby reducing this art to an exact science, was Professor Abbe of Jena.
    For grinding, the big lens is cemented with pitch to an iron plate mounted on a spindle. Emery or some other abrasive is sprinkled on the glass and another iron plate placed above, which is shoved hither and thither over the surface. Finer and finer grinding materials are used, and for the final polishing cloth covered tools and rouge. Very frequent tests of the glass are made. Pieces are chipped off and examined with the spectrometer for their indices of refraction. It is upon these measurements that the mathematical calculation of the curvature is based.
    As already shown, a telescope objective consist of two lenses: one convex and made of crown glass, the other concave and made of flint glass. Therefore two lenses must be ground simultaneously and fitted together with the utmost precision. In the final grinding and polishing, tools having an exactly opposite curvature to that required in the lens itself are employed. In testing the accuracy of this curvature, the lens is placed in a "test glass" also of exactly opposite curvature. Between the two surfaces of glass will be a very thin film of air which will give rise to a color effect just as is observed in soap films viewed in sunlight. When the thickness of this air film is everywhere the same, the color appearing will be uniform over the whole surface.
A standard microscope
In this manner deviations in curvature of only one two-hundred-fifty-thousandths of an inch may be detected. Each lens is then centered in a lathe and the edges ground, after which the two lenses are placed together and mounted for final testing in the telescope tube.
Uses of the Lens
    Try to imagine for a moment what this world would be without spectacles, the camera, the microscope, binoculars, field glasses and the stereopticon, and you will only just begin to appreciate something of the immense significance of the lens in the affairs of men.
Larva of mosquito
Courtesy P. O. Gravelle, South Orange, N.J.
Magnification 14 diameters
    The purpose of a lens always is so to bend the rays of light passing through it as to form an image of some object, or to assist in the formation of an image. The ordinary hand magnifier affords one of the simplest illustrations. We hold it a certain distance from an object, and a magnified image appears. This is a virtual image. It could not