The New Student's Reference Work/Spectacles
|←Specific Heat||The New Student's Reference Work (1914)
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Spec′tacles are instruments used for aiding the sight. They are said to have been invented in the 13th century. The first ones were very clumsy, and there was very little improvement until the beginning of the 19th century. Spectacles are worn for strengthening the vision, when it is weakened by old age, or to remedy any natural defect, such as nearsightedness, and also to protect the eye from too strong light. The glasses or lenses used should be made of the best of glass and carefully ground, though sometimes they are made from rock-crystal and called “pebbles.” The glasses should be very carefully selected to accomplish the purpose intended, it being often necessary to fit each eye separately. For short sight or nearsightedness the lens is concave, and the weakest glass that will permit distant objects to be seen should be used. For the weakness of old age, which is farsightedness and is first noticed by one’s holding a book farther off in reading etc., the lens is convex, and the strongest glass should be used.