Читать книгу Inventors at Work, with Chapters on Discovery - George Iles - Страница 45

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In this field a recent minor improvement is worthy of mention. Benjamin Franklin many years ago made a pair of spectacles in which the upper half of each glass was ground for far seeing, the lower half for near seeing. To-day such bi-focal spectacles are not made in halves, with an unpleasant broken line across them. In each of the new eyeglasses toward the base a small lens of dense quality is enclosed; through this lens a wearer looks at objects nearby; through the upper part of the eyeglass he looks at distant objects. The joining of the three parts is effected so skilfully as not to be discernible.

Bi-focal lens for spectacles.