| 摘要 |
PROBLEM TO BE SOLVED: To provide a lens for learning whose focal distance is varied and which becomes a concave lens as well as a convex lens. SOLUTION: By pouring water 6 into space between two soft transparent plates 2 and 3 whose periphery is fixed on a frame 1 and varying the quantity of the water by a piston 5, internal pressure is changed, and the transparent plate is recessed or swollen to become the concave lens or the convex lens, then the focal distance of the lens is changed by the degree of the pressure.
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