Submarine Portholes
Portholes on submarines are generally made of acrylic plastic. In the case of deep diving submarines, the portholes can be several inches thick. The edge of the acrylic is usually conically tapered such that the external pressure forces the acrylic window against the seat. Usually such windows are flat rather than spherically dished. This decreases the area that can be viewed, but eliminates distortion associated with curved glass.
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