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• In optics any distance of over twenty feet, rays of light from which are practically parallel.

1898 - American pocket medical dictionary
By Willam Alexander Newman Dorland

• A term frequently employed by ophthalmologists to designate the position of an object the rays of light coming from which are parallel. Practically it is about 20 feet, as the rays emanating from such a point are so slightly divergent as to be regarded as nearly parallel.

1916 - Appleton's medical dictionary
By Smith Ely Jelliffe