In 1886 William Gurley (1821-1887) obtained a patent (#353,406) for a Locke level with
the vial inside the telescope tube. He described it as "a short, compact, cheap, durable, efficient,
easily adjustable, and well-incased telescopic leveling-instrument, having a very large and clear
field, and requiring but one eye to be used by an engineer or surveyor in quickly taking
preliminary or approximate levels with the instrument held by hand." Maria Gurley, William's
wife and executrix of his estate, obtained a patent (#360,805) for a binocular version of this
instrument in 1887. Here the tube on the right holds the usual lenses of an opera glass, while the
tube on the left holds the level vial, prism, and cross wires of the level. The monocular was soon
selling for $12, and the binocular for $15, and both remained on the market for about 30 years.
The Interstate Commerce Commission transferred this instrument to the Smithsonian in 1962.
Ref: W. & L. E. Gurley, A Manual of the Principal Instruments Used in American
Engineering and Surveying (Troy, N.Y., 1893), pp. 225-226.