Optical Distance Measurement instruments.The instruments in this section are distance measurement devices used to measure distances from a vessel to distant objects. These devises were made along different design lines, two of which are currently shown here. They were used to determine the distance between vessels when sailing in convoy, to get distances to enemy vessels, or simply to the shore.
In all cases known lengths or heights of objects were used to determine one's distance to it, by superimposing the end of the known feature on top of each other by coincidence.
The Schick Stadimeter does this in a sextant way of measurement by using double reflection, while the Cotton Range Finder does this by deviating the light paths using a linear lens combined with a short prism.
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1943 U.S. Navy stadimeter Cotton Type Range Finder