Optical rangefinders were an essential tool in all forms of artillery.
This video explores the basic mechanics of how they work.

If you are interested in learning more, these links below provide fascinating insights. Courtesy to them for their valuable research and insights.

http://en.wikipedia.org/wiki/Coincide...
http://www.navsource.org/archives/01/...
http://mathscinotes.com/wpcontent/up...
http://www.eugeneleeslover.com/USNAVY...