It was somewhere outside Barstow when "R.H."
wrote:

http://puzzlephotos.blogspot.com/

303 Surveying gadget. You stare into it and you can effectively
"see sideways" at accurately 180° to each image. Then you put this
gadget on top of a staff and walk between two other staves. Looking
at the images of the staves and moving around until they line up, you
can place your new staff accurately on the line between the initial
two.

The old way involved two people, one moving the staff and the other
standing at one end of the line and hand-signalling them back and
forth. This way can be done single handed.
--
Smert' spamionam