From the instructions on the thing, I get the impression that it is a pretty fat line too. If it was as thin as the regular lines on the optics, using one side of the line as reference would probably be a non issue in practicality I think.
I'm also pretty sure it wasn't holographic from the description. It was probably literally a plain black metal bar that would move across the periscopes eyepiece at a rate determined by the gyrocompass, periscope bearing, and submarine speed.
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