NavList:

Alexandre Eremenko you wrote: "Stadimeter has scale of smaller range, and higher scale resolution: No drum or vernier is required."

With an ordinary two mirror sextant the angle measured is twice the angle between the mirrors.  With this stadimeter the velocity ratio is much greater, though not constant.  How is this achieved?  I can think of gears, a torque pointer, some sort of linkage of levers and springs, but none of these are obvious in the photograph.  Can it be achieved with mirrors and light rays alone?

After playing around with diagrams, I’m still struggling, so I’m taking a more logical approach.  With the mirrors perpendicular to the line of sight and parallel you would see two images next to each other.  If you moved the arm (for want of a better name) until the top of one image is inline with the bottom of the other, the amount the mirrors move apart or together will be a function of 'a known', the height of a lighthouse say, and 'an unknown', the distance off.  If you take care of the known value first by winding it onto that drum thingy.  Then the movement of the arm appears to give you the distance off directly.  Any better explanation or set of explanatory drawings would be greatly appreciated.

Incidentally it seems that picture editors at the Washington Post and the Daily Mail, or perhaps the syndicator doesn't know the difference between a sextant and a stadimeter either. See:

https://www.washingtonpost.com/news/checkpoint/wp/2015/10/13/after-two-decades-sailing-by-the-stars-is-back-at-the-naval-academy/  
https://www.dailymail.co.uk/news/article-3273519/US-Naval-Academy-returns-celestial-navigation-amid-fears-computer-hacking.html  DaveP