NavList:

Thank you Greg, I think I’m beginning to see it.  I’ve just been experimenting holding my compasses over a horizontal mirror and looking at the reflection.  Let’s suppose there was a mark on the bottom of the Sun. The oil reflected ray through the horizon glass would be inverted so that spot would now appear on the top of the Sun seen through the horizon glass. Kissing this spot with the same spot on sun seen through the index glass, which wouldn’t be inverted, would be like shooting the bottom of the Sun. Therefore, all I have to do is divide the height sextant by two and add 16 minutes.  I would use the real height of the bottom of the Sun to get the artificial refraction.  A few years ago, I could have drawn this, and worked it out for myself, but I’m afraid the little grey cells are getting a bit rusty.  Steady for Astro.  Dave