A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2020 Jun 28, 09:47 -0700
Frank Reed you wrote: Early octants often had two sight peeps separated by 1-1.5 cm horizontally. A metal shade could slide over one or the other of them. This was equivalent to the riser on sextant scopes (both historically and today). The line of sight (line of collimation) was still expected to be parallel to the frame but at two different levels.
Were the holes different sizes; it's not easily seen from photos on the web? Also 2x the area being only 1,42xthe diameter make size difference less obvious. Did the peep plate have to slide in or out depending upon which peephole was in use? Is riser the name for the telescope holder on a sextant?
You also wrote: The small size of the peephole on early octants served as "poor man's eyeglasses". If you are somewhat near-sighted and normally wear glasses or contacts, try poking a small hole in a bit of cardstock, maybe 2-3 mm across, and look through it. Notice how your visual acuity greatly improves!
Yes, this seems to work. Presumably there are similarities with pin-hole cameras, depth of focus of a 'steam' camera being dependent upon lens aperture, and makeshift emergency snow goggles as per winter survival manuals. DaveP