NavList:

..............the
slightly eccentric thing about both is the huge white plastic (nylon?) Knob
on the micrometer. Seems stylistically out of place on a traditional 1930s
style,clunky  piece of  British metal engineering? However, very functional
and easy to read accurately.
Thanks for your pics and information. Do you get acceptable nav results?

Francis

As I said, I thought it might be more nautical to leave my 1941 Mates sextant in TIKI for once, so I’m working from memory.  I’d always liked to imagine the micrometer drum was ivory (dreamer!), but of course looking at its underside on my original 5.5mb snap, I can see that it was moulded, probably pressed and cured, so it must be plastic.  It’s not worth sacrificial testing to see if it’s thermal or thermal setting plastic, but for the 1941 model I would guess that it was thermal setting.  It’s possibly melamine formaldehyde, which was discovered by William F Talbot around 1936, or possibly Catalin developed in 1927, which is a clear version of phenol formaldehyde (Bakelite, which is normally brown or black).  Being white Melamine enabled various different coloured hard plastic items to be made.  Catalin could also be made in light colours, but white Catolin eventually yellowed.  By how much, I’m not sure, but most sextants living in their boxes are well protected from daylight, so maybe not that much.  The handle on my 1941 mates is black plastic, probably Bakelite like the handles on most Hughes WW2 aircraft sextants. 

Incidentally, I just spotted this monstrosity https://www.ebay.com/itm/Vintage-Huson-Sextant-in-Nice-Wood-Case-H-Hughes-Son-London-Made-for-Kelvin-Wi-/113403498581  .  There ought to be a Society for the Prevention of Cruelty to Sextants. 

Re navigation, the nearest to the real sea it's been with me is sitting in a shelter above Littlehampton Beach.  With Mrs P doing the timing and recording, and me doing the observing, we quickly got down to about one minute of arc accuracy.  However, the chap I bought it from used it throughout his sea going career.  You can just make out SS City of Bristol on the scrap of the receipt I told you about in the bottom right of the photo I posted. DaveP