NavList:

Tony.  For British sextants getting the size of bubble you want in the conditions you want is usually a bit simpler, at least for the navigator, than Brad suggests. You just need to know where the knob is to change the size of the bubble and how to hold the sextant to make the bubble appear or disappear and how to trap it in the cylinder.  It’s a bit like riding a bike.  Once learned, never forgotten.  Attached is a photo of me showing someone how to make a bubble in a Hughes MkIXA sextant.  You can play to your hearts content until you get the size of bubble you prefer.  I seem to remember 1/3 of the distance between the tramlines was the size advised.  Bigger than that was hard to aim with, smaller tended to be rather sluggish.  At night, all you saw was two red spots of light reflected off bubble, so you might also wish to adjust the size to get the required separation.  Also below is a picture of a very old and dirty Hughes bubble plus tramlines (The bubble appears from the side around 10.00 O’clock.  Start by tilting the sextant so 10.00 is at 12.00 while you make the bubble.  Then trap it in the cylinder by going back to the vertical.  Oh air navigation was such fun!), a picture of the bubble assembly, and a picture of the relevant page in the manual.  Many American sextants have similar but different arrangements, and others have bubble units which aren’t normally adjustable in the air; however, there was sometimes space in the case for a spare bubble chamber in case the first one got really bad.  I’ll leave it to others to say which had which. DaveP

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