NavList:

Whilst most bubble sextants allow the bubble size to be altered in the air, twas never ever so.  The earliest attempts to make a bubble horizon really did begin with a simple spirit level, except even ‘simple’ requires further explanation.  You can’t just strap a spirit level onto a marine sextant and start taking sights.  Light from the body is focussed at infinity, whereas the bubble will be no more than six inches in front of your face; eyes don’t like that.  Therefore, you have to install collimating lenses to make it look as if the bubble is also at infinity.  A double reflection marine sextant is not affected by tilt in the direction of viewing.  This is not so with a ‘simple’ spirit level and the bubble must be kept in line with fiduciary marks.  This problem can be overcome as described below.  

Although the marine sextant is impervious to tilt in the direction of observation, it is affected by lateral tilt.  The mariner corrects for this by rocking the sextant gently and taking a reading when the body is at its lowest point and just kissing the horizon.  You can’t do this with an artificial horizon comprising a single spirit level in the direction of observation, so a second spirit level at right angles to the first was often installed.  The Admiral Gago Coutinho sextant is a prime example.  Maintaining the body in the longitudinal bubble, possibly between fiduciary marks, while keeping the lateral bubble correct, was quite a task for eye and brain.  The solution was the ‘bubble chamber’, proposed by Louis Fave in 1906 and patented by Booth and Smith from RAE in 1919, comprising a single bubble in a circular chamber.  If the body is to move with the bubble when the sextant is tilted, the radius of curvature of the roof of the chamber should be equal to the focal length of the collimating lens¹.  However, the size of the bubble varies with temperature and pressure, so to ensure the bubble could be adjusted to the ideal size as the aircraft flew higher or into different climates, a bubble making diaphragm controlled by a screw soon had to be included adding to the complexity.

Concerning temperature changes, although the inside of an aircraft can get uncomfortably cold (or hot!) for someone sitting relatively immobile for several hours, it rarely gets as cold as it is outside at high altitude.  The open cockpit navigators experienced outside temperatures, but they rarely flew very high.  With astrodomes, the early problems were more to do with combating condensation, which might also turn to frost, until they improved dome ventilation.  Turning to periscopic sextants, which were used well into the tropopause, the Smiths periscopic sextant mounting was supplied with electrical heating to prevent the sextant freezing in its tube and some of this power was fed to the sextant to provide lighting and heat to the viewing window to prevent frost forming on the outside and moisture on the inside.  As far as I can see the Kollsman periscopic sextants didn’t have heaters but relied on being sealed full of dry nitrogen and upon silica gel crystals.  DaveP

1.  Francis M Rogers.  Precision Astrolabe. P161 & 162 http://www.fer3.com/arc/imgx/Bubble-sextants-Precision-Astrolabe.pdf