28 05 2021


A few years ago, I bought a Mark IX bubble sextant for parts from a local auction web site. I had vaguely noted an excrescence had been added to the top, and when it arrived I found that it contained a ninety degree prism. The bubble unit had been removed and in its place was a half-silvered mirror angled at 45 degrees. Someone seemed to have attempted a conversion so that the sextant could be used at sea, using the natural horizon. This in itself is not a bad idea, as the Mark IX series of bubble sextants represents a cheap way of getting an instrument with a digital read out, as shown in Figure 1.

Figure 1: Right-hand side of sextant.

The index mirror is rotated in steps of 10 degrees(actually 5 degrees but the light ray is deflected through 10) by depressing the knob and rotating it, when on releasing it it clicks into the selected value. Pushing up the 5 degree knob adds five degrees rotation to the ray deflection. The fine adjustment knob adds up to 8 degrees fifty minutes to the value set by the 10 degree knob.

Upon further exploration of the excrescence at the top it turned out to contain a right angle prism to divert light entering it directly downwards into the instrument. A variable polaroid filter can be swung to filter the incoming light. I suspect the fitting was canabalised from something like a WWII drift meter. A half-silvered mirror with a silvered aperture of only 18 x 7.5 mm diverted the down-coming rays into the eye hole, joining the rays from the observed body, as in a normal sextant.

Figure 2 shows the interior of the sextant. The gearbox, as well as counting off the degrees and minutes, also rotates the index mirror. The shades control allows seven combinations of three shades of increasing total density

Figure 2: Interior mechanism.

Figure 3 shows the light path of the conversion. Horizon rays are shown yellow, body rays are shown red and the mirror as dashed green.

Figure 3: Light path

It is not my intention to provide detailed instructions on replicating this instrument layout, as much will depend on the skills and workshop resources of the reader. However, a few details may help.

Figure 4 shows the prism mounting from the side. The prism is cemented to a back plate which is held against three points by two springs. The three points are a fixed nipple and two screws for adjusting index and side error.

Figure 4: Side view of prism mounting.

Figure 5 shows that the prism backplate lies between two guide rails, soldered to a further plate which forms the rear of the mounting and which is threaded for the adjusting screws.

Figure 5: Apical view of prism mounting.

I felt that the view through the half-silvered mirror was rather restricted and also discovered that it was not well-centred on the light rays, so I replaced the mirror and its mounting with one carrying a 50% beam splitter measuring about 25 x 20 mm. This is mounted in the left half of the instrument frame, as shown in Figure 6 with the light path.. By carefully adjusting the permanent height of the mounting above the frame I was able to get the horizon view to occupy roughly half of the field of view, while the body rays occupy the whole field of view. The horizon f.o.v. is somewhat brighter, as it has passed through fewer air-glass interfaces.

Figure 6: Beam splitter position.

For those who are not familiar with the Mark IX series of bubble sextants, the two halves mate together, located by two dowel pins and four screws a shown in Figure 7. Those interested in restoring A Mark IX, IX A or IX BM to working order as a bubble sextant will find my restoration manual a useful guide. See: My Bubble Sextant Restoration Manuals for details.

Figure 7: The assembled instrument.

Mercury amalgam mirrors

6 05 2021

For years I have wished to reproduce the method of re-silvering sextant mirrors using the process which was common until the mid 19th century, when chemical methods of depositing silver on to glass were invented. For several centuries prior to that, tin foil was dissolved in mercury to form a two-phase amalgam of tin-mercury crystals. A ready source of tin foil used to be tea chests, but if they exist at all nowadays, they will be lined with aluminium foil, which does dissolve in mercury but immediately decays to an oxide-rich powder.

For a sextant mirror, only a few drops of mercury are needed and high purity tin foil can be obtained in small quantities from China on e-bay. Mercury metal is fairly harmless stuff in itself, but its vapour poses health risks and many of its organic compounds are very toxic. For our purposes, we do not need to wear personal protective equipment, wear rubber or nitrile gloves as well as protective eye wear, nor do we need to use respirators with cartridges approved for use with mercury vapour, as the amount of vapour released by the tiny amount we need must pose negligible risks.

Mercury metal is a bit hard to get hold of, as many carriers refuse to handle it because of its highly corrosive effects on aluminium and often groundless health fears, but an old mercury in glass thermometer or two may contain all we need. I am fortunate to have been given a litre by a retired chemistry teacher who had received several litres of it when an old DC power station closed down.

The first step is to remove the old silvering. Classically, it was coated with sealing wax, which is mainly shellac with a colouring substance, so it can be soaked overnight in alcohol and then rubbed off with a finger. The old silvering may come away with it, but if not, it can be dissolved in concentrated hydrochloric acid, often sold as “spirits of salt” for cleaning concrete. The vapour from fuming hydrochloric acid is irritant, so cover the container while the mirror soaks. The glass can then be thoroughly cleaned with alcohol to remove all traces of oil or grease.

The next step is to smooth a piece of tin foil about the same size as the mirror with a margin of a few millimetres all round. This can be done on a smooth glass or other clean surface, using the finger tip or a scrap of chamois (“shammy”) leather (Figure 1).

Once this is done make sure it has not stuck to the glass. I then transfer it to a piece of cartridge paper with folded-up sides to catch any stray mercury.

Figure 1: Smoothed foil.

Then add a drop of mercury and spread it evenly over the foil with a finger tip to give a brightly glistening surface . Add another drop of mercury, to give an excess, which brings any dross of mercuric oxide and dust to the surface (Figure 2).

Figure 2: Mercury excess.

Then place a slip of clean paper on top of the mercury, followed by the glass and holding the glass with a light downward pressure, slide out the paper and, with luck, the dross (Figure 3).

Figure 3: Removing dross.

Tilt the glass and let excess mercury drain off ,steadying it so that it does not slide off (Figure 4). Note the brightly reflective result.

Figure 4: Draining mercury 1.

The mercury tends to collect at the bottom edge and can be encouraged to drain by adding a slip of tin foil (Figure 5).

Figure 5: Draining mercury 2.

At this point, I slide a slip of paper between the foil and the glass substrate to prevent the two from sticking together, and allow a day or so for all the mercury to drain away. Though now containing tin, it can be collected in a separate vessel and used again if one does much silvering. Figure 6 shows the back of the new mirror after trimming excess tin.

Figure 6: Mirror back.

And Figure 7 shows the front. Tin is less reflective than silver, but the eye can just about detect a doubling of brightness, and I certainly cannot detect any difference between modern mirror glass and the few ancient sextant mirrors I have renovated.

Figure 7: Mirror front.

I use acrylic paint to protect the back and continue the paint around the sides to ensure a waterproof seal. If “authenticity” is desired and you can find sealing wax you can dissolve it is “spirits of wine” (alcohol) and use that as a sealant, or colour some shellac.