A Nautical Sextant Bubble Horizon

2 09 2010

This post is preceded by “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  “Refilling Mark V/AN5851 bubble  chambers” ,  “Overhaul of MkV/An5851 bubble chamber” ,  “AN5851-1 : jammed shades carrousel” ,  “A Byrd sextant restored” and “Update on Byrd Aircraft Sextant”

A little while ago on e-bay I saw an adaptation of an A10-A bubble unit to a nautical sextant fail to reach its reserve at over $300, even though it was offered with a copy of my overhaul manual for the A10 series aircraft sextant. I recalled that a couple of months previously, I had made a very similar adaptation for a friend who lives in Paris, where natural horizons are not easily to be found. Since my means are relatively limited, I am always looking for ways of paying for my addiction to nautical sextants, so I decided to make another and this time to offer it for sale on the internet.

Most aircraft bubble units are of Second World War vintage, and after sixty five years, the fluid has leaked out of nearly all of them. The exceptions in my experience are the British Mark IX series, which were sealed with shellac and solder. US instruments sometimes sealed the glasses with shellac, but closed the filling hole with a taper pin or, as in the case of the A12, a ball bearing forced down upon its seat with a grub screw. Others used seals of lead or plastic and almost without exception, they leaked sooner or later. In the case of the A10A bubble unit, there were no fewer than six places where it could leak: two holes sealed with taper pins, one for filling and the other to allow a passage to be drilled btween the bubble and reservoir chambers, the top and bottom glasses, the joint between the diaphragm and the body of the unit and the joint between the reservoir and the body of the unit.

It is not possible to re-seal the A10-A units with shellac without damaging or destroying the Lucite illuminating ring. O rings had been patented by Niels Christensen in 1937 and during WWII the patent was taken over by the government in the national interest, but, curiously, did not find their way into sextant bubble units. It may be that, as most of them were filled with xylene, the elastomeres of the day were not equal to the task, but the A10-A units were, according to the official overhaul handbook, filled with relatively benign alcohol, just like the units in the German SOLD sextants and the later Russian copies of the SOLD. Although I have resealed units using home-made lead washers, it is much easier to remove the old seals and replace them with standard O rings if re-filling with alcohol or with Viton (fluorocarbon) O rings if using xylene.

So, having cleaned a bubble chamber and  resealed it with O rings I addressed the matter of attaching it and its optical attachments to a nautical sextant. Figure 1 shows the light path.

Figure 1 Light path through unit

The bubble lies at the focus of a spherical mirror, so that the rays that make up the image of the bubble reflected from the mirror are parallel and the bubble appears to be at infinity. These rays are intercepted by a partially reflecting surface or beam splitter and diverted into the eye. The eye also sees the image of the heavenly body, whose light rays, also apparently at infinity, pass straight through the beam splitter, so the images of the bubble and the object can be superimposed by adjusting the sextant. In daylight, the bubble is illuminated by the light from the sky and at night by a lamp that conducts the light through a Lucite strip that surrounds the top glass. Providing that the reflected rays from the spherical mirror are at right angles to the plane that contains the bubble, a line of sight through the centre of the bubble will always be horizontal. The mirror mounting allows it to be adjusted to this condition, and I give full details in my restoration manual. Providing it is collimated in this way (from the Latin collimare, which would have meant “to put in line” if a medieval scribe had not mis-copied collinare) it can be mounted on the nautical sextant without further adjustment.  A small index error may remain and have to be determined by observations from a known position.

The unit is attached to the sextant by a rising piece that I make using a shaping machine, the machine tool par excellence for cutting one-off vee ways. Rather than drill more holes into the unit, I removed the shouldered screw that held the shades and the top of the two  screws that limited their movement. I cut off the bottom screw short and used it to blank off the hole. I drilled out the holes and tapped them 4 BA. It is as well to dismantle the unit completely to avoid damage to internal parts when doing this. Instructions for dismantling are again given in my manual.

In day time the bubble is illuminated from above via a ground-glass diffuser screen that can be moved aside to view the bubble when adjusting its size. At night, a tiny bulb throws light onto the ends of a Lucite (UK : Perspex) strip that surrounds the top glass and the light is conducted around by total internal reflections. These bulbs are becoming hard to find nowadays, so I have experimented with using  a high-intensity red light emitting diode instead and it works quite well. The main difficulty with the adaptation is in reducing the diameter of the LED to fit the existing fitting. It is relatively simple to solder the LED to the base of a defunct bulb. The brightness of the lamps, incandescent or LED, is controlled by a potentiometer in the battery box. Incidentally, the Lucite strip does not seem to make a lot of difference to the quality of the lighting if for some reason it disintegrates or has to be dispensed with.

Here is another view of a bubble unit, from the rear of the sextant:

Figure 2 Rear view of unit



Update on Byrd Aircraft Sextant

11 08 2009

This post is preceded by “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  “Refilling Mark V/AN5851 bubble  chambers” ,  “Overhaul of MkV/An5851 bubble chamber” ,  “AN5851-1 : jammed shades carrousel” and “A Byrd sextant restored”

Since writing the previous post about the Byrd sextant (or should it be the de Florez sextant?) I became disatistfied with the restoration and, for those interested in air navigation intruments I decided to add to the photographs. You will need to read the previous post first.

As noone followed my hint to donate me a set of index shades, I have had to make them myself. For the outline, it was simple enough to scribe around a genuine shade on to a sheet of 2.5 mm brass four times, drill and ream the mounting holes and saw out the blanks using a piercing saw. This tool is a little like a small fret saw and takes very fine blades having as many as 40 teeth per inch. It cuts on the down stroke and is used with the blade held vertically. Here is a picture of me cutting out a clock wheel blank. Note that it helps to have lots of light and vision…

Copy of 100_0066

I could then bolt the four blanks together with a close-fitting screw through the mounting holes and files the outlines to shape. It is actually easier and quicker to do the straight bits using a vertical milling machine – if it is already set up – and just to file the rounded corners by hand. The block of four could then be mounted and centred in the four jaw chuck of a lathe and all four drilled through and bored to 22 mm diameter. The outermost blank was then counterbored to 24 mm to a depth of 2 mm and removed, then the next counterbored and so on for all four, taking care to loosen and tighten the same pair of chuck jaws each time. This is not good practice, but it saves time when accurate centring is not vital.

It is hard to discover sources of neutral density glass, so I made a trephine out of mild steel and cut out discs of plastic Cokin filter material. This is used in photography and seems to be flat and parallel enough for a sextant likely to have observational errors of the order of minutes. I made the fit so the discs just popped into the brass frames and saved myself the trouble of having to swage them into place, though they might have looked more “genuine” if I had. Making the shouldered screw that holds them all in place was staightforward turning and I made the Belleville spring washer by making a thin brass washer, sitting it on the end grain of a block of hard wood and hitting it hard with a ball bearing. The finished set of shades shows well in the next photograph, as does the semi-circular lens that allows the level to be seen in focus.

Copy of 100_2948

I wasn’t happy with the fiducial line in my original restoration. It was simply a piece of fine thread wrapped around the level vial and secured in place with clear varnish. Since it is wrapped around a curved surface, it can be viewed only from one angle and still be seen as a straight line. In any case, it was rather too thick, so I scribed a thin line on some perspex and then cut and filed and drilled a tiny piece to size, securing it to the vial carrier with two 12 BA screws. These are only 1.3 mm in diameter and I was greatly relieved to have tapped the two blind holes without breaking my only tap of this size. The next photograph shows this small but important part. The flash has made it look dustier than it was in reality. I have no idea what was used in the original models. The Smithsonian Museum has two examples, but both are incomplete and I have only web photographs to look at.


Making the case from African mahogany needed only normally careful woodworking. Dovetail joints for the corners had by the 1920s given way to comb (finger) joints, but as some later American aircraft  cases used corner rebates, which are much easier to make without special machinery, this is what I used, with brass pins across the joints to prevent disaster if the very strong glue should fail:

Corner rebate

I copied the hook latches from a Hughes and Son sextant case and the handle is a very close copy of the handle used for a Brandis Aeronautical Sextant Mark 1 Mod 4 of 1931. I am not good at sheet metal work, so will gloss over the battery box, with its belt loop. The pick for the two capstan headed screws was simple to make and the mirror-adjusting wrench required only the ability to convert a small round hole to a small square hole using a file. It remained  to dismantle the instrument to its component parts and spray-paint them using a satin finish paint that, while not perfectly imitating the original finish, at least has the merit of pleasing its owner.

The final photograph shows the sextant in its case with its furniture and fittings. It is certainly not an easy sextant to use on land, but since these latest retoration efforts, there has been little clear sky around for me to make a serious assessment. If there is some particular aspect of this instrument that you would like discuss or to see illustrated, do contact me.


A Byrd Sextant Restored

30 05 2009

This post is preceded by “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  “Refilling Mark V/AN5851 bubble  chambers” ,  “Overhaul of MkV/An5851 bubble chamber” and “AN5851-1 : jammed shades carrousel”

I recently acquired  a Brandis nautical vernier sextant without case, telescope, or any shades. It appeared to have an extra mirror in front of the horizon mirror and I recognised it as an early bubble sextant of the type used by the then Commander Richard Byrd on his claimed flight to the North Pole in 1926. There are several magazine photographs extant that show Byrd in posed pictures, using a similar sextant, this one, for example:


K Hilding Beij, writing in the Bureau of Standards Report 198, Astronomical Methods in Aerial Navigation in about 1926, refers to the sextant as a “Byrd sextant”, though Luis de Florez, a prolific  inventor, claimed priority. He had filed for a patent for exactly this type of bubble sextant in March 1919 and he was granted US Patent number 1,536,286 in May 1925. My sextant looked like this when I received it:


The one Byrd is using is a full-size Brandis vernier quintant with an arc of 180 mm radius reading to 30 seconds, whereas my example is unusually small for a vernier quintant, having an arc radius of only 140 mm, also reading to 30 seconds, so my hopes of possessing an historic instrument were disappointed (but see postscript). Nevertheless, it is a rare and early instrument dating from about 1920 and I felt it was worthwhile  to restore it to working order. As I have no access to original instruments, I did not set out to make exact copies of the attachments that make a nautical quintant into an aeronautical one, but I did follow the same principles, while retaining all the original parts. Needless to say, I dismantled it completely to begin with, and cleaned all the individual parts. A photograph of the restored instrument will perhaps best help to explain its workings.


The optical path for the heavenly body is as usual, via the index mirror and silvered half of the horizon mirror. An ordinary spirit level vial is held in a carrier and viewed via an auxiliary mirror set at 45 degrees above it, through the plain half of the horizon mirror. The image of the bubble would be out of focus viewed directly through the x 2 Galilean telescope and so an extra, semicircular, lens is interposed in the light path to bring it into sharp focus. The auxiliary mirror may be swung downwards to allow direct view of the natural horizon by pressing a spring-loaded catch.

The sensitivity of the vial has to be carefully chosen. If too sensitive, it is never at rest when the instrument is held in the hand and if not sensitive enough it is not possible to get meaningful results. I settled for one where the bubble moves 2 mm for 6 minutes change in level. This is of the same order of sensitivity as most other bubble sextants.  The bubble is illuminated from one end and, to try to get even illumination, the vial is painted white over most of its surface, including the end distant from the lamp. This has parallels in the lighting of some circular bubble cells, where an attempt is often made to conduct the light around the periphery of the cell with some sort of light guide. The next photograph shows a view of the lighted vial through the telescope (the view is somewhat more extensive than this in reality).


In sextants with circular bubble cells, one usually aims either to centre the body in the bubble or to align it with its equator, but this cannot be done with a linear cell, so a more-or-less central datum line is used and the sextant adjusted so that when the bubble is centred, the datum line lies on the horizontal. As is usual for a Galilean telescope, each half of the objective lens “sees”  its own half of the field of view (compare this with the inverting or astronomical telescope, where obscuring half the objective simply cuts out half the total light). Thus, the auxiliary, semicircular lens in the telescope attachment sees the left half-field and  is used to bring the bubble into focus by sliding back or forth.

The heavenly body is seen on the other half-field and, as is usual, there is a narrow band of overlap where both the sky and the horizon or bubble may be seen together. I have not made any significant trials as yet, but it is relatively easy on dry land to align a star, the moon or the sun with the datum line while trying to keep the bubble centred. However, the instrument gives no indication of lateral tilt and in an aircraft the results must have been very uncertain.

The scale lighting is particularly good. A shaded lamp shines via a standard diffusing screen on to the scales, which are viewed through a simple magnifier. Unlike many such systems it gives a very even illumination so that the main and vernier scales are seen with equal contrast, making reading relatively easy and rapid.


The handle seems to be a modified early Brandis battery handle. Power is now from an external source. Scale lighting is via the original press switch, while the bubble unit is switched by a rather crude rotary switch on the front of the handle, seen in the general arrangement photo above. The following photo shows the wiring layout, pretty well as found except for the decayed silk covered wire.

Byrd handle

I have coated the parts that I have made – telescope and attachment, and vial carrier – with modern paint. The original paintwork on the rest of the instrument is careworn and I cannot help but feel that it would look better for a fresh coat of paint. I would repaint a more modern instrument in the same condition and wonder how readers might feel about that. Is it sufficiently “historic” to preserve it as found? Would repainting it devalue it in some way.”? After all, noone is likely to mistake it for a modern fake, repainted or not.

The index shades, by the way, I borrowed from another Brandis sextant, so that if anyone has a spare set of Brandis or US Navy Mark II index shades that I can beg or buy, I should be glad to hear from them.

Post script, 11 September 2015.

When I wrote this post, I thought Byrd was holding a larger sextant than mine, but an enquiry led me to look more closely at the structure of the sextant he is shown holding and I now believe that he is holding the same type of sextant that I describe, viz.a survey sextant of 140 mm radius.

AN 5851-1. Jammed shades carrousel

14 05 2009

This post is preceded by “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  “Refilling Mark V/AN5851 bubble  chambers” and “Overhaul of MkV/An5851 bubble chamber”

I have been asked about the carrousel that holds the various shade glasses in the AN5851 bubble sextant. This is one of my least favourite bubble sextants, heavy (2.5 kg), awkward to hold and the optical path complicated by the inclusion of a horizon prism. The near indestructability of the case was bought at the expense of more weight (6.4 kg!).

My correspondent reported that his carrousel was jammed and wondered whether its associated spring could be the cause. Here’s how you get at it.

1) Remove 4 screws (circled in white, below) from the black plastic cover and remove it. Take the opportunity to clean the copper electrical contacts inside it, that carry current to the scales-illumination lamp.

Carrousel 0

2) Loosen, but do not remove, the two screws that hold the black shade cover over the carrousel. Then dislocate it upwards and pull it outwards.

Carrousel 1

Carrousel 2

 This reveals the shouldered screw that holds the carrousel on to a complicated little alloy casting that also mounts the astigmatiser. There is a brass spring washer between the head of the screw and the alloy carrousel body. Removing the black shade cover also loosens the leaf spring’s attachment, and in the unlikely event that a disordered spring is the cause of stiffness, this should allow the carrousel to revolve freely.

Carrousel 3

3) Corrosion between the central screw and the carrousel is much the most likely cause of stiffness. It is not easy to get at the screw slot without removing the index prism (best left alone) or improvising a bent screw driver from a piece of steel strip or grasping the head of the screw in a narrow pair of pliers. However, if you swing the astigmatiser out of the way, you will be able to get at the two screws that hold the casting into which it is inserted, to the frame of the sextant. If there is corrosion, and you need to use a lot of force, you may break off the screw where it enters the casting or damage the casting. It is much safer to apply a little thin oil between the screw head and the carrousel. Allow a few hours for it to penetrate and then gently work the carrousel free. Be prepared patiently to repeat the process several times. Remove the central screw only if it loosens easily.

Replacement is the reverse of dis-assembly.

Overhaul of Mk V/ AN5851 sextant bubble chamber

20 12 2008

This post is preceded by “Bubble Illumination of Mk V and AN 5851 Bubble Sextants” and “Refilling Mark V and AN5851 Bubble  Chambers”

The preceding post dealt only with refilling a bubble chamber that was otherwise clean and tidy. This post covers dismantling for cleaning and refitting the glasses. You will need an electric iron, a pencil, a screwdriver, some flake shellac (from a decorating shop or French polisher), some xylene, a syringe (preferably glass) and needle or a fine pipette.

Start by removing the bubble chamber assembly from the sextant as in the previous post. Remove the taper plug. Remove the air chamber (4 screws from underneath), carefully preserving the gasket if you can. The control unit unscrews by using a wrench on the octagonal metal rim. At this point, it is wise to be wearing an apron with its skirt clipped to your bench or table, as there is a ball bearing with a hole pierced through its centre just  waiting to fall onto the floor and be lost for ever. Dont let this happen to you. It is difficult to drill holes through small balls. Empty the control unit into the bubble chamber by repeated pressing and shake out as much xylene as you can. You should now have reached the state shown in the following photograph:


Also remove the four tiny countersunk screws on the underside and carefully prise the illuminating annulus free. The paint beneath is radium paint and, while it no longer glows in the dark, the radioactivity from the contained radium is still present and is best removed, before replacing it with modern photo-luminescent paint.

See https://sextantbook.com/2008/12/09/bubble-illumination-of-mk-v-and-an-5851-bubble-sextants/  for some advice on how to do this.


Meanwhile, your electric iron, carefully secured in some way with the sole upermost, can be warming up. The maximum setting seems to be just the right temperature. For those lucky people with laboratory hotplates, the temperature of my iron cycles between 160 and 182 celsius. Place the stripped down bubble chamber on the hot iron and do something else for five minute while it heats up and softens the shellac that is holding everything in place. You can check whether it is hot enough by touching the chamber somewhere with a flake of shellac to see whether it melts.

When the right temperature  has been reached, hold the chamber in one hand with a thick towel or heavy leather glove and unscrew the bottom retaining ring. For this you can use the correct tool, a pin wrench, or a piece of 1.5 mm metal strip previously filed to fit the slots. Then rap the chamber sharply on a wooden bench and with luck the bottom glass will fall out. When your hand has stopped hurting from holding a very hot object,  turn over the chamber and unscrew the other retaining ring. While everything is still hot, push out the top lens with a pencil from underneath.

Allow everything to cool down and then soak the glasses and retaining rings in alcohol while you chip away as much old shellac as you can from the chamber body. If you are patient, you could instead soak all the parts overnight in alcohol and rinse in the morning with clean alcohol. Everything should now look something like this:


There is a very tiny passageway between the air and bubble chambers and you should make sure that this is perfectly clear of shellac, as it enter the bubble chamber partly under the bottom glass and can easily be completely blocked by stray shellac.

Return the bubble chamber to the hot iron with the top of the chamber uppermost and when it is hot enough, carefully apply a ring of shellac to the flat lens seat. Do this by picking up flakes about 5 to 10 mm long in a pair of fine forceps and smear as you dab. When the ring is complete, align the etched line of the top glass with the fore and aft line of the chamber (the bubble control is aft and the air chamber to the left) and drop the lens into place. You will get perhaps one chance at realigning it if you get it wrong, before the glass gets too hot to touch. The lens goes in convex side up. The concave side has a flat annulus ground on it and this goes downwards.

The official manual instructs one now to apply pressure to the lens with a jig until the shellac has cooled somewhat, when the retaining ring is screwed into place. If you have the means, you can improvise a jig like mine, shown in the next photograph, or use a Mark I pencil as shown in the one after,  to apply a little downward pressure to make sure there is a complete ring of shellac beneath the glass. Having tried both on a variety of bubble chambers, I now use a pencil and screw down the retaining ring while everything is still nice and hot, and I bet the WW II technicians did too.



Allow the chamber to cool down and check the inside for loose flakes or little teardrops of shellac on the underside of the top glass. The latter can be chipped off easily and the job completed with a wipe of alcohol. The cleaner the inside is, the better the bubble will move and the clearer the view, so do it now, as you won’t be able to after the next step, which is to fit the bottom glass, following the same procedure as for the top. When applying the bottom ring of shellac, take special care to keep it away from the mouth of the capillary passage to the air chamber, but make sure there is some more peripherally to complete the seal.

When everything has cooled down again, chip away shellac from the outside of the glasses, clean up and refit the control and air chamber, the reverse of dis-assembly. Take care that the hole through the ball lines up with the passage into the bubble chamber from the control.

The last photograph actually shows an A10 vapour pressure chamber being resealed. The original seals were lead washers and don’t work for me when re-used. You can mess about with O rings that cause alterations in the optical path length and sometimes dissolve in xylene, or search for the ideal material with which to replace the lead washers, or you can make a really good job of it with shellac.

I hope to have an e-book overhaul manual for the A10/10A ready by the end of February, 2009. Meanwhile, you could enjoy reading The Naked Nautical Sextant and its Intimate Anatomy (but first you have to buy it).

Refilling Mark V / AN5851 bubble chambers

18 12 2008

This post is preceded by “Bubble Illumination of Mk V and AN 5851 Bubble Sextants”

Mark V is the US Navy designation for the AN5851 sextant. Early models were fitted with vapour pressure bubbles, which are hard, but not impossible to refill. This post will deal with only the somewhat easier air bubble models, which are distinguished by having a cylindrical air chamber projecting from the top left hand side of the chamber and, because I want at least to start to answer a request for details before Christmas intervenes, I’m sticking to the simplest case. This is where there is a bubble of some sort present and the fluid and glasses appear clear. If there are particles floating around in the fluid, or it has completely dried up leaving a powdery film inside the chamber, a full overhaul will be needed.

At the front of the chamber, you will see projecting a peg about 2 mm in diameter. This is a tapered pin that occupies a tapered hole through which the chamber was filled. To remove it, remove the chamber and eye lens assembly as described in the previous post. Then grasp the end of the pin in a pair of pliers and pull directly in line with it while twisting. It should come out without too much trouble, unless you pull and twist out of line, when it may break off and leave you in deep trouble, the solution for which will be covered in a future post.


Once the pin is out, you can refill the chamber with xylene using a hypodermic syringe and needle, if you can locate a glass syringe. You can use a disposable plastic one, but the xylene attacks the plastic and the syringe rapidly becomes very stiff to operate. An alternative is to use a glass pipette with the end drawn out to a fine tube of about 1 mm diameter. This is what I now use. Some decorating shops stock xylene in bulk and may be prepared to sell you 500 ml or so. It will last you a long time. An alternative suggested, with which I have no experience, is Zippo or Ronsonol cigarette lighter fluid. You should not use alcohol, as the glasses  have been sealed with shellac, which is soluble in alcohol.

When the chamber is full, temporarily replace the taper pin with a pushing and screwing motion,  hold the chamber with the air chamber downwards and press repeatedly on the control knob until no more air comes into the bubble chamber. This displaces air from the bellows into the bubble chamber. Then refill the bubble chamber . At this point you can  replace the taper pin using a firm pushing and screwing motion. Do not tap it in with a hammer. It may be necessary to experiment a little to get the right amount of fluid. It should be possible to remove the bubble altogether, so that sights using the natural horizon can be taken. If the bubble cannot be entirely removed, remove the taper pin and add a little more xylene.

To empty the air out of the bubble chamber to reduce the size of the bubble, hold with the air chamber upwards while pressing repeatedly on the bubble control knob until you have a bubble of the desired size. Pressing forces air into the air chamber through a tiny capillary tube and when you release, xylene passes into the bubble chamber. To get a larger bubble, hold with the air chamber downwards while operating the control. 

If you cannot get a bubble at all, the air chamber may be completely full of xylene and you will have to remove a little, the capillary passage between the bubble and air chambers may be blocked by debris( clear with a piece of fine wire), or the hole in the ball that seals the bellows unit to the bubble chamber may be misaligned (see next post).

Do let me know if you find this post useful and ask if something is unclear. Consider, too,  buying yourself a gift of The Nautical Sextant…

Bubble illumination of Mk V and AN 5851 bubble sextants

9 12 2008

A recent posting to the Sextants@yahoogroups.com asked about renewing the illumination of  the US Navy Mk V bubble chamber. I think I am correct in saying that this is identical to the AN 5851 chamber illumination. A little oddly, there was no attempt to have electrical illumination. Instead, self-luminous radium paint was used. Now radium has a half life of nearly 1600 years, so plainly it is not the radium that has decayed, but the luminescent compound mixed with it. Replacing the paint with a modern photoluminescent paint is not difficult, but  some minimal commonsense precautions should be taken to avoid swallowing or breathing in the old paint.

The bubble chamber, incidentally, is one of the easier ones to service, as the manufacturer sensibly provided a plug in the form of a taper pin to allow refilling. The top and bottom of the inside of chamber need to be scrupulously clean before refilling and this is not so simple.

Let us assume, however, that you have a bubble in a chamber that is free from debris. The first step is to remove the chamber together with the eyepiece prism by removing the three screws shown in the following photograph:


Removal of the eyepiece prism is completed by inverting the bubble chamber and removing two more screws, as shown in the next photograph:


The radium paint is contained beneath the black washer-like disc that is secured by four tiny countersunk screws. Removing these allows it to be carefully prised from its seat. The paint can then be removed from the underside of the washer and from the underside of the bubble chamber bottom glass by softening  and dissolving the paint with acetone. Certainly you should take every precaution not to inhale or swallow any. How you dispose of the paint is up to you. The level of radioactivity is low and there is only a small quantity, so disposing in the sea is unlikely to do any harm to anyone.  A local university physics department or the radio therapy unit of a hospital might be prepared to advise.

Replace the paint on the underside of the washer with, first, a layer of white paint and then a layer of modern photoluminescent paint. A layer of the latter (but not the white paint) can also be applied to the bottom glass beneath the washer – the thicker the layer of the luminescent paint, the better the illumination. Re-assembly is the reverse of dis-assembly.

To charge the photoluminescent paint, shine a bright light, say, a 40 watt bulb, onto the index prism for 5 minutes or so. How long the light lasts depends on how long you charge the paint for and on the grade of the paint.

You might wonder whether overpainting the radium paint with a paint containing copper-doped zinc sulphide would renew the radioluminescence. The answer seems to be “No” because the alpha particles emitted by the radium penetrate poorly and the ZnS.Cu needs to be mixed intimately with the radium.

Post me a comment if you would like me to deal with cleaning and refilling the bubble chamber. It is not as simple as replacing the paint, but needs no very special tools, notwithstanding the instructions given in the original overhaul manual.