British Admiralty Vernier Sextant

23 06 2011

Previous posts in this category cover:  “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” , “Restoring a C. Plath Drei Kreis Sextant” , “Heath Curve-bar sextant compared with Plath” , “A Drowned Husun Three Circle Sextant”, ”Troughton and Simms Surveying Sextant” , “A Sextant 210 Years On” and “A fine sextant by Filotecnica Salmoiraghi.”

The Admiralty ordered sextants from Henry Hughes and Son and bought them provided that they met Admiralty specifications as determined by the National Physical Laboratory at Teddington. Pre-World War II they were given on Admiralty Form 575, but I have not been able to find a copy to discover exactly what the specifications were. Presumably they were exacting and conservative.

Figure 1 : Inspection certificate of sextant number 222**

In the late 1930’s  a batch of vernier sextants was ordered from Hughes and Son and in the early 1950’s another batch, was ordered, this time of micrometer sextants. Except for the rack and micrometer mechanism, they had many things in common: the form of the frame, the mirrors and their mountings , the shades and their mountings and their telescopes. The frame is the classical A type frame, of bronze, heavily ribbed and  the shades are conventionally mounted, but the mirrors are circular and sealed and the telescopes have coarse interupted threads that allow them to be rapidly mounted and dismounted. The sextant can be returned to its case with the index arm set in any position except at the extremes, said to be an  important practical point if a question about a reading occurs when calculating a position line (though most people would try to average several sights of the same body, when gross errors would be noticed).

A few weeks ago, I came upon an Admiralty pattern vernier sextant in good condition and enjoyed restoring it to a near-new condition, at the same time observing points of difference with other instrument types.  Figure 1 shows a general view of the instrument, with the star telescope fitted. In the background is the inverting telescope with an extra, high-power eyepiece ,  two eyepiece shades and an adjusting pick. Note the unusual mirror mountings and magnifier mounting.

Figure 1 : Admiralty Pattern sextant and accessories

 
Telescopes and their mounting
The sextant was issued with a 2 x 40 mm Galilean star telescope and a 5 x 25 mm inverting telescope, the latter provided with an extra eyepiece to give a magnification of about x 10. Normally, this high powered eyepiece would be provided with a pair of parallel cross hairs, so that the collimation of the telescope could be checked, but only traces of these remain. Possibly they were removed as being unnecessary, as it is not possible to adjust the collimation on this instrument.
 
Many Galilean telescopes are provided with an eye lens that restricts the field of view, but in this instrument (Figure 3) , the eye lens is 19 mm in diameter, so that even spectacle wearers can take advantage of  the exceptionally wide field of view that I measured as 19 degrees.  Opinion of the time seems to have been divided on the power that the star telescope should have, as, for a given diameter of objective lens, the field of view decreases as the magnification increases. Heath and Co., Hughes’s main competitor in England, noted that the diameter of the  fully dilated pupil of the eye at night was 8 mm and that this regulated the amount of light that could be received by the eye. There was no advantage, they said, in having the 20 mm diameter pencil of the low-powered telescope, as the eye could not take advantage of it. It was their view that if the emergent pencil and the pupil diameter were about the same, all the light gathered by the objective lens would enter the eye and “both the star and the horizon would appear comparatively brightly illuminated.”  Hughes’s standard star telescope was 2.5 x 30 mm as against Heath’s 3 x 30mm, so they were not too far apart. If the Admiralty wanted a 2 x 40, it was presumably felt that the wide field of view and exit pencil made quite certain that the brightest possible image of the horizon could be obtained even if some of the entering light got wasted.
 

Figure 3 : Galilean star telescope

 
In daytime, the amount of light entering the telescope is not a critical factor, and a higher magnification allows one to judge better when the image of the sun, moon or, occasionally, Venus is touching the horizon. The bodies are brighter too, so that finding them with a restricted field of view is easier. However, with a small field of view, it is easy to lose the body in bringing it down to the horizon. The inverting telescope (Figure 4)  has a measured field of view of 7.6 degrees, about the same as a 6 x 30 monocular, and bringing the body down in rough weather without losing it is much easier. For practical purposes, the high powered eyepiece would seldom be used at sea, its main purpose being for checking collimation and perhaps ocasionally when using an artificial horizon on dry land in an out-of-the-way place.
 

Figure 4 : Inverting telescope

 
As I have noted above, there is no provision for adjusting collimation. Instead, the robust triangular rising piece is manufactured so that it is square to the plane of the arc of the sextant  (Figure 5) and likely to remain that way, as wear in the area should be negligible.
 

Figure 5 : Checking squareness of face of telescope ring.

 Figure 6 (below) shows the internal structure of the mechanism. The socket is firmly attached from behind the frame by three screws and sits in a machined circular pocket that ensures that it is square to the frame (Figure 7). The socket is made in two parts : one with a triangular holes broached through it and a disk with a round hole for the shank of the feed screw. The two are silver-soldered together and it is impossible to see the joint.  It would not be possible to make the triangular hole without this artifice. The feed screw is held captive in the disc, between the triangular flange and the knob, which has the familiar square. This allows adjustment to remove backlash by tightening the axial securing screw.
 

Figure 6 : Rising piece mechanism.

 

Figure 7 : Attachment of rising piece mechanism to frame.

Figure 4 shows the external interupted thread on a telescope and the internal thread can be seen in the interior of the very substantial telescope ring in Figure 5 .  Alternate one-sixths of the thread are machined away, so that the telescope can be inserted with the white dots lined up and secured in place by rotating through less than one sixth of a turn. The Admiralty  may have decided on  this device by analogy with  the interupted threads of the breech closing mechanism of naval artillery, but Hughes also used it in their higher class sextants made for civilian use.
 
Index mirror mounting
The index mirror is sealed inside its bracket and is not adjustable. Like the telescope mounting, it was manufactured correctly in the first place and, short of breakage, would remain square to the plane of the arc throughout its life. The rear of the bracket bears the statement ” Pat. No 472814, which was accepted in 1935. Figure 8, extracted from the patent document, shows how the silvered rear of the mirror was protected from the effects of salt water by holding it against a “resilient washer” by means of a threaded ring with a lip that bears on the periphery of the mirror.
 

Figure 8 : Extract from patent on sealing mirrors

 
Horizon mirror mounting
 This is altogether more complex, since light has to pass through it. Figure 9 shows the mounting exploded. In the left half of the figure the mirror can be seen. A disc of optical glass is cemented to the back of the mirror to protect the silvering and the edges of the mirror and glass where they join is bevelled, so that there is a thin wedge of cement to prevent infiltration of water. There are four dots of silver on the rear of the mirror (Figure 10), presumably to ensure that the cement layer was of uniform thickness , to prevent any prismatic effect. The sandwich is contained in a cell and retained there by a threaded ring and washer.
 

Figure 9 : Horizon mirror mounting exploded.

Figure 10 : Rear of horizon mirror sandwich.

 
The right half of the photograph shows how the cell is attached flexibly to the bracket, by means of two slightly curved leaf springs, attached at their ends to the cell and in their middle to the bracket.  The usual two adjusting screws bear on the cell to adjust out side and index error, and are provided with protective screw-on covers. 
 
The bracket bears the caption “Patent number 30340/34”, but this is in fact a patent application number, one higher than the application number for the index mirror patent. I have not been able to trace the patent itself and assume that one was never granted, possibly because some important aspect had already been anticipated.
 
Magnifier mounting
 
Many manufacturers contented themselves with mounting a magnifier at the end of a swinging arm, centred about two thirds of the way down the index arm, with the centre of the lens approximately over the junction of the arc and vernier scales. Obviously, at each end of the vernier scale, the lens centre would be a little out of line with this junction.  When a Ramsden type of compound magnifier was used, this was of little consequence, as the Ramsden provides a relatively flat field, but many cheaper instruments were provided with a simple plano-convex lens, which suffers from quite severe off-axis distortion.
 
While Hughes provided a Ramsden type magnifier in their top-of-the line pre-WWII vernier sextants, in this Admiralty pattern sextant, there is only a simple magnifier, but it is carried in a substantial mounting that allows the lens to follow the curve of the scales (Figure 11).
 

Figure 11 : Magnifier mounting

 A curved bed is mounted on two pillars and a short dovetail slide with keeper underneath carries the magnifying lens mounting. An elegant little brass knob completes the assembly.
 
Tangent screw and clamp
 The tangent screw mechanism is one that was (and is) commonly used in theodolites for obtaining slow motion. I touched on it briefly in my description of a Troughton and Simms sextant and I repeat the description here (see Figures 12 and 13).
 
A block that can slide in guides on the back of the lower end of the index arm can be clamped to the limb. A tongue or lug projects from the sliding block and is sandwiched between the end of the tangent screw and an opposing spring, both of which are contained in a tubular frame that is secured to the lower end of the index arm. When the clamp is released, the index arm is free to move over the arc. When it is secured to the limb by the clamp, the tangent screw can be used to make fine adjustments, and the spring inside the spring box provides motion in the opposite direction. It also takes care of backlash, which can be an annoyance in a vernier instrument, even though it does not affect the accuracy of the reading.  Other makers were content to leave the ends of the dovetail slide open, but in this instrument they are closed off with end pieces.
 

Figure 12 : Tangent screw mechanism in situ.

 

Figure 13 : Tangent screw mechanism exploded.

 
 
 Finally, Figure 14 shows the instrument in its refurbished substantial mahogany case. Like all Hughes sextant cases, the handle is on the right hand side, so that the case is not set down on its hinges, and the recessed hook latches face to the left, so that gravity keeps them closed when the case is carried by the handle. The handle is of the type commonly found in military chests and is usually let into the wood, but that detail has been omitted in this case. A “belt and braces” approach to securing the sextant has been adopted, with a pocket and boxwood retaining latch for the handle, while three felt covered pads in the lid hold the instrument secured when the lid is closed and latched. The corners have box-comb joints and top and bottom are, as in most sextant cases, glued and screwed on with counter sunk brass screws.
 

Figure 14 : Sextant in case

 
 If you have enjoyed reading this account, you will, I am sure, enjoy reading my book The Nautical Sextant, available through booksellers, from Amazon or direct from the publishers, Paradise Cay and Celestaire. Intending buyers in Australia and New Zealand may find it interesting to Contact me, as I am able to offer them a discount on the published price.
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2 responses

31 08 2016
stewart135

Thank you for this. I have read it with great interest. I have a Kelvin Bottomley and Baird Ltd Ser no.1965 with a Teddington certificate from 1926, Vernier sextant. It was a gift to my great great uncle from his father as he went to sea having passed his Merchant exams about then. He went on to be a Capt, did the Atlantic convoys and then ran one of the Glasgow docks during WW11. Quite a character, I have been told. I still use the sextant although these days it is a backup only. Still, a skill I like to practice. I’m currently in French Polynesia so am getting lots of time to keep my hand in. I’d like to clean it back to its best at some point as well. In the meantime I am starting to have a little difficulty reading the vernier and was wondering what you would suggest best to clean that with? Grateful if you could use my low bandwidth email which is and use text only to answer. Regards Stewart Henderson

31 08 2016
engineernz

It’s good to hear that your old sextant is seeing some use. On no account use silver polish or any other abrasive, including cigar ash(!) to clean the scales of a vernier sextant. Best for the silver arc is some dilute (50%) ammonia solution on a finger tip, and rub gently to remove the tarnish. It gets your finger tip clean too. Follow up with a little black boot polish or black artists oil paint to make the graduations a little clearer.

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