Octagonal and Decagonal scopes

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It’s very important to a collector to actually have an example of one of these multi-sided telescope designs in any collection, as they are some of the oldest, and in some ways most interesting. So I have usually bought any that I saw that were a reasonable price: but because they are old and desirable, they were not cheap!

The story was told in the previous article about the Dollond 8-Sided telescope from 1860. The construction of this one used eight identical strips of mahogany, about 7/8” wide, with the inner edges filed down, so that they can be glued together in an octagon. This is then held together with the brass rings at each end, retained by small screws.

This style was obviously the “one to have”, so that possibly later, and maybe smaller telescopes copied the design by taking a round length of wood, drilling out the centre, then planing flats onto the external surfaces. I would suggest that most of the smaller hand-held decagonal telescopes, ie the ones with 10 sides, and ones with any larger number of flats, would be made this way, as it would be more difficult to get 10 narrow strips to glue together neatly, and it is maybe easier to plane off the round barrel into ten sides. Having said that it does seem that planing the unit into 8 flats would be easier for me, as the angles of each flat would be simpler to get right.

So let’s look at these other four multi-sided telescopes to see what emerges.

The telescope examples

  1. Small Oak-barrelled unit:

DSCN2435This one, pictured at the bottom of the group above, is an un-named oak barrel, eight-sided telescope: the wooden barrel is 12” long, 1.375” wide from flat to flat. It has a bell-shaped eyepiece, a very narrow diameter objective, at 0.625”, and the single brass draw has three joints to mount the internal lenses, plus one at the very top of the bell: this lens is designed without any protection (like a covering slide). The objective does have a sliding cover. The telescope gives a really good magnification, say x12.

This unit is constructed from a single length of wood, drilled out with a smooth straight bore. The eight joints between the flats are relatively sharp edges, simulating the appearance of a unit made from strips. Perhaps unusually for this design the draw tube is trapped in the main barrel, so does not pull out completely in use.

Reference #182.

  1. Medium-sized Mahogany unit:

DSCN2429bThis is the same design as the oak unit, but bigger, built from mahogany – and it has ten flats on the outside of the main barrel. Here the lens closest to the eye of the user is positioned as if recessed a little into the bell housing, and there is a slot for a slide to act as a cover: the actual slide is missing. The other three lenses in the barrel are mounted at two screwed splits along the length, and the last is at the objective end of the draw, which pulls out freely, it is not retained in the barrel. If I saw the machined groove at the end of this draw on a modern piece of equipment I would say it was designed for an ‘O-ring’. This might indeed have been used to give some resistance to pulling the draw right out, and it could be squashed through in the assembly of the unit – there’s no other way of getting it inserted in practice.

Left is this telescope: right is a typical example of a legible signature!

Left is this telescope: right is a typical example of a legible Dollond signature!

At the objective end, the lens aperture is 1”, and the glass is protected by a sliding cover. The remains of a signature are visible, engraved into this cover, and it corresponds with “Dollond London” in a script form that corresponds with that seen on other Dollond telescopes.

The barrel is interesting, in that the inside bore is circular, and the outside is cut with ten sides, but the transition between each of the slides is very much smoother over, consistently along the 24.5” length. The telescope actually has a split which extends maybe a third of the way along the barrel, which follows the grain and moves from one flat side to the next. Possibly this split was caused when someone was hand-engraving his initials “E+P” on one of the flats, which remarkably is followed by “1781” which is presumably the date of this engraving work. [To put this into context, Captain Cook discovered and mapped the east coast of Australia in his voyage from 1768 to 1771, and then was killed in Hawaii in 1779. Dollond’s patent on the two-part  objective lens was dated 1760]. The distance across the flats at the eyepiece end is 44mm, whereas at the objective end this distance is reduced, to 39mm. This is effectively a reverse taper in the outside size of the barrel.

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Inside, the bore of the hole is constant at 30mm, from the objective back to the orifice half way along the barrel, which has a diameter of 19mm. From there the internal diameter increases towards the single draw and the eyepiece, so mirrors the effective reverse taper seen on the outside. The brass draw is about 30mm diameter, and the brass sprung slide that holds it in place is around 3” long, inserted inside the barrel.

Magnification is only around x6. Reference #59

  1. Gaitskill Mahogany telescope.

DSCN2429aAnother longer mahogany 10-sided unit, similar to #59, but here the barrel is visibly tapered in the normal way, with the larger diameter at the objective lens. The barrel is 24.5” long, including the brass end fittings. But the flats are flat, without the obviously rounded corners at the edges of the flats seen on #59.

At the eyepiece end, the barrel is 43mm across the flats, outside, and at the objective end, this measurement is 48mm. Each flat is about 14mm wide. Inside, the internal bore is straight half way down the tube at the eyepiece end, and then slowly increases towards the objective lens position. But this is not to allow the use of a large objective lens, as very close to the objective (~20mm) there is an aperture/orifice, with a hole diameter of only 16mm. The brass housing holding the objective is 50mm diameter, and the lens itself has a visible diameter of around 30mm: but this lens appears to have a coating of some form around the edges, which might even be glue. The effectively useful diameter of the central part of the objective is around 21mm.

The objective does not appear to unscrew, to remove the lens holder from the brass screwed to the end of the barrel. So since the screws here do not look original, these were taken pout to inspect the lens. The result seems to indicate that the outer edge of the lens doublet has a different focal length to the middle section. The apparent colour difference is possibly that only the centre section is ground to the correct radius to nestle into the second lens, and where the two do not touch gives an impression of a coating, because of the different reflectivity. Having such an objective means that the close aperture inserted in the barrel would have been essential so as not to allow the outer edges of the objective to transmit light down to the eyepiece.

DSCN2431Again the single draw pulls out of the barrel completely, just after the last lens in the eyepiece set of four, so the barrel has two screwed splits. Similar machining on the end is not a groove, but a raised rim on a section of the draw tube which has a smaller diameter than the rest of the tube, so maybe it was wrapped in some material or felt to push through and make an end stop. The two draws are almost interchangeable between #59 and this one, which is reference #189. Here though the draw has the words Gaitskill, maker, Wapping, London engraved on the very first section of the draw tube, on the left, ie with the ‘G’ of ‘Gaitskill’ adjacent to the eyepiece, which is the early style. The slide over the eye lens has lost its retaining pin, so is in danger of pulling out completely and getting lost.

When focussed, the single draw is only pulled out by around 2.5”. Magnification is slightly better than the Dollond #59, at maybe x7.5. Reference is #189. Joseph Gaitskill operated as a Ship Chandler, Compass and Mathematical Instrument maker from various addresses in Wapping from 1778-1811, but was called Gaitskill & Co from 1805. So this telescope also dates from the 1790s approx.

  1. A restoration project!

It’s all there, except for one bit that’s missing, that is. The missing bit is the objective lens, and carrier: it was screwed into the end of the barrel, but has either been knocked off, or fallen off, either overboard, or otherwise it was lost.

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The barrel is externally 10-sided, mahogany, with worn French polish. A third of the way down there is a brass band around the barrel, which is screwed down at one point, maybe holding the barrel together, as there is a split along this end of the barrel running down half the length. Externally the barrel is not tapered, and is about 42mm flat to flat. The internal bore is round, and is also a constant diameter, at about 36mm, but 3.5” from the objective there is an internal orifice with about an 18mm aperture diameter. So the ten sides are produced by planing down the round barrel into a series of flats – again the transition from one to the next is relatively smoothed over.

The single draw has three screwed splits, but the second split going towards the eyepiece has a cartridge containing two separated lenses: overall there are five lenses in this single draw, which pulls straight out of the barrel. While this single draw fits into the other telescopes described above, very loosely, the optical properties of this draw and the other objectives are not compatible. The lenses used do not appear to be incorrect in any way, so they are all there, but as the saying goes, “not necessarily in the right order”.

It just looks like a real problem to understand! Project ref #165.

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