Prototype variable magnification scope by Banks

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This three draw telescope is engraved as made by “Banks”, of “441 Strand, London”. It also has the letters “INVᵀ”, presumably indicating it was an inverting telescope. But it isn’t. Maybe it means an “Inverting lens” has been added to make the image the right way up! Note that the engraving is on the “left side”, ie the first letter, the B of Banks, is closest to the eyepiece. This normally indicates an early date, typically around 1800, as by 1810 the standard had changed, and the engraving was the opposite way round. The engraving also looks a little crooked, so maybe it was indeed when Banks first started making such instruments.

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In a slot in the first draw, there is the capability of moving the cartridge which holds the third lens in the scope sequence (starting from the eye) backwards and forwards, to adjust the viewed size of the viewed image (the magnification, in modern parlance). This third lens is centrally mounted in this cartridge, and it does not seem to be removable. The cartridge was presumably moved by a screw or pin positioned in the slot, attached to the cartridge: this was missing, so at the moment its place is taken by a piece of a wooden cocktail stick.

 

Does anyone have any suggestions as to what was attached to those two holes at the end of the slot? (Bearing in mind that the whole tube has to slide into the second draw, which means there is no protrubrance allowed above the OD of the tube….)

I found this very interesting, as I have mostly used pancreatic scopes, which use a similar approach, separating the two lens cartridges by a different amount, and therefore increasing the magnification. Changing the magnification also changes the focus point, so requires a slight focal adjustment. In fact, this was how my first telescope, the N&B Petrel, worked.

But what this one does is not quite as easy to define.

Banks in the Strand.

Robert Banks, or Bancks, was working at 441 Strand from 1805 to 1830. So assuming the telescope dates within this period, it would definitely pre-date all of the pancreatic scopes I have ever seen. The earliest I have found, which works very effectively, was that produced by Spencer, Browning & Co in about 1850: – see the story about the wreck of the ‘Eagle’ in 1859 on here. This later telescope is labelled as ‘Patent Pancratic’, in the engraving. Spencer, Browning & Co worked from 1840-1870.

Banks had taken a different approach to the problem, which is maybe why Spencer, Browning managed to obtain their Patent, later. Banks introduced a moveable third lens in the first draw tube, ie one of the lens pair making the second cartridge. Spencer, Browning use the separation of the two lens cartridges ie moved the third and fourth away together, further away from the eyepiece to change the magnification.

Does this work?

The Spencer, Browning variable magnification system works brilliantly, and many modern scopes use this as standard, On the other hand, I have never seen one like this Banks model before! This Banks telescope came without the pin in the sliding section. That is essential to position the third lens correctly. It also came with a 4th lens that seemed to be a substitute lens, for one that had maybe been broken in use.

So while the telescope works, but not very well, it could be much improved with the right lens in this 4th position. Or maybe it explains why this system was not adopted more widely as a variable magnification or different style of pancratic type of telescope.

Possible redesign

If the 4th lens were to be attached at the far end of the moveable cartridge, instead of in the threads provided, it would be possible to move that back and forth: then this would be the same as the pancratic principle. However, this would be a little more difficult to work with, since the focus point would move significantly as the second cartridge is moved to change magnification. At the moment, the first draw pulls out of the second, as the lens assembly substituted for the 4th lens is not big enough to make a stop within the sleeve: maybe this thread was just used for such an end stop, and the fourth lens did move with the sliding cartridge? I still need to test this theory with some practical lens examples.

The components of the first draw are shown below:

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(The Blu-tak is to hold that lens assembly in place, it does not fit the screw-thread)

Description

DSCN6599I should mention something to give you an idea of the telescope size. As said before, it is a three draw scope, with a mahogany body. The original screws, and there are lots of them, are all still present. There are eight screws holding the objective holder into the mahogany part!

 

The first and second draws have engraved arrows, pointing towards the objective. The collar on the eyepiece end of the barrel has an engraved arrow, pointing towards the eyepiece: so these are almost certainly not giving dismantling instructions, as some people have suggested.

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Extended the scope is 28” long, and the objective lens housing is around 47mm OD. Closed up it would be 9” long. The eyepiece itself has a movable flap cover, which instead of a hole can position a ruby coloured glass over the eye hole, for when looking at the Sun presumably.  (See the picture up above).

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Medium sized Ramsden 3 draw telescope

This telescope is a  product from a major telescope maker from the 1790s. So, bought on Ebay, it came looking sad and unloved, held together with masking tape. But with everything in place, more or less: maybe not in the right places however.

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As received, c/w masking tape and grime

Taking it to pieces, the first draw, the eyepiece, did not work as an inverting microscope, which is always a bad sign. Then the view through the unit was rather how I imagine tunnel vision to be, a view down a long narrow tube. No real field of view available.

The name Ramsden

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The engraved first draw, after polishing

However, the telescope is a Ramsden, made maybe 1790, engraved properly (on the left side, as was the standard in 1790), and is complete with the objective lens cap. The objective lens is clean and complete, the barrel in good condition. Inside, the lenses in the first draw looked a lot the worse for interference, one stuck in the threads, and with gripper marks round one of the brass lens mounts as evidence of a lot of previous stress. It also appeared that they had been shuffled, so did not work well optically. Indeed the first draw worked as a microscope only when reversed. Then one lens was found to be jammed into the cartridge with an unusually positioned aperture assembly pressed inside the cartridge.

On removing this aperture, an unsupported unmounted lens is found: it is the necessary to start to sort them out. In this scope unusually the threads all seem to be much the same size, for the lens mounts, so that makes it easier to get them mixed up. Later the manufacturers “keyed” the lens threads to make things easily located. It transpired, after hours of trying, that this bare lens must have been a substitute for a broken one, and it was just not right. The lenses and cartridges, with their integral apertures, were returned to what seems to be the correct orientation and position by comparing the design with three other Ramsden scopes in the collection. I then realised that this one is almost identical to #51, a Ramsden bought many years ago, and reported on this website already. That report, and others, also gives the history of Ramsden in the 1790s.

Eventually the lens was replaced by a slightly smaller lens and mount from a J Webb similar scope (#263), and the whole thing fell into operation properly.  There were two ‘John Webb’ instrument makers reported by Gloria Clifton, one from 1792, and his son from 1800-1847. Telescope #263 was engraved as from 408 Oxford Street, which is where Clifton says the son was based – for a short period – in 1808. Telescope #263 has other separate problems, such as a broken objective lens, so the transfer of this number 2 lens to the Ramsden is not a restriction. However the search continues for a suitable lens and holder that might fit the Ramsden thread, as the J Webb lens is small and only wedged in position.

Renovation

The brass polished up well, with Brasso. The main barrel is mahogany, and the old French polish scraped off easily to reveal the wood, which has a couple of longitudinal cracks, but nothing serious. This is scheduled for repolishing. The objective holder has one small screw missing, which needs to be replaced. The objective lens end-cap is present, and in good condition.  As yet the eyepiece sliding lens cover is still stuck firmly in the open position.

The pictures that follow show images before the renovation, as it was received.

 

 

 

Then these pics are after polishing, but before any French polish added on the barrel.

 

 

 

In the last picture the second lens cartridge from the bottom of the first draw can be seen: the lens facing the eye is the one that has been attacked with some form of plier grip. Also on the right, the slider on the first draw is seen, where the thread is adjacent to the knurled shoulder. Better made telescopes a few years later would have had the threaded section at the opposite end of the slider.

The telescope has ended up a fairly well made, neat and effective telescope, with really good optics, as you would expect from Ramsden.

Owners and applications?

We do not know any of this. This smaller size unit (7.5″ compacted, and 22″ extended) could have been used by a cavalry officer, or on board a sailing ship, or indeed just by a country house owner. It has not been bashed about, and appears undamaged apart from the one broken internal lens, which has had to be replaced. Overall diameter is about 1.8″ at the largest part over the objective.

Equally we have no knowledge of the ownership of the scope even recently. The Ebay seller lived in Barnard Castle in County Durham: a lot of sailors came from the northeast – Capt Cook came from Middlesbrough after all!