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.
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.
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:
(The Blu-tak is to hold that lens assembly in place, it does not fit the screw-thread)
I 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.
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).
The scope was bought in July 2017, expensively for current Ebay price levels, at £78. Seller was Bramblewood3. The seller did not understand the true purpose of the slot either. But it is certainly unique!