When trying to date very old telescopes, the obvious starting point is anything associated with a maker’s name engraved on there – often on the first draw, or eyepiece of the telescope. The plan then is to go to historical records, to see when that maker was active and creating instruments. First stop is the “Directory of British Scientific Instrument Makers, 1550-1851”, by Gloria Clifton.
Sometimes that does not help, as the maker, or his sons and heirs, worked over a long period: for example Dollond, the most prolific and long lasting name in the industry!
The next clue is where the maker’s name is engraved: if it is written with the initial letter nearest to the eyepiece, on the first draw, then this was the style adopted up until about 1800, plus or minus 10 years depending on the maker. If the last letter of the makers name is closest to the eyepiece, then the scope is probably later than 1800.
If there is no name
The design of the scope is the only pointer if there is no name. Often this produces a debate between the various ‘experts’. Areas of discussion include the shape of the eyepiece cover, the diameter of the objective; the presence of apertures inside the barrel; and whether the objective seems to be a dual/compound lens or a single convex lens. But consider the basics: what is the telescope made from? Earlier than 1800 there were mainly wooden barrels, and mostly a single brass draw: larger diameter brass tubing was not readily available in this period, so telescopes were small in diameter, and typically had one draw only. Mostly the lenses were in pairs at either end of the first draw: an alternative was to have splits in the first draw at each of three lens positions along the draw, to give access, as well as a lens at the eyepiece (and the objective).
The major discussion about the objective is whether it is a compound lens, as Patented by Dollond. Made of one convex lens (crown glass) and one concave lens (flint glass), or it is a single convex lens.
Another reference book, “Collecting and Restoring Scientific Instruments” by Ronald Pearsall, gives some useful reference dates.
1729: Chester Moor Hall combined two lenses of opposite powers, to produce an objective lens that overcome the chromatic aberration present in single lenses, particularly at higher powers. This was the birth of the achromatic lens, although the term was not applied until 1766.
1752: John Dollond and his son Peter set up in business, and applied for a patent on the use of the compound lens in telescopes, to avoid chromatic aberration.
1758: Peter Dollond, a very astute businessman, persuades/encourages John to apply for a Patent based on Moor Hall’s (unpublished) work (nearly 30 years earlier).
1761: The Patent is granted, but John Dollond died: Peter Dollond carried on in the business. Frances Watkins, a “partner” in the Dollond business, left the business and joined many of the other independent instrument makers to petition to have the Dollond Patent revoked: these included James Champneys, Francis Watkins, Addison Smith and Henry Pyefinch. Note that supplies of the imported flint glass were difficult to obtain, typically were often low quality, but expensive.
1762: Jesse Ramsden sets up his own business, and works as a subcontractor to Peter Dollond: however the “Eyes Right” record of the history of D&A suggests that Jesse was an “assistant” to Dollond. Jesse was not one of the people who joined the petition to have the Patent revoked.
1763: Peter Dollond introduces a triple lens objective, to overcome another chromatic aberration problem.
1765: Jesse Ramsden marries Sarah Dollond, Peter’s older sister: part of his dowry was a share in the patent revenues. Ramsden arguably took the lead in telescope development towards the end of the C18, and his protegées, like Matthew Berge (Berge was his foreman, and he took over the business when Ramsden died in 1800), Thomas Jones and William Cary continued the dynasty. [There was also a John Berge apprenticed to Dollond]. Berge’s apprentice Nathaniel Worthington took over from Berge when the latter died in 1819. Ramsden also worked for George Adams at this time: Adams had joined the petition against the Patent.
1766: Peter Dollond moves to new premises (59 St Paul’s Churchyard) and is joined by brother (John). Berge’s apprentice Nathaniel Worthington took over from Berge when the latter died in 1819
1766: When the petition to revoke the Dollond Patent failed, Peter Dollond sued those who had been selling achromatic telescopes using the dual lens objective. These included James Champneys, Francis Watkins, Addison Smith, Francis Matthews and also Henry Pyefinch (who had not been a party to the petition).
1783: The Dollonds start to use brass draw-tubes, based on a Patent taken out in 1782 by Joshua Martin. By this comment I think Pearsall means the tubes became more easily available to produce two and three draw telescopes economically. He suggests this allowed Dollond to no longer use paper covered vellum tubes. (Previously it had been my impression that such paper based tubes had been phased out about 30 years earlier than that!- Ed). Interestingly, pictures of Dollond refractors dated as in 1744 and 1760 show them with square tubes and (in 1760) two square draws. Certainly a Dollond “pocket” telescope of 1785 is pictured with a brass barrel and a single drawtube.
1790s: Increased demand for telescopes for naval warfare, with the rise of Napoleon: first major sea battle is the battle of the Nile in 1798, where Nelson is instrumental in destroying most of the French fleet, when they were invading Egypt.
There are other reference books giving the history of various telescope makers, but these have not yet been consulted in reference to this paper.