![]() Made of one convex lens (crown glass) and one concave lens (flint glass), or it is a single convex lens. The major discussion about the objective is whether it is a compound lens, as Patented by Dollond. 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). 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. 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. Often this produces a debate between the various ‘experts’. The design of the scope is the only pointer if there is no name. If the last letter of the makers name is closest to the eyepiece, then the scope is probably later than 1800. 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. 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! First stop is the “Directory of British Scientific Instrument Makers, 1550-1851”, by Gloria Clifton. The plan then is to go to historical records, to see when that maker was active and creating instruments. 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.
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