This telescope was a recent purchase on Ebay. For some reason I seemed to be the only bidder! There were two John Gilberts listed on Tower Hill, in London: they were father and son, covering the periods 1719-1749, and then 1751-1791. Both were quoted as located in Postern Row, Tower Hill.
The scope is classic C18 construction, a single draw, with a small diameter objective (single) lens, and a split draw with three eyepiece lenses. The single draw is retained in the barrel, so possibly indicating a later date in C18.
The engraving on the first draw is oriented with the first letter close to the eyepiece: it states ” J. GILBERT – Tower Hill – LONDON, and the three lines use three different fonts.
The body is a reddish mahogany, fairly heavy: the two brass end pieces are retained with some form of nail rather than a grub-screw, as was adopted later. Neither the objective assembly nor the opposite end supporting the single draw appear to unscrew. On the objective, which is maybe a lens of about 1cm diameter, there was once a sliding cover, but this has been lost, maybe when the mounting was damaged.
The overall diameter is 4 cms, and the closed length just over 13″. When extended, the total length is 20.5″. Within the barrel the three lens eyepiece has the lenses equi-spaced: this presumably makes it a Schyrlean eyepiece, as developed by a Capuchin Monk, Schyrleus de Rheita (1597-1660).
The Gilbert family
John, the son quoted above, had more sons: one was also called John (III), and also William. Both were trained as apprentices in the business, and William transferred to be apprenticed to Dollond in 1769. William and his sons eventually worked in the Navigation workshop at 148 Leadenhall Street, amongst the experts there.
Date and Purpose
There is no way to tell exactly: it is pre-1780, probably, but maybe not earlier than 1750. It was well made, solid, and has survived well – it is a good quality telescope. Probably used on board a ship of some form. It is interesting to compare this scope with the octagonal one from James Chapman, as they are much the same in sizing.