I came across the following entry in Samuel Pepys’ diary dated 19th August 1666, in which he writes of observing Jupiter with some friends later that evening after dinner. Pepys notes that “We did also at night see Jupiter and his girdle and satellites very fine with my 12-foot glass, but could not Saturne, he being very dark.” It struck me that Pepys refers to Jupiter’s girdle in the singular; was only one belt visible due the observation coinciding with a possible disappearance of the south equatorial belt? Are there any other contemporary observations which could possibly confirm this? Also, by “12-foot glass” was Pepys referring to an actual focal length of 144 inches, or just 12 inches? If the former, it must have been an extremely substantial instrument!
As I understand it, telescopes of this era were always measured by focal length, rather than aperture. So “12 feet” does indeed refer to the length (similarly, Herschel’s “40-foot” telescope had that focal length). The instrument apparently cost Pepys the small fortune of £9.
The article also points out that Pepys’ activities with optical glass went beyond astronomy… 26 May 1667… “I did entertain myself with my perspective glass up and down the church, by which I had the great pleasure of seeing and gazing at a great many fine women…”