19. Transit telescope by J. Sisson
London, 1739
Jonathan Sisson (1690?-1747)
brass and wood
length of telescope 85 cm
[Inv. MdS-122]

In the Inventario delli Ordegni Astronomici ... fato l'Anno 1746 we read:

"Transit telescope with two lenses for observing Meridian transits, to the lower part of the Telescope is joined a brass semicircle divided in degrees, to the upper part a brass cylinder with a ball that serves as counterweight, another ball attached to the tube at the eyepiece which also serves as counterweight;
A level is attached to this Telescope to make the axis perpendicular to the telescope horizontal;
Another Axis, or wooden spindle for blocking the Telescope at different inclinations.
Both the Axis perpendicular to the Telescope and the wooden spindle have their own brass supports, two fork-shaped for the former, and two for the latter consisting of two transversal plates, all made in England.
This instrument is supported by a sandstone Pedestal adorned with marble, the whole thing very elegant.

Its designer, Jonathan Sisson, was the best pupil of Graham, maker of the instruments ordered by Halley for the Greenwich Observatory. His instruments were famous for their detailed perfection, all the more incredible if we bear in mind that in those days there were no machine-tools and all the work was completely manual.
As with the large mural quadrant and the movable quadrant built by the same craftsman [files 14 and 17] the movable part of this instrument is identical, down to the smallest detail, to the transit telescope described in the Encyclopédie of Diderot and d’Alembert.
Of particular note is the large reversible level, still filled with its fluid. Only a few years before, in fact, had the technique of reaming glass tubes allowed bubble-levels to be produced sensitive enough for setting up astronomical instruments.
It was reinstalled in 1979 between the rebuilt columns which, according to Zanotti, were "sufficiently Doric in proportion as to satisfy needs and elegance" (Handwritten note by Zanotti in Reg. Sp. Ist. Scienze Bo., vol. VII, dated 16 August 1742, Arch. Dip. Astron. Bo.). The supports built into the columns and the wooden bar connecting the eyepiece of the instrument to the lower spindle, ensuring right standing in elevation, were also restored in the same year. The micrometer wires that were found broken were replaced along with the reflector which, though not named in the inventory description mentioned above, seems to be original. The tube bears the words: Jonathan Sisson London. On the object lens is written JaS, Mann Fecit 1739. This is probably James Mann Jr. (c. 1717-1749), heir to a well-established workshop in London (sign Archimedes and two pairs of golden Spectacles) where achromatic lenses were also made. Some lenses of the artisan Mann family can be found in Oxford and Cambridge (Daumas, op. cit.).

The volume of the Registri della Specola di Bologna (Arch. Dip. Astron. Bo.) marked "1815", date 30 May, contains a note referring to its replacement by a new transit telescope, built by Reichenbach, Utzschneider und Liebherr [file 20].

E. Baiada, A. Braccesi (1983), pp. 118-119 and 106, fig. 17.
M. Daumas (1953), p. 301.
D. Howse (1993), p.608.