Like AST0983 this is a small portable transit instrument designed and patented by Latimer Clark and retailed by J. [or A. J.?] Frost. This is a slightly later, slightly cheaper version of the same type of instrument (retailing in 1882 at £7, 10s) again following Latimer's philosophy of providing cheap transit instruments so they might become as common a domestic instrument as the camera.

A transit instrument is a telescope pivoted on a stand so that it can only move in one plane. Normally this would be aligned north-south so that as the Earth turns, every star visible from the latitude of the telescope can be seen to rise and fall over the course of a year. These types of telescopes can be used to create star charts, since every star can be viewed and so plotted. The time at which each star crosses the north-south line, or meridian gives one co-ordinate, the angle the telescope must point at to see it (its angular height) gives the other. It can also be used to find the time by the stars.

Telescope: Telescope has an aperture of 38mm (1.5 inches). The instrument is made of brass and glass with the main part of the telescope tube painted black.

Mount: This telescope is mounted as a transit instrument and so is pivoted. The base can be screwed onto a window-sill (hence the name). The instrument can by adjusted by means of 4 levelling screws on the base and the level mounted over the base.

Box: The box is part numbered AST0990.1 and can be accessed via the WHOLE/PART toggle.