Page 5 – ‘Notes of a trip’ by Robert Jameson

circular wooden structure of 15 feet, diameter, surmounted by a lightwood and canvas dome, with wheels fitting on to the wallplate, and capable, of being revolved horizontally, so as to direct the door of the dome to any part of the heavens. In this building is mounted the equatorial, a ‘fine’ six-inch refractor, by Mr. Howard Grubb of Dublin (the maker of the great Vienna 27-inch refractor, and the new 28-inch in course of construction for Greenwich). This instrument is mounted, with clockwork, to follow any object under observation of its own accord as long as it may be desired. The length of telescope from object glass to eye-piece measures a little over 10 feet, with a diameter ail given above of six inches.’ The main tube is of strong brass, painted a very light French grey, while all the mountings are of polished brass, and the draw tubes near eye-end, nickel, or silver-plated. The whole is mounted on the equatorial mounting, which again is placed on a hollow cast iron pillar, with a diameter of two feet at base, tapering upwards. This iron pillar is bolted to a solid pier of masonry, to keep the whole instrument perfectly steady, and free from tremor. The telescope is fixed to one end of the declination axis of equatorial -i.e., the axis of the instrument at right angles to the axis of the earth – on which it may be moved north and south. To indicate the amount of this motion a graduated circle, divided to 10 minutes of arc on a silver band, is fixed to this axis, with its plain at 9 degrees to the equator. This circle is read by means of a venier down to 3 seconds of arc. To the other end of declination axis is fixed a self-adjusting lamp, which illuminates the instrument inside, as well as the declination circle, by means of series of cleverly arranged mirrors. From the eye-end the observer may read the declination, without stirring front his seat, by means of an ingeniously arranged” reader’s microscope,” which really reads” round a corner.” The telescope is balanced by means of counterpoise weights screwed on to the declination axis, and although a considerable weight, so nicely is it adjusted, that a child may move it with its little finger. The big tube itself is likewise counterpoised in a similar way. The telescope with the declination axis, lamp, and balanced weights are all fitted on to another axis, the polar axis, round which they turn bodily. The polar axis is situated exactly parallel with the earth’s axis, and thus has its ends pointing directly to the poles of the heavens. This adjustment has to be made by each observer for his particular station, and is found by actual observations. To-the northern end of the polar axis there are two brass circles, with a silver band round each, on which are graduated the hours of right ascension, from 1 to 24, down to two minute intervals, each of which may be read to two seconds of time. The one circle is connected to the polar axis, with its vernier indicating the meridian, while the other revolves round the polar axis, and may thus be moved round directly to show the right ascension of the object observed, by setting it to read the sidereal time over against the meridian vernier. To the eastern side of the equatorial room there is another small wooden structure, opening into the large room. This contains a small transit instrument, also bolted to a stone pier. The transit is used for showing stars the moment they pass the meridian, and the time note on a large clock, also mounted opposite. Sidereal time is all-important in an observatory. Alongside of the clock, on a shelf, there is a student’s chronograph-an instrument by which the observer may register on a ribbon of paper, electrically, the precise time on any event – in fact it is an electrical assistant. The electric light has been fitted to the equatorial, but owing to many difficulties to be overcome it has not been quite successful hitherto, the primary batteries used being too weak. The position of Rolfontein, at 5800 feet above sea level, with its splendid climate and clear sky, is indeed very favo urable for astronomical observations-so that it offers many advantages to an observer able to give all the time to his work.