Orwell Astronomical Society (Ipswich)
The Dipleidoscope: A Solid State Transit Instrument
The word dipleidoscope is derived from the Greek and means double image viewer. A dipleidoscope consists of a hollow equilateral prism with a clear glass front face and the two rear faces internally silvered. The long axis of the prism is approximately aligned with the Earth’s polar axis. The prism thus produces two images of a celestial object, one by direct reflection from the front face that moves in the opposite direction to the object, and the other reflected via the two rear faces that moves in the same direction as the object, but at double speed. At one point these two images coalesce. When the dipleidoscope prism is correctly orientated this conjunction of images occurs as the object transits the local meridian.
The chronometer maker Edward John Dent, FRAS, Associate ICE, (1790-1853) worked for some time in the mid-nineteenth century to simplify the transit telescope originally developed by Ole Rømer (1644-1710) in Denmark. Dent was motivated to make it easy for the non-astronomer to ascertaining the correct time easily. He was approached by James Mackenzie Bloxam, a barrister from Denbighshire (born circa 1814, died 1857), with an already working device. The two went into partnership with the first dipleidoscopes going on sale for two guineas (£2 2s or £2.10, equivalent to £161 at 2010 prices when adjusted for inflation) in March 1843. Bloxam appears to have been afraid of compromising his professional standing as a barrister with much of his work being published only after he died. To secure their respective rights over the device, a patent was sought by Bloxam and was granted on 20 June that year (United Kingdom Patent no. 9793). A French patent was applied for on 24 July 1844. After Dent died his stepson Frederick William Dent took over manufacture.
The dipleidoscope as manufactured by Dent used three separate pieces of glass. These had to be precisely and securely placed and, in 1928, Sir Charles Vernon Boys FRS (1855-1944) promoted the use of a solid prism which cannot go out of alignment. The mounting of Dent's instrument, which is made of brass, is 2⅛ inches (52 mm) long by 3⅜ inches (85 mm) wide and 2⅜ inches (60 mm) high, and around 1lb 15oz (1 kg) in weight. The front aperture of the prism is ⅞ inch (22 mm) long by ½ inch (12 mm) wide. A tightly fitting lid is provided to protect the optics from the weather as the instrument was designed to be mounted outdoors with a clear southern meridian aspect, although portable versions were also available. The lid bears the name E. Dent (or F. Dent for models produced after 1853) along with the business address. To aid observation of the moment of transit, a viewing telescope was sometimes fitted. The brass was painted matt as, if it were polished, solar observation would be very difficult, due to the mounting also reflecting the Sun. The length and width of the prism is sufficient to allow, as a minimum, correct observation of any ecliptic object. Prospective owners in the tropics were invited to supply the latitude where the instrument was intended to be installed, so that the correctly angled mounting could be supplied.
The instrument was a success and, within a year, Dent was importing additional parts from France at 3s 6d (17½p, equivalent to £13.40 at 2010 prices when adjusted for inflation) per dipleidoscope in order to keep up with demand.
Dent had a stand at the Great Exhibition of 1851, held in the Crystal Palace in Regents Park. Two dipleidoscopes were displayed in class X (philosophical, musical, horological and surgical instruments) entry 55, numbers 29 (an ordinary dipleidoscope) and 30 (an equatorially mounted dipleidoscope).
To help dipleidoscope owners align their instruments, Dent produced a 28-page instruction booklet. Additionally, he initially offered to dispatch a competent person with a chronometer to carry out the adjustments. The employee’s work was charged at the cost of travelling expenses plus ten shillings (50p, equivalent to almost £40 at 2010 prices when adjusted for inflation) remuneration per day. By 1862 this service was replaced by a double orthogonal spirit level and magnetic compass accessory that was made available at no cost. It was to be returned to the Dent Company within a specified time as, once the dipleidoscope was correctly set, the accessory was of no further use. The booklet ran through at least eight editions and was still being published in 1868, some twenty-five years after dipleidoscopes were first offered for sale. Interestingly, there is a complete section devoted to whether the dipleidoscope owner should arrive at his local station earlier (eastern longitude) or later (western longitude) than the local time obtained from the dipleidoscope to catch a desired train (running according to Greenwich Time).
In taking a transit of either the Sun or Moon, Dent recommended three observations to increase accuracy: the first as the two limbs initially touch, second as the two images overlap exactly, and finally as the two images separate. Averaging the time of the three events gives the best estimate. A table in the booklet gives solar semi-diameter values throughout the year allowing for reconstruction of possible missing values of a triple. For solar timing Dent recommended either using the viewing telescope fitted with a dark filter (of the type fitted to sextants) or capturing a projected image using a sheet of paper, held about two feet away from the dipleidoscope. The time taken for the two solar images to pass over each other varies through the course of a year, with a maximum of 2 min 22 sec in mid-December and a minimum of 2 min 7.6 sec in mid-September. A well adjusted dipleidoscope has a claimed accuracy of less than one second.
A "universal" model was also produced with the dipleidoscopic prism being placed on an adjustable miniature equatorial mounting; this enabled the user to make timing observations at any latitude and at up to 45° (three hours of time) either side of the meridian passage of an object. This had the great advantage of enabling the observer to capture an observation at his convenience rather than waiting for the chosen object to transit, and helped to overcome the vagaries of the weather.
The advent of telegraphic, telephone and radio time signals made amateur dipleidoscopes (and transit telescopes, for that matter) redundant. However, dipleidoscopes can still occasionally be found in antique shops and on-line auction sites. As these instruments are solid state, with no moving parts to wear, unless the optics has been interfered with, they can still operate as well as when they left Dent's workshop over a century ago. A clue to the date of manufacture of the instrument is given by the name on the lid ("E" for Edward, "F" for Frederick) and the business address specified (the business opened additional shops over the years). Each dipleidoscope also has a serial number stamped on the back.
Bill Barton, FRAS