Duplex mirror [from Mance 5-inch Mk V Heliograph]
Object detail
Accession number
1978.454.3
Description
Signalling mirror/Heliograph - round mirror encased in a solid metal black frame. The frame pivots on its central axis in a U-shaped stand with a projection rod at its centre. There is a wing-nut at one end which can be tightened to hold the mirror firmly in any suitable position. The reflective part of the mirror has a small white rectangular mark (with one end curved). Against the white background are three parallel black lines with one black line bisecting them. There is a black dot at the start of this line. On the reverse side of the mirror which is completely blackened, a white arrow has been inscribed into the metal.
Brief History
This is a duplex mirror from a Mance 5-inch Mark V Heliograph. Heliographs were a wireless optical telegraph popularly used in military and surveying work between the 1870s until mid-way through the 20th century. A heliograph conveys messages to a receiving station by redirecting sunlight. Mirrors movements or shutters manipulate sunlight and create flash signals, usually in Morse code, and this is the message directed to the receiving station. A second ‘duplex’ mirror was attached to a heliograph when the angle between the sun and the distant station was greater than 90º. This second mirror was oriented to intercept the sunlight reflected from the main mirror and redirect it to the receiving station.
Media/Materials
Credit Line
Duplex mirror [from Mance 5-inch Mk V Heliograph], 1978.454.3. The Museum of Transport and Technology (MOTAT).
Public comments
Thank you for sharing this extensive information.
- MOTAT Curatorial Research posted 3 years ago.
This object is the Duplex mirror from a Mance Heliograph, 5-inch Mark V [1,2]. The Mance heliograph was a wireless telegraph that signaled by modulated flashes of sunlight. It was used from the 1870s through at least World War 2 ( Photographs of the heliograph in use during World War 2 are at [3]). The duplex mirror was used when the angle between the sun and the distant station was greater than 90º. A thumbnail photo of one, captioned "Secondary Mirror" is on the Mance Type Mk V Heliograph website[2], which links to a larger photo at [4]. I've supplied excerpts from [1] describing the duplex mirror as [5]. The keyword "Heliograph" should be added to the "Classification" section for this device. [1] Signal Training, Volume III, Pamphlet No. 2., Heliograph, 5-inch, Mark V., 1922, London, HMSO, 20pp. The duplex mirror is discussed on pp 4, 7,9,10,16 and others http://www.prc68.com/I/pdf/1922_08_Heliograph_5in_MarkV_med.pdf [2] Mance Type Mk V Heliograph website https://www.prc68.com/I/MkVHeliograph.shtml [3] World War 2 images of heliographs at the Australian War Memorial: https://www.awm.gov.au/advanced-search?query=heliograph&collection=true&facet_type=Photograph&facet_related_conflict_sort=10%3ASecond%20World%20War%2C%201939-1945 [4] Large image of duplex mirror https://www.prc68.com/I/Images/MkVHeliographSecMirb.jpg [5] In [1], page 4, in the parts list: W. Duplex mirror with frame in U-arms(T) X. Sighting vane on ditto Y. Butterfly and pivoting screw Page 7 - in Diagram 3, the duplex mirror is shown at right Page 9: "xii. The duplex mirror (W).– in the "duplex mirror" there is a portion left unsilvered ; on this a paper vane is fixed (on the back of the mirror), which serves the double purpose of enabling the signaller to effect and alignment and to see that his light is properly adjusted. The vane is marked with lines in a similar manner of the vane on the sighting rod. The frame of the duplex mirror has no trunnions, but the mirror is pivoted in the "U" arms by means of two screws which pass through the ends of the arms into the fram. A butterfly screw is provided ot clamp the mirror at any desired elevation. The stem on its "U" arms can be clamped in the socket at the end of the sight arm." Page 10 "2. When the sun and the distant station are in opposite directions it becomes impossible to reflect the light from one mirror alone in the required direction and a second mirror, called the duplex, must be placed so that the light from the sun is reflected from the signalling mirror on to the duplex, and thence to the distant station. As the light striking the duplex does not come direct from the sun but from the signalling mirror which is moved as necessary in accordance with the apparent motion of the sun, it is not necessary to move the duplex mirror in order to keep the light on the distant station. The duplex mirror must always be used when the angle between the sun and the distant station is greater than 90°." Page 16 "4. When using the duplex mirror it may be taken as a rough guide that the sight arm should be placed at an angle of 45° to the signalling mirror. In order to get the i aximum amount of light reflected to the distant station, the sight arm should be placed as near as possible to an imaginary line joining the sun and signalling mirror, but it must be placed sufficiently far to the side to prevent the duplex mirror R intercepting the sun’s rays from the signalling mirror B, and to prevent the latter intercepting the light reflected from the duplex mirror to the distant station Z (see Diagrams below)."
- Richard Fowell posted 3 years ago.