The lens and mirror are mounted in the turret of a darkened room - the image being projected onto a viewing table. The distance from the lens to the table would need to be adjusted to bring objects at different distances into focus but by mounting the camera on some hill there will be no near objects thus a single setting will suffice.
This was not the first such camera obscura in Douglas as Brown's 1876 guide reports:
Situated in and surrounded by a large garden, tastefully laid out, and provided with every appliance for out-door amusementcroquet, quoits, swings, roundabouts, and (as Artemus Ward would say) " other refreshments"will be found the Victoria Tower, erected by Mr Stokes (to whom sufficient praise cannot be given) with the laudable desire of adding to the limited number of open-air attractions in the Island. In the tower is a camera, upon whose mystic surface all the beauties of the adjacent neighbourhood, both land and sea, may be seen faithfully and clearly reproduced. Nor is this all; for added to this may be seen a collection of transparencies, telescopical, magnetical, galvanical, and electrical wonders truly surprising in so small a space. A very pleasant after-noon may be spent here.
[William Stokes, English-born 1810, described himself as 'pleasure ground keeper' in 1881 census - the pleasure grounds were up on Victoria Road, the 1868 O/S plan shows an observatory which may be the same building and is presumably that which can be seen above the trees in many late victorian photos of Douglas bay]
Another camera was established at Port Soderick in the late 1890's.
First appearance on the Head was in 1887 when Douglas Town Commissioners approved the construction - Manx Sun 28 May 1887 reported that the Obscura 'as recently seen at the Liverpool and Manchester Exhibitions' was being erected next to the Toboggan Slide. However at the end of the season on 26 Oct 1887 it was reported that the wooden building, owned by Mr Hicks who also lived on the premises, had burned down. The next reference would appear to be in 1891 when a plan by Mr J. R. Fielding for a proposed enlargement of the Camera Obscura was submitted to the Douglas Town Commissioners Improvements Committee but turned down owing to a dispute between J.S. Goldie Taubman and the council over access to land owned by him (he also owned much of the Head). In March 1892 the council announced that they were willing to rent the land but only on an annual lease with the rent 'as for the present camera' and that 'the new camera was to be in accordance with the plans submitted'.
John Richard Fielding (described as 'Machinist' in the patent application) of Roach House, 2 Grosvenor Road Douglas, obtained a patent (#19,597) in August 1892 (application dated November 1891) and according to Hammond the camera described was almost exactly that installed on Douglas Head. There is a possibility that John Richard Fielding was related to the Fielding Family long established in Douglas (this has given rise to the 'from Rochdale' tag but evidence is lacking).
The next reference was in 1896 (Manx Sun 22 Aug) when it was reported that Mr William's Camera Obscura at Douglas Head had been 'entirely remodelled and constructed'.
Hammond describes the Douglas Camera Obscura as 'perhaps one of the most remarkable camera obscuras still in operation' and 'should be regarded as an historical monument'.
Inside are 11 tables arranged in a circle separated from its neighbours by a partition with each image formed by its own lens and mirror installed in its own dormer window set in a conical roof.
It may of course be purely accidental that it overlooked the Port Skillion bathing place - it soon acquired a reputation for 'spying' on courting couples!
In 1907 the building was sold to John Heaton, also from Rochdale, a one-time employee of Fielding - in the photo it is described as 'Heatons Grand Union Camera'.
For many years the Camera was enclosed in an unattractive protective shuttering whilst Douglas Corporation decided what to do with it - in 2005 it was restored and is now again open to the public during the holiday season (manned by IoM Vic Soc volunteers).
Still Obscure IoM Vic Soc Newsletter #32 Autumn 1992
Getting Less Obscure IoM Vic Soc Newsletter #44 Autumn 1996
John H. Hammond The Camera Obscura: A Chronicle Bristol:Adam Hilger (ISBN 0-85274-451-X) 1981
R. Kelly Great Union camera obscura Manx Life Nov/Dec 1976.