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steamships sailing towards Victoria Harbour.
Hong Kong's first lighthouse
In 1875, the Government Gazette announced that 'a light will be exhibited on Cape D'Aguilar' on and after the 16th April next. The illuminating apparatus is fixed Dioptric of the First Order showing a white light. The focal plane of the light is 200 feet above mean sea level, and in clear weather it should be seen at a distance of 23 nautical miles. The tower is round, of stone, 30 feet high, with a total height from its base to the lantern vane of 57 feet.'
This official announcement was meant for the marine community with a series of technical terms. In laymen's language, 'fixed' means the showing of a continuous or steady light. Most of the lights of the time were of this type. They are brighter and simpler technically speaking. The disadvantage was that fixed lights were sometimes confused with lights of ships or with neighbouring shore lights. Later, improved types could show their distinctive characteristics and were known as "flashing" or "occulting" depending on the shorter or longer duration of light respectively.
The Dioptric system adopted by the Cape D'Aguilar Lighthouse aimed at concentrating and intensifying the light. It used lens and prisms to concentrate the light from the burner into beams of parallel rays directed to the horizon, just like an overhead projector which takes a light bulb and forces the light forward in one direction. This kind of lens was first designed by Augustin Fresnel in 1822 and was named after him as the Fresnel lens.
Another older system, which was first employed in the mid 1700s, is called the catoptric system which involves using a parabolic metallic reflector behind the light, similar to that used in flashlights. A combination of these two systems is also available called the CataDioptric System.
Lights of the Dioptric system are classified according to their power which depends on the focal distance from the centre of the burner to the lenses. According to the British system the focal distance in diameter of the first Order is 1,840 mm, the second Order 1,400 mm, the third