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On the catoptric system, with revolving lights, each lamp was furnished with a chimney, which passed out at its upper extremity, through a small hole in the reflector into a fixed central hollow shaft, which served the purpose of a ventilating chimney to all the lamps.
'Will you kindly place yourself with your back to the window? he said. 'I am going to apply what is called the catoptric test. You have probably heard of it? 'My ignorance of scientific matters is fathomless. The other smiled, and at once offered a simple explanation of the term.
The reflectors in general use measure over the tips twenty-one inches as applicable to stationary, and twenty-live inches for revolving lights. The Catoptric or reflecting system was first adopted under the direction of Borda, at the Corduan Lighthouse, probably about the year 1780.
A covered passage runs between them, in the centre of which are the residence and offices attached to the towers, so that the keepers can communicate without being exposed to the fierce gales of winter. Each of the white towers is sixty-one feet high, and contains a brilliant fixed catoptric or reflecting light.
The lantern consists of a brilliant catoptric fixed light, produced by nineteen Argand lamps. It was built in 1793 by a Mr Smith. Before the granite blocks of which it is composed were brought to the rock, they were hewn out and put together at Sennen Cove. The stones are dovetailed one into the other, and are secured by oak trennels strongly cemented.
A considerable time was occupied in inquiry and negotiation, when at length, on the 26th October, 1836, the light at the Isle of May was changed from the catoptric to the dioptric system, and a committee of the Royal Society of Edinburgh met at Dunbar, a distance of thirteen miles from the lighthouse, to make observations on the two lights, which were exhibited in contrast.
Hence by dioptric means the consumption of oil necessary for the fourteen reflectors will produce almost as powerful a light as that which would require the oil of twenty-four reflectors in the catoptric system, and consequently there is an excess of oil equal to that consumed by ten reflectors, or four hundred gallons in the year against the Scotch system.
Imperfect Illumination of the old Lighthouses First Improvements The Argand Lamp and Reflecting Mirrors Revolving Lights The Catoptric System Varieties of Lights The Dioptric System Its Details Introduction of this Method into Great Britain Comparison of the two Methods The Drummond and Voltaic Lights Gurney’s Lamp Captain Basil Hall’s Experiments Ventilation of Lighthouses.
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