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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.
On the dioptric system, the remedy was simple: it was merely to erect a tall chimney over the central lamp, and lead it out at the roof; by which means, the draught of the lamp was improved, and all the products of combustion carried off.
Everything is based on the focal plan, and on the rotation of the octagon drum, formed of eight wide simple lenses in range, having above and below it two series of dioptric rings; an algebraic gear, secured from the effects of the beating of winds and waves by glass a millimetre thick , yet sometimes broken by the sea-eagles, which dash themselves like great moths against these gigantic lanterns.
It consists of alternate and opposite sectors of dioptric spherical mirror, MM, and of Fresnel's fixed light apparatus, AA. By the revolution of this composite frame about the burner, the same immediate alternation of light and darkness is produced, the first when the front of the fixed panel, and the second when the back of the mirror, is presented to the eye of the sailor.
In August 1840 the raft was found to have completely settled down, the piles were as firm as if they had been screwed into clay; a lighthouse was therefore erected within the short space of three months; and on the 16th February, 1841, a dioptric fixed light was exhibited off this dangerous spot, and was visible ten miles off in all directions.
I refer to the use of dioptric media which correct the diminished refracting power of the humors of the eye, in other words, spectacles. I don't use them. All I ask is a large, fair type, a strong daylight or gas-light, and one yard of focal distance, and my eyes are as good as ever. But if YOUR eyes fail, I can tell you something encouraging.
There was one of Stevenson's new cages of dioptric prisms around the flame, and once every minute it was turned by clockwork, flashing a broad belt of radiance fifteen miles across the sea. All night long that big bright eye was opening and shutting. "BAGUETTE!" said Thibault, "it winks like a one-eyed Windigo."
The Dutch were the first to adopt Fresnel’s system. In the year 1834 the Commissioners of Northern Lighthouses sent Mr. Alan Stevenson to Paris to inspect the system, and his report was so favourable, that the reflecting apparatus of the revolving light at Inchkeith was removed, and the dioptric instruments substituted.
The new light was exhibited on the evening of the 1st of October, 1835, and so great was the satisfaction afforded, that a similar change was made at the fixed light of the Isle of May. The Trinity-House of London followed next in adopting the improved system, and a revolving dioptric light of the first order was erected at the Star Point in Devonshire.
Much also has been said in favour of the Drummond and Voltaic lights, which, on account of their prodigious intensity would appear to be most desirable; but the uncertainty which attends their exhibition renders it at present impossible to adopt them: but there is a yet more fatal objection the smallness of the flame renders them wholly inapplicable to dioptric instruments, which require a great body of flame in order to produce a degree of divergency sufficient to render the duration of the flash in revolving lights long enough to answer the purpose of the mariner.
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