ILLUMINANTS AND BURNERS.--Mineral oil has been introduced, and special burners designed in order to burn it. The largest of the old colza oil burners (known as Fresnel burners) has an intensity of 230 candle units, or 23·6 candle units per square inch, while a similar burner for paraffin has been reported as affording 415 candle units, or 44·3 units. The latest developed Trimley burner of Sir James Douglas, with nine concentric wicks, gives 49·8 units. The oil used has a flashing point of 154° F. The average cost of mineral oil is one-fourth that of vegetable oil. Gas is also freely employed, and in the 108-jet burner of Mr. Wigham it has attained a power of 2,433 candles, and in the 10-ring burner of Sir James Douglas, a power of 2,619 candles. Beyond the highest reach of oil or gas burners, say 2,500 or 2,600 candles, is the electric arc giving an intensity of beam of ten millions of candles. At the other end of the scale is the duplex burner, which, with the new Chance lenses, is available in the smallest apparatus for a horizon of from seven to ten miles.
The Spon Lane Lighthouse Works have been extended greatly since the last visit of the Association. The unique methods of grinding and polishing the optical glass have not been materially changed, but the number and variety of machines have been largely increased. In the mechanical shops there is now a complete _outillage_ for turning, drilling, shaping, screwing, &c., applicable to all the old designs and processes, and also the newer branches introduced since 1865. There are about 200 workpeople employed, many with consummate skill and intelligence, as may be assumed from the precise and highly finished work in which they are engaged.
At this date about 260 complete sea lights and about 300 complete harbour lights have been sent out, together with about 200 smaller port lights and about 370 of the recently designed ship lights.
Very striking specimens of optical science have been contributed by Messrs. Chance, as exemplified in the list of dioptric lights they have supplied around our own coasts, of which The Longships, the Wolf Rock, the Flamborough Head, the Hartland Point, the Start Point, the South Stack Rock, The Casquets, Bull Point, Anvil Point, the Eddystone, and many others are examples.