By this arrangement each lens transmits to all the points of the horizon in succession a light equivalent to that of from three to four thousand lamps with double currents, and eight times greater than the light produced by the silver parabolic reflectors; it is, according to Arago, the same amount of light as would be obtained if it were possible to bring together the third of the whole number of gas-lights which illumine the streets, the shops, and the theatres of Paris; and this wonderful result is obtained from a single lamp. This lamp has four concentric burners, which are defended from the action of the excessive heat produced by their united flames, by means of a superabundant supply of oil, which is thrown up from the cistern below by a clock-work movement, and constantly overflows the wicks. A very tall chimney is necessary in order to supply fresh currents of air to each wick with sufficient rapidity to support the combustion. The carbonization of the wicks is not very rapid; and after they have been burning a long time, the flame is not sensibly diminished, as the great heat evolved from the mass of flame promotes the rising of the oil in the cotton. In the year 1820, in the course of some investigations connected with the Trigonometrical Survey of Great Britain, and conducted by a deputation of scientific persons from London and Paris, M. Fresnel exhibited from the French side of the channel, by means of his lens and a large lamp, a powerful light which was observed by the English across the channel. The brilliancy of this light so struck Lieut.-Col. Colby, of the Royal Engineers, who was engaged in these observations, that he immediately corresponded with Mr. Stevenson as to its probable use upon the Scottish coast. 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. In their report they conclude:-- ‘1. That at a distance of thirteen miles the mean effect of the new light is very much superior to the mean effect of the old light (perhaps in the ratio of two to one). 2. That at _all_ distances the new light has a prodigious superiority to the old, from the equality of its effects in all azimuths. 3. That the new light fulfils rigorously the conditions required for the distribution of light to the greatest advantage. 4. That at distances much exceeding thirteen miles, the new light must still be a very effective one, though to what extent the committee have not observed. The light is understood to be still a good one, when seen from Edinburgh at a distance of about thirty miles.’