While engaged in the survey of Ireland, Thomas Drummond in 1826 devised apparatus for signaling many miles, thus facilitating triangulation. Distances as great as eighty miles were encountered and it appeared desirable to have some method for seeing a point at these great distances. Gauss in 1822 used the reflection of the sun's image from a plane mirror and Drummond also tried this means. The latter was successful in signaling 45 miles to a station which because of haze could not be seen, or even the hill upon which it rested. Having demonstrated the feasibility of the plan, he set about making a device which would include a powerful artificial light in order to be independent of the sun. In earlier geodetic surveys Argand lamps had been employed with parabolic reflectors and with convex lenses, but apparently these did not have a sufficient range. Fresnel and Arago constructed a lens consisting of a series of concentric rings which were cemented together, and on placing this before an Argand lamp possessing four concentric wicks, they obtained a light which was observed at forty-eight miles. Despite these successes, Drummond believed the parabolic mirror and a more powerful light-source afforded the best combination for a signal-light. In searching for a brilliant light-source he experimented with phosphorus burning in oxygen and with various brilliant pyrotechnical preparations. However, flames were unsteady and generally unsuitable. He then turned in the direction which led to his development of the lime-light. In his first apparatus he used a small sphere of lime in an alcohol flame and directed a jet of oxygen through the flame upon the lime. He thereby obtained, according to his own description in 1826, a light so intense that when placed in the focus of a reflector the eye could with difficulty support its splendor, even at a distance of forty feet, the contour being lost in the brilliancy of the radiation. He then continued to experiment with various oxides, including zirconia, magnesia, and lime from chalk and marble. This was the advent of the lime-light, which should bear Drummond's name because it was one of the greatest steps in the evolution of artificial light. By means of this apparatus in the survey, signals were rendered visible at distances as great as one hundred miles. Drummond proposed the use of this light-source in the important lighthouses at that time and foresaw many other applications. The lime-light eventually was extensively used as a light-signaling device. The heliograph, which utilizes the sun as a light-source, has been widely used as a light-signaling apparatus and Drummond perhaps was the first to utilize artificial light with it. The disadvantage of the heliograph is the undependability of the sun. With the adoption of artificial light, various optical devices have come into use.