between Folkestone and Calais, Fresnel's compound lenses, then new to science, were used at night, and to Colby's notes thereupon, communicated to his friend Robert Stevenson, the engineer of the Bell Rock lighthouse, we owe the adoption of these lenses in British lighthouses (A. Stevenson, Treatise on Lighthouses, ii. 5, in Weale's series).
In 1824 a survey of Ireland was ordered after a very careful consideration of the subject before a select committee of the House of Commons, which recommended that the work should be entrusted to the ordnance (Parl. Reports, 1824, viii. 77, 79). The Duke of Wellington, as master-general, selected Colby to plan and execute the survey, and left the number and selection of persons to be employed thereon entirely to him (Wellington Supp. Despatches, iv. 219, 333). Into this, the great work of his life, Colby forthwith entered with all his energy and skill. Being intended to facilitate a general valuation of property throughout Ireland, with a view to secure a more equal distribution of local taxation, the survey was required to be so precise that the accuracy of the details should be unquestioned, while yet the cost was to be kept within reasonable limits. Colby determined to make it dependent on chain measurement, controlled by a very complete system of primary, secondary, and minor triangulation, allowing of the fixation of a trigonometrical point for each four hundred statute acres. He also decided to have the work carried on under direct official supervision, instead of by contracts with civil practitioners, a practice then largely followed in the ordnance survey of England. For this reason he adopted a military plan of organisation, and obtained the Duke of Wellington's approval of a plan for raising three companies of sappers and miners to be trained in survey duties. The cost of these three companies of 105 men each, who could at any time be made available for the ordinary service of the country, was defrayed out of the annual parliamentary grants for the survey. Later, as the work progressed, he subdivided the duties into so many different branches, serving as mutual checks, that he was enabled to avail himself of the natural aptitude of the lower orders of Irish, large numbers of whom were employed on the survey. The Irish survey was begun by Colby with a small party of sappers on Divis mountain, near Belfast, in 1825. Not approving of the appliances used or proposed for base-line measurements, Colby instituted a series of experiments on the expansion and contraction of metal bars under variations of temperature, guided by which he eventually devised a dual arrangement of brass and iron, called by him a 'compensation bar,' which, with an ingenious arrangement of connecting microscopes, forms the beautiful apparatus known by his name, and since used in base-measurements in all parts of the world (Mem. of Colby, pp. 268-72). With this apparatus a base-line, eight miles long, was measured under Colby's personal superintendence, on the southern side of Lough Foyle, in 1827-8, an account of which was published long afterwards by order of the board of ordnance (An Account of the Measurement of the Lough Foyle Base, 1847). Colby ordered two ten-foot iron standard bars to be constructed, to serve as a permanent record of the length of the compensation bars and of the base-measurements therewith at a temperature of 62 Fahr.; likewise two three-foot bars, which in March 1834 he caused to be compared by a committee of experts with the parliamentary standard yard, and which bars formed part of the evidence on which the parliamentary committee had to rely for the restoration of the standard yard, after the latter was destroyed by the fire which burned down the houses of parliament in the autumn of the same year. Colby's biographer also claims for him that he was the first to point out the collateral advantages to be derived from combining with the national survey researches and collections illustrative of the geology, natural history, statistics, and antiquities, especially as regards local names, of the country. His ideas on this subject were overruled by financial considerations, but have since borne rich fruit in many quarters.
The great difficulty at the outset was the want of a trained staff, training in such duties being a work of time. Hence the progress made was slow and unsatisfactory, and an idea arose that the methods adopted were too refined for the particular purpose in view (Wellington Supp. Despatches, iv. 331, 333). These representations led to the appointment of an engineer committee, with Sir James Carmichael Smyth at its head, which, after vexatious inquiry, recommended the adoption of more rapid but less accurate methods than those in use.
In 1828 Colby married Elizabeth Hester Boyd, second daughter of Archibald Boyd of Londonderry, sometime treasurer of that county. By this lady, who was a descendant of the Errol and Kilmarnock families, and on her mother's side of the Earls of Angus, he had a family of four sons and three daughters. After his marriage Colby removed from London to Dublin, residing at first in Merrion Square, and afterwards at Knockmaroon Lodge, at the gates of Phoenix Park, within
