Optician more important and dignified. Far-sighted or aging colonial Americans imported spectacles from Europe. Spectacles were primarily for the affluent and literate colonists, who required a valuable and precious appliance. Benjamin Franklin in the 1780s developed the bifocal. Bifocal lenses advanced little in the first half of the 19th century. The terms bifocal and trifocal were introduced in London by John Isaac Hawkins, whose trifocals were patented in 1827. In 1884 B. M. Hanna was granted patents on two forms of bifocals which become commercially standardized as the "cemented" and "perfection" bifocals. Both had the serious faults of ugly appearance, fragility,
Anglo-French Survey (1784–1790) the time of his death he was correcting the final proofs of his report and the work was brought to a conclusion by Isaac Dalby, a senior civilian employee of the Board of Ordnance who had organised the calculations of the triangles. Anglo-French Survey (1784–1790) The Anglo-French Survey (1784–1790) was the survey to measure the relative situation of Greenwich Observatory and the Paris Observatory. The English operations, executed by William Roy, consisted of the measurements of bases at Hounslow Heath (1784) and Romney Marsh (1787), the measurements of the angles of the triangles (1787–1788) and finally the calculation of all