"We simply note that if, in fact, the Greeks were able to start fires at long distances, they could not have done it with a single mirror. This is the relevant point to the subject of adaptive optics. Archimedes would necessarily have had to devise a method of accurately pointing a number of independent reflectors to multiply the effect. Because it was not until the latter part of the twentieth century A.D. that analogous techniques were realized in practice, such as the array of 63 flat mirrors comprising the French Pyrenees solar concentrator system, Archimedes' achievement is truly fantastic. In any case, the method, if used, did not prevent the Romans from ultimately sacking the city and killing Archimedes."

-- Robert K. Tyson and Peter B. Ulrich, "Adaptive Optics," The Infrared and Electro-Optical Systems Handbook, Volume 8.