To present a full account of the developments in the manufacture of large lenses one needs to address wider issues rather than just provide a catalogue of technological progress. The advances in glass manufacture and improvement in optical techniques have to be considered in relation to the cultural, social and economic factors that have determined where, how and why large lens manufacture developed in specific countries. The challenge facing historians trying to tackle this technological theme, is that it is often poorly documented and little is preserved in the historical records. Until relatively recent times, opticians have concealed their methods, trade secrecy being an important economic strategy. To provide an example, it should be noted that although William Herschel produced the best optics and telescopes of the day, he published practically nothing about his methods. What has been gleaned of his techniques has only been uncovered by careful study of surviving manuscript sources and measurement of his surviving optics. Such was William's personal knowledge, that his son John had to take instruction from his father to refurbish William Herschel's 20-foot telescope. This training gave John tacit knowledge of William's methods and allowed him to successfully undertake his cape observations in the Southern Hemisphere. In spite of the shortcomings of the historical record, historians can give a measured account of the developments of lens optics by studying surviving telescopes and their optics.