When I resided at Bath I had long been acquainted with the theory of optics and mechanics, and wanted only that experience so necessary in the practical part of these sciences. This I acquired by degrees at that place where in my leisure hours, by way of amusement, I made several two-foot, five-foot, seven-foot, ten-foot and twenty-foot Newtonian telescopes, beside others, of the Gregorian form, of eight, twelve, and eighteen inches, and two, three, five, and ten feet focal length. In this way I made not less than two hundred seven-foot, one hundred and fifty ten-foot, and about eighty twenty-foot mirrors, not to mention the Gregorian telescopes.

Ch.4 "Life and Works" Footnote: At least one of these telescopes had the principal mirror made of glass instead of metal. Philosophical Transactions of the Royal Society of London (1803). - Sir William Herschel: His Life and Works (1880)