A corrected reprint of the ?rst edition appeared in 2000. It was a requirement that the pagination remain unaltered, but nevertheless, apart from minor and format corrections on 17 pages, a number of corrections or additions of substance could be incorporated. These included minor corrections to Figs. 1. 3 b) and 2. 8 and to the text of Fig. 5. 18. The most important change of all was probably the complete revision of the historical treatment of Cassegrain in the Portrait Gallery, due to the superb research of Baranne and Launay on his identity, published in 1997. Additions of substance were text on pages 21, 323 and 487 (Portrait Gallery Mersenne) and corrections on pages 117 2 4 (y to y ), 174 (concerning the scale of Fig. 3. 37), 263 (Fig. 3. 96 instead of 3. 97 in the text), 341 (sign in the text equation below Eq. (4. 36)), Table 5. 2 (concerning UKIRT), Table A. 15 (?rst symbol), pages 505 (Ref. 3. 71) and 531 (Brown and Cassegrain). Several of these errors were pointed out by readers, to whom I express my gratitude. The present 2nd edition contains all the material of the ?rst edition, - changed apart from some further corrections, but with 25 pages more of - ditional explanations or new material, including 5 new ?gures (2 in Chap. 4, 2 in Chap. 5 and 1 following the Portrait Gallery).