From the above table it will be seen, that to construct, for example, a 30 inch compound object-glass, the radius of the anterior side of the crown must be 7-1/2 inches, and that of the posterior side 11.63 inches; the radius of the anterior surface of the concave 10.428, and that of the posterior 20.856 inches. It may be proper to observe, that in these computations, the radius of the anterior surface of the concave is less than the posterior side of the convex, and consequently admits of its approach, without touching in the centre--a circumstance which always requires to be guarded against in the combination of achromatic glasses. The following table shows the radii of curvature of the lenses of a _triple_ object-glass, calculated from formula deduced by Dr. Robison of Edinburgh. -------+--------------------+---------------------+------------------- Focal | Convex lens of | Concave lens of | Convex lens of length.| crown glass. | flint glass. | crown glass. -------+--------------------+---------------------+------------------- Inches |Inc. Dec. Inc. Dec. |Inc. Dec. Inc. Dec. |Inc. Dec Inc. Dec. 6 | 4. 54 3. 03 | 3. 03 6. 36 | 6. 36 0. 64 9 | 6. 83 4. 56 | 4. 56 9. 54 | 9. 54 0. 92 12 | 9. 25 6. 17 | 6. 17 12. 75 | 12. 75 1. 28 18 | 13. 67 9. 12 | 9. 12 19. 08 | 19. 08 1. 92 24 | 18. 33 12. 25 | 12. 25 25. 50 | 25. 50 2. 56 30 | 22. 71 15. 16 | 15. 16 31. 79 | 31. 79 3. 20 36 | 27. 33 18. 25 | 18. 25 38. 17 | 38. 17 3. 84 42 | 31. 87 21. 28 | 21. 28 44. 53 | 44. 53 4. 48 48 | 36. 42 24. 33 | 24. 33 50. 92 | 50. 92 5. 12 54 | 40. 96 27. 36 | 27. 36 57. 28 | 57. 28 5. 76 60 | 45. 42 30. 33 | 30. 33 63. 58 | 63. 58 6. 40 The following table contains the proportions of curvature, said to be employed by the London opticians.