8. Sir John Herschel, in his work on the telescope, gives the following table of reflective powers:—
| After transmission through one surface of glass not in contact with any other surface | 0·957 |
| After transmission through one common surface of two glasses cemented together | 1·000 |
| After reflection on polished speculum metal at a perpendicular incidence | 0·632 |
| After reflection on polished speculum metal at 45° obliquity | 0·690 |
| After reflection on pure polished silver at a perpendicular incidence | 0·905 |
| After reflection on pure polished silver at 45° obliquity | 0·910 |
| After reflection on glass (external) at a perpendicular incidence | 0·043 |
The effective light in reflectors (irrespective of the eyepieces) is as follows:—
| Herschelian (Lord Rosse’s speculum metal) | A. | 0·632 | |
| Newtonian (both mirrors ditto) | B. | 0·436 | |
| Newtonian (small mirror or glass prism) | C. | 0·632 | |
| Gregorian or Cassegrainian | D. | 0·399 | |
| { | A. | 0·905 | |
| The same telescopes, all the metallic | { | B. | 0·824 |
| reflections being from pure silver | { | C. | 0·905 |
| { | D. | 0·819 |