Note 8 — Chapter XII · THE MODERN TELESCOPE.
Reading edition
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