There is one great point of advantage in the use of the reflector in physical work,—the absence of secondary spectrum; but it is by no means certain that stellar photography will not be more easy with refractors. ----- Footnote 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 0·957 with any other surface After transmission through one common surface of two glasses 1·000 cemented together After reflection on polished speculum metal at a perpendicular 0·632 incidence After reflection on polished speculum metal at 45° obliquity 0·690 After reflection on pure polished silver at a perpendicular 0·905 incidence After reflection on pure polished silver at 45° obliquity 0·910 After reflection on glass (external) at a perpendicular 0·043 incidence 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 BOOK III. _TIME AND SPACE MEASURERS._ CHAPTER XIII. THE CLOCK AND CHRONOMETER. I. THE RISE AND PROGRESS OF TIME-KEEPING. When we dealt with the astronomical instruments of Hipparchus, we saw that although the astrolabe which that great observer used was the germ of our modern instruments, the time recorded by Hipparchus and those who lived after him down to the later times of the Roman Empire was, as they measured it, a time which would be entirely useless for us. The ancients contented themselves with dividing the interval between sunrise and sunset, regardless whether this was in summer or winter, into twelve equal hours. Now, as in summer the sun is longer above the horizon than in winter in these northern latitudes, we have more time during which the sun is above the horizon in summer than in winter, and if that period of time is to be divided into twelve hours, the hours would be much longer in summer than in other seasons.