Again, the refractor has the advantage over the other in not being of so large aperture when of equal power, so that the disturbing effects of air currents is considerably less, but the method of making the tubes of open lattice-work materially reduces this objection.
We have mentioned the difficulty of mounting mirrors, especially of large size, but this has now been got over very perfectly. This difficulty does not occur in the mounting of object-glasses of sizes at present in use, but when we come to deal with lenses of some 30 inches diameter, the present simple method will in all probability be found insufficient.
On the other hand the cost of mirrors is of course much less than that of object-glasses, a matter of considerable importance. The late M. Merz, on being asked as to price of a 30-inch object-glass, estimated that, if it were possible to make it, its cost would be between £8,500 and £9,000.
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.
[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 |