Reflectors and Refractors.—Two prin cipal forms of astronomical telescopes are in use today, reflectors and refrac tors. In the first the rays are brought to a focus by falling on a concave mirror; in the second by passing through a sys tem of lenses. In this connection it is worth noting that in French the word telescope generally refers to reflectors, lunette denoting refractors. Though re fractors were first invented, no very important astronomical discoveries were made, as we look at telescopic discov eries today, till the invention and use of reflectors by Herschel. There are several forms of reflectors, the Herschelian, in which the mirror is tipped a little in the tube, so that the rays are brought to a focus at the upper end near one side, the observer looking down the tube; the Newtonian, in which a small plane mir ror is introduced at an angle of 45°, and reflects the beam out at the side of this tube; and the Gregorian, or Casse granian, in which the large mirror is pierced in the center and the rays are reflected back through it from a second flat, concave, or convex mirror part way up the tube. In the first two forms the observer must work in an awkward posi tion at the top of the telescope; in the latter he works at the bottom, as in a refractor, and the telescope is much shorter. Reflectors are cheaper, easier to construct, and can be rendered per fectly achromatic, so that they are equally good for optical, photographic, and spectroscopic work. Here, however, their superiority ends. They are awk ward to handle and use, their reflecting Dower constantly diminishes by tarnish so that they must be frequently repol ished or resilvered, they require more accurate figuring at first, they do not give so much light for the same aperture, even in their best reflecting condition, and, most important of all, any distortion through flexure—they are very heavy— or through local effects of temperature, causes almost infinitely more distortion in the images than the same degree of flexure or distortion of a lens; for when a lens bends, one side becomes more con vex and the other more concave, and these nearly counterbalance each other, while the effect of bending or distorting a mirror is to deflect the rays just twice the whole amount. This is why a re
flector has to be humored and handled in the most careful manner, and special precautions taken in the mounting and support of the mirror; while a refractor, once finished, can be roughly used, and still it will give better definition than the average reflector. A refractor corrected for optical work cannot be used with good results for photography unless a third lens is attached in front of the objective, or one of the lenses of the objective turned over and their distance changed; and, with any construction whatever in a refractor, the spectroscope must be ad justed to a different focal length for dif ferent wave lengths of the spectrum. With respect to the distortions produced by flexure, etc., in reflectors, it should be noted that any form of refractor which uses a mirror as a part of its construc tion is open to the same objection. It is for this reason that telescopes of the form of the equatorial-coude, in which there are two reflections from mirrors, are not likely to be in demand where the best definition is a point to be sought after. This form of telescope makes an excellent form for a rapid and convenient comet-seeker.
With the growth in size of telescopes their use and management becomes more difficult, and the mounting has become as important a party as any in the con struction of large modern telescopes.
In 1918 the second largest telescope (72 inch) was mounted on Little Saanich Mountain, Vancouver Island, B. C. There are three 60-inch telescopes in the Western Hemisphere; one is at the Na tional Observatory, Argentina, another was constructed for Harvard University, at Ealing, England, and another was erected at Mt. Wilson, U. S. A., in 1908.
See LENS: OBSERVATORY: LICK OBSERV ATORY: YERICES OBSERVATORY.