A great improvement was introduced by Browning more than a half century ago, in the support shown in Fig. 34. Here the ring _A_, (_B_, Fig. 31) carries three narrow strips of thin spring steel, _B_, extending radially inward to a central hub which carries the mirror _D_, on adjusting screws _E_. Outside the ring the tension screws _C_ enable the mirror to be accurately centered and held in place. Rarely, the mirror is replaced by a totally reflecting right angled prism which saves some light, but unless for small instruments is rather heavy and hard to obtain of the requisite quality and precision of figure. A typical modern reflector by Brashear, of 6 inches aperture, is shown in Fig. 35, complete with circles and driving clock, the latter contained in the hollow iron pier, an arrangement usual in American-made instruments. [Illustration: FIG. 34.—Support of Diagonal Mirror (Browning.)] Recent reflectors, particularly in this country, have four supporting strips instead of three, which gives a little added stiffness, and produces in star images but four diffraction rays instead of the six produced by the three strip arrangement, each strip giving a diametral ray. In some constructions the ring _A_ is arranged to carry the eyepiece fittings, placed at the very end of the tube and arranged for rotating about the optical axis of the telescope. This allows the ocular to be brought to any position without turning the whole tube. In small instruments a fixed eyepiece can be used without much inconvenience if located on the north side of the tube (in moderate north latitudes). Reflectors are easily given a much greater relative aperture than is practicable in a single achromatic objective. In fact they are usually given apertures of _F_/5 to _F_/8 and now and then are pushed to or even below _F_/3. Such mirrors have been successfully used for photography;[9] and less frequently for visual observation, mounted in the Cassegranian form, which commonly increases the virtual focal length at least three or four times. A telescope so arranged, with an aperture of a foot or more as in some recent examples, makes a very powerful and compact instrument. [9] An _F_/3 mirror of 1_m_ aperture by Zeiss was installed in the observatory at Bergedorf in 1911, and a similar one by Schaer is mounted at Carre, near Geneva. [Illustration: FIG. 35.—Small Equatorially Mounted Reflector.] This is the form commonly adopted for the large reflectors of recent construction, a type being the 60-inch telescope of the Mount Wilson Observatory of which the primary focus is 25¼ feet and the ordinary equivalent focus as a Cassegranian 80 feet.