Brackets shown in dotted lines at _O_, _O_, carry the usual finder, and a hinged door _P_ near the lower end of the tube enables one to remove or replace the close fitting metal cover that protects the main mirror when not in use. Similarly a cover is fitted to the small mirror, easily reached from the upper end of the tube. The proportions here shown are approximately those commonly found in medium sized instruments, say 7 to 10 inches aperture. The focal ratio is somewhere about _F_/6, the diagonal mirror is inside of focus by about the diameter of the main mirror, and its minor axis is from ⅕ to ¼ that diameter. [Illustration: FIG. 32.—Reflector with Skeleton Tube (Brashear).] Note that the tube is not provided with diaphragms. It is merely blackened as thoroughly as possible, although stray light is quite as serious here as in a refractor. One could fit diaphragms effectively only in a tube of much larger diameter than the mirror, which would be inconvenient in many ways. A much better way of dealing with the difficulty is shown in Fig. 32 in which the tube is reduced to a skeleton, a construction common in large instruments. Nothing is blacker than a clear opening into the darkness of night, and in addition there can be no localized air currents, which often injure definition in an ordinary tube. [Illustration: FIG. 33.] Instruments by different makers vary somewhat in detail. A good type of mirror mounting is that shown in Fig. 33, and used for many years past by Browning, one of the famous English makers. Here the mirror _A_, the back of which is made accurately plane, is seated in its counter-cell _B_, of which a wide annulus _F_, _F_, is also a good plane, and is lightly held in place by a retaining ring. This counter cell rests in the outer cell _C_ on three equidistant studs regulated by the concentric push-and-pull adjusting screws _D_, _D_, _E_, _E_. The outer cell may be solid, or a skeleton for lightness and better equalization of temperature. Small specula may be well supported on any flat surface substantial enough to be thoroughly rigid, with one or more thicknesses of soft, thick, smooth cloth between, best of all Brussels carpet. Such was the common method of support in instruments of moderate dimensions prior to the day of glass specula. Sir John Herschel speaks of thus carrying specula of more than a hundred-weight, but something akin to Browning’s plan is generally preferable. There is also considerable variety in the means used for supporting the small mirror centrally in the tube. In the early telescopes it was borne by a single stiff arm which was none too stiff and produced by diffraction a long diametral flaring ray in the images of bright stars.