From the experiments I have made in reference to these instruments, it is demonstrable, that _a tube is not necessary_ in the construction of a reflecting telescope--at least on the principle now stated--whether it be used by day or by night for terrestrial or celestial objects; for I have frequently used these telescopes in the open air in the day time, without any inconvenience from extraneous light. Therefore, were a reflecting telescope of 50 or 60 feet in length to be constructed, it might be fitted up at a comparatively small expence, after the expense of the metallic substances, and of casting, grinding, and polishing the speculum is defrayed. The largest instrument of this description which has hitherto been constructed is the 40 feet reflector of Sir W. Herschel. This complicated and most unwieldy instrument had a tube of rolled or sheet iron 39 feet 4 inches in length, about 15 feet in circumference, and weighed about 8000 pounds. Now, I conceive that such enormous tubes, in instruments of such dimensions, are altogether unnecessary. Nothing more is requisite than a short tube for holding the speculum. Connected with one side of this tube (or with both sides were it found necessary), two strong bars of wood, projecting a few feet beyond the speculum end, and extending in front as far as the focal length of the mirror, and connected by cross bars of wood, iron or brass--would be quite sufficient for a support to the eye-piece, and for directing the motion of the instrument. A telescope of 40 or 50 feet in length, constructed on this plan, would not require one fifth of the expense, nor one fourth of the apparatus and mechanical power for moving it to any required position, which were found necessary in the construction of Sir W. Herschel’s large reflecting telescope. The idea here suggested will perhaps be more readily appreciated by an inspection of fig. 71, where A is the short tube, BC and DE the two large bars or arms, connected with cross bars, for the purpose of securing strength and steadiness. At I and K, behind the speculum, weights might be applied, if necessary, for counterbalancing the lever power of the long arm. F represents the position of the eye-piece, and GH the joint and part of the pedestal on which the instrument is placed. With regard to telescopes of smaller dimensions, as from 5 to 15 feet in focal length--with the exception of the expense of the specula and eye-pieces--they might be fitted up for a sum not greater than from 3 to 10 or 15 guineas. [Illustration: _figure 71._]