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was first practically obviated by Mr. Dollond , who by a propercombination of glass, having a different dispersive power, for hislenses constructed achromatic telescopes, which in a great measureremoved the indistinctness arising from the different refrangibilftyof light. And for the construction of such telescopes several tedi-ous calculations have been made and theorems invest igated, and theseagain much simplified for the accommodation of the artist. Butthough it has often been attempted to construct telescopes accord-ing to the results of these calculations, yet the diversity of glasses,and the uncertainty of the workman’s producing the very curva-tures which he intends, is so great, that the object-glass turns outdifferent from bis expectation. The artist who makes great num-bers acquires a pretty certain guess at the remaining error; andhaving many lenses, intended to be of one form, but unavoidablydiffering a little from it, be tries several of them with the other two,and finding one better than the rest, he makes use of it to com-plete the set. The great difficulty in the construction is to find theexact proportion of the dispersive powers of the crown and flint-glass. The crown is pretty constant; but there is hardly two potsof flint-glass which have the same dispersive power. Even if con-stant, it is difficult to measure it accurately ; and an error in thisgreatly affects the instrument, because the focal distances of thelenses must be nearly as their dispersive powers. A telescope con-structed so as to remedy the delects arising from the different re-frangibilitv of light is called the Achromatic Telescope, which is aname given to the refracting telescope, invented by Mr. JohnDollond , in which were two object-glasses of crown-glass, and oneof flint, the eye-glasses concave. The improvement was completedby his son Peter Dollond ; who, conceiving that the same methodmight be practised with success with convex eye-glasses, found,after a few trials, that it might be done. Accordingly he finishedan object-glass of five feet focal length, with an aperture of 3|inches, composed of two convex lenses of crown-glass, and one con-cave of white flint-glass. But apprehending afterwards that theapertures might be admitted still larger, he completed one of 3-£feet focal length, with the same aperture of 3$ inches. In the 17-inch improved achromatic telescope, the object-glass is composedof three glasses, viz. two convex of crown-glass, and one concaveof while flint-glass ; tire focal distance of this combined object-glassis about seventeen inches, and the diameter of the aperture two in-ches. There are four eye-glasses contained in the tube, to beused for land-objects; the magnifying power with these is nearfifty times; and they are adjusted to different sights, and to differentdistances of the object, by turning a finger-screw at the end of theouter tube. There is another tube, containing two eye-glassesthat magnify about seventy times, for astronomical purposes. Thetelescope may be directed to any object by turning two screws inthe stand on which it is fixed, the one giving a vertical motion,and the other a horizontal one. To describe all the varieties ofshape and accommodation which may be given to a telescope,would be a task as trifling as prolix. The artists of London andof Paris have racked their inventions to please every fancy, and tosuit every purpose. The chief consideration is to have a steadyview of the distant object. This is unattainable, unless the axis ofthe instrument be kept constantly directed to the same point of it;for when the telescope is gently shifted from its position, the objectseems to move in the same or in the opposite direction, accordingaff the telescope inverts the object or shews it erect. This is owingto the magnifying power, because the apparent angular motion isgreater than what we naturally connect with the motion of the tele-scope. This does not happen when we look through a tube with-out glasses. All shaking of the instrument therefore makes theobject dance before the eye ; which prevents us from seeing it dis-tinctly. But a tremulous motion, however small, is infinitelymore prejudicial to the performance of a telescope, by making theobject quiver before us. A person walking in the room preventsus from seeing distinctly ; nay, the very pulsation in the body ofthe observer sufficiently agitates the floor to produce this effect,when the telescope has a great magnifying power, and every partof the object is diffused over the whole space of the angular tremor.The more rapid this motion is, the indistinctness is the morecomplete. Therefore the more firm, elastic, and well bound to-gether, are the frame-work and apertures of the telescope, the morehurtful will this consequence be. A mounting of lead, were itpracticable, would be preferable to wood, iron, or brass. This is one

jeat cause of the indistinctness of the very finest reflecting tele-scopes of the usual constructions, and can never be totally removed.In the Gregorian form, it is hardly possible to damp the elastictremor of the small speculum, carried by an arm supported at oneend only, even though the tube were motionless. The tube of a veryfine telescope, made by James Short, was laid on a great lump ofsoft clay, pressing it firmly down into it. Several persons lookedthrough it, and read a table of logarithms at the distance of 310yards. The telescope was then put on its stand, and pointed tothe same object; none of the company could read a greater dis-tance than 23 j yards, although they could perceive no tremour.'I'hey thought the vision as sharp as before; but the incontroverti-ble proof of the contrary was, that they could not read at such a dis-tance. The high magnifying powers of Dr. Herschel’s telescopesmade all the usual apparatus for their support extremely imper-fect. But his judgment, ingenuity, and fertility in resource, areas eminent as his philosophical ardour. He has contrived for hisreflecting telescopes stands, which have every property that can bedesired. The tubes are all supported at both ends. The motions,both vertical and horizontal, are contrived with the utmost sim-plicity and firmness. We cannot more properly conclude thisarticle than with a description of his 40 feet telescope, the noblestmonument of philosophical zeal and of princely munificence thatthe world can boast of. Fig. 1, Plate CLXXII L represents aview of this instrument in a meridional situation, as it appearswhen seen from a convenient distance by a person placed to the’south west of it. The foundation in the ground consists of twoconcentric circular brick walls, the outermost of which is 42 feetin diameter, and the inside one 21 feet. They are two feet sixinches deep under ground ; two feet three inches broad at the bot-tom, and one foot two inches at the top, and are capped withpaving stones about three inches thick, and 12J broad. The bot-tom-frame of the whole apparatus rests upon these two walls by 20concentric rollers III. and is moveable upon a pivot, which gives ahorizontal motion to the whole apparatus, as well as to the tele-scope. The tube of the telescope A, though very simple in itsform, which is cylindrical, was attended with great difficulties inthe construction. This is not to be wondered at, when its size,and the materials of which it is made, are considered. Its lengthis 39 feet four inches; it measures four feet ten inches in diame-ter ; and every part of it is of iron. Upon a moderate computation,the weight of a wooden tube must have exceeded an iron one atleast 3000 pounds; and its durability would have been far inferior.It is made of rolled or sheet iron, which has been joined togetherwithout rivets, by a kind of seaming well known to those who makeiron-funnels for stoves. Very great mechanical skill is used in thecontrivance of the apparatus by which the telescope is supported,and directed. To command every altitude, the point of supportis moveable ; and its motion is effected by mechanism, so that thetelescope may be moved from its most backward point of supportto the most forward, and, by means of the pulleys GG, suspendedfrom the great beam H, beset to any altitude, up to the very zenith.The tube is also made to rest with the point of support in a pivot,which permits it to be turned sidewise. The concave face of thegreat mirror is 48 inches of polished surface in diameter. Thethickness, which is equal in every part of it, remains now about 31-inches; and its weight, when it came from the cast, was 2118pounds, of which it must have lost a small quantity in polishing.To put this speculum into the tube, it is suspended vertically by acrane in the laboratory, and placed on a small narrow carriage,which is drawn out, rolling upon planks, till it comes near thehack of the lube ; here it is again suspended and placed in the tubeby a peculiar apparatus. The method of observing by this tele-scope is by what Dr. Ilerschel calls the front-view ; the observerbeing placed in a seat C, suspended at the end of it, with his backtowards the object he views. There is no small speculum, but themagnifiers are applied immediately to the first focal image. Fromthe opening of the telescope, near the place of the eye-glass, aspeaking pipe runs down to the bottom of the tube, where itgoes into a turning joint ; and after several other inflections, it atlength divides into two branches, one going into the observatoryD, and the other into the work room E. By means of the speak-ing-pipe the communications of the observer are conveyed to theassistant in the observatory, and the workman is directed to per-form the required motions. In the observatory is placed a vatu-