are subject to what is called spherical ab- that a one-horse power would be fully suffierration, that is, the edge both of specula cient for executing at the same time three or and lenses has a shorter focus than the four specula six inches in diameter. For centre. In lenses this may be diminished such sizes Lord Rosse conceived that a or even removed by the opposite aberration day would suffice for completing the proof a concave lens; but this remedy cannot cess, and that a machine on the scale be applied to specula. It therefore occur- shown in his drawing, "would be suffired to Lord Rosse, that the first step to- ciently large to grind and polish a specuwards the improvement of the reflecting lum of three feet diameter, or perhaps larg telescope, was to diminish the spherical ab- er." In this interesting communication erration. With this view he formed the Lord Rosse suggests what he afterwards speculum of three parts, a central speculum, accomplished, that the motion for produca ring, inclosing the central speculum, and ing a parabolic curve, "might be imitated an outer ring. These three portions were by means of the eccentric guides, and the cemented together, and ground and polish- slow circular motion of the speculum, and ed as one speculum. They were then com- with this advantage, that, were it found bined by an ingenious piece of mechanism, really successful, the same result would so that the first and second rings could be probably be always afterwards obtained." advanced each a small fraction of an inch, Before the year 1830, Lord Rosse had in order that their foci should accurately made still further advances towards the coincide with the focus of the central spec- great object he had in view. He found ulum. Lord Rosse's first attempt did not from many experiments that he could not succeed to his wishes, owing to a defect in cast a speculum of the moderate dimensions the mechanism, which required frequent of 15 inches, without reducing the composiadjustments, as the smallest shock displaced tion considerably below the highest standthe images. He then tried to combine one ard, that is without using so much copper ring only, 1 inch thick, with a central as to produce a soft and yellowish metal. metal 1 inches thick, the two together All the specula cracked in annealing when forming a speculum of six inches aperture, the proper composition was employed. In and two feet focal length. This combination was more successful, as it "remained in perfect adjustment even after very violent shocks." In these combinations Lord Rosse did not perceive the ill effects which he had apprehended from contraction and expansion; and it remained to be seen, from future trials, if they did appear, whether or not they could be removed. "On my return from Parliament, (June 1828,) says Lord Rosse, "if other avocations do not interfere, I propose to construct a speculum in three parts, of 18 inches aperture, and twelve feet focal length;-this will be giving the experiment a fair trial on a large scale." This proposal was accordingly executed, and he found the speculum superior to a solid one of the same dimensions.

order to get over this difficulty, he tried to cast the specula in different pieces, and to unite them by tinning their surfaces; but though this was practicable, he abandoned it for the following plan. He found that an alloy of copper 2.75 parts, with 1 of zinc, expanded and contracted with a change of temperature in the same degree as speculum metal, and was an alloy malleable, ductile, and easily worked. With this alloy he cast a speculum 15 inches in diameter, with a rim and ribs behind. It was turned smooth and flat on one side, and tinned. Six pieces of the highest speculum metal, 1th of an inch thick, were then placed on the flat tinned surface, so as to complete a circular disc 15 inches in diameter, and when soldered to it, composed a plated speculum. When ground and polished, it formed an excellent teles

In order to grind and polish large specula, Lord Rosse soon perceived that a steam-cope of twelve feet focal length. Upon the engine and appropriate machinery were necessary. He accordingly invented a machine of this kind, and transmitted an account of it to the writer of this article, who published it in the Edinburgh Journal of Science for October, 1828. The engine which his Lordship actually constructed and used was one of two horse power, though from some rude trials with it he inferred

same plan Lord Rosse constructed a speculum two feet in diameter, for a telescope twenty-six feet long. Hitherto it had been believed by opticians, that a fine polish could not be given to specula, unless when the polisher became dry and hot; but Lord Rosse at this stage of his researches found out a method of polishing a cold metal upon a moist polisher, an object of very

great importance, as a speculum should be polished at the same temperature at which it is to be used.

cool it with sufficient rapidity; it requires to be warm, so that there may be no moisture deposited upon it from the sand; it may be cooling power. The metal should enter the heated to 2120, without materially lessening its mould by the side, as is usual in iron founding, but much quicker, almost instanteously; one second is sufficient for filling the mould of a

perature of the metal, this can best be ascertained by stirring it with a wooden pole occawhen the carbon of the pole reduces the oxide sionally, after it has become perfectly fluid: on the surface of the metal, rendering it bril

When the metal has become solid in the ingate or hole through which it enters the mould, the plate is to be removed quickly to an oven heated a little below redness, to remain till in diameter, should be three or four days at cold, which, where the plates are nine inches least."-Phil. Trans., 1840, p. 511.

When the nine inch plates are properly scraped and cleaned, much attention is necessary in soldering them upon the tinned surface of the alloy speculum. Care must be taken that until the tin on the speculum is fused, the melted resin must not be poured in between the plates.

The next step in Lord Rosse's progress was to make a plated speculum, three feet in diameter. The proportions of copper and tin, which he found to be best, were the definite ones of four atoms of cop-nine inch plate or speculum. As to the temper to one of tin, or 1264 parts of copper to 58.9 of tin, or 32 of the one to 1491 of the other. After preparing the alloy speculum, which was to be plated, and turning it to a radius of 54 feet, Lord Rosse pro-liant like quicksilver, the heat is sufficient. ceeded to cast the small plates of speculum metal, about 9 inches square. In doing this he encountered great difficulties, owing to their extreme brittleness, arising, no doubt, from the too rapid cooling of their edges, and the consequent state of tension. In order to remove this evil, he sawed the plates with a circular sawing disc of iron, immersed in emery and water, and he so far succeeded that he obtained plates with which he composed a two feet speculum. He also used the same plates originally for the three feet one, but before the combination was completed, he discovered the true process of casting specula of all sizes. In The great success which attended this order to produce uniformity of cooling, new method of casting these nine inch he tried two ways of constructing the specula, induced Lord Rosse to try it on a mould. The first was to make the lower surface of the mould, containing the liquid speculum, absorb the heat rapidly, and the upper retain it; and the second was to cool the lower surface, while the heat of the upper surface was undiminished. The first plan did not succeed; but the second did, by making the lower surface of the mould of iron, and the upper of sand; but though the castings were sound, there was this defect, that bubbles of air were entangled between the iron disc and the speculum metal, producing cavities which it was troublesome to grind out. Hence he was led to replace the iron disc, by one made of pieces of hoop iron, placed side by side with their edges up, tightly packed in an iron frame, the surface, thus composed of edges, being smoothed to the proper curvature, by filing or turning By this most ingenious process he constructed a metallic surface every where open, as the closest plates allowed the air to pass freely between them.

large scale, and he accordingly proceeded with one twenty inches, and another three feet, which on the first trial were cast perfect. The crucibles which he employed were made of cast iron, and cast with their mouth upwards; and the fuel used was peat or wood, which are both preferable to coke.

A perfect speculum being thus obtained, the next object to be accomplished is to work it, by grinding and polishing, to a perfect spherical figure. The machine for this purpose, which we have already described, was improved and enlarged so as to work a speculum three feet in diameter, and after several years' experience, during which specula have been ground and polished with it many hundred times, it has been found to work large surfaces with a degree of precision unattainable by the hand. The peculiarity in this process, introduced by Lord Rosse, and as we conceive essential to success, is, that the polisher works above and upon the face of the "So successful was this expedient," says speculum to be polished, and one singular Lord Rosse, "that of sixteen plates cast for the three feet speculum, not one was defective. advantage of this arrangement is, that the The following particulars require to be attend-figure of the speculum can be examined as ed to. The disc of hoop iron should be as thick the operation proceeds, without removing as the speculum to be cast upon it, so as to the speculum, which, when a ton weight, is VOL. V.-No. I. 5

no easy matter. The contrivance for do-stroke 21 times slower, and equal to 0.27 ing this is so beautiful, and has proved so of the same diameter, measured on the edge useful, that we must briefly explain it. The of the tank, or 1.7 beyond the centre of the machine is placed in a room at the bottom polisher. 3d. A rotation of the speculum of a high tower, in the successive floors of which trap-doors can be opened. A mast is elevated on the top of the tower, so that its summit is about 90 feet above the speculum. A dial-plate is attached to the top of the mast, and a small plane speculum and eye-piece, with proper adjustments, are so placed that the combination becomes a Newtonian telescope, and the dial-plate the object.

performed in the same time as 37 of the first strokes; and 4th. A rotation of the polisher in the same direction about sixteen times slower. If these rules are attended to, the machine will give the true parabolic figure to the speculum, whether it be six inches or three feet in diameter. In the three-feet speculum, the figure is so true, with the whole aperture, that it is thrown out of focus by a motion of less than the thirtieth of an inch," and even with a single lens of one-eighth of an inch focus, giving a power of 2592, the dots on a watch dial are still in some degree defined.

specula are not used they are preserved from moisture and acid vapors by connecting their boxes with chambers containing quick lime, an arrangement which Dr. Robinson had applied for several years to the Armagh reflector.

During the operation of polishing the larger specula, a variety of difficulties occurred, but they were all surmounted by the ingenuity and patience of Lord Rosse. At first, in order to allow a lateral expan- The twenty-six feet telescope thus exesion of the pitch, it appeared necessary to cuted, has a general resemblance to that of increase the thickness of the bed of pitch Ramage, but the tube, gallery, and vertical as the diameter of the speculum was in- axis of the stand are counterpoised. It is creased. This proved a failure, and the used as a Newtonian telescope, with a small lateral expansion was provided for by mak-plane speculum, to prevent the image being ing grooves in the pitch; but these grooves, deformed by oblique reflection, which is though there were two sets at right angles the effect of the front view. When the to each other, and only two inches distant, were with difficulty kept open, and the polisher lost its figure. All these evils, however, were removed by furrowing the polisher itself, so as to divide it into definite and insulated portions. The effect of this improvement was so great, that the When this telescope was completed, it plated or divided three feet speculum de- became an object of high interest to ascerfined better with a power of 1200 than it tain its performance. In doing this, Dr. had previously done with a power of 300. Robinson had, as he remarks, "the advanIn place of pitch, Lord Rosse used, as his tage of the assistance of one of the most celpolishing surface, a mixture of common ebrated of British astronomers, Sir James resin and turpentine, and this composition South;" but the weather, the state of the air, was laid on in two strata of different degrees and the light of the moon, between the 29th of hardness, the outer one being the harder, October and Sth November 1840, were unthe subjacent softer layer expanding later-favorable. The following is the substance ally, so as to preserve the figure of the of Dr. Robinson's report :polisher. The speculum being placed in a cistern of water, the polishing process is then effected by using peroxide of iron and water, of about the consistence of thin

"Both specula, the divided and the solid, seem exactly parabolic, there being no sensible difference in the focal adjustment of the eye-piece with the whole aperture of 36 inches, or one of twelve; in the former case there is more flutter, but apparently no difference in definition, and the eye-piece comes to its place of adjustment very sharply.

"The solid speculum showed a Lyræ round and well-defined, with powers up to 1000 inclusive, and at moments even with 1600; but the air was not fit for so high a power on any telescope. Rigel, two hours from the meridian, companion separated by more than a diameter with 600, was round, the field quite dark, the of the star from its light, and so brilliant that it would certainly be visible long before sunset.

"Orionis, well defined, with all the powers | J. H.'s, 86,) and in particular, having prolongafrom 200 to 1000, with the latter a wide black tions brighter than the others in the direction separation between the stars; 32 Orionis and of the major axis, longer than the ring's 31 Canis minoris were also well separated. "It is scarcely possible to preserve the necessary sobriety of language, in speaking of the moon's appearance with this instrument, which discovers a multitude of new objects at every point of its surface. Among these may be named a mountainous tract near Ptolemy, every ridge of which is dotted with extremely minute craters, and two black parallel stripes in the bottom of Aristarchus.*

"There could be little doubt of the high illuminating power of such a telescope, yet an example or two may be desirable. Between ¿1 and 2 Lyræ, there are two faint stars, which Sir J. Herschel (Phil. Trans. 1824) calls debilissima,' and which seem to have been, at that time, the only set visible in the 20 feet reflector. These, at the altitude of 189 were visible without an eye-glass, and also when the aperture was contracted to 12 inches. With an aperture of 18 inches, power 600, they and two other stars (seen in Mr. Cooper's achromatic of 13.2 inches aperture, and the Armagh reflector of 15 inches) are easily seen. the whole aperture, a fifth is visible, which Dr. R. had not before noticed. November 5th, strong moonlight.

With

"In the nebula of Orion, the fifth star of the trapezium is easily seen with either speculum, even when the aperture is contracted to 18 inches. The divided speculum will not show the sixth with the whole aperture, on account of that sort of disintegration of large stars already noticed, but does, in favorable moments, when contracted to 18 inches. With the solid mirror and whole aperture, it stands out conspicuously under all the powers up to 1000, and even with 18 inches it is not likely to be overlooked.

Among the few nebulæ examined were 13 Messier, in which the central mass of stars was more distinctly separated, and the stars themselves larger than had been anticipated; the great nebula of Orion and that of Andromeda showed no appearance of resolution, but the small nebula near the latter is clearly resolvable. This is also the case with the ring nebula of Lyra; indeed, Dr. R. thought it was resolved at its minor axis; the fainter nebulous matter which fills it is irregularly distributed, having several stripes or wisps in it, and there are four stars near it, besides the one figured by Sir John Herschel, in his catalogue of nebulæ. It is also worthy of notice, that this nebula, instead of that regular outline which he has there given it, is fringed with appendages, branching out into the surrounding space, like those of 13 Messier, (Sir

* Dr. Robinson, in his address to the British

Association, on the 24th August, 1843, stated, that in this telescope, a building the size of the one in which they were assembled would, under favorable circumstances, be easily visible on the Lunar surface.-Athenæum, Sept. 23, p. 867.

breadth. A still greater difference is found in 1 Messier, described by Sir John Herschel, as 'a barely resolvable cluster,' and drawn, fig. 81, with a fine elliptic boundary. This telescope, however, shows the stars, as in his figure 89, and some more plainly, while the general outline, besides being irregular and fringed with appendages, has a deep bifurcation to the south."*

In a Paper entitled "Observations on some of the Nebula," communicated to the Royal Society on the 13th of June last, Lord Rosse has given sketches of five of the nebulæ in Sir John Herschel's Catalogue,† numbered 88, 81, 26, 29, and 47, as seen in his three feet specula, and as soon as this paper is printed, the comparison of these drawings with those of Sir John Herschel, will exhibit the power of the new telescope.

Fig. 26 of Sir J. Herschel's Catalogue (Messier, 27) called the Dumb-bell Nebulæ, from its supposed resemblance to a dumbbell, is shown by Lord Rosse's telescope to be a cluster of stars, or rather two clusters in close proximity, and, indeed, to a certain extent, blended together, and without the exact elliptical terminations of Herschel's figure.

Fig. 81 of Sir J. Herschel's Catalogue (Messier, 51) seen as an oval nebula by both these astronomers, is found to be a cluster of stars remarkable for its singular appearance, the ramifications from its southern extremity extending to a distance equal to its major axis, and giving it the appearance of a scorpion.

is a perfectly circular planetary nebula ; Fig. 45 of Sir J. Herschel's Catalogue but Lord Rosse has discovered it to be an annular nebula like the elliptical annular nebula in Lyra (29, Sir J. Herschel's Catalogue, and 57 Messier) but very much more difficult to be seen.

Fig. 49 of Sir J. Herschel's Catalogue is represented as a remarkable round planeach of the three vertices of an equilateral etary nebula, containing three stars, one at triangle: Lord Rosse's telescope shows this as a long irregular patch, with about seven stars in it, grouped unsymmetrically.

These are a few interesting examples of the manner in which the new telescope has

ed that symmetry of form in globular nebulæ, upon which was founded the hypothesis of the gradual condensation of nebulous matter into suns and planets.

resolved nebulæ into stars, and has destroy- | equal area, and strongly framed so as to be stiff and light. There are twelve of these in the outer rim, nine in the next, and six sectors at the centre. Each of these pieces is supported at its centre of gravity on a hemispheric bearing, at the angle of a triangle of cast iron, these triangles being in their turn similarly supported at the angles of three primary triangles, which, again, are supported at their centres of gravity by three screws which work in a strong iron frame, and serve for adjusting the mirrors. This frame carries also levers to give lateral support to the speculum, in the same difThis frame, which con

Such is a brief account of the construction and performance of a telescope which Dr. Robinson characterizes as the most powerful that has ever been made. Its superiority to all other instruments must have been very gratifying to Lord Rosse, and might have justified him in resting from his labors, and enjoying the honor of having triumphed in so noble an undertaking: but the instrument was scarcely out of his hands fused manner. before he resolved upon attempting the contains the speculum, is attached to an imstruction of another reflector, with a spec- mense joint, like that of a pair of comulum six feet in diameter, and fifty feet passes moving round a pin, in order to give long! This magnificent instrument was the transverse motion for following the star accordingly undertaken, and within the in right ascension. This pin is fixed to last month has been brought to a successful termination. The speculum has six feet of clear aperture, and therefore an area four times greater than that of the three-feet speculum, and it weighs nearly four tons! The focal length is 53 feet. It was polished in six hours, in the same time as a small speculum, and with the same facility; and no particular care was taken in preparing the polisher, as Lord Rosse intended to repolish it as soon as the focal length was ascertained to be correct; but upon directing it to a nebula, the performance was better than he expected, and he therefore has suffered it to remain in the tube for the present. The second or duplicate specuJum, not yet finished, is in every respect the same in size. It was only three weeks in the annealing oven, and is reckoned very good.

the centre piece between two trunnions, like those of an enormous mortar, lying east and west, and upon which the telescope has its motion in altitude. To the frame there is fastened a large cubical wooden box, about eight feet a side, in which there is a door through which two men go in to remove, or to replace the cover of the mirror. To this box is fastened the tube, which is made of deal staves, hooped like a huge cask. It is about 40 feet long, and 8 feet diameter in the middle, and is furnished with internal diaphragms, about 6 feet in aperture. The Dean of Ely walked through the tube with an umbrella up!

This enormous tube is established between two lofty piers or walls of castellated architecture, about sixty feet high, one of which carries an iron semicircle, against which the tube bears when in the meridian. The casting of a speculum of nearly four The declinations will, therefore, be given tons must have been an object of great in- in this case by a circle and level, as in terest, as well as of difficulty; but every Troughton's Transit Instruments. The difficulty was foreseen and provided against. celestial object is followed in right ascension In order to ensure uniformity of metal, the by drawing the telescope from this plane blocks from the first melting, which was through a range of fourteen feet, with a long effected in three furnaces, were broken up, screw, moved either by hand or by a clock, and the pieces from each of the furnaces with a rate variable with the declination. were placed in three separate casks, A, B, The hour angle will, in such cases, be oband C. Then in charging the crucibles tained by another circle and level. The for the final melting of the speculum, suc-other pier carries the galleries for the obcessive portions from cask A were put into servers, which, for fear of producing tremor, furnaces a, b, and c, from B into b, c, d, Lord Rosse was unwilling to attach to the and so on. tube. The galleries will consist of three In order to prevent the metal from bend-stages, with some help from ladders, each ing or changing its form, Lord Rosse has stage being pushed forward in succession introduced a very ingenious and effective from the top of the piers. support. The speculum rests upon a surface of twenty-seven pieces of cast iron, of

This immense mass of matter weighing about twelve tons, requires to be counter