SPECULUM for reflecting telescopes, is made of a kind of white copper consisting of 32 parts fine red copper, one of brefs, 15 of grain-tin, and three of white arsenic. The process given by the late J. Edwards, who was rewarded by the Board of Longitude for disclosing it to the public, was published in the Speculum Nautical Almanack for 1787, and is as follows: Melt the copper in a large crucible, employing some black flux, composed of two parts of tartar and one of nitre: when melted, add to it the brefs and the silver. Let the pure tin be melted in another crucible, also with some black flux. Take them both from the fire, and pour the melted tin into the fused mass in the large Cronfield's crucible. Stir the whole well with a dry spatula of birch, and pour off the fused metal immediately into a large quantity of cold water. The sudden chill of the water will cause the fluid metal to divide into an infinite number of small particles, which will cool instantly.

If the copper be completely saturated, the fracture of one piece of this mixed metal will appear bright, and of a gloomy look, resembling the face of pure quicksilver. But if it is of a brown reddish colour, it wants a little more tin. To ascertain the required proportion, melt a small quantity, known by weight, of the mixed metal, with a known very small part of tin; and, if necessary, repeat the trial with different doses, till the fracture of the new mixture looks as already described. Having now ascertained the necessary addition of tin that is required, proceed to the last melting of the whole metal, together with the additional proportional dose of tin; fuse the whole, observing the same cautions as before; and you will find that the mixture will melt with a much less heat than that for the first fusion. Have ready as many ounces of white arsenic in coarse powder as there are pounds in the weight of the metal; wrap up the arsenic in a small paper, and put it, with a pair of tongs, into the crucible; stir it well with the spatula, retaining the breath to avoid the arsenical fumes or vapours (which however are not found to be hurtful to the lungs) till they disappear; take the crucible off the fire, clear away the dross from the top of the metal, pour in about one ounce of powdered rosin, with as much metal, in order to give the metal a clean surface, and pour out the metal into the moulded flasks.

The speculum should be moulded with the concave surface downwards, and many small holes should be made through the sand upwards, to discharge the air. The moulding sand from Highgate near London, used by the founders, is as good as any for casting these metallic mirrors. The cast metal should be taken out from the sand of the flask whilst it is hot, or else it may happen to crack if left to cool within. See TELESCOPE.

But in addition to what has now been said, we must notice some other information relative to the grinding, polishing, and other important circumstances connected with the method of preparing the most perfect speculum for telescopes. The metal being taken out of the flask, as already noticed, and this should be done as soon as it has become solid, and while it is yet red hot, care must be taken to keep the face downwards to prevent it from finking. Holding it in that position by the git, force out the sand from the hole in the middle of the mirror with a piece of wood or iron, and place the speculum in an iron pot, with a large quantity of hot ashes or small coals, so as to bury the speculum in them a sufficient depth. If the sand is not forced out of the hole in the manner above directed, the metal, by finking as it cools, will embrace the sand in the middle of the speculum so tight, as to cause it to crack before it becomes entirely cold. And if the metal be not taken out of the sand, and put in a pot with hot ashes or coals to anneal it, the moisture from the sand will always break the metal. Let the speculum remain in the ashes till the whole is become quite cold. The git may be easily taken off by marking it round with a common fine half round file, and giving it then a gentle blow. The metal is then to be rough ground and figured.

But before we proceed to describe that process, it may be proper to give an account of another composition for the speculum of a reflecting telescope, which has been employed with great success, by Rochon director of the marine observatory at Brest. Of this composition the principal ingredient is platina; which, in grains, must be purified in a strong fire by means of nitre and the salt of glafs, or that flux which in the English glafs-houfes is called by the workmen sandifer. To the platina, when purified, add the eighth part of the metal employed in the composition of common specula; for tin without red copper would not produce a good effect. This mixture is then to be exposed to the most violent heat, which must be still excited by the oxygen gas that disengages itself from nitre when thrown into the fire. One melting would be insufficient: five or fix are requisite to bring the mixture to perfection. It is necessary that the metal should be in a state of complete fusion at the moment when it is poured into the mould. By this process I have been enabled (says the author) to construct a telescope with platina, which magnifies the diameters of objects five hundred times, with a degree of clearness and distinctness requisite for the nicest obervations. The large speculum of platina weighs fourteen pounds: it is eight inches in diameter, and its focus is fix feet. Though the high price of platina will, in all probability, forever prevent it from coming into general use for the speculums of telescopes, we thought it proper to notice this discovery, and shall now proceed to the grinding of the speculum.

For accomplishing this object, a very complicated process is recommended in Smith's Optics, and one not much more simple, by Mr Mudge in the 67th volume of the Philosophical Transactions; but according to Mr Edwards, whose speculums are confessedly the best, neither of these is necessary. Besides a common grindstone, all the tools that he made use of are a rough grinder, which serves also as a polisher, and a bed of hones. When the speculum was cold, he ground its surface bright on a common grindstone, previously brought to the form of the gage; and then took it to the rough grinder.

The tool is composed of a mixture of lead and tin, or of pewter, and is made of an elliptical form, of such dimensions, that the shortest diameter of the ellipse is equal to the diameter of the mirror or speculum, and the longest diameter is to the shortest in the proportion of ten to nine. This rough grinder may be fixed upon a block of wood, in order to raise it higher from the bench; and as the metal is ground upon it with fine emery, Mr Mudge, with whom, in this particular, Mr Edwards agrees, directs a hole or pit to be made in the middle of it as a lodgement for the emery, and deep grooves to be cut out across its surface with a graver for the same purpose. By means of a handle fixed on the back of the metal with soft cement, the speculum can be whirled round upon this grinder fo rapidly, that a common labourer has been known to give a piece of metal, four inches in diameter, so good a face and figure as to fit it for the hones in the space of two hours. The emery, however fine, will break up the metal very much; but that is remedied by the subsequent processes of honing and polishing.

When the metal is brought to a true figure, it must be taken to a convex tool, formed of some stones from a place called Edgedon in Shropshire, situated between Ludlow and Bishop's Castle. The common blue hones, used by many opticians for this purpose, will scarcely touch the metal of Mr Edwards's speculums; but where they must be employed for want of the others, as little water should be used as possible when the metal is put upon them; because it is found by experience that they cut better when but barely wet, than when drenched with water. The stones, however, from Edgedon are greatly preferable; for they cut the metal more easily, and having a very fine grain, they bring it to a smooth face. These stones are directed by Mr Mudge to be cemented in small pieces upon a thick round piece of marble, or of metal made of tin and lead like the former composition, in such a manner, that the lines between the stones may run straight from one side to the other; so that placing the teeth of a very fine saw in each of these divisions, they may be cleared from one end to the other of the cement which rises between the stones. As soon as the hones are cemented down, this tool must be fixed in the lathe, and turned as exactly true to the gage as possible. It should be of a circular figure, and but very little larger than the metal intended to be figured upon it. If it be made considerably larger, it will grind the metal into a larger sphere and a bad figure; and if it be made exactly of the same size, it will work the metal indeed into a figure truly spherical, but will be apt to shorten its focus, unless the metal and tool be worked alternately upwards. On these accounts Mr Edwards recommends it to be made about one twentieth part longer in diameter than the speculum, because he has found that it does not then alter its focus; and he earnestly diffuses the use of much water on the hone pavement at the time of using it, otherwise, he says, that the metal in different parts of it will be of different degrees of brightness.

The metal being brought to a very fine face and figure by the bed of stones, is ready to receive a polish, which is given to it by the elliptical rough grinder covered with pitch. With respect to the consistency of this pitch, Mr Mudge and Mr Edwards give very different directions. Whilst the former says that it should be neither too hard nor too soft, the latter affirms that the harder the pitch is, the better figure it will give to the metal. Pitch may be easily made of a sufficient hardness by adding a proper quantity of rosin; and when it is hardened in this way, it is not so brittle as pitch alone, which is hardened by boiling. Mr Edwards advises to make the mixture just so hard as to receive, when cold, an impression from a moderate pressure of the nail of one's finger. When the elliptical tool is to be covered with this mixture, it must be made pretty warm, and in that state have the mixture poured upon it when beginning to cool in the crucible. Our author recommends this coating to be made everywhere of about the thickness of half a crown; and to give it the proper form, it must, when somewhat cool, be pressed upon the face of the mirror, which has first been dipped in cold water, or covered over with very fine writing paper. If it be not found to have taken the exact figure from the first pressure, the surface of the pitch must be gently warmed, and the operation repeated as before. All the superfluous pitch is now to be taken away from the edge of the polisher with a pen knife, and a hole to be made in the middle, accurately round, with a conical piece of wood. This hole should go quite through the tool, and should be made of the same size, or somewhat less than the hole in the middle of the speculum. Mr Edwards says, that he has always found that small mirrors, though without any hole in the middle, polish much better, and take a more correct figure, for the polisher's having a hole in the middle of it.

The polisher being thus formed, it must be very gently warmed at the fire, and divided into several squares by the edge of a knife. These, by receiving the small portion of metal that works off in polishing, will cause the figure of the speculum to be more correct than if no such squares had been made. Mr Mudge directs the polisher to be firewood over with very fine putty; but Mr Edwards prefers Colcothar of vitriol. Putty (says he) gives to metals a white lustre, or, as workmen call it, a silver hue; but good colcothar of vitriol will polish with a very fine and high black lustre, so as to give the metal finished with it the complexion of polished steel. To know if the colcothar of vitriol is good, put some of it into your mouth, and if you find it diffuses away it is good; but if you find it hard, and crunch between your teeth, then it is bad, and not well burned. Good colcothar of vitriol is of a deep red, or of a deep purple colour, and is soft and oily when rubbed between the fingers; bad colcothar of vitriol is of a light red colour, and feels harsh and gritty. The colcothar of vitriol should be levigated between two surfaces of polished steel, and wrought with a little water; when it is worked dry, you may add a little more water, to carry it lower down to what degree you please. When the colcothar of vitriol has been wrought dry three or four times, it will acquire a black colour, and will be low enough, or sufficiently fine, to give an exquisite lustre. This levigated colcothar of vitriol must be put into a small phial, and kept with some water upon it. When it is to be used, every part of the pitch-polisher must be first brushed over with a fine camel's hair brush, which has been dipped in pure water, and rubbed gently over a piece of dry clean soap. The washed colcothar of vitriol is then to be put upon the polisher; and Mr Edwards directs a large quantity of it to be put on at once, so as to saturate the pitch, and form a fine coating. If a second or third application of this powder be found necessary, it must be used very sparingly, or the polish will be destroyed which has been already attained. When the metal is nearly polished, there will always appear some black mud upon its surface, as well as upon the tool. Part of this must be wiped away with some very soft wash leather; but if the whole of it be taken away, the polishing will not be so well completed.

With respect to the parabolic figure to be given to the mirror, Mr Edwards assures us, that a very little experience in these matters will enable any one to give it with certainty, by polishing the speculum in the common manner, only with cross strokes in every direction, upon an elliptical tool of the proper dimensions.