CHAPTER VII.
ON THE METHOD OF GRINDING AND POLISHING OPTICAL LENSES AND SPECULA.
I originally intended to enter into particular details on this subject, for the purpose of gratifying those mechanics and others who wish to amuse themselves by constructing telescopes and other optical instruments for their own use; but, having dwelt so long on the subject of telescopes, in the preceding pages, I am constrained to confine myself to a very general sketch.
1. To grind and polish lenses for eye-glasses, microscopes, &c.
First provide an upright spindle, at the bottom of which a pulley is fixed, which must be turned by a wheel by means of a cord and handle. At the top of the spindle make a screw the same as a lathe-spindle, on which you may screw chocks of different sizes, to which the brass tool in which the lens is to be ground, may be fixed. Having fixed upon the breadth and focal length of the lens, and whether it is to be a plano, or a double convex—take a piece of tin-plate or sheet copper, and, with a pair of compasses, draw an arch upon its surface, near one of its extremities, with a radius equal to the focal distance of the lens, if intended to be double convex, or with half that distance, if it is to be plano-convex. Remove with a file that part of the copper which is without the circular arch, and then a convex gage is formed. With the same radius strike another arch, and having removed that part of the copper which is within it, a concave gage will be obtained. The brass tool, in which the glass is to be ground, is then to be fixed upon a turning-lathe, and turned into a portion of a concave sphere, so as to correspond to the convex gage. In order to obtain an accurate figure to the concave tool, a convex tool of exactly the same radius is generally formed, and they are ground one upon another with flour emery; and when they exactly coincide, they are fit for use. The convex tool will serve for grinding concave glasses of the same radius—and it should be occasionally ground in the concave tool to prevent it from altering its figure.
The next thing to be attended to is, to prepare the piece of glass which is to be ground, by chipping it in a circular shape, by means of a large pair of scissors, and removing the roughness from its edges by a common grind-stone. The faces of the glass near the edges should likewise be ground on the grind-stone, till they nearly fit the concave gage, by which the labour of grinding in the tool will be considerably saved. The next thing required is to prepare the emery for grinding, which is done in the following manner. Provide four or five clean earthen vessels; fill one of them with water, and put into it a pound or half a pound of fine emery, and stir it about with a stick; after which let it stand 3 or 4 seconds, and then pour it into another vessel, which may stand about 10 seconds; then pour it off again into the several vessels till the water is quite clear; and by this means, emery of different degrees of fineness is obtained, which must be kept separate from each other, and worked in their proper order, beginning at the first, and working off all the marks of the grind-stone; then take of the second, next of the third, &c.,—holding the glass upon the pan or tool with a light hand, when it comes to be nearly fit for polishing. The glass in this operation should be cemented to a wooden handle, by means of pitch or other strong cement. After the finest emery has been used, the roughness which remains may be taken away, and a slight polish given by grinding the glass with pounded pumice-stone. Before proceeding to the polishing, the glass should be ground as smooth as possible, and all the scratches erased, otherwise the polishing will become a tedious process. The polishing is performed as follows: Tie a piece of linen rag or of fine cloth about the tool, and with fine putty, (calcined tin), or colcothar of vitriol (a very fine powder, sometimes called the red oxide of iron) moistened with water, continue the grinding motion, and in a short time there will be an excellent polish.
In order to grind lenses very accurately for the finest optical purposes, particularly object-glasses for telescopes—the concave tool is firmly fixed to a table or bench, and the glass wrought upon it by the hand with circular strokes so that its centre may never go beyond the edges of the tool. For every 6 or 7 circular strokes, the glass should receive 2 or 3 cross ones along the diameter of the tool, and in different directions; and while the operation is going on, the convex tool should, at the end of five minutes, be wrought upon the concave one for a few seconds, in order to preserve the same curvature to the tools and to the glass. The finest polish is generally given in the following way. Cover the concave tool with a layer of pitch hardened by the addition of a little rosin, to the thickness of 1/15th of an inch. Then, having taken a piece of thin writing paper, press it upon the surface of the pitch with the convex tool, and pull the paper quickly from the pitch before it has adhered to it; and if the surface of the pitch is marked every where with the lines of the paper, it will be truly spherical. If any paper remains on the surface of the pitch, it may be rubbed off by soap and water, and if the marks of the paper should not appear on any part of it, the operation must be repeated, till the polisher or bed of pitch is accurately spherical. The glass is then to be wrought on the polisher by circular and cross strokes with the putty or colcothar, till it has received a complete polish. When one side is finished, the glass must be separated from its handle, by inserting the point of a knife between it and the pitch, and giving it a gentle stroke. The pitch which remains upon the glass may be removed by rubbing it with a little oil or spirits of wine. The operation of polishing on cloth is slower, and the polish less perfect than on pitch; but it is a mode best fitted for those who have little experience, and who would be apt, in the first instance, to injure the figure of the lens by polishing it on a bed of pitch.
2. On the method of casting and grinding the Specula of Reflecting Telescopes.
The first thing to be considered in the formation of reflecting telescopes, is the composition of the metal of which the specula are made. The qualities required are—a sound uniform metal, free from all microscopic pores—not liable to tarnish by absorption of moisture from the atmosphere—not so hard as to be incapable of taking a good figure and polish—nor so soft as to be easily scratched, and possessing a high reflecting power. Various compositions have been used for this purpose, of which the following are specimens:—Take good Swedish copper 32 ounces, and when melted, add 14½ ounces of grain tin to it; then, having taken off the scoria, cast it into an ingot. This metal must be a second time melted to cast a speculum; but it will fuse in this compound state with a small heat, and therefore will not calcine the tin to putty. It should be poured off as soon as it is melted, giving it no more heat than is absolutely necessary. The best method for giving the melted metal a good surface is this: the moment before it is poured off, throw into the crucible a spoonful of charcoal-dust; immediately after which the metal must be stirred with a wooden spatula and poured into the moulds.—The following is another composition somewhat similar. Take 2 parts copper as pure as it is possible to procure; this must be melted in a crucible by itself. Then put, in another crucible, 1 part of pure grain tin. When they are both melted, mix and stir them with a wooden spatula, keeping a good flux on the melted surface to prevent oxidation, and then pour the metal quickly into the moulds, which may be made of founder’s loam.
The composition suggested, more than half a century ago, by the Rev. Mr. Edwards, has often been referred to with peculiar approbation. This gentleman took a great deal of pains to discover the best composition, and to give his metals a fine polish and the true parabolical figure. His telescopes were tried by Dr. Maskelyne, the Astronomer Royal, who found them greatly to excel in brightness, and to equal in other respects those made by the best artists. They showed a white object perfectly white, and all objects of their proper colour. He found, after trying various combinations, the following to be the best: namely 32 ounces of copper, with 15 or 16 ounces of grain tin, (according to the purity of the copper) with the addition of one ounce of brass, one of silver, and one ounce of arsenic. This, he affirms, will form a metal capable, when polished in a proper manner, of reflecting more light than any other metal yet made public.
The Rev. J. Little, in his observations on this subject in the ‘Irish Transactions,’ proposes the following composition, which he found to answer the purpose better than any he had tried, namely—32 parts of best bar copper, previously fluxed with the black flux, of two parts tartar and one of nitre, 4 parts brass, 16 parts tin, and 1¼ arsenic. If the metal be granulated, by pouring it, when first melted, into water, and then fused a second time, it will be less porous than at first. In this process, the chief object is, to hit on the exact point of the saturation of the copper, &c., by the tin. For, if the latter be added in too great quantity, the metal will be dull coloured and soft; if too little, it will not attain the most perfect whiteness, and will certainly tarnish.[35]
When the metal is cast, and prepared by the common grind-stone for receiving its proper figure—the gages and grinding-tools are to be formed in the same manner as formerly described for lenses, with this difference, that the radius of the gages must always be double the focal length of the speculum, as the focus of parallel rays by reflection is at one half the radius of concavity. In addition to the concave and convex tools—which should be only a little broader than the metal itself—a convex elliptical tool of lead and tin should be formed with the same radius, so that its transverse should be to its conjugate diameter as 10 to 9, the latter being exactly equal to the diameter of the metal. The grinding of the speculum is then to be commenced, on this tool, with coarse emery powder and water, when the roughness is taken off, by moving the speculum across the tool, in different directions, walking round the post on which the tool is fixed, holding the speculum by the wooden handle to which it is cemented. It is then to be wrought with great care on the convex brass tool, with circular and cross strokes, and with emery of different degrees—the concave tool being sometimes ground upon the convex one, to keep them all of the same radius, and when every scratch is removed from its surface, it will be fit for receiving the final polish.
When the metal is ready for polishing, the elliptical tool is to be covered with black pitch about 1/20th of an inch thick, and the polisher formed in the same way as in the case of lenses, either with the concave brass tool or with the metal itself. The colcothar of vitriol should then be triturated between two surfaces of glass, and a considerable quantity of it applied at first to the surface of the polisher. The speculum is then to be wrought, in the usual way, upon the polishing tool, till it has received a brilliant lustre, taking care to use no more of the colcothar, if it can be avoided, and only a small quantity of it, if it should be found necessary. When the metal moves stiffly on the polisher, and the colcothar assumes a dark muddy hue, the polish advances with great rapidity. The tool will then grow warm, and would probably stick to the speculum, if its motion were discontinued for a moment. At this stage of the process, therefore, we must proceed with great caution, breathing continually on the polisher, till the friction is so great, as to retard the motion of the speculum. When this happens, the metal is to be slipped off the tool at one side, cleaned with soft leather, and placed in a tube for the purpose of trying its performance; and if the polishing has been conducted with care, it will be found to have a true parabolic figure.[36]
It was formerly the practice, before the speculum was brought to the polisher, to smooth it on a bed of hones, or a convex tool made of the best blue stone, such as clockmakers use in polishing their work, which was made one fourth part larger than the metal which was to be ground upon it, and turned as true as possible to a gage. But this tool is not generally considered as absolutely necessary, except when silver and brass enter into the composition of the metal, in order to remove the roughness which remains after grinding with the emery.
To try the figure of the metal.—In order to this, the speculum must be placed in the tube of the telescope for which it is intended; and, at about 20 or 30 yards distant, there should be put up a watch-paper, or similar object, on which there are some very fine strokes of an engraver. An annular kind of diagram should be made with card-paper, so as to cover a circular portion of the middle part of the speculum, between the hole and the circumference, equal in breadth to about 1/8 of its diameter. This paper ring should be fixed in the mouth of the telescope, and remain so during the whole experiment. There must likewise be two other circular pieces of card-paper cut out, of such sizes, that one may cover the centre of the metal, by completely filling the hole in the annular piece now described: and the other such a round piece as shall exactly fill the tube, and so broad as that the inner edge just touches the outward circumference of the middle annular piece. All these pieces together will completely shut up the mouth of the telescope. Let the round piece which covers the centre of the metal be removed, and adjust the instrument so that the image may be as sharp and distinct as possible. Then replace the central piece, and remove the outside annular one, by which means the circumference only of the speculum will be exposed; and the image now formed will be from the rays reflected from the exterior side of the metal. If the two images formed by these two portions of the metal be perfectly sharp and equally distinct, the speculum is perfect and of the true parabolic curve. If, on the contrary, the image from the outside of the metal should not be distinct and that it should be necessary to bring the little speculum nearer by the screw, the metal is not yet brought to the parabolic figure; but if, in order to procure distinctness, we be obliged to move the small speculum farther off, then the figure of the great speculum has been carried beyond the parabolic, and has assumed the hyperbolic form.
To adjust the eye-hole of Gregorian Reflectors.—If there is only one eye-glass, then the distance of the small hole should be as nearly as possible equal to its focal length. But in the compound Huygenian eye-piece, the distance of the eye-hole may be thus found:—Multiply the difference between the focal distance of the glass next to the speculum, and the distance of the two eye-glasses, by the focal distance of the glass nearest the eye; divide the product by the sum of the focal distances of the two lenses, lessened by their distance, and the quotient will be the compound focal distance required. Thus, if the focal distance of the lens next the speculum be 3 inches, that of the lens next the eye 1 inch, and their distance 2 inches, then the compound focal distance from the eye-glass will be (3 - 2 × 1)/(3 × 1 - 2) = ½ inch.—The diameter of the eye-hole is always equal to the quotient obtained by dividing the diameter of the great speculum by the magnifying power of the telescope. It is generally from 1/25th to 1/50th of an inch in diameter. It is necessary, in many cases, to obtain, from direct experiment, an accurate determination of the place and size of the eye-hole, as on this circumstance depends, in a certain degree, the accurate performance of the instrument.
To center the two specula of Gregorian Reflectors.—Extend two fine threads or wires across the aperture of the tube at right angles, so as to intersect each other, exactly in the axis of the telescope. Before the arm is finally fastened to the slider, place it in the tube, and through the eye-piece (without glasses) the intersection of the cross wires must be seen exactly in the centre of the hole of the arm. When this exactness is obtained, let the arm be firmly riveted and soldered to the slider.
To centre lenses.—The centering of lenses is of great importance, more especially for the object-glasses of achromatic instruments. The following is reckoned a good method:—Let the lens to be centered be cemented on a brass chuck, having the middle turned away so as not to touch the lens, but near the edge, which will be hid when mounted. This rim is very accurately turned flat where it is to touch the glass. When the chuck and cement is warm it is made to revolve rapidly: while in motion a lighted candle is brought before it, and its reflected image attentively watched. If this image has any motion, the lens is not flat or central; a piece of soft wood must therefore be applied to it in the manner of a turning tool, till such time as the light becomes stationary. When the whole has cooled, the edges of the lens must be turned by a diamond, or ground with emery.
For more particular details in reference to grinding and polishing specula and lenses, the reader is referred to Smith’s ‘Complete system of Optics’—Imison’s ‘School of Arts’—Huygenii Opera—Brewster’s Appendix to ‘Ferguson’s Lectures’—‘Irish Transactions,’ vol. X., or ‘Nicholson’s Journal,’ vol. XVI., Nos. 65, 66, for January and February, 1807.