The simple addition to an ordinary lathe for the purpose of grinding telescope specula has been of so much benefit to the author that it was thought, if made known through the medium of the Society of Arts, it might become useful to others interested. Up to about fifty years ago the whole of the specula of reflecting telescopes were made of what was termed speculum metal, a composition of copper and tin in the proportion of their chemical equivalents. This metal, when highly polished, is stated to reflect only about 65 percent of the incident light. Just about fifty years ago Liebig made known his method of reducing nitrate of silver to the metallic form by means of grape sugar. The silver thus thrown down is puie, and when polished, auihorities state it reflects over 90 per cent of the incident light.
A mirror having upon it a film of pure silver will be seen to possess a very great advantage over one of speculum metal. A mirror made of speculum metal may in the course of time lose its luster and polish, and require to be again polished. This may result in the original figure being destroyed and lost. When made of glass and silvered on the face by I.iebig's process, the silver may, and no doubt does become oxidized in the course of time; but the silver has only to be dissolved off, and the mirror resilvered as often as may be necessary, without affecting in any way its original figure.
The construction of mirrors made of glass very soon became general alter Liebig's process was known. The first to make them was Dr. Steinheil of Munich, in the year 1857, and about the same time Foucault of Paris, and Draper of New York, also made glass specula. The construction of glass specult has now become very general among amateur astronomers, and it is with the view of showing how very simple an addition to a lathe may be effective for that purpose that the present communication is made.
Before describing brietly the machines used in grinding mirrors, it may be well to indicate the first step of all. Take, for example, the tool necessary to construct a speculum whose diameter is 61/2 in., and focal length, say, 5 ft. 6 in. This is a size to which those beginning such work might do well to restrict themselves. The first thing is to make two grinding tools-a convex and a concave-each having a radius of curvature of 11 ft.
A templet must be struck by means of a long wooden rod, through one end of which a nail or bradawl is passed into the floor, and at the distance of 11 ft. another nail or cutter of some sort makes a circular mark or cut upon a piece of zinc or brass.61/2 in. broad, lying on the floor. When the metal is cleanly separated by clipping and filing at the mark, we have two templets-a convex and a concave. A piece of board is then placed on a lathe chuck and turned on its opposite faces to correspond to the above templets, and 61/2 in diameter. Two castings are then obtained e.ther in iron, brass or zinc, Horn this pattern and worked upon each other, the convex surface of the one into the concave sui lace of t he ot her, until either by turning, filing or grinding, they lit each other perfectly.
The convex surfa e of the one is cut into squares with a file, the grooves so cut being ab out 1/2 in. apart, the depth of the grooves being immaterial. This is all the preparation necessary previous to beginning th,e operation of grinding. The most perfect system of grinding is that in which the whole operation is done by the hands alone. But hand-grinding is so laborious, slow and fatiguing, that almosteveryone desires the assistance of a machine of some kind to lighten his labor. Now, it may be stated that no machine has ever been made or can be made, to grind perfect speculum by itself. Machines require constant alteration of the stroke during the process, and it is with the view of showing why this is nece-sary that reference to them requires to be made before describing the simple method devised by the author.
There are three essential things which require to be kept prominent ly in view to insure success in grinding mirrors, either by hand or by a machine. The first is that in grinding, a true spherical surface must be got. (Sir Howard Grubb states in one of his articles that a true spherical surface is only got by chance.) The second is the length of stroke used, while the third is the side stroke. According to Sir John Herschel. the second and third stem to be essential. A beginner, will have much difficulty with the first, less difficulty with the second, but the third is the most important of all.
Reference is here only made to the case in which the tool is ma le to work over the speculum either by strokes entirely straight, or partly straight and partly circular. While the speculum is slowly revolving on the machine, the grinder i caused to move across the speculum at a short distance from its center; this movement constitutes the ' side stroke." If this were not done, and the center of the grinder pass invariably acros the center of the speculum, a truly sperical curve could not he obtained, and the center would be ground down much more than it oaght. to l>e.
Another point requires to he mentioned in reference to machine grinding. The motion of a machine is regular, and at certain times the strokes of the grinder recur oftener at certain places than at others, giving rise to nodes and. of course, causing a circular groove or zone in the speculum at these points There may be one or more of these zones. Thus, if the machine causes its strokes to meet with regularity at a definite point, there will be a depression at that zone all round the mirror, and instead of a circular curve there will be a wavy form ; a speculum with such a surface will naturally be of no use. It is diflicult to see these defects until the mirror is partly polished, when they are at once detected by reflected light. By a skilful and judicious use of the side stroke, a mirror can be made without any zones.
It occurred some months ago to the author that this difficulty of the side-stroke could be got lid of in the caseof small specula up to, say, 81/2 or 10 in. in diameter by using jointly the lathe and the hand motions, and he believes that he has been successful. The drawing of the simple addition to a lathe is snbjoin-d. It will be seen to consist of a board 1 in. thick, 3 in. broad, and 15 in. long, firmly bolted down to the lathe bed. A short upright piece is attached to the off-end by two strong iron hinges which allow it to swing for wards and backwards through the action of a wooden rod attached to a crank-pin fixed to a chuck on the lathe mandrel. The other side of this upright piece has a wooden rod which engages with a pin on the back of the grinding tool. A cord passes from the small groove on the lathe-wheel, which is ft. indi-ameter, to a 6 in. wheel in the mandrel, and when the lathe is worked the grinding tool is made to move across the speculum by means of the two wooden arms.
The crank is set so as to produce a motion of one-eighth the diameter of the speculum, which rests upon a piece of t hin wood somewhat larger than its diameter, and while the motion of the lathe continues this piece of wood is turned round more or less by the left hand, either backwards or forwards, thus giving an irregular motion of the very best kind, superior in every way to a mechanical one.
Now, if this were all, any number of zones would be the result, and this has been found to he so in actual practice. These are got rid of in a very simple way. A piece of string is hooked on to a na I in the middle of the wooden rod which drives the grinder, while the other end is attached to some fixed point. The suing allows the center of the griuder to pass over the center of the speculum; but to get the side-stroke the the pointer finger of the right hand is pressed on the string more or less, and the grinder in tins way earn be moved while the lathe is running, to the necessary extent off the center of the speculum, and thus obtain the necessary side stroke in the very simplest possible way.
The stroke is a straight one; but notwithstanding this, the motions given by the actions of the two hands entirely eliminate the zones, and the result is a nearly spherical surface if, indeed, it is not as true a one as can he desired. Two specula, each of 6 1/4 in. diameter have been ground in this way, and the results given by them are exceedingly good. They are still unsilvered and, of course, reflect somewhat under 5 per cent of the incidental light; hut they undoubtedly show both by trial on stars, on the moon and Jupiter, that the movement forms a means of obtaining a nearer approach to the spherical surface than can be obtained in any other way.
It may be thought that too much has been said about obtaining a spherical curve; but it seems to be the foundation, and the correct one, for getting the necessary parabolic one. The difference between a parabolic and a spherical surface is so small that it is assumed by most that the conversion to the parabolic form is got in the polishing. Mr. Ritchey gives in his Memoir the difference in regard to some mirrors, and these are in decimals of an inch :
1. His own 2 ft., of 03 in. focus .0004
2. Lord Rosses 6 ft. of 60 ft. focus .0001
3. The Yerkes 4 ft. mirror of 25 ft. focus .0006 It will be seen from these figures that in the first one the difference is only 4-16 000 of an inch, an amount which it seems could without difficulty be removed in the polishing after a true spherical surface has been obtained.
The lathe runs so easily with the grinder and acts so rapidly with the motion that when it is making 120 strokes a minute a 6 1/2 in. polished speculum can be put on the machine, the top portion ground with flour of emery and made ready for polishing again in about an hour. The two 6 1/2 in. specula which are at present used were treated in that way.