"JUDGING from the letters I receive from men who have made their mirrors, and are now contemplating or actually making their eyepieces seem that an increased interest has sprung up among amateurs who desire to complete their entire telescopes with their own hands. I have for years urged mirror makers to try their hand at small lens making, telling them that the difficulties are no greater, and that the fun is just as great. I have picked up a few more ideas on the subject since writing the chapter on eyepieces (page 66, 'Amateur Telescope Making,') and am here offering them to the 'fraternity,' along with a description of the spindle I have been using here at Pasadena the past four years.

"Figure 1 shows the general set-up, the spindle and motor bolted to a cast iron slab. The motor is 1 h.p. (1725 r.p.m.) giving ample power for the larger work running up to 2-1/2" diameter. The spindle is 6" long, 7/16" in diameter and runs in ball bearings. The dish A is removable, and a horizontal stud at B allows turning the spindle easily into a horizontal position for centering work. The dish can be clamped, C, in any azimuth so as to bring the lever arm D to the most comfortable position. The lever arm is provided with a universal joint E, and the rod itself can be slid sidewise and rocked, so as to bring the pin P, carrying the lens, to just the right place over the lap. Two pulleys on the motor and two on the spindle, of 5/8", 1", 1-1/2", and 2" diameter, allow a wide range of speeds for the different sized lenses.

"I experienced trouble with the belt, trying different materials in order to obtain the smoothest action and longest life. The one on the machine now is the most satisfactory. It is leather, round, and has no joint, and was cut from a strip of 4" belting (a, Figure 6). Slots in the base permit taking up the stretch. My last addition to the machine is locating the starting and stopping switch F within easy reach-for the left hand is usually occupied holding the lever that controls the spinning lens-also the provision of a brake G for quickly bringing the motor armature to rest.

'The amateur, in attempting his own machine, may well disregard certain features shown in my design, for this instrument was made with the resources of a large machine shop. The patterns are all expensively cored, the castings aluminum. But the essential features should be retained-namely, a smooth running spindle (plain bearings will do, but must be kept carefully oiled and protected from grit), a smooth running belt, a detachable dish, and provision for bringing the spindle to a horizontal position.

"With the machine so described it is unnecessary to depend on a lathe for turning up the various curved surfaces on the brass laps, for you are virtually already provided with a speed lathe set up on end. A very little practice with a hand tool made out of an old file, using the lever arm as a steady rest as shown in Figure 2, will form a lap in a few moments so that it fits its templates.

"Of course, for good work, the laps should be made in pairs-male and female-and ground together. Such time will be saved in holding the laps to their correct curvature, if the bulk of the glass has first been removed from the glass blank before placing it on the machine. (See A.T.M., p. 67, Figure 53b.)

"I am now using four grades of abrasives: Nos. 90, 1F, and 600 Carborundum, and 305 emery. An addition of an equal amount of talc to the emery will almost surely prevent scratches in the final grinding. It leaves the glass with a surface that comes to a polish in a few moments. Scratches usually show up with 600 Carbo. A fruitful cause is letting the lap become too dry. It doesn't take long for the rapidly rotating tool and spinning lens to move the lubricant away from their central areas. Should the worst happen the lens will seize to the pitch, the pin on the lever arm will jump out of its pivot, and the lens may fly off into the dish and very likely suffer a chipped edge. Perhaps a hardwood dish, or a dish of some material that will cushion such a blow, would be advisable.

"I find it pays to do some thorough house cleaning when changing from one grade to one finer, also that it pays to put down fresh newspapers, to scrub the hands well with a scrubbing brush and clean the finger nails. My four grades of abrasives are kept in four glass cups-caster cups-(at the five-and-ten, at a nickle each). I ground their edges on a sheet of window glass, then cut up the window glass into cover plates.

"IN polishing I cover the tool, or lap, with only about 1/16" of rather hard pitch and, while still warm and rotating on the spindle, smooth it up to shape with its mate (wetted). The prepared fine-ground lens will do about as well. The rouge polishes faster if the pitch is rechanneled often. It only takes a moment with the blade of a pen knife, and Figure 3 shows the way I do it. The pitch at the center of the tool can well be removed, for it permits the pitch under pressure to flow toward the center as well as the edge.

"The tyro must work out his own salvation until he knows just how far to let his spinning lens move away from the center of the rapidly revolving tool, in order to have the polish come up evenly on the glass. He will notice that with the center of the lens directly over the center of the tool, they are both going at the same r.p.m.-they are as one- and no action is taking place. Theoretically, and assuming perfect contact and also assuming that the tool retains its shape (which it never does), a position for the lens center from the tool center will be found when the lens ceases to spin and comes to rest, and beyond that critical point it will start spinning in the other direction. But pitch, as mirror makers know to their sorrow, is queer stuff, and possessed of seven devils, and one must find for himself about how far the lens should pass across the tool.

"What is actually taking place between the glass and pitch surfaces is rather complicated. In Figure 6b, with the tool rotating anti-clockwise as shown, the areas on the tool at A and C will tend to give the lens the same rotation as the tool. But around B the tendency of the tool is to turn the lens in the opposite manner, namely, clockwise. Moreover, heat is being generated, the pitch is flowing, and the slope of the surfaces is continually changing with the stroke.

"In general it can be said that enlarging the central cavity in the tool will bring more of the polishing action to the outer parts of the lens, and trimming down the size of the tool to less than that of the lens will have the opposite effect. The length of the stroke is, of course, important.

"As with mirrors, it is desirable to remove the lens as few times as possible. I use a match-stick to apply a drop or two to the lap when necessary. By carefully moving the spinning lens so as to expose as much of the center of the tool as is safe, the rouge is applied at the center and allowed to work outward by centrifugal force.

"The danger of producing zones by paring away the tool at center or edge might argue for a full-sized tool, uniform through" out, the exact counterpart (obverse) of the lens surface itself. Zonal irregularities become apparent when two lenses with contact surfaces-say flint and crown of an achromatic doublet-are viewed under monochromatic light. The interference rings are not evenly spaced, there are too many of them, and they depart from circles as the lenses are moved eccentric to each other.

"CENTERING comes next. When the lenses are polished, the dish is removed, the clamping nut H (Figure 4) unscrewed and the spindle brought horizontal. For centering I use an attachment (Figure 4) that slips over the seat formerly occupied by the dish. This gadget comprises the rod K carrying the edging plate L and a screw M acting against an arm of K, and the guard N. The right adapter (A in c, Figure 6) is pressed on the tapered spindle end and a little hot pitch daubed on the flange at B. With the flame of a bunsen burner (Figure 5)-an alcohol lamp will do as well-the adapter and spindle end are given a warming and the lens cemented against the flange B (Figure 6c). By giving the spindle a few turns and looking at the lens the reflections from the lens (of different parts in the room, lights, and so on) will be seen to wabble. The lens is then moved a trifle on its seat and the spindle again given a turn. If you have reduced the wabble-if the reflections show less movement -you moved the lens in the right direction. If not, and it has increased, you have made a bad matter worse. A few tries and the secret is out and you know which way to move the lens to reduce the wabble and finally to wipe it out altogether. What you see in the lens as it rotates remains fixed. The axes of the spindle and the lens coincide. Probably the pitch has cooled off before the job is done, but a few passes of the flame allow the adjustment to be carried on indefinitely.

"We are now ready to edge the lens. On goes the edging plate L (Figure 4), and the screw M is advanced until the edging plate just touches the lens. The dish is placed as shown, with some 1F Carbo and water in it, and a spoon. The motor is started, the guard N dropped and the Carbo spooned on to the edging plate. As the plate is brought to bear on the lens under the action of screw M, and the Carbo is dragged under the lens edge, the ear detects a vibration of the plate due to the pounding cam action of the lens. In a short time, as the glass wears down, this pounding disappears and one has only to watch with an outside calipers (O, Figure 5) for the lens to come down to the required diameter. The lens is removed under a little heat, placed in kerosene (or gasolene) over night, when it can be washed clean with soap and water.

"What I have here described (as well as in Chapter XI, Part I, A.T.M.) are the wrinkles and methods worked out by myself without being prejudiced by a knowledge of professional practice. Undoubtedly some of them will appear crude and amusing to the professional. For example, it may be that a better way of making a lens run true on a spindle is to use a fork (d, Figure 6) and, as explained on pages 69-70, A.T.M., I have used them both but I remember a bad scratch that developed when I used the fork. There may be other simpler and more orthodox ways, but it's been lots of fun working out one's own technique.

"To me a le s is a wonderful and beautiful object, and I shall be well repaid if what I have described will start others of our now large following of telescope makers to try equipping their instruments with their own oculars."-Pasadena, California, December,1934.

WE nominate Mr. Porter's article, W above, for inclusion in the new "A.T.M. Supplement."

And what is the new "A.T.M. Supplement"? It is something that, as yet, isn't. It is something that is to be. We plan to publish, next fall, a companion volume to A.T.M., similar to it in binding, format and typography but thinner-perhaps 128 pages. Into this we shall pour a general collection of items long and short, which we either have on hand or can think of, such as reprints of articles on telescope optics, odd data, more "contributions from advanced amateurs," and general material of use to telescope makers, all brought together in one book, an omnium gatherum. Reprinted items from past numbers of the present department may also be included. This book will be sold either with A.T.M. or separately, as desired. We have already started to assemble the ingredients of this book, but we want also to get the ideas of our readers concerning what they think ought to be put into it to make it a round peg in a round hole.

Above all, we want this to be a practical book-no articles on purely theoretical optics or about the pretty stars. Tentatively, we think there should be a good, solid section on telescope mounting design. Whom do you nominate that is qualified to contribute that section? A short section on making setting circles; we have one or two small things on this already, but who has further material? Who can furnish a compact chapter on clock drives-not merely about them but telling exactly how to make them? What other subjects need coverage and who will cover them? Gentlemen, name your poison. We solicit your advice but do not guarantee to take it! And we must move with celerity. Write to us.

LAST month, after your scribe had read and passed, supposedly to the printer, the final proofs of the pages for this department for the March number, a nefarious gang of his fellow editors (or fellow clowns) secretly conspired to insert his photograph in these pages, under a caption which read "The Mentor of the Amateur Telescope Makers." In an accompanying note they accused him of modesty, an unscientific statement which is also libelous, and they even called him a gentleman. The very picture itself was a libel, being one which they are believed to have scavenged from a waste basket after your scribe threw it there because it too closely resembled that of Al Capone. In brief, the only accurate statement connected with this whole deep-dyed plot was the assertion that your scribe would not see the picture in the magazine until too late; and this is the very assertion that, with sad, wet tears in his voice, he is dead certain nobody on earth will ever, ever believe-no, never. And did not the conspirators know that very fact? Aye, they even admit it, and they actually laugh about it. Your scribe's most fervent disclaimer, they say, will merely be regarded as a part of his original put-up job, and so will all further disclaimers, right on out to infinity and beyond.

In these heart-rending circumstances, with his spirit crushed, and in a hopeless fix, your despairing scribe has picked out a nice hole, is now about to crawl into it and pull the hole in after him. Goodbye, goodbye-alas, goodbye forever!-The Dementor of the Amateur Telescope Makers.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.