NOW AND THEN THIS DEPARTMENT LEARNS of a telescope mirror which, while its performance on the stars apparently is not affected, exhibits under the knife-edge the entire outline of the lap on which it was cold pressed. George A. Kennedy, mining engineer, of Denver, before he died, described a mirror figured for him by C. S. Walton, 5975 W. 44th Ave., Wheatridge, Colo. (after a very noted old optical firm in New England had re-figured it for $100 and done a botch job). The lap had often been left for many hours on the mirror, under a seven-pound weight. At the end of figuring, the Foucault test made clearly visible the pattern of the lap. "It does not seem to interfere in the least with the excellent performance of the mirror on the stars," Kennedy wrote. Yet his curiosity was aroused.

A. W. Everest commented as follows: "I've seen pitch that would etch glass, and I know of several cases where ordinary city water ate a hunk right out of it. Leo Scanlon has a mirror that looks like a perfect pitch lap under the knife-edge test, yet with a perfect visual polish. He blames it on the water in the Monongahela River. Kennedy's, however, sounds like that other monster, pitch flowing into the glass, with long pressing. Glass molecules are rather loosely packed, and pitch will readily flow in between them if the water film is missing. When the lap is removed, generally only after a fight, this pitch is sheared off and stays there, making the pattern of the lap harder than the rest of the surface. You can't see it at first, but, since this part of the surface polishes slower than the rest, about the time you think you are finished, there the thing will stand out in relief under the knife-edge test. Several hours' hard polishing would cut through this hard skin."

"Why, however," Everest continues, "does anybody press any lap more than an hour or so with only enough weight to get it into contact? The lap doesn't fit the mirror ten seconds after you start to use it, anyway."[See Everest's chapter on advanced mirror making technique, in "A.T.M.A."-Ed.]

Referred to Kennedy, these comments elicited the following: "The lap was never in contact with the glass dry for a moment. Also, acid etching would seem to be the most probable answer if the image on the glass were all criss-crossed, because of the numerous times the lap had been replaced in a different position during pressing and polishing. But it is not. The lines of the facets are all clear and unbroken."

This appears to eliminate the effect described by Everest, in connection with this particular mirror, but the effect, with its odd cause, is well worth remembering.

The correspondence was shown to Leo Scanlon, who passed it to Dr. Lee Devol, at the Mellon Institute of Industrial Research, who is interested in glass. He 11 commented as follows: "Glass does many strange things which we have not yet learned how to explain. I do not believe any of us can say exactly what took place; in the transfer to the glass. We do know that glass is readily attacked by different chemicals. Even pure water can be guilty of the attack. There was moisture in the lap, together with a large number of compounds whose action it would be impossible to predict. The remaining surface -may be more resistant to polishing because it is more resistant to chemical attack by the water in the rouge, or it may be that the chemical nature of the glass surface is such that it is less inclined to attach itself to rouge than it would have been before exposure to the lap. In this case there should be, for the portion of the glass which has been covered by the lap, a lower coefficient of friction in polishing."

Well, so what? This: We have hypotheses but no final answer. Can anyone else provide the definitive one in this odd puzzle?

ONION SACK or any kind of coarse-mesh netting may be used for cold-pressing sub-facets, or facetettes, into the ordinary facets of a pitch lap (Figure 1). This has long been common among professionals. There is no mystery about it and it is simple to do. The fabric (wet) is laid on the pitch lap, and the mirror is then laid on the fabric, and cold pressing is done as usual.

Commenting on the use of onion sack, Cyril G. Wates, Edmonton, Alberta, says, "I wouldn't go back to the plain lap. You can lift the mirror right off the subfacetted lap with no suction at all. The objection, sometimes heard, that the

facetettes soon press out seems without much foundation. They remain after 24 hours' cold pressing (without weigh). I also find them a complete remedy for the mottled effect known as 'dog biscuit.'" F. B. Ferson, Biloxi, Miss., who also uses onion sack, says, "My experience is as Wates says, that the facetettes take, quite a long time to press out. I like facetettes from onion sack because the lap seats almost at once. I use common onion sack from a grocery store-first washing it, of course. The lap hardness and pressure of polishing govern the sinking speed of facetettes. They should sink out in not over two hours of actual we of polishing by hand."

Your scribe years ago used marquisette for the same purpose. If you don't know this word, ask the feminine side or, simpler still, yank down a window curtain (justified, of course, in the name of sacred science). Figure 1 shows a 6" lap w facetettes pressed in with some such material, as used by H. Lynn Bloxom, 1425 Fourth Ave., N., Fort Dodge, Iowa.

AS IS WELL-KNOWN, after several years' effort to teach his invention, the HCF lap, perfect manners, A. W. Everest decided that it probably would never become a true smoothie. He, therefore, revised his chapter on the HCF lap in "A.T.M." (4th ed.), recommending its use through the polishing and gross figuring stages alone, and a pitch lap for final figuring. The reason for this was the uneven surface texture HCF gives.

Others found that HCF will not always make a perfectly fitting lap: It is often rather inflexible. William A. Rhodes, 1206 E. Garfield Street, Phoenix, Ariz., obtains a perfectly fitting HCF lap softening the beeswax tetrahedrons with radiant heat and pressing before they cool.

First, the tool is primed by rubbing pieces of HCF or other beeswax into its pits. Next, the HCF is applied, the mirror laid on it, and the protruding HCF trimmed off.

Then, with a bowl type of electric floor heater, the lap is warmed to the exact temperature "at which, by touching the very edge of the HCF with the finger, it will feel as if about to collapse, but no higher." Rhodes points out that, if carried a whisker higher, it "will suddenly disintegrate into a puddle even before one can withdraw the lamp. What is wanted is a mushy condition."

In the meantime the mirror has been warmed, and rouge mixture has been swabbed on it.

The warm mirror is placed immediately on the lap and loaded with about ten pounds extra pressure, and the two are left to cool to room temperature.

"Instead of the tetrahedrally embossed HCF surface it will look as if buckshot had been pressed into it. This gives 50 percent added surface at the start of polishing and the surface will fit the mirror."

This, of course, is now essentially a plain beeswax lap, not an HCF lap, but many prefer beeswax laps anyway, and this is one way to make one.

Rhodes roughs out his mirrors with crushed steel and makes a separation of glass particles from the particles brushed off the tool, for re-use, by means of a magnet, which picks up the steel grains. These are then pulled off the magnet by hand.

THE FOLLOWING communication written by F. W. Bubb, Professor of Applied Mathematics at Washington University, St Louis, Mo., appeared in the Journal of the Optical Society of America (Vol. 32 p. 400), and is related to the note on page 319, "Amateur Telescope Making" (Prof. R. W. Wood's experiments).

"It is well known that a liquid rotating in a vessel with constant angular velocity develops a central cavity in the form of a paraboloid of revolution. Such z liquid surface formed upon mercury was used by Professor R. W. Wood as thc reflector of a telescope.

"The thought naturally comes that, if one could solidify a liquid after rotation bad been set up, one might get a very perfect parabolic mirror-possibly of great size and of any desired focal length.

"A number of synthetic resins are now available in liquid form, which harden into solids upon heating. Some of these solids are remarkably strong, having a tensile strength of about 15,000 pounds per square inch.

"May it not be possible to rotate a vessel containing one of these liquids, heat it while maintaining the speed of rotation constant, and thus harden the plastic with its perfect parabolic cavity? Such a surface could then be coated with a reflecting metal film.

"Certain questions arise as to the practicality of this scheme. Would the rather high viscosity of these liquids permit the parabolic surface to form before hardening sets it? After hardening begins, does the volume of the plastic change-thus a1tering the shape of the surface? Will evaporation from or absorption of moisture into the body of the plastic alter the shape? Might not these objections- as well as those which others will find-be overcome?"

Your scribe referred this question to some of those in charge of the 200-inch telescope. It will be recalled that, before testing glass for the mirror, a variety of other substances were carefully studied.

It proved that the trouble with this idea was the "mosaic" structure that forms on the surface of the synthetic resins.

TEST for turned down edge on a mirror is offered by William M. Grace, Jr., 304 S. Foster St., Dotham, Ala. Place the mirror on edge on a table so that the image of a naked lamp bulb, behind the observer, appears on it. The lamp bulb would be far enough from the eye (via the mirror) to appear about one fourth the mirror's diameter. Look at the image and move the head until it shifts to the mirror's edge. If the bulb appears to flatten just before its image runs over the edge, the edge is turned down. If it appears to be sucked over the edge, the edge is turned up.

Grace finds this test more delicate than the eyepiece test-which, however, isn't nearly so sensitive, according to Harold Lower, as the Ronchi and diffraction tests, especially if the eyepiece is a cheap one. But it would show a grossly turned edge, and such a test is useful to beginners who don't yet feel sure of their own interpretation of other tests.

THERMAL effects caused by evaporation I of the water used in polishing are explained by Everest in "A.T.M.A." page 27. To obviate this, J. R. Haviland suggests a trial of Prestone or a saturated solution of magnesium chloride and water-neither of which liquids will dissolve pitch or wax and, being non-volatile, will not evaporate.

HARKS who can handle optical design, S including its mathematical equations, will find in the May number of the Journal of the Optical Society of America a 15-page article by W. M. Stempel of Stevens Institute of Technology, entitled "An Empirical Approach, to Lens Design " The design of a Huygens eyepiece is the example used for setting forth this empirical method.

HAVE you a copy of Conrady's "Applied Optics and Optical Design' which is not at present in use, and which you will be willing to place in the hands of war production workers in optics who need this book? It is now (temporarily, we hope) out of print and apparently unobtainable anywhere. You could probably obtain from sale whatever you originally paid, even for a worn copy. Other books on optics are out of print, one being Martin's "Applied Optics" which, it is said, will not be reprinted for the duration.

NEW BOOK entitled "Prism and Lens N Making," by F. Twyman of Adam Hilger Ltd., England, embodies that author's lifetime experience in this work and will soon be reviewed in this magazine. It contains much that would interest precision opticians in any kind of production .(Advance notice.)

TROPICAL sky maps are not common. In an English scientific journal (Nature, March 27, 1943) we discover a brief listing as follows: "Watson, H. E., compiled by. The Tropical Sky; maps of the constellations visible in the latitudes of the West Indies, Guianas, Nigeria, E. Africa, Ceylon; Malaya, etc. (5° to 10° N. or thereabouts) and of the planets and Solar System, showing how and where to identify them in the starry background. Pp. 27 plus three maps. Georgetown, British Guiana, Daily Chronicle, Ltd., 1942. 3s 6d. net." These maps are not available from Scientific American, and interested readers must order direct from the publishers. We have not seen the maps. The above are all the data we have concerning them.

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