The mirror was made on the machine shown in Figure 4, where the disk, 2-1/2" thick, is shown while being perforated. Central spindle, 3 r.p.m., side cranks 40 r.p.m and 1 r.p.m. Figure 3 shows the preparation of a plaster-backed polishing lap for an f/2.5 sphere for testing the secondary. The lap was made by the Ritchey method-pouring pitch between 1/4" strips of wood laid on newspaper. Cross marks were pushed into the strips before the pitch cooled, and the strips were broken into squares on hardening. The squares were later heated enough to detach the newspaper and applied to the disk, which was already covered with HCF, and others were similarly applied to the main mirror (Figure 5). An excellent method of supporting a mirror for test devised by the late Henry H. Mason of Florida and once illustrated in these columns and used by Jones, is shown in Figure 6. A crank behind the board permits rotation of the spool and belt to bring any diameter of the mirror to a vertical.

Jones spent a year planning this telescope, making 15 detailed drawings. To make the main mirror took one month of day and night work. Iron tools were used for grinding. Polishing took 27 hours, figuring 30 hours, and at no time did the mirror misbehave. The secondary is a 5-1/2" convex.

FROM Cyril G. Wates, 7718 Jasper Ave., Edmonton, Alberta, Canada, we have received the following item pertaining to the telescope making art:

"With the Foucault test nearly eighty years old, it is hardly likely that anything connected with it is actually new, but speaking for myself, there was always something lacking in the precision of the test until I made the discovery that by sliding the knife-edge along the optical axis instead of across it, I could watch the shadows change from right to left, and thus determine the exact focal center of any zone.

"In order to slide the knife-edge along the axis with absolute precision an adjustable guide must be provided with means for moving the edge laterally without affecting the parallelism of the guide and the axis. I built a rather elaborate machine with micrometer adjustments, right-angled prism, and so on, but nothing of the sort is really necessary. A strip of wood screwed down to the table, a large block to slide along it, and a knife-edge mounted on this block in such a way that, by turning a screw, the knife can be moved in and out-that is all. The essential point is that the whole thing must be movable exactly along the optical axis.

"In actual testing, I start inside the focal center and work toward me, then start outside and work in. I find that there is rarely more than 0.01 of an inch difference, even on central zones. I have completely refigured my 9" mirror, using this method, with the result that I can easily split double stars which were formerly quite hopeless. The whole point of the method is that you watch the shadow continuously and can easily determine the point at which it reverses, while the customary method requires the observer to judge a fixed appearance. It is the same difference as there is between a motion picture and a series of photographs."

FOLLOWS a second item by Wates:

"The contributions of J. H. Hindle and others in ATMA on the construction of diagonal mirrors for Newtonian telescopes leave little to be desired, but for the benefit of those who, like myself, hesitate to attempt the edging and figuring of an elliptical flat, the following considerations are submitted.

"Diagonal mirrors, are generally made either elliptical rectangular. In either case, the blank must be edged to size and then provided with a 'surround' of similar glass before attempting to figure the surface to a true plane. To these shapes may he added a third, viz., circular with straight edges. Let us see what is the loss of light due to the use of each of these shapes; taking a 12" mirror with a focus of 96" as an example.

"The correct size for an elliptical diagonal will be 2" by 2.83". The area of parallel rays intercepted will be that of a circle 2" in diameter, or 3.14 sq. inches. Assuming glass 3/8" thick, a rectangular diagonal will intercept a rectangle of rays 2" by 2.3", allowing for the protruding lower edge, which could, of course, be ground off if desired. This area will be 4.60 sq. inches, which is 1.46 sq. inches greater than that of the ellipse. The effective area of the O.G. being 110 sq. inches (approx.), there will a loss of light of about 1.3 percent, which is equal to masking a strip around the edge of the mirror .038" wide-or about 1/32"

"How about a circular diagonal? Again following for the squares, the rays intercepted will take the form of an ellipse, 2.83" by 2.30", with an area of 5.11 sq. inches. As compared with the elliptical mirror, this represents a loss of light l.8 percent, equal to a strip .052" wide around the edge of the O.G. As compared with the rectangular diagonal, the circular one causes a loss of light of only 0.5 of 1 percent, or an amount equal to grlh8~away 14thousandths from the edge.

"These differences are so small that one is led to the conclusion that the labor involved in constructing a truly elliptical diagonal is sheer waste of energy, and that a circular mirror which can be polished and figured without elaborate complications is perfectly satisfactory in all but very short focus telescopes. Certainly the difference in performance between a 12" mirror and one being a diameter of 11.948" is hardly justification for the work involved."

FIVE men in Providence, Rhode Island- Prof. C. H. Smiley, mathematician-astronomer of Brown University, Donald S. Reed, Harry A. MacKnight, Paul Eberhart and Frederick C. Hoffman-have made a 6" Schmidt camera shown in Figure 7. The tube is shown inverted, Hoffman (right) has his thumbs on the mirror cell, his little finger on the film supporting ring, and Eberhart (left) his fingers on the ring for the correcting plate. Figure 8 shows the special machine by which these workers solved the problem of the difficult, irregular, knock-kneed, bow-legged, psychopathological curve on Schmidt correcting plates. Asked to describe this creation Reed wrote:

"Mr. MacKnight designed and built the grinding and polishing machine for the Schmidt correcting plate or lens. The plate was cemented in the Bakelite ring on the turntable and turned slowly by motor. A fine emery wheel rotating at right angles was used in grinding, and a similar wheel of wood with pitch on its edge was used with rouge for some of the polishing. The handle at the end of the long screw for radial movement moves the grinding, wheel from center to edge of the lens. The depth of the cut is adjusted by a micrometer.

"The shape of the lens face was tested with a dial indicator measuring to .0001". Ring laps with No. 600 Aloxite and rouge were used for final grinding and polishing. As you suggested, we used a Borium tool to rough out the deep curve of the spherical mirror and its tool on a lathe."

In Figure 8, MacKnight is nearest the reader, adjusting the abrasive wheel of the machine, Hoffman sits behind, while standing at the back, with the f/1 mirror in his hands, is Smiley who has led the project- not as a professor but as an amateur. (How to designate in this column men of different vocations who make telescopes is often a problem. As amateurs in a hobby they are all equals and so they are all simply "Smith", "Jones", "Brown" and so on whether billionaires or paupers' old or young, doctors, professors or water boys which we believe to be as they would best like it.)

RESIDENT in China is C. N. Joyner 11 address "P.W.D., B.M.C, Tientsen", an American civil engineer who built the telescope shown in Figure 9 and with made photographs of the moon, and with these illustrated a book reviewed elsewhere in this number. Figure 10 is the homemade camera with which these lunar photographs were taken. It consists of a film-pack adapter connected to a Thorndyke shutter, the whole screwing into the eyepiece holder of the telescope. Joyner has two mirrors, a 10" made by himself and a 12" Hysil mirror made by H. E. Dall of England, each of 100" f.1. His own four previous mirrors were roughed out using oil instead of water. The telescope tube is of plywood; the base has a run off shelter. Beside Joyner in the photograph is his son Nicolas whom he describes as the "Gastronomer" or "Assistant Director of the Observatory in Charge of Gastronomy". (At about the same hungry age your scribe's initials were parodied within his family as "All Gone Inside".)

FIGURE 11 is an 8" made by C. E. Raible, Seavey Road. Milvale, Pa. The mounting based on an illustration of one of the designs that were proposed for the 200" telescope. Raible made the drawings, patterns, castings, and did the machining.

FIGURE 12 is a mounting by C. E. Mielke, 235 Princeton Avenue, West View, Pa. It is made of pipe fittings but not ordinary pipe fittings. Mielke describes it thus: "The castings are standard l-1/4" flanged pipe fittings faced but not drilled. This size is suitable for a 6" or 8" telescope mirror. For a 10" mirror 2" fittings should be used. As they are rough cast on the inside, the two tees are bored 1-1/2" deep at each end and bronze bushings driven in. This gives the shaft a bearing at each end and also the large surfaces of the flanges are in contact, which makes for smooth operating and rigidity. The polar axis is a drive fit in the tee, so that the shaft turns with the tee."

YEARS ago W. H. Pickering and others described mysterious and periodic changes in size, shape, and color of certain lunar markings. Amateurs watched these but in later years observation largely lapsed. Now three excellent summaries of this subject, in the June-July number of Popular Astronomy (Northfield, Minnesota), bring it to the front again, and definitely describe observations that amateurs can make with plain equipment. Who knows-there may be marigolds on the moon, after all!

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.