"We did not make the objective lens. The objective is of 8" aperture with a focal length of 130". The tube is seamless and made of a magnesium aluminum alloy 1/8" thick, which makes it light and rigid. The fittings are of bronze.

"The equatorial is made of steel, fabricated by the electric arc. Both the declination and polar axis shafts are carried on Timken precision bearings. The circles are bronze, 16" in diameter. The hour circle is graduated to l^m and the vernier reads to 5^S. The declination circle is graduated to 15' and the vernier reads to 1'.

"The equatorial is mounted on a 10" extra heavy pipe which goes into the ground about five feet in about four yards of concrete. There are provisions in the head for adjustment both ways for setting the polar-axis.

"The electric drive, which is of our own design, consists of a box made of 1" cold drawn steel 7" by 7" by 15", with two partitions, making three compartments in which are mounted in a series three 40-to-1 worm drives, and one variable speed friction disk, which are mounted on Timken bearings.

"The power is a 1/8 h.p., 1800 r.p.m., 1-phase synchronous motor driving the first worm reduction. There is then interposed the variable speed friction disk. The driver is a steel disk held in contact with the edge of the driven Formica disks by spring tension. The driven disk slides on its shaft to give the variation in speed, and is controlled by a graduated dial on the outside of the box. All the gears run in oil.

"On the last worm gear shaft, which extends out through the end of the drive, is a bronze drum with a half round thread cut on it of the same pitch as the diameter of the Monel cable, which runs up over a grooved sheave on the worm shaft on the equatorial, and then down through the column, with a weight on it. The drum has a ratchet for winding the cable on it, and holds enough cable for about 4 hours' running.

"You will note from the photograph that we have a polarizing eyepiece for solar observation. This screws in place of the regular eyepiece holder. This we made from aluminum castings, mounting four black glass flats inside on a 57 degree angle.

"Another thing is a sidereal clock which we made, a picture of which I am enclosing. We bought a No. 68 Seth Thomas movement, took off the train of gears which moves the hour and minute hands, and cut another set of gears with a compound gear 1-to-2 for the hour hand drive. These we mounted on a brass plate which we screwed to the front frame of the movement and which gave us a 24-hour clock. The face is aluminum and the figures and graduations are filled with black Duco. The case is arc welded steel sheet with screwed bronze doors, which makes it practically dust or air tight."

JUDGING from letters received, also from your scribe's own recollection, most beginners find that the knife-edge test leaves them with severe headaches from eye strain. For the first year or so this test is generally nerve- wracking, after which it seems to flatten out so that looking at the shadows as long as one wishes to do so is no harder on the eyes than looking at the scenery at a bathing beach or at a pound of butter. Perhaps the following hints will help:

The first thing is to place one's self in a completely comfortable, relaxed position, and get into a placid, limp, easy-going frame of mind. To make physical relaxation better possible, find out the height of your eye when sitting in a slouched position in a chair, and then place the test stand at the same height. Sprawl out your feet and arms when testing, and have your head well back, with no knots tied in your neck-relax. If possible, rig something for your left elbow to rest on, while manipulating the knife-edge, and have it at "just right" height, but this rest ought not to be on the same base as the knife-edge. Adjust the mirror so that the cone of returning rays is high enough above the table or board on which the knife-edge rests to permit you to get your chin on top, or else have the knife-edge come so near the edge of the table that your chin is not a factor.

The worst source of headache and nervous tension is trying to close the other eye when making the test, just as it is when using a telescope or microscope. Leave the other eye entirely open, as all microscopists and astronomers do; you will soon learn to forget or ignore the extraneous image on its retina-it is on the eye but not in the brain, as it were. Don't squint or screw up the eye you look with-just look with it as you would look off down the street, wide open, easy and naturally.

A third factor is psychological: At first you do not seem to see all you had hoped to see and, thinking that all others see everything, you become irritated at yourself and thus waste nervous energy-like a bunkered golfer. It may, therefore, help if you are told that no one sees everything in the test at first and, in fact, mighty few ever do become well enough trained to extract all the juice from it. Insight comes gradually, hence you should not expect to pick it all up, or even a quarter of it, within a month or so. You will see enough to work with, after playing with the test for a few sessions, but the education of eye and brain does on practically forever.

If you are still doubtful about all that has just been written, as applied to your own case, perhaps it will encourage you to hear that the writer had all of these trouble for months, and then none of them.

A YEAR or so ago we decided to find out, if possible, about the matter alleged local figuring of optical surfaces by means of the heel of the hand, the thumb or the fingers, which some said was a myth and others said was a recognized method the old-timers. We referred the question to a professional, and he told us the tradition was based on a myth, and we believed his stating that belief in this department. Pretty soon, however, we began receiving reports from various amateurs who were actual doing it. First it was Clyde Tombaugh, who has his name "writ large" on Pluto. He wrote, "One's thumb is an excellent tool rub glass. The method is supreme for central hills." Then came S. H. Sheib of Richmond, telling us he used the middle finger of his hand and that the method worked well-too well, in fact, until he found that a little of it went quite a way. Next, Wallie Everest said he used the method regularly-fingertips, in his case; following which J. H. White of Cranford, N. J., mentioned his recollection of many years a watching Alvin Clark working on a lens with a very broad thumb.

It began to look as though the ayes have it, and so we tried the method on a mirror which had a raised ring in the outer zone: mirror inverted, thumbs dipped in rouge mixture, and worked on the zone with short strokes as the pedestal was circumnavigated, knuckles used as a gage and control of distance in from edge. The zone came down and the method proved to be capital. Thus another fine theory is blown to ribbons by a mere fact; it may be true that a live meat local polisher ought not work, but apparently it does. One thing remember is that a little of this method goes a long way-otherwise your raised zone becomes a valley and your central hill as well. A little preliminary practice and gradual "feeling out" of the method will reveal its safe limits before any serious damage is done.

Horace E. Dall, 166 Stockingstone Road, Luton, Beds., England, submits the following suggestion for a direct method of measuring the angular diameter and physical appearance of certain stars. "So far as the writer is aware," he points out, "the following suggested method has not been put forward before and, although limited in application to about 10 percent of the sky, it may prove to be capable of giving information concerning the diameter and distribution of light on star disks unobtainable by any other known means.

"It is of course well known that the stars occulted by the moon apparently disappear instantaneously, due to their exceedingly small angular diameter. Actually, in the case of a certain number of stars, the time interval of occultation is by no means beyond the powers of modern electrical methods of recording. The method proposed is to direct the light from the star approaching the dark limb on to a sensitive photoelectric cell of the type which responds practically instantaneously to light variations. The small electromotive force generated would be conveyed to an amplifying unit employing thermionic valves, and the current from these would in turn be conveyed to a cathode-ray deflector and recording device similar to those employed for measuring and recording explosion pressures in guns. Alternately the current from the amplifier could be recorded photographically by an Einthoven string galvanometer having an extremely short vibration period or by an electro-magnetically operated vibrating mirror.

"The interpretation of the photographic record of an occultation (or reappearance) so obtained should present no special difficulty. Due allowance having been made for the inertia of the photo-electric cell and recording device, and for the eclipse of the aperture of the telescope, the remaining time interval and light curve will give a measure of the diameter and light distribution curve of the stellar disk.

"From a study of the theoretical occultation curve it does not seem unreasonable to assume that the method is capable of measuring stars down to an angular diameter of 0.0001", i.e., 100 times smaller than the limit of the 50-foot interferometer. This corresponds to the size of our sun at a distance of 93 parsecs (at which distance it 1 would appear 9.7 mag.) or to a star of Aldebaran's size at a distance of 7500 parsecs. It may be contended that irregularity of the lunar horizon will introduce errors; this is certainly true-but the probability of steep slopes is not great, and several occultations can be observed of each star. Furthermore, an analysis of the shape of the occultation curve may reveal slight irregularities of the lunar limb which can be allowed for. Another problematic point is the effect of our own atmospheric tremors; but the writer would suggest that this may not be a serious factor unless the total light received from the star is very rapidly and irregularly fluctuating.

"The possibility of causing artificial terrestrial occultations (from mountains and so on) is probably ruled out because of atmospheric tremors on both sides of the occulting plate. Although other planetary bodies would not be suitable for measuring star disks, owing to the nature of their surfaces, the method may be inverted and be made to supply information regarding the surface, or, for example, in the case of Saturn, the nature of the rings."

Here, then, is another chance for someone who is interested in electronics to work out something that will make him famous as a contributor to science. Three of our amateur fraternity are working on a similar problem in photo-electric telescope guiding-something which, if satisfactorily solved, will amount to a whole lot.

HERE is a note R. W. Porter recently sent in: "Dr. Anderson called my attention today to the fact that the depth of a telescope mirror is always equal to the distance between the foci of the mirror's central and marginal zones.

"For instance (in the figure) if knife-edge and pinhole are together at a" for an even cut-off of light reflected from around a, and are at c" for light reflected from c, then a"c" is equal to the mirror's sagitta ac'. This applies to all apertures or focal ratios.

"It arises from a fundamental property of the parabola, viz., that the subnormal for any point on the curve is a constant. Hence, in the figure, c'c" equals b'b", leaving ac' equaling a"c", or ab' equalling a"b".

"These differences of focal length in the neighborhood of the mirror's center of curvature, found with knife-edge and pinhole moving together, are derived from but if the pinhole alone is fixed (as is customary among amateurs) .

"It had never occurred to me that a fellow could find this quantity quite independent of the formula, by simply laying a straightedge on his mirror and measuring its depth. I had never seen the point alluded to in any book on applied optics."

THANKS to Yankee ingenuity there are something under 1,000,000 ways to cut out the facets on pitch laps. The one in the accompanying drawing is by William A. Mason, 1303 Lakeview Ave., Lorain, Ohio. He says, "Do not try to make the groove full depth at the first stroke. Pull this tool through the pitch rather than push it. Do it slowly. With this tool there will be no chips or flakes." In the redrawn sketch the lap thickness was inadvertently exaggerated.

ABOUT once a month, for years, we have received letters much like the following: "Will you kindly advise me the names of books or publications containing data on the theory and construction of spectroscopes, also on their operation." So far as is known, there is no such book, and we wish somebody would write one covering just that ground in an all-around manner. Physicists pick up their knowledge of the use of spectroscopes by word of mouth, from older physicists, not from anything in print. Of course, it may be that such a book wouldn't pay its way, unless a very high price were charged for it, but we believe many an amateur would lay down five simoleons for one if it were practical, understandable and interesting. A physicist we recently spoke to about the matter stated his belief that it would be possible to drop out the deep stuff (higher physics) which is so closely hooked up with spectroscopy, and still write a good book, but be was too busy to do so. The fact probably is that few who understand the use of spectroscopes (except manufacturers) could tell us how to make them. Maybe it would best be a two-man book, then, but we don't know the names of the logical candidates to write it.

LAST month we gave you a little advance dope on the forthcoming fourth edition' of A.T.M. Various things, including our annual vacation and a little attack of the tummyache, have held us up and, at present writing (Oct. 14), the second proofs have just been returned to printer. So it looks as if the book ought to be ready by another month. It will be no larger but certain out-of-date parts have been replaced by what we think is better stuff, a lot of small corrections have been made, and quite a few new notes substituted older ones.

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