Right photo (Eeny): "Templets and tools used in making the lenses for the two eyepieces shown," Taylor states. "Templets were machined to .001" curves on tools are equally accurate. The field lens is of Chance Bros. crown and the smaller lens is a cemented doublet of barium crown and flint. First class performance from both eyepieces."

Meeny: "From left to right: A block of glass from which two cylinders have been ground; the trepan that did the grinding; a cylinder of glass 3/4" in dia. x 1-3/32" long, ground out in one hour without forcing; six brass tools for grinding curves, and the finished Hastings type D eyepiece. In the foreground are the precise templets with which to check curves. The background shows a group of drill-rod tools with holder, used for fine thread cutting and work on small parts."

Miny: "A 1-1/2'' eyepiece completely disassembled. The 'skirt' is steel (less likely to scratch from brass adapter tube). The four cell parts are of tobin bronze. Smaller lens is a cemented doublet of barium crown and flint."

Mo: "A 3/4", type D Hastings eyepiece completely disassembled. The first two parts are the cell which holds the lens. Next is the complete lens, showing the tiny cap lens which is cemented to the small end of the cylinder. Then comes the front or eye end made of tobin bronze, and at right the skirt of steel. All parts are precisely centered."

Figure 2, also by Taylor, shows his drill press setup for grinding cylinders from a block of glass. "Note the dam of wax which forms a well to hold the abrasive mixture," he writes. "This arrangement keeps the trepan from overheating, reduces the necessary grinding time to one third, is economical on abrasive and makes a cleanly performance, as can be seen from the photograph. This was taken immediately after releasing the cylinder, and with no cleaning up except to wash the cylinder. The latter shows in the foreground, washed free of the grinding goo. Tools for grinding the curves of a Hastings D type lens are shown lined up, in order to include them in the photograph."

IN one chapter of "A.T.M.A." Dall of England describes the construction of a camera obscura, consisting of a movable flat projecting above the roof of a house and feeding downward into an objective lens which throws its image vertically down on a viewing table in an attic. Figure 3 is a terrestrial photograph made through this instrument. "I was trying," Dall writes, "to take an infra-red photo of a little church which stands up in the sky-line 17 miles from here, in Buckinghamshire. I have got best results so far with visible light and a red filter instead, because of the too lengthy exposure with infra-red, also because of enhanced diffraction effect of the longer wave. Infra-red long-distance photography tends to defeat itself, as far as detail is concerned, because of the reduced resolving power of the long wavelength. A 2" telescope will give as good an image at 5000 A.U. (average of spectrum) as a 4" telescope at 10,000 A.U. (infra-red), although the infra-red does penetrate atmospheric haze. Objective diameter 4-1/2", plus eyepiece, plus enlargement. Red screen an Ilford Panchro plate. Exposure 2 minutes (This house won't 'stay put' for a long exposure. A temperature difference of a degree or so will cock the house over enough to give a band image of a line at this big magnification.) at 8 P.M., last July. Beclouded sun immediately above and slightly to right. E.f.l. of final result about 1400" or 115'. The curve drawn on the photo show the size of the setting sun or moon at the same scale. The little drainage trough just visible [It shows on the original but not on the half-tone, hence we added an arrow.--Ed.] sticking out of the distant tower would be about 6 inches across. It is a coincidence that my viewpoint shows two churches, on distant 17 miles, the other 5-1/2 miles, within 1 minute of arc of one another."

Foreground objects in Dall's photograph, under-exposed as he explains, are rather dim and the half-tone reproduction renders them still more so. On the original the details of the foreground, consisting of treetops, are visible, and the picture has considerably more life than in the half-tone

Any readers of "A.T.M.A." who may be planning camera obscuras will gladly be hooked together by ye scribe if they care to send in their names-a sort of "Camera Obscura Club" would thus be formed. It is also expected that the publication of Haviland's detailed instructions for making objective lenses by something more like professional methods than any instructions previously available, will result in a trend in that direction, as "A.T.M.A." has already reached more than 1200 persons, assuming but one reader to a copy-quite an assumption, for some copies are known to be read by a dozen amateurs.

THOSE of us who observe in the East, or even in parts of the West, will envy Clyde W. Tombaugh ("A.T.M A.", page 639) of the Lowell Observatory in Arizona, where seeing is really seeing. He writes:

"I have been having some fine views of Venus, Mars, and so on, through my 12". On the 5th of March, during the early twilight of the evening, the seeing was ripping good -No. 8 on a scale of 10 (10 giving a perfect diffraction pattern for a 5" object glass) and I finally put on a power of 742. The image of Venus was huge, perfectly sharp, clear cut, and there was scarcely a quiver.

"On the morning of March 11, I had some No. 6-7 seeing with my 12", power 300, and saw the minute northern polar cap (now at minimum size) of Mars well; it was only about 1/2 by 1/4 second of arc, down at the N. limb of the planet. I saw the oasis Isminii Luci, cloud areas, Dawes Forked Bay, Syrtis major, portions of the Protonilus and Deuteronilus "canals", and glimpsed the Hiddekel and Gehon "canals"! The colors of the markings came out beautifully in the morning twilight. At that time, the angle subtended by the disk of Mars was only 9.8 seconds of arc (equal to that of a 3" ball placed a mile away) Isn't that enough to thrill anybody? At moments of very good seeing, the whole disk of the planet came out very hard and sharp (when is when one sees the canals).

"The time to really see things on planets is during the twilight, both morning and evening, when there is a lull. We find that our seeing in regard to steadiness is generally 2 to 3 points lower during the middle part of the night. Soon after the sun gets up in the morning the seeing begins to dance again. Most people think they should wait until it is good and dark before looking."

THERE is evidently much interest in the 1 Richest-field Telescope described in the final chapter of "A.T.M.A." The only misprints yet found in this book which could really cause trouble are in that chapter. On page 635, bottom paragraph running over to the next page, please change the capital letters M in five places to m. On page 637 line 2, make log delta sub m read simply log delta, and in line 3, make delta m/a read delta over a squared. Of less importance, on page 140, line 2, change "opposite" to "preceding." On page 155, third line, change plano-convex to plano-concave. Page 215, Figure 1 was accidentally turned on end. Page 334, shift footnote star in line 5 to page 335, line 34, after "l4". Page 348, line 11, change "remaining" to "ramming." Page 583, line 18, insert after "image," "as full as the eye can accept." Please report any other errors found.

AMATEURS who collect practical data on telescoptics (how do you like that word?) may wish to obtain a pamphlet entitled Abrasive Grain Sizes. Write Supt. of Documents, Washington, D. C., and ask for R118-36.

AUTOMATIC guiding again: In August 1934, James T. Barkelew, of Los Angeles, described his attempts to solve this intriguing problem and last January Wilbur Silvertooth of Long Beach, California, described the efforts he too has made. Further efforts toward solution of the same problem are now outlined in the March number (Vol. 8, p. 78) of the Review of Scientific Instruments (Lancaster Pa.) by A. E. Whitford and G. E. Kron, of Washburn Observatory. University of Wisconsin, Madison, Wis. The following is from their article:

"The principal difficulty in designing an automatic guider for a telescope is the extremely small amount of light available to actuate the mechanism. Most photo-electric control devices operate on 10^-2 to 10^-3 lumens, but an automatic telescope guider must work on 10^-9 to 10^-10 lumens to be successful.

"The plan of operation of the automatic guider in its present form is show schematically in Figure 4. The control is exercised in one coordinate only. The similarity with the original design of the Hardy color analyzer is evident. A 90 degree roof-prism aluminized on its upper surfaces is set in the focal plane of the telescope and divides the light of the star into two parts. These two beams are reflected by mirrors, pass through converging lenses and are reunited on the sensitive is surface of a photo-electric cell. A light-chopper nearly in the plane of the lenses occults the two beams alternately. If the star image is not centered exactly on the knife-edge, unequal amounts of light will pass through the two lenses and there will be an alternating intensity on the photocell. The alternating voltage thus generated is amplified many times and applied to the armature of the correction motor. The field of the motor is supplied with current of the same frequency from a commutator on the same shaft with the light-chopper. The correction motor drives the telescope or a sliding plate-holder in the focal plane in such a direction as to correct the error. When the star image is centered exactly on the knife-edge the two beams are equal, there is no flicker and the motor stops. When the star image moves from one side of the knife-edge to the other there is a change of 180 degreesin the phase of the armature current with respect to the field current and the motor reverses its direction of rotation.

"The advantages of this system of control are: (l) Because there is only one photocell and because a.c. amplification is used, the balance condition is dependent on geometrical considerations only, and is independent of drifts in steady-state currents in vacuum tubes and changes in cell sensitivity. (2) The control is exercised entirely by electron tubes without any mechanical relays. (3) As long as the error is less than half the diameter of the star image (and an error larger than this is certainly beyond the limit of tolerance) the correcting impulse is proportional to the need for it. As the balance point is approached, the motor current gradually goes to zero. This permits rapid correction of an error without the oscillation about the mean position common with simple on-and-off methods of control.

"This arrangement was tried out on the 10" photographic telescope of the Mount Wilson Observatory, and later at the Cassegrain focus of the 60" reflector." The results are shown in three star photographs not here reproduced but the authors describe them as follows: "In all three cases the guider corrected a cumulative error in right ascension and produced round star images of about the same diameter as 1-second exposures of bright stars.

"In the preliminary tests of the guider's usefulness made thus far it has been considered sufficient to confine the control to one coordinate. In order to have control in both coordinates, the outfit would have to be duplicated. Two knife-edge prisms set at right angles could be used, with separate guiding stars for each, or the light of a single star could be divided by a half-silvered mirror. Perhaps the best system would be to use a four-sided reflecting prism."

The four-sided prism method was the one proposed by the amateur, Barkelew. Just how far this latest bit of work, following the work previously described by amateurs, has gone in the direction of a final solution, cannot yet be said, but the results appear to be promising.

IN April, 1936, we showed how W. B. Hiner, 123 Cleaves Ave., San Jose, Calif., attached a simple "Barlow" lens fixedly to the end of an eyepiece. Figure 5 shows a later arrangement of his which has variable magnification. As the largest lens is moved toward the diagonal the eyepiece must be moved away from it, as provided by his mechanism. Such a lens is, of course, not achromatic and with higher powers it gives pronounced color on the planets, but this is less noticeable on the moon, he states.

Answering several inquiries about achromatic Barlows, J. R. Haviland, author of the chapter on objective lens design in "A.T.M.A.," says: "Use same formula as for achromatic O. G. Start with focus, say, minus 8 or 10", take account of signs in formula, ~ and results will give foci of two components on from there same as for positive lens. When used as a Barlow flint is first, that is, away from the eye and toward the sky."

FIGURE 6 shows what James W. Lillico, 10 Bayley Ave., Yonkers, N. Y., thinks a perfectly good bathtub is for. "Living in an apartment and having a wife," he writes, "I could not see the possibility of having a barrel full of stones parked in the kitchen." Years ago, when R. W. Porter took up mirror making, that is what he did and Mrs. Porter says he tracked a heavy, red ring of rouge around a pedestal on her kitchen floor. Once his rouge was accidentally mistaken for paprika and shaken on baked potatoes.

AND now comes a note from Russell W. Porter, whom we have frequently begged to send us, for this department, any homely little scraps of information or practical sidelights on the progress of the 200" at Pasadena until it is finished. "In order to show members of our fraternity the care exercised in safeguarding the 200" mirror from scratches, I am sending you a sketch (Figure 7) taken at the wicket door where all who enter the large room are required to remove their shoes and to don rubber sneakers. Before this rule was laid down, sweepings from the optical shop floor showed innumerable pieces of steel chips that had found their way into the 'holy of holies,' clinging to the soles of shoes of workmen- or visitors-who thus unconsciously imperiled the surface of the big disk.

"As the 200" telescope is the No. 1 exhibit here on the campus now, the visitor's gallery is well patronized by tourists from all over the states."

The sign on the doors reads: "No Admittance. Please do not open these doors until the outside doors are closed."

WHAT TN can read Czech? Each month the Czechoslovak Astronomical Society sends us a neat little journal named Rise Huezd, but we can't read it. One page is evidently devoted to amateur telescoptics, some of which may call for translation and presentation here but it's all Czech to us.

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.