"THE mirror is a Chance disk, 3-1/2" thick, with a focal length if 120". The aperture is 30" (f/4). It is floated on a system of triangular supports, contacting the underside of the mirror at 18 points. [Hindle's well-known system, described in the book 'Amateur Telescope Making,' page 229.-Ed.]. A cross-section of the mirror cell, with reference to the edge support, is shown in Figure 1. The annular ring, R, is divided into 12 segments, each of which has vulcanized fiber inserts, I, which come against the mirror. Two whole fiber rings are turned, dovetail shape, then sawn into 12 sections, and forced circumferentially into position. Each segment of ring, R, is forced by means of two screws to its final position against the inclined surface, S, and in this position the fiber inserts are bored out a few thousandths larger than the mirror itself. In this way an adequate edge support is provided, and it is impossible to pinch the mirror by tightening the screws. When these are released, however, and the segments backed off or lifted out there is ample clearance for the extraction of the mirror.

"The tube is constructed from solid-drawn steel conduit, such as is used for electrical wiring purposes, and the lengths are threaded through circular rings of heat-treated, high tensile aluminum alloy. These rings are securely clamped to each tube by means of a taper fitting such as is used for attaching ball bearings to a straight shaft (Figure 2). The diagonal bracing is of high tensile steel wire, which is pulled taut by means of screws at the bottom end of each tube, near the mirror mount. A perfectly rigid tube of minimum weight is thus obtained.

"The four arms of the spider carrying the diagonal are of phosphor bronze sheet, mounted on the extreme end ring, with provision for concentric adjustment.

"The diagonal mirror is 7" minor axis, and about 1-1/2" thick, mounted in an aluminum and brass cell which has a tubular stem, so that electrical means of slightly raising the temperature of the back side of the plane, to avoid dewing, can be adopted, as described in 'ATMA' by Dr. Steavenson.

"The primary mirror is not perforated. It is collimated with reference to the brass sleeve which forms the center of the spider returning its reflection precisely concentric with a ground spot, exactly in the center of the mirror itself. The eyepiece support can be readily detached from the tube if necessary, and there is a considerable range of adjustment longitudinally. Being supported at three points, the final adjustment-that of setting the eyepiece tube exactly on the optic axis, and parallel to it- is comparatively simple.

"The polar axis is built from a 5" steel tube, with 1" thick walls, and the ends plugged solid. It is supported in ball bearings, one immediately below the fork, and the other immediately below the worm wheel. The general design of the mount latitude of 51-1/2 degrees is clearly seen from Figure 3, the fork itself being a substantial on casting of 'U' section, split where slips on the top of the polar axis, to which it is firmly attached by compression bolts. A segment of a worm wheel the trunnion of the tube enables the escape to be adjusted in declination, through the medium of a worm and spur gear, with a flexible joint to a broom handle within reach of the observer. When the inclined steel handle and its nut are released, the tube can be moved to any approximate position by hand.

"The drive to the polar axis is possibly to some extent novel. The worm wheel on the polar axis is definitely fixed to it the usual clutch being dispensed with. That enables the wheel itself to be more precisely mounted, and the engagement of the worm and wheel more definitely adjusted. The primary drive is by gramophone motor, through a 60-to-1 worm gear, which, in turn, through a friction driver rotates the main worm shaft and the wheel on the polar axis, the latter reduction being 360-to-1 (Figure 4). Particular attention is directed to the small worm gear, which is carefully assembled and runs in ball journals to reduce friction to a minimum so that the gramophone motor is not overloaded. The pitch of worm and wheel is 10 D.P., that is /10, and the pitch-circle diameter of the worm is 0.75", so that the angle of the thread is 7-1/2 degrees, which practically gives irreversibility. The compression springs on the friction drive enable the necessary power, with an ample margin, to be transmitted to the main worm shaft.

"The gramophone motor is to be independently mounted so that no vibration or electrical hum can be transmitted to the telescope. At a speed of about 60 r.p.m., the normal speed of the polar axis is secured, and this is capable of permanent and delicate adjustment. Energizing a small A.C. electro-magnet entirely releases the brake on the governor of the motor, and allows it to run at top speed. This occurs when the accelerating button is depressed, to overtake an object in the field of view. The retard button simply interrupts the gramophone motor circuit, allowing the object to overtake the telescope.

"Rapid adjustment in right ascension is effected by means of a separate 1/4 h.p. motor, which runs in either direction as desired. Due to the greater power of this motor' the friction drive loses control, and the polar axis makes a complete revolution in about a minute. Owing to the limitation of power that can be transmitted by the friction drive, and the practical irreversibility of the small worm gear, the gramophone motor is quite unaffected by the rapid adjustment taking place, and continues to run on as usual. Immediately therefore, after the 1/4 h.p. motor is disconnected, the friction drive instantly resumes its functions without any lag or loss of time whatever. During normal working the 1/4 h.p. motor rotates idly, very slowly."

Weights of individual parts of the instrument are as follows:

Mounting, including fabricated R. S. C. foundation, baseplate, polar axis, fork, bracket, and footstool castings 2399 lbs

Tube with flat and its cover 577 lbs

Cell and mirror supports 385 lbs

Mirror 196 lbs

Total 3557 lbs

This ends Hindle's description. Reader of "ATM" and "ATMA" will recognize him as a co-author of those books. By vocation he is a manufacturer of looms-those very heavy ones of 30 tons or so such as are used in weaving the dryer felt of cotton, thick and wide, used by paper makers. The same readers will recall the picture of a 30" mirror on a grinding machine, on page 245 of "ATM" (fourth edition, of 1935): this is the same mirror. Hindle enjoys making telescopes. Having made them, he gives them away! However, lest about 10,000 readers obey that impulse to rush applications for free 30" reflectors by cable, we hasten to add that he hasn't exactly given away his two large telescopes, a previous 20-1/2" and this 30", in quite that way. He has, however, commendably arranged to place them where they will work hard for astronomy. The 20-1/2" (see its photo in "ATM," page 453) has been in the hands of Dr. W. H. Steavenson, a prominent English variable star observer, more widely known by hundreds of astronomical friends in Britain and America as "Steave," who contributed the chapter on dewing of optical surfaces, in "ATMA." The new 30" is being mounted in the grounds at Cambridge University Observatory by arrangement with Sir Arthur Eddington, where it will be available for the use of the Professor's staff. Later Dr. Steavenson hopes to be in a position to make regular use of the newer instrument himself.

LAST month in this department there was a long article by Hendrix and Christie, of the Mt. Wilson Observatory, on the Schmidt camera, and now we have the following comments on that article written by Prof. C. H. Smiley, Director of the Ladd Observatory at Brown University and a leading exponent of the Schmidt camera:

"It is with some hesitancy and considerable distaste for the task that this criticism of the article by Hendrix and Christie in last month's Scientific American is written. I am deeply indebted to Mr. A. H. Joy of the Mt. Wilson Observatory and to a lesser extent to Mr. Hendrix for valuable assistance and advice in 1937 in connection improvements made on the Ladd Observatory Schmidt camera which had just been completed. I am well aware that Mr. Hendrix has made more Schmidt cameras than any one else on earth. If I were asked where the outstanding authorities in astronomical optical work were located, Mt. Wilson Observatory and Hendrix and Christie would come first to my mind.

"Even so, one of the sentences early into the article by Hendrix and Christie seems to me to be distinctly unfair-'Several articles have been written about the Schmidt camera since the inventor set forth its in principles in 1931, but little that is new has been included in these discussions.' It is true that Schmidt was a genius and that he did state most of the important facts in his original article. However he did not give a mathematical design for the correcting plate, he did not give any method of testing nor was his method of making a correcting plate revealed to the public until after his death. These important deficiencies have been taken care of by papers by F. B. Wright, B. Strömgren, H. A. Lower, Y. Vaisala, A. DeVany, and others.

"One might overlook a single ungenerous remark in the early part of the article but a considerable amount of material presented in the latter part of the article has previously been published by others, yet the article does not indicate this. A recent letter from Christie suggests that Hendrix and he knew all these things before others published them. I do not question the veracity of this statement. However, priority in scientific matters is usually established by publication in a journal generally avail able to others. On this basis, F. B. Wright and B. Strömgren should receive credit for their publication of the mathematical design of the correcting plate, and other acknowledgments of priority in publication should have been made.

"In brief, I feel that the ungenerous and misleading statement concerning earlier publications and the failure to mention the people who have made information on the Schmidt available generally are unfortunate, particularly so in view of the fact that Hendrix and Christie have had most of the information presented in this article, for the past five years and have withheld it from publication during that interval."

RULE for finding sagitta of a mirror, expressed directly in sixteenths of an inch, offered by Joseph Dwight, Hyannis, Mass.: Divide aperture by focal ratio.

To measure depth of sagitta on a mirror while roughing out, Lloyd Anderson, 11909 Vincente Blvd., West Los Angeles, Calif., puts a small drop of soft pitch at the center and places a steel straight-edge across the mirror, pressing the pitch down. Then, with a calipers, he simply measures the thickness of the pitch. Automatic feeler!

FOREIGN telescoptics is one of the hobbies of R. L. Beardsley, 2515 W. 21 St. Los Angeles, Calif., who reads about 769 foreign languages. He says G. O. Bjordal, Box 111, Askim, Ostfold, Norway, is one of five TNs in that town who would like to receive photographs of American TN's telescopes. They read English.

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.