Figure 1 shows a typical measuring engine of this kind, a "coördinate comparator" made by Gaertner for professional astronomers' use. First; the carriage as a whole is slid up or down the sub-base, to place the microscope in a comfortable position for the user. On this carriage is mounted the circular stave, showing clearly in the figure. Within it but obscured by other parts, is the plate, set in a square recess. This stage, its periphery divided into angles, may be rotated by means of the little hand crank shown, and it is rotated until the two star images successively a fall under the sliding microscope. The scales are read at each setting and then the angles and position are determined from the readings. The microscope is mounted on a slide and runs on a long, precise screw beneath the slide, a screw having in this instance a if periodic error of less than 0.001 mm, or about 1/25,000 inch. On the left end of this screw is a drum with a scale and this gives direct readings to 0.001 mm. The microscope reticule consists of two hairs set parallel and one at right angles to these. Angular separation of the stars may be calculated because the scale of the plate is known or may be computed. However, such an apparatus, costing as much as a fine car, is out of the amateur's reach. The plate measuring engine shown in Figure 2 was therefore made by Lawrence A. Cox, one of the Cox brothers living at 47 Upper Green, Mitcham, Surrey, England, whose telescopes and observatory have previously been described here, and is described by Harold W. Cox. They have omitted the circular stage, and they measure by means of a two-stage carriage arrangement, obtaining rectangular coordinates, probably working out the hypothenuse by trigonometry.

"Basically," Cox writes, "the instrument consists of a frame or plate holder 1, to hold the negative or plate, which can be moved accurately in one direction along round rails 2, 3, by means of the fine adjustment provided by micrometer screw 4. These rails are made from lengths of l/2" silver steel rod purchased in very straight lengths. Motion at right angles to this is made possible by allowing the microscope 5 to slide on round rail 6 and flat rail 7, it can be moved more accurately by means of micrometer screw 8. The microscope not alone travels on round rod 6 but is hinged on it also, and may furthermore be swung back out of the way when required thus

"The micrometer screws 4 and 8, of which 9 and 10 are the extensions, are ordinary heads from L. S. Starrett metric type machine micrometers and permit an adjustment range of 1/2"; actually, only 1/3 cm of movement is used. Extension 9 is kept forced against 4 by a spring thrust, 10 against 8 by gravity. When measurements are made between star images more than 1/2 cm apart we make use of a glass reference plate or reticule 11, ruled with fine lines, which divides it up into sections 1/2 cm square. The plate is a photographic copy of a master ruled plate at Greenwich. Both the microscope and the plate holder can be released, from the threaded micrometer extension moved in position to bring star and microscope coincident, and then locked in position by milled-head clampscrews 12, 13. Position is then read by counting off the lines on the reference plate or reticule and interpolating the rest by adjustment of the micrometers. The use of the reticule serves to reduce errors which would be cumulative if a long micrometer thread were used instead of the 1/2 cm movement, also errors caused by curvatures in the rails that carry the moving parts.

"Finally, the microscope itself has a cross-reference, in the form of very fine scratches on glass on a piece of microscope slide cover made with the sharp edge of a piece of broken quartz rod. Even after several such scratches have been made with the same edge the scratches obtained are finer than those obtainable by most other hand methods. Cross-wires would be much too coarse as this apparatus has a direct reading accuracy of 0.0001" (inch) on the micrometer scales and, by interpolation, to 0.00001".

"The reticule is always in position and the photograph for inspection is laid on top. The two are illuminated from below by a lamp in a white enameled base. Looking through the eyepiece of the microscope one can see the cross-lines in the microscope, the star images, and the lines of the reticule, all in focus at the same time. Sometimes the cross- lines in the microscope are removed and a fine metric scale engraved on glass is used instead. The lamp in the box 14 is so arranged that direct light does not illuminate the plate, but reflected light from the white walls gives an even illumination over the whole plate. As shown in the photo, the plate holder takes 1/2" plate size, but with an extra fitting dropped into position any smaller plate can be measured.

"Owing to the fact that any other person making a plate measuring machine would have his own ideas about details I don't think it is worthwhile including them here."

Regarding this, we agree with Cox and the author of page 84: few wish to copy details slavishly.

A SCHMIDT camera (Figure 3) having 9" mirror and 6-1/2 " correcting plate (hence f/1.5) has also been made by the Coxes, and stellar photographs taken with it and submitted to Lower, of San Diego, California, for opinion elicited the significant response, "the Coxes have made a Schmidt!" The images were remarkably fine and round.

"Now follows the sad tale of a self-inflicted joke on this department. A little later the two Coxes sent the photograph shown in Figure 4. On the same day came Professor Russell's manuscript for his September article entitled "How Hot is the Sun?" Calculation showed that something was needed to make it fit the usual two-page space-what better filler than this Cox picture? Study closely the familiar solar rice grain background and all its other details. The picture was inserted and given the legend "Sunspots photographed with a 12" reflector, by L. and H. Cox." The present reproduction is from the same half-tone plate that was shown in September.

Well, as it turned out, the "sunspots" weren't quite that-not precisely. Instead, they were a 140 times enlargement of two star images at the edge of a 16 degrees field taken with the Schmidt camera shown in Figure 3! The photograph had been sent to demonstrate the quality of the Schmidt. Note particularly the absence of coma in the round, clean-cut star images. However, there were signs of coma elsewhere for your scribe went straight into a coma, especially when it turned out that the photo had been correctly described in the original Cox letter. Hereafter, this department is wearing two pairs of specs.

Several months have elapsed since the misnamed photograph was published. No reader has written the editor to apprise him of the error. No doubt all you readers noticed it, of course! For kindly forbearing to write me about it all are thanked. Their innate tenderheartedness has been quite touching.

So your scribe's face is red, red, red, almost infra-red, and now at sight of a sunspot picture he simply breaks down and sobs.

MORE recently the Coxes have sent Figure 5, showing their No. 2 Schmidt job, an f/1.5 with a 12" f/0.75 primary and a 6-1/" correcting plate. It covers a full 20 degree field. The cell for the correcting plate is all metal and alone weighs 120 pounds. Two doors, similar to those in Figure 3, show slightly on top of the tube; these bed down on rubber seatings.

To the Journal of the British Astronomical Association, Vol. 48, No. 8, the Coxes have contributed a 5 page article describing their work in figuring the correcting plate and making the film holder. Too long to reprint here, your scribe will lend it for a brief time to any who seriously aspire to make Schmidts.

FOURTH Cox item is a chronograph (Figure 6) "for use in our observatory," as they describe it. "We experimented with various forms of recording, one of them being that of burning holes in the paper by an electric spark. The voltage of the ordinary house supply was stepped up by a transformer, about 700 volts proving best for the paper we were using, and one side of the output was connected to a platinum point fixed to the armature of the magnets and the other side to the metal track over which the paper runs. The impulse from our clock, a Synchronome having an accuracy of about one second per month actuated the magnets, bringing the platinum point into contact with the paper, which was not sufficient to act as an insulation from the metal track.

''However, '' they continue, ''this method was abandoned, as the sparks made interference for the radio used for giving clock checks against Greenwich. (This we did by connecting the radio receiver in place of the personal switch and then comparing recordings of our clock with that of Greenwich, and by this method we can check to an accuracy of about 0.1 seconds every other day.) We are now recording with the two siphons shown in Figure 6, fed from a small tank of ink fixed to the inside of the track. One of these is fed from the half-minute impulses and also from the personal switch''.

PRINCIPLE of a simple micrometer for measuring angular diameters, also for relative measurements of detail on Jupiter and other planets, is shown in Figure 7, redrawn from a sketch by F. L. Frazine, 1016 17th Ave. N., St. Petersburg, Fla. Adapter tube for connection to telescope. Brass plate soldered to this. To that, in turn, is soldered a bracket to support eyepiece, the eyepiece alone being sketched in on the drawing. Rider on plate.

Lap one edge of plate straight. Drill 3/4" hole in center. Cement silk fiber or spider web across hole and at small angle with straight edge of plate. Second fiber is cemented to copper wires shown, and adjusted parallel and precisely over the first by bending these wires. At same time rider is at right-hand stop. "As I have constructed it," Frazine says "the full scale reading equals 50 seconds and the scale is in 25 divisions. I find the micrometer is accurate to about 1 second of arc. To calibrate it, I timed a star from web to web when the webs were set wide apart.'

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