"The tube is of the square plywood type, which I favor on account of freedom from air current trouble. The mounting uses an 18" face plate of rollers-very rigid and sweet running. I did the optics-all the lathe and instrument work, including the circle engraving, slow motion clamps, and so on. A local friend, Eric Perry, did the woodwork, concrete and cast iron work, patterns, and so on. The R.A. circles are 20" and the decl. 12" in diameter. The telescope is delightful to use-extremely crisp images-in and out of focus star disks dead alike. The figure was perfect to my measures and no scratches. A parson in Norfolk (Eng.) now uses it."

THE photograph in Figure 3 is that of Max Papkoff, 382 N. 19 St., Salem, Oregon, and his 8" square, wooden tube telescope. Mr. Papkoff states that he cemented a disk of 3/8" plate glass to a thick piece of wood and made his mirror thus. By all rules, laws and sacred traditions, such a mirror ought to be badly flexured, especially when the wooden backing warps due to intake of moisture. Yet he says it gives sharp, distinct star images with a half-inch eyepiece. Several amateurs are similarly known to be flirting with thin mirrors, backed and unbacked in type, and we hope later to publish a few statements from some of them (more are solicited).

This question may not be so simple as it appears. Perhaps it is not one to be settled merely by citing "rules." It is suspected that the thin disks sometimes stand up on their own account, regardless of supposed assistance from the wooden, metal, or glass backing to which they are attached either rigidly or the reverse. The 1:6 or 1:8 ratio usually demanded does, of course, allow for a big factor of safety. When a thin disk performs well, would it have performed equally well without the backing? Unscrambling the several complex considerations involved in this whole question, and making a really scientific analysis, would be a nice job for someone. No general deduction made from one, two or three or less than a dozen or so of mirrors made of different glass would seem to be very safe. Hence, a big job. A writer in the Journal of the Royal Astronomical Society of Canada, July-August, 1935, seemed pretty dogmatic about the matter, attacking the 1:8 ratio recommended in "A.T.M." and another Canadian has requested us to answer his article, "taking the other side." But we do not wish yet to take any side in this argument, for it is possible that there is insufficient data a yet to back either side very positively. This taking sides and arguing business too often obscures matters that ought to be handle with more light than heat. We need more light.

The wooden tube telescope (Figure 4) is hexagonal and was made by Ellsworth Mebold, a commercial artist, Marine, Illinois.

Recruits are wanted for the "Wooden Tube Club."

A FEW numbers back we suggested that someone work out a dingbat by means of which an old hand at mirror testing would coach a rookie, the two (or any other two) being able to watch and discuss the same shadows at one time. William Scott, Glascock Reynolds, and Louis Mobley, Westminister Drive, Atlanta, Georgia, sent in the photograph shown in Figure 5. It shows a typical test rig, plus a diagonal sheet of flat glass at a 45 degree angle, placed between mirror and knife-edge. The glass is very thinly silvered. (This is easy-just set out to get a thick coat.) These three complain that they have to place their cheeks "affectionately close" in order to see well. Their whiskers became tangled. Chester Silvernail, 5151 Bristol Road, San Diego, California, proposed a similar set up, to be combined with a Ronchi rig like the one in "A.T.M.," page 266, Figure 3.

THE following is from J. J. Ruiz, M.S., D.Eng., 1065 Park Ave., Schenectady, N. Y. "Many an amateur telescope maker, while walking around his barrel, must have speculated, at one time or another, about the size and shape of his Carborundum and of the pits and holes on his glass. Being an amateur microscopist as well as a telescope maker, I set myself the task of finding out, and here are some photomicrographs showing what I found. Figure 6 shows what the abrasives look like under a magnification of approximately 80 diameters. They are Nos. 80, 280 and 400 Carbo, and the last is No. 800 emery. Note that the emery does not show the sharp splinters and corners of Carbo which explains why the final fine grinding with emery gives a smoother surface. Figure 7 shows the appearance of glass under vertical illumination when ground with No. 80 emery, No. 280 Carbo, and No. 800 emery. The magnification is approximately 100 diameters. [For the time being. the photo in the lower right-hand corner may be ignored.-Ed.]

"Using the method suggested in a previous number [First focusing the microscope on the level parts between the pits-after a small amount of polishing-and then on the bottom of the pits, and measuring the distance between these two focal planes by means of a micrometer attachment.-Ed.] the depths of the pits left by the different grades of abrasives were measured. The microscope was provided with means for vertical illumination, and a dry 3-mm. objective was employed. The averages of several measurements give the following:

"The pits left by No. 80 emery are not proportionally so large as those left by 220 Carbo. This is probably because the large grains break down easily under pressure into finer grades. Note in this connection the appearance of the ground surface shown in Figure 7."

An attempt was made to correlate the figures which Mr. Ruiz gives for pit depth with those for abrasive size, given on page 493 of the new (fourth) edition of "A.T.M.' While more data on more sizes of abrasive would perhaps better define a rule, it was apparent that pit depth is at least approximately proportional to grain diameter. The big professional manufacturers have (secret?) data on this.

Mr. Ruiz continues: "When we come to rouge we find that it is in a class by itself. The lower right-hand photomicrograph in Figure 7 shows rouge under a magnification of approximately 800 diameters, or ten times that of the previous photomicrographs. The diameter of the rouge grains is about 0.7 micron (0.000026 inch) which is only a thousand times (more or less) the diameter of a water molecule. A suspension of rouge in water can be used to show the Brownian movements caused by the random impacts of the water molecules. Every amateur telescope maker who has access to a microscope with an oil immersion lens ought to perform this experiment."

MR. RUIZ next takes up the question of the actual nature of the polishing operation-sub-microscopic planer work or molecular flow (the "butter" theory-which is discussed in "A.T.3I.," pages 326-331, but which has never been settled).

"The process of polishing." he says, "is different from that of grinding. Years ago Lord Rayleigh advanced the theory that polishing is essentially a process of plastic flow of molecules from the hills into the valleys, although undoubtedly there is some removal of material which is not deposited. Figures 8 and 9 show the results of an experiment by the author which bears out this theory. Figure 8 is a photomicrograph of an ordinary piece of plate glass which has been scored with a splinter of Stellite; the magnification is approximately 100 diameters. The glass was then polished with rouge on pitch, using considerable pressure, until all the scoring had completely disappeared when examined under the microscope. Chemists tell us that a substance on which energy has been spent is less stable, so that if we etch the glass with hydrofluoric acid, we may expect that the acid will attack and remove more readily the material deposited in the grooves. Figure 9 was taken after the glass had been etched with the acid, and it will be seen that the original pattern of the scoring is brought out more or-less, The scoring was not of uniform depth, which explains why some of the lines have disappeared almost completely. The distortion of the lines shows that there has been plastic flow of the material. The numerous random light scratches were cause by carelessness in polishing."

These results are in favor of the butter theory. Most scientists who have investigated favor that theory-most practical opticians the other.

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