"Editor Amateur Telescope Dept.:

"At the time the articles on telescope making first appeared in the SCIENTIFIC AMERICAN, I was teaching machine shop and physics in the Industrial College of Guatemala City, Central America. I sent to you for the book, 'Amateur Telescope Making,' and obtained two six-inch glass disks, some abrasives and rouge. I went through the book carefully several times before commencing work, translating most of it into Spanish for the benefit of the boys in my physics class. The work of grinding and polishing was done in an adobe house with walls 18 inches thick. This proved ideal for the job as the temperature change did not usually exceed five degrees in the 24 hours.

"It was impossible to obtain pitch in Guatemala, so resin was used. Instead of adding turpentine to bring the lap to the right consistency, we found it necessary to boil the strained resin a few minutes to make it harden properly. About a dozen laps were made before a good one was obtained. The polishing went much better than we anticipated and there was no trouble from scratches. The shadow testing was difficult, but a week's squinting along the razor edge gave us enough experience to interpret correctly the shades in terms of curves. The mirror w as finally 'brought to a uniform sphere and then parabolized to a little more than half the conventional correction.

"Then came the silvering. It cost us nearly five dollars in chemicals to learn that the book meant EXACTLY what- it said about avoiding an excess of ammonia. After that the six-inch mirror, as well as several small flats, were silvered.

"Altogether, the instrument was very satisfactory for a first attempt and greatly stimulated the boys' interest in the heavens. Since that time I have made another six-inch mirror and two ten-inch mirrors.

"The first ten-inch mirror was made for and with the help of Dr. J A. MacKnight, Director of the American Academy of Guatemala City. This is the instrument shown in the picture. The tube is built up of half-inch mahogany strips bound at the ends with cast aluminum hoops. The cell is of the same material and is double, with four leveling screws. The mounting was cast and machined in the school shop and is arranged to be placed on a concrete column. In the picture it is shown on a temporary support for the first trial as it came out of the shop."

THE newly discovered methods of grinding and polishing we adverted to at the start are: (1) The so-called "Biron" method invented by Mr. C. H. Biron. a member of the amateur fraternity who lives in Pittsfield, Massachusetts. Mr. Biron's clear elevation drawing is virtually self-explanatory. The curve produced is not a paraboloid but a catenary. (2) A method brilliantly conceived by the Tel. Ed. while eating a typical New York commuter's swallow-and-run breakfast. In this method a hot waffle is employed for the lap and the polishing agent, which is retained in the ready-made indentations in the lap, is Vermont maple syrup (we used this out of loyalty to Vermont where amateur telescope making took its recent rebirth, and we are loyal to our origins if nothing else).

The second method has been given only preliminary trial as yet but it was found that by cleverly combining Method 1 with Method 0, and replacing the left-hand cat with a Pomeranian poodle of substantially equal mass, it was possible to turn out a completely parabolized mirror in less, even than the two or three hours required where Mr. A. W. Everest's new honeycomb foundation lap, recently described, is used.

As we go to press research is being directed toward making the new method virtually automatic so that the amateur may leave it running and retire, awaking in the morning to find the completed mirror ready to run off under its own power. Thus far, some difficulty has been experienced in bringing the alternations of the cat and dog into proper phase, and practical suggestions are solicited. -The Tel. Ed.

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