The first professor is Dr. M. de K. Thompson of the Massachusetts Institute of Technology, an electrochemist in the Department of Physics. Here is what he writes:

"According to the invitation contained in 'Amateur Telescope Making' to those who have made a telescope, I enclose a photograph of one I have just finished.

"The mirror is a six-inch disk of Pyrex glass, one half inch thick. These cost only 95 cents each. The time required for roughing out was six hours and 40 minutes, showing that Pyrex is about twice as hard as plate glass. Fine grinding took three hours and ten minutes.

"After a total of four hours' polishing I found that the edge was turned up, with a high region in the center and a ring around this region. A six-inch stroke brought this to a sphere in one hour, and I parabolized in 20 minutes more with an overhang stroke. The shadows looked exactly like those shown on page 225 of 'Amateur Telescope Making', and the radius of curvature was one-tenth inch less at the center than at the edge.

"I had great difficulty in cleaning the glass properly for silvering. Scrubbing with cotton tied to a glass rod and wet with nitric acid did not at first leave the surface so that water drained evenly without separating into drops. This is the usual test for clean glass. Then hot caustic followed by hot chromic acid was tried. Neither produced the desired result. Finally, I got a pair of rubber gloves and rubbed with cotton and nitric acid until the surface drained evenly. Over half an hour's vigorous rubbing was required. The silvering then was successful.

"I had a wooden equatorial mounting made as described in 'Amateur Telescope Making,' and made a tripod. This is a 15 pound iron casting drilled to take a one-inch steel pipe, and three half-inch steel rods as feet. As a finder, which is quite necessary, I use a strip of brass with a V-notch at each end. The notches are painted with luminous paint. The ends of the rod are bent up so that any object in line with the bottom of the notches is in the field. I find a piece of silvered windshield, tested for flatness and ground to an ellipse by sticking on the end of a wooden rod one inch in diameter and sawed off at 45 degrees, very good as a diagonal.

"I have been using a Ramsden eyepiece of one-inch equivalent focal length from a microscope, and have had fine results looking at the moon."

Pyrex is made by the Corning Glass Works, Corning, New York. We are glad Professor Thompson rubs in well the necessity of cleaning the glass thoroughly before silvering. The criterion of "clean enough" which a male worker usually applies to dish washing (if he ever does any) won't do at all. This has been emphasized in numerous places but confirmation from outside has added weight.

PROFESSOR of anatomy, George W. Corner, M.D., of Rochester University, Rochester, N. Y. writes as follows:

"In accordance with your request that persons who have made telescopes under the influence of SCIENTIFIC AMERICAN should send in pictures of the instrument, I enclose a photograph of a six-inch reflector which I constructed. The work was guided entirely by your book, 'Amateur Telescope Making.'

"I have been able to see two or three of the zones across the disk of Jupiter, the ninth magnitude companion of Polaris, and other objects, which makes me feel that the result is up to the average. As you can see, there is nothing remarkable about the instrument. The axes were made from two front hubs of a model T Ford. The total cost was about 30 dollars, exclusive of eyepieces. I have used microscope eye-pieces from my laboratory for the lower powers with complete success.

"I succeeded perfectly with the silvering on the first trial for the reason that Brashear's method is quite similar to the Bielschowsky method of staining connective tissue reticulum, a technique with which I am quite familiar. It might help those who have difficulty with silvering if it were insisted, even more than in the book, that the critical point is that at which the brown precipitate of alkaline silver is cleared by the addition of more ammonia. Great care at this point is the secret of success."

Professor Corner's comment on the similarity of silvering and staining connective tissue reticulum reminds us of a recent experience at the dentist's. We discovered the dentist silvering one of our molars and, it turned out, this is a well-known dental procedure for plugging up minute cracks.

FROM Ypsilanti, Michigan, comes a letter written by F. R. Gorton, professor of physics and astronomy. No institution was named, but presumably it is the Michigan State Normal College, which is at Ypsilanti. Dr. Gorton says the telescope he describes was produced in connection with college work and, he adds, "I believe that the interest developed in the making of this mirror turned one of our best students to the field of astronomy. He has now completed his work for the master's degree in our state university and receives a scholarship for continuing work in the astronomical field.

"The photograph shows an eight-inch reflector which was made at a very low cost. The instrument is portable, and the eyepiece is easily reached while the observer stands on the ground. The mirror was ground in the usual manner from an eight inch disk, and the polishing was carried over a long period to remove a few defects from the surface. The definition is very good.

With a base of the kind Professor Gorton describes it should be possible even to adjust a telescope accurately in the meridian, using a small pinch bar or pry and expect it to stay put if left out-of-doors, yet not have to attach it. This kind of base might well be adapted to flat roofs. The two holes are ingenious, permitting a pair of "pall bearers" to pick up even a heavy telescope without difficulty and walk off with it.

W. A. CALDER of Beaver Dam, Wisconsin, graduated at the University of Wisconsin and took his M. A. degree in physics, but became exposed to telescope making, and now see what has happened: the world has lost a potential physicist, but gained an astronomer, for Mr. Calder is now at Harvard College Observatory (Cambridge, Massachusetts) doing graduate work in astronomy.

"The snapshots," he writes, "show my 'Unfinished Symphony' of telescopes. The little draw telescope started the ball rolling when I was in the grades. I made an equatorial mounting for it, using an old water motor for the polar axis, and peanut butter jar covers for graduated circles. The three-and-a-half-inch Gaertner refractor was bought from the college I attended, through the help of a very kind-hearted professor. It has a fine clock and has made me partial to refractors.

"I made the six-inch mirror last winter according to 'A. T. M.', but not according to Ellison who insists on a cellar workshop or one whose temperature it not subject to rapid variations. It was made in a clothes closet so small that there was barely room to walk around the barrel. Galileo said, "Give me a lever long enough and strong enough and I can lift the world." I would like to add, "Give me a pound of optical rouge and I can paint it red." Those were happy evenings of grinding, with the radio going and the recently acquired wife helping by furnishing popcorn and lemon.

"After finishing the six-inch, I began the twelve-inch mirror shown in the picture. I never had much more than an hour and a half at a time. Cordon Harder, who became interested in telescope making while I was working on my six-inch, volunteered to finish the job. The equatorial head, shown holding the six-inch reflector, deserves special mention. I was fortunate in being able to get Mr. Henry Harder, an expert machinist, to make this. The castings were made to order and I believe the job beats Ford axle, or pipe-fitting jobs. The polar axis turns on ball bearings, so that the motion is very smooth."

IN the April number mention was made 1 of the aging of reducing solution. Later the following statement was made by Mr. Ferdinan Ellerman, astronomer on the staff of the Mount Wilson Observatory, who has charge of silvering the 100-inch mirror. "While the solution needs to age after mixing, in a similar manner to dry plates, the aging process improves both. But the dry plates reach a maximum sensitivity and after that begin to deteriorate. The silvering solution seems to reach a maximum and remain in that condition. As a rule we do not keep our reducing solutions more than two years, but we have a test bottle which is five years old and appears to be in as good condition as it was two months after it was made."

So that seems to be that. From the final clause of Mr. Ellerman's letter it may perhaps be deduced, incidentally, that he regards two months as about the right time to age the reducing solution before it is used.

No one has advised us yet concerning any accidents due to the explosion of spent silvering solution. It will be remembered that we commented on the recent Circular of the Bureau of Standards No. 389, "The Making of Mirrors by the Deposition of Metal on Glass," in which the risk involved in silvering without protective goggles was emphasized, and requested any amateur who knew of such an accident to advise us. It would seem that, with several thousand amateurs of all shades of experience working on the silvering of mirrors, a few at least ought to get their heads blown off if the risk is more than a minimum. It may be, however, that several hundreds have suffered this form of inconvenience without informing us, as a man without a head would tend to write fewer letters than one with head remaining in normal attachment. Once we interested one of our readers in amateur archeology and he ventured into the open places of Nebraska in search of ancient man. A blizzard came up and he froze to death. What share of responsibility for this sad accident we must shoulder is a question, but we should feel as bad if some amateur telescope maker were to be injured by an explosion of spent silvering solution, unless he had been warned in advance. On the other hand it ought not to be necessary to chase everybody out of the house and warn the neighbors before starting to silver a mirror, as we recall doing some years ago after taking the usual warnings very much to heart.

THE amateur telescope makers and users 1 who constitute the Astronomical Section of the Academy of Science and Art of Pittsburgh have revealed to us that the First Annual Assembly of Amateur Astronomers and Telescope Makers of the Pittsburgh District will be held Saturday and Sunday, August 8 and 9. The program of activities includes a visit to the optical shops of J. W. Fecker, successor to John A. Brashear, where will be seen, among other interesting things, the 69-inch mirror cast at the Bureau of Standards for the Perkins Observatory of Ohio Weslyan University which is there being figured. Also an evening will be spent at the Allegheny Observatory where Director Jordan has granted permission to use the 13-inch refractor. By way of "trimmings" there will be a dinner in a park near the Observatory and a visit to Valley View Observatory, headquarters of the local organization just mentioned. (This homemade observatory is scheduled to be described in a special article in next month's number.)

Anyone interested in attending this new gathering of enthusiasts can obtain further information by addressing the Secretary of the Astronomical Section, Valley View Observatory, 106 Van Buren Street, Observatory Post Office, Pittsburgh, Pa. It will have much in common with the informal gatherings annually held at Stellafane, near Springfield, Vermont. Everybody comes that wants to, and all have a good time and a fine chance to swap experiences with other telescope makers and users. Better plan to drive to Pittsburgh for the August 8,-9 week-end.

This announcement has no known bearing on the annual get-together held in Vermont. No announcement has been made with regard to the exact date of the latter. Why not take in both shows?

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.