"The covering is of light sheet iron, each piece cut to fit from one rafter to the next. The dome has no track to run on, other than the circular plate, but is kept from running off by means of eight castings having wheels at the end. One of these can be seen in the picture.

"The clock drive, too, is home-made, being patterned after one of the many small clocks used for this kind of work.

"Scarcely anything built by the amateur can be done without using some parts of a Ford, and my equatorial mounting is made of two Ford hubs. These afford me ball bearings and work very nicely."

TO show what kind of telescopes are made by amateurs in France-though not many make them there-a picture from L'Astrornomie, the monthly magazine of the Société Astronomique de France, amateur society of some 5000 members, is reproduced. This telescope has a 6-1/2 inch mirror of 51 inches focal length and was made by Monsieur H. Cocquelet, artiste musicien a l'orchestre de l'Opera de l'Comique, 50 rue de la Justice, Paris (XX^e) The diagonal is a prism. There are two finders, one of which is the half of an opera a~ glass. The other features, except the central complex which shows poorly in the original, explain themselves.

HERE is a new-old method of silvering mirrors outlined in a letter from Prof. H. L. Johnston of the Department of Physical Chemistry at the Ohio State University, Columbus, Ohio. It appears to have some points of superiority over the version of the same (Brashear) process now used by telescope makers, but doubtless will require modification, since it has apparently been used mainly for silvering the inside surfaces of thin glass flasks. Amateurs who are already familiar with silvering are invited to adapt it to telescope mirror needs and report results. Beginners are warned that it may ruin a good mirror unless used with judgment and intelligent modification based on, experience and due respect for HF and ice.

1st, Scrupulously clean the object to be silvered. (Very important in any silvering process.) I usually do this by washing thoroughly with 50 percent nitric acid; rinsing thoroughly with distilled water; covering the object with concentrated ammonium hydroxide for a period of one to three hours; rinsing this off thoroughly, and then allowing the object to stand immersed in distilled water until ready to apply the silvering solution. (The surface must remain wet.) In extreme cases dilute HF may be used in place of HNO₃ for the initial cleaning.

"2nd. Prepare the following solutions: A. 25 grams NaOH in 350 cc. distilled water. B. 35 grams AgNO₃ in 350 cc. distilled water. C. 35 grams dextrose in 700 cc. distilled water.

"3rd. Mix A and B and nearly, but not quite, clear with ammonia. I usually save out a portion of B with which to 'back titrate' in case I overshoot the ammonia. The reaction is slow and, even with constant shaking, one is apt to overdo the ammonia. This will ruin the job. The proportion of B is not very critical. I sometimes overdo the ammonia, in which case I simply make up some additional A_gNO₃ solution. Even if I thus exceed the original specification by 30 or 40 percent it seems to make little difference in the results. The final solution of the ammoniacal mixture of A and B must be cloudy with a brown cloud of Ag₂O. An excess of ammonia will produce a black cloudiness which will ruin the results.

"4th. Add approximately a liter of cracked ice to the ammoniacal solution of A and B and also to C. This will usually shift the equilibrium in A and B so as to clear or nearly clear that solution. This is all right and is perhaps desirable; that is, the excess of Ag₂O must not be too great, but the proportions are not critical.

"5th. Mix the two ice cold solutions, and immediately pour over the object which is to be silvered (filtering out excess ice). The reaction is sufficiently slow to permit reasonable time in handling. If the object being silvered is so constructed that it can be warmed with a stream of tap water at room temperature, that is desirable as it localizes the early silvering reaction on the desired surface. If this can not be done one simply has to let the process take its course. When the silver coat is sufficiently thick everything is washed off with distilled water. If the exposed silver surface is to be used as the mirror (as in optical instruments) it is best to cover with 95 percent alcohol in order to arrest oxidation of the moist silver surface. The alcohol can be poured off at once and the polishing process be carried out within a few minutes. This silvering formula has the disadvantage of yielding a rather heavy, flocculent precipitate during the silvering reaction. For an optical surface, it is necessary to swab the silvered surface with a soft cotton swab continuously during the silvering process, in order to prevent the attachment of this flocculent material to the newly formed silver surface.

"I have used this formula primarily for silvering Dewar flasks.

"I also used the formula for optical surfaces, in silvering some small mirrors for a reflecting telescope in use on this campus. It appeared to give satisfactory results. I told Professor Stetson of the formula and he used it in silvering their large reflector in the new telescope, at the Perkins Observatory. He tells me that he found the method superior to the standard methods in use for astronomical reflectors and with which he had had previous experience. Of course, the flocculent precipitate is a disadvantage that must be considered in carrying out the process.

"Several months ago Brickwedde (and Scott?) of the U. S. Bureau of Standards published an article on silvering mirrors in which they employed substantially the same method I have here outlined to you. If you print this, I think that you should mention the fact that the formula had been used in the Chemistry Department of the University of California for a number of years (and probably somewhere else before that)."

On October 24, as proofs of this page are passed, it is doubtful whether the new edition of A. T. M. will be ready in mid November, as announced in other pages prepared earlier. If not, the delay cannot be more than a few days or weeks.

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