THE first is by George Croston who usually signs his name Xton) of La Grande, Washington, secretary of the Amateur Telescope Makers and Astronomers of Tacoma (also see A.T.M., page 444), who writes:

"My portable compound telescope was built for service on Mt. Ranier. It is a triple telescope, so constructed that it can be changed quickly to a Newtonian, a Cassegrainian, or a Gregorian, as desired.

The six-inch mirror works as a Newtonian at f/3. The Cassegrainian secondary and the Newtonian diagonal are both mounted on a turret, an extension of which projects through the aluminum tube and is rotated by means of a knurled knob, in order to bring up the desired combination. To use the telescope as a Gregorian this turret combination is removed as unit, leaving the Gregorian secondary which is permanently in place.

"The weight, including the tripod is 24 pounds. The upper section the telescoping tripod legs is of airplane steel tubing and the lower duralumin. Each upper section of the tripod has a small steel cone brazed on it to fit a corresponding recess in the outer edge of the mirror cell and thus the three legs are drawn and held tightly in place by means of three center-connected airplane turnbuckles. This provides a rigid assembly for transportation, also protection for the tube and a convenient handle for carrying, as is shown above.

"The tripod head is made to such a diameter that the open end of the tube will nest in it, providing a tight cover that protects all the optical surfaces from dust and possible damage in transit, as well as a convenient base on which to store the telescope.

"This instrument was made mainly for terrestrial observing but it performs well in astronomical work on the Cass and Greg combinations. The Newtonian, having such a short effective focal length, corresponds in design characteristics to a very large field glass, giving a brilliant wide field of low power. Vibration is the principal disability experienced with this type of telescope when used in astronomical work. However, this one soon comes to rest. Vibration is a price which one must expect to pay for portability and extreme lightness combined with high power.

"The secondaries are 1-5/8 inches in diameter. By using the Kirkham direct focal test (A.T.M. page 271-Ed.) they were accurately figured. Their small diameter would have made this a difficult job by any other method. They have micrometer adjustments in three directions, accuracy in this respect being absolutely necessary to good performance.

"The photographs used in the new edition of A.T.M. to illustrate the Ronchi test were made from the mirror used in this instrument. This is also the first mirror to be figured and tested by this method. (Messrs. Kirkham and Croston live near one another and often co-operate.-Ed.

A NEAT portable telescope is that made by Don H. Johnston of The Humphrey Company, Cleveland Ohio, who writes briefly:

"This is an 8-inch portable job. Full equatorial mounting with slow motions. Pipe fittings are used, and the rest of the parts came from scrap heap. The outfit is very rigid and steady and, owing to the short tube, it can be carried outside and set up easily and quickly. Highest magnification is 187 diameters. The finder is adjustable and eyepiece holder has rack and pinion movement. Focus 37-1/2 inches."

THE Sellers portable altazimuth mounting, made by F. J. Sellers, F.R.A.S., M.E., of 42 Church Crescent, London, N. 10, England, is shown in an accompanying drawing, which nearly explains itself. A⁴ is a universal joint. A³ is a screw for slow motion in altitude. B² represents two rods which may be rotated by the handgrip in order to move the telescope in azimuth. This tight-fisted control is a very positive one. Quick approximate adjustment is made by spreading the bi-pod sidewise or forward, or both. Both controls are within convenient reach of the eyepiece. The mounting is light in weight. Mr. Sellers is the author of Chapter X, Part X, of "Amateur Telescope Making" and is prominent in amateur astronomical circles in England.

ON page 410 of the new edition of the "Amateur Telescope Making" the two mentions of caustic soda, in lines 14 and 15, should be changed to caustic potash. On page 233 in Editor's Note change Ohio State to Ohio Wesleyan. Please don't forget to hunt for further errors in A.T.M. as you read, and report them to us. Ten thousand pairs of eyes are better than one, and we are anxious to locate the mistakes -and we don't say "if any," since it is common experience that newly published matter usually contains them.

PRELIMINARY plans are being worked out for a national get-together of amateur telescope makers, to be held in Chicago, probably in June. The following note concerning it is from Arthur Howe Carpenter (A.T.M., page 380), president of the Amateur Telescope Makers of Chicago: "This organization," he writes, "will be host to all members of the telescope making fraternity in the nation and we should like the event to take place in June, concurrently with the meeting of the American Astronomical Society, which is set for June 21-25. We should like to hear from prospective visitors as soon as possible, in order that we may make plans. Address; 811 Bell Avenue, La Grange, Illinois."

As you will gather, the plans for this national convention have not yet fully matured, but you will be informed through this department regarding any changes. At present the American Association for the Advancement of Science also proposes to hold its summer meeting in

Chicago between June 19 and 30. The sessions of this great meeting are open to anyone interested in science and willing to pay small membership fees (less than five dollars). The Chicago group hopes to have a headquarters booth open throughout the summer, where amateur visitors may hobnob with them and actually work with them. So here are two added inducements and three great conventions of scientists!

A national organization of amateurs will also be effected at this meeting.

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.