"A general description of this machine will interest any amateur who has made a reflecting telescope.

"In Figure 1, l is the table on which the disk will remain throughout the grinding, polishing, and figuring-in fact it is the cell of the telescope tube itself, and the mirror will never leave it after it is finished. The individual supporting units are not shown. The table is on trunnions that allow the mirror to be brought to vertical for testing.

"The grinding and polishing tools are controlled by a rotating spindle or driving pin, attached to the trolley 2. This trolley moves back and forth transversely on the bridge 3. This bridge in turn travels longitudinally on tracks 4, attached to the main frame of the machine.

"The driving mechanism for both the trolley and bridge motions is the scotch yoke. Figure 2 looks down on the trolley yoke, and Figure 3 is a side view showing one of the pair of yokes that move the bridge. The tracks supporting the bridge are extended at the rear of the machine so that polishing tools may be well away from the table for warming the pitch facets preparatory for hot pressing, and also for washing off the grinding tools.

"At 5, Figure l, is the control station where the operator has complete electrical control of all the various motors and motions.

'The maximum stroke is 6 feet for the trolley, and 6 feet for the bridge. When used together, the tools may be made to move in a variety of strokes. Speeds on the track are from 8 to 25 feet per minute.

"Figure 4 shows the full size tool in process of being covered. When ready for grinding it will be covered with 1928 glass facets cemented to its convex surface. As is generally known the 200-inch disk was cast in an open mold and hence its surface is approximately flat. Some 3-1/4 tons of glass will have to be removed to bring the mirror to its spherical shape. The sagitta or depression amounts to 3.8 inches. Small size cast-iron tools will be used for roughing out the curve before using the full size tool.

"The first operation on the disk will to place it face down on the table (which is covered with 2 inches of sponge rubber and carefully calibrate the positions of the 36 holes destined to receive the supporting units that will carry this 17 tons of glass internally all inclinations to gravity. The holes will then be ground out, the back ground, and the edge ground. The disk will then be turned over and the rough grinding of its surface commenced.

"The 120-inch Hat disk received earlier has a machine of its own, similar to the machine here shown but smaller and with a different driving mechanism. The disk at present is fine grain and flat to 0.00012 of an inch (concave) Its flatness was controlled during grinding by testing its surface with a stretched piano wire, but sufficient reflection was obtained at grazing incidence to allow a optical check test which confirmed those of the piano wire, Polishing and figuring this ten-foot flat will not be started until the 200-inch mirror is figured spherical the reason being obvious to mirror make who have made flats for testing at the focus.

"The room in which the work is do is 165 feet long, 52 feet wide, and 39 feet high. The walls are covered with cork insulation. There are no windows. The room is illuminated from overhead by electric lights in the roof, and by mercury vapor lamps around the walls. The air is conditioned, washed, tempered, and given the requisite amount of humidity. The opticians like to work in a rather high humidity and a temperature of about 70 degrees.

"A visitor's gallery is provided, separated by a glass partition, so that the public may see what is going on at any time. I never go into the optical shop-and I go there quite often-without seeing several faces glued to that glass partition, evidently wondering what it is all about. And you will find them there for the next four years-the scheduled time for completion."

NUMEROUS readers have asked us why we did not, beginning several years ago, publish the data about the design of the 200" telescope. The reason was that, while the project was announced several years ago, the design was not settled-it had only been begun. It will be understandable why those who have had the most to do with it did not wish to rush into print at each successive stage of the constantly changing evolution. Now that most of the design has been settled and actual construction begun, we hope occasionally to give our readers some of the homely details, written not as the average new reporter would describe them for a general audience but for the telescope maker.

AMONG amateur telescope makers and doers of practical things in England is F. J. Hargreaves, Director of the Photography Section of the British Astronomical Association-an amateur association which we have previously urged American amateurs to join in order to obtain the a monthly Journal, which is a live one. Mr. Hargreaves now sends us from Mirastelle, Woodland Way, Kingswood, Surrey, England, the following communication:

"The purpose of this letter is to enlist your help in a matter which I and some of my friends have very much at heart. I refer to the systematic observation of the surface features of Jupiter. This subject also supplies an answer to the question in the second paragraph on page XI of 'The Book.'

"The systematic observation of Jupiter seems to be indigenous to this country. Observers elsewhere make occasional observations, sometimes of great value, but nowhere else, so far as I know, do observers watch the planet on all available occasions, for hours on end, timing the transits of markings across the central meridian.

"The trouble, however, is that England lies in high northern latitudes, and when Jupiter is below the equator the opportunities for observing it here fall off grievously. Last apparition, for example, my total of transits was only 220, as against 1500 and more in previous years. The present apparition will be worse.

"There is nobody in the southern hemisphere interested in this work, Unfortunately; although we have made many efforts to rouse our brothers in South Africa and Australia to action, there has been no adequate response,

"Now the United States extends a good way south, and conditions even in the northern states are much more favorable than in England when the planet is in low southern declination, as at present. Largely owing to your efforts, there must now be hundreds of people in America who have made good telescopes and who would be glad to know of some useful astronomical work that they could undertake. Any telescope of 6 inches aperture and upwards is suitable; I have obtained as many as 60 transits in a single evening and also many detailed drawings with a 6-1/2 inch reflector.

"Apart from latitude, there is the scarcely less important matter of longitude. Even under the most favorable conditions we can observe only for about 10 hours; for the next 14 hours the planet is below the horizon. As the rotation period is just under ten hours, you will appreciate that there are necessarily large gaps in the records of observers grouped on a small island. If we had similar observers in a widely different longitude, these gaps would be filled.

"The observations themselves require, apart from a telescope, only a clock or watch showing Greenwich Mean Time to the nearest minute, or local time if the observer will correct for his longitude. The observer must learn the names of the various belts and zones, and of course must have a good planetary eye and sound judgment. Men who see Mars covered with cobwebs (but cannot see the large markings on that planet) are not as a rule useful as Jovian observers. The observer notes that a dark marking of some kind on the North Temperate Belt, for example, is a little to the right of the central meridian. As it moves towards the left there will be a time when it appears to be exactly central of the belt. It is then on the central meridian, and the time is noted thus:

D spot N T B 8^h 24^m,

Or whatever it was.

"If it is a hump on the south edge of the North Equatorial Belt it would be described as

Hump S edge N E B

The list of such observations can be diversified by sketches of details, or whole-disk drawings. If the separate observations are numbered serially throughout the apparition, the objects in the sketches can be identified by a reference to the serial number.

'Apart from actual observing, the observer will also be required to reduce his observations to longitude (zenographic) by the aid of tables in the 'Handbook' of the B.A.A. This is tedious, but requires nothing more of mathematics than simple addition and subtraction. Finally, observers undertaking this work would be welcomed as members of the B.A.A., although naturally this is not a requirement but only an invitation.

"Observations such as these, however and simple they may seem, are of very great importance for a study of the movements of the various currents on Jupiter. They are interesting in themselves, as the observer can easily follow the various currents and their changes by plotting his own observations on squared paper, and if several observers are in touch with one another by correspondence the sporting spirit is easily aroused and they vie with one another in the matter of numbers of observations.

"The present Director of the Jupiter Section is B. M. Peek, of Sohan, Silhill Hall Road, Solihull, Warwickshire, England. I am writing for him, because I have already been in communication with you and am an A.T.M. He will supply all the necessary information to anyone interested."

ON page 65 of the earlier editions of "A.T.M." is a somewhat old picture of Vard B. Wallace, and now from the same Mr. Wallace who writes on the letterhead of the "Vard Mechanical Laboratory-Scientific Instruments," 3135 Blanche St., Pasadena, Calif., we receive a description of a neat engine for dividing setting circles and making protractors, made in his laboratory.

"The table of the engine (See Figure 5) is a little more than 14" in diameter and has 360 teeth cut on the edge. The table is split through the center line of the teeth, so that the upper half may be rotated with respect to the lower half. After the teeth were hobbed (in the Astrophysics shop at 'Cal Tech') the two halves of the gear were shifted and lapped 144 times in different positions. The ultimate result was that there is no visible error in the teeth in any position when viewed through a glass.

"In Figure 6, at the left you will note a pair of cams on the same shaft as the crank. These cams actuate two pistons in small hydraulic cylinders The liquid from these two cylinders flows through the tube to the tracelet mechanism. One tube connects with a cylinder that moves the trace let back and forth. The other raises the tool on the back stroke so that it is free of the table while the latter is turning. This will seem like a needlessly elaborate device till you consider that it is necessary to shift the whole tracelet mechanism laterally on the bridge to accommodate large or small protractors. It is required that the tracelet be raised and lowered to take thick or thin work. On top of all that it is also necessary to be able to rotate the tracelet so that bevel or even cylindrical work may be done. With the hydraulic system, all this is accomplished with no difficulty other than the slight bending of the tubing. To do the same thing mechanically is quite a task.

"The machine is almost silent in operation The engine will scratch 360 lines on a disk in about 14 minutes, putting long and short lines in their proper places, and upon completing the disk it will shut itself off and ring a buzzer for attention. You will notice that the tracelet is inclined at a slight angle in the pictures. This was for some bevel work that we do in production."

FROM time to time we receive inquiries about apparatus for making setting circles. The Vard apparatus, just described, is of course very far too fine an instrument to construct just to make circles for a telescope or two-in fact it is a finer instrument than most telescopes themselves, and then some. The one in Figure 7 goes still farther being the circular dividing engine at the National Bureau of Standards and about the last word in such things. It can be used to graduate circles up to a meter in diameter Such a machine costs about 10,000 dollars or more, and will graduate so that the errors are only about one second of arc and in some cases even less. This is better than a millionth of a circle.

In good machine shops dividing heads are used as attachments to milling machine for all sorts of fancy purposes. Figure 8 reproduced by courtesy of the Cincinnati Milling Machine Company, shows how these work. The handle at the right has a spring pin that drops into holes evenly spaced around a circular index plate. Through the gears shown in section it actuates a worm and worm wheel, the work being attached to the shaft of the latter. Such rigs cost well up into the hundreds of dollars.

Figure 9 shows a dividing head plus a divider that divides to one second of arc, and is made by Kearney and Trecker. The cost is about the same as that of a medium-priced motor car. Most amateurs are lucky even to get a look at one, like a cat and a king. Amateurs desiring to divide setting circles may, however, rig up their own dividing head`. (To be continued in "A.T.M.," Vol. II.)

MEMBERS of the Louisville Astronomical Society want a 20" telescope and the Corning Glass Works will reduce price of Pyrex disks to about $80 each, provided five can be made at same time. Walter L. Moore of that society, who is Associate Professor of Mathematics at the University of Louisville (summer address Box 163 R.F.D. 1, Coral Ridge, Ky.), wishes to get the necessary five buyers together. Write direct to him, please, and enclose stamp.

Stellafane get-together, Saturday, August 8. Porter coming east for it. Last call.

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.