"The present figure of the 200" mirror approximates closely that of a sphere with a radius of curvature of 111 feet. The disk was first shaped on front, back, and edges by grinding with coarse Carborundum and the center was then hollowed out to a depth of about 3-1/4" with the aid, first of a 50" tool, and then of successively larger tools up to the full size of 200". As the spherical figure was approached, finer grades of Carborundum were used, so that when optical tests became necessary only a few hours of polishing were needed to give the surface sufficient reflecting power. During the first stages of work upon the disk, the cylindrical holes on the back of the mirror and the large areas between the supporting ribs were filled with plaster of Paris, and after the face and back had been made parallel the 40" opening at the center of the disk was closed with a glass plug weighing about one ton. The plaster of Paris was removed when the surface had reached nearly the required form, and the mirror was placed upon rubber pads which rest on the frame of the support system of multiple counterweighted levers. The friction of these pads was required to neutralize the strong lateral pull of the grinding and polishing tools making contact over so large a surface, and for this reason the horizontal plates of the supporting system were not engaged. The optical tests, however, were made with the mirror vertical and balanced upon the support system just as it will be when in use on the telescope. Future optical work, including the parabolizing, will be carried on with the mirror resting upon the rubber pads; but these will be removed when the figuring is completed.

"From the first the optical tests of the mirror proved to be most satisfactory. No change was seen in the figure when the mirror was tipped repeatedly from a horizontal to a vertical position or when it was rotated. Also no local deformations could be detected at the points of contact of the 36 individual lever supports. In fact, the support system was found to be performing quite as well as had been hoped by its designers. The length of the radius of curvature proved to be about 2" less than had originally been planned, but this slight difference, amounting to about l" in the focal length, is far within the limits allowed for in the design of the telescope, and it is probable that no attempt will be made to reduce the amount through polishing. The surface was found to be reasonably free from zones, but a small amount of astigmatism was present amounting to about 0.1" in the optical cut-off at the center of curvature. This proved to be due to a slight deformation of the disk caused by the horizontal pads upon which the mirror rests. [Supplementary note added in letter from Dr. Adams to Scientific American, to bring article up to a later date: "Through suitable changes in the supporting pads this astigmatism has now (March 15) been eliminated by a small amount of fine grinding and polishing. The zones have also been considerably reduced and the entire mirror surface is nearing an accurate spherical figure. Work will soon be commenced upon the final figuring of the 120" plane mirror to be used in testing the parabolization of the 200" surface.'']

"Although the final parabolic curve of the surface of the mirror will be only 0.005" deeper than the spherical curve, the area is so great and the process of polishing so slow that it is planned to make most of the change through fine grinding and to leave but the final stages of the figuring to the polishing tool. This procedure has already been found successful in figuring the spherical curve.

"Accurate tests of the parabolic figure of the 200" mirror will require the use, as an auxiliary plane, of the 120" mirror. The completion of this mirror will be one of the next major undertakings in the optical shop. It is at present shaped on front, back, and edges, and the surface has been ground approximately flat. It is planned to figure the three auxiliary hyperbolic mirrors, one for the Cassegrainian and two for the coudé combination, by the method devised by Hindle, without the use of the 200'' mirror. Four spherical mirrors, each about 40" in diameter and with a radius of curvature of 25', are being prepared for this purpose, two being very nearly completed. The mirrors will be used side by side in a cloverleaf pattern and provided with adjustments for bringing their centers of curvature into accurate coincidence.

"A few figures are of interest as indicating the scale of the optical work on the 200" mirror. About five tons of glass have been removed in the process of shaping and figuring, and about 20 tons of Carborundum have been used for grinding. During the polishing, about 50 pounds of rouge an hour is the average consumption with the full-sized tool, and but a small fraction of this material can be salvaged. As a result, rouge has been purchased in quantities hitherto quite unfamiliar to dealers in optical supplies."

IF you can train your eyes to perform suitably, you can superpose the two images shown in Figure 1 and obtain a three-dimensional view of the 200" mirror and of the lettered image. You are standing in the lofty visitors gallery in the Optical Shop at the California Institute of Technology, in Pasadena, California, looking through the windows that divide the gallery from the room in which the big mirror lens," as newspaper writers still call it) is being completed. Axis of the mirror is about 2' below your floor level. About 6' below that, there is a kind of super-pinhole about a foot in diameter, with a lettered transparency over it and a powerful light source behind it. The rays go to the mirror and are reflected back to a focus about 6' above the axis, at your approximate eye level and 2' or so in front of the window. In Russell Porter's words, "you'd swear the image was material. Startling! " It seems to hang in space and you feel the urge to feel of it, others comment.

Thus far, however, there is nothing stereoscopic about it; in fact, the stereoscopic part has no connection with the apparatus or mirror but was obtained by taking two photographs from the visitor s gallery, the second one made with the camera shifted sidewise about the same distance as the separation of the average man s eyes. These photographs were taken by David O. Woodbury, an amateur telescope maker who, after spending several months in Pasadena and Mt. Palomar, has written a book about the 200" telescope, one chapter of which was recently pre-published as an article in Reader's Digest (the entire book is due to appear in October).

There are three ways to get the three-dimensional effect: (1) use a mounted pair of prismatic lenses; (2) hold a long strip of cardboard between the end of your nose and the center of the pair of pictures, so that the right eye sees the right-hand picture alone and the left eye the left-hand picture alone, and then try to drift off into a focus on infinity (I think dreamily about that brunette,: or (3) command your eyes to do the trick without any cardboard-this can he done by many, after a bit of practice.

The square object in the immediate foreground is a rack to carry the knife-edge, when the mirror is tested à la Foucault.

RECENTLY, in one of the innumerable newspaper items which describe a mirror as a "lens," your scribe was further enlightened by the statement that a "lens" had been "polished with face powder!" It isn't difficult to figure out how this howler happened, but then followed this thought: "Could a passable polish be had with face rouge, and just what is in face rouge, anyway?" Will some face-rouge-experienced reader provide the answers?

ANNOUNCEMENTS: Stellafane convention will be held on Saturday, July 22- R. J. Lyon, Secretary, Springfield Telescope Makers. 252 Summer St., Springfield, Vermont.

CARL GROSSWENDT, 31 W. 87th St., New York, Secretary of the Optical Division of the Amateur Astronomers Association, New York City, states that the Optical Division (which is simply the former New York Telescope Makers) cordially invites all amateurs who visit the New York World's Fair to inspect its workshop deep in the bowels of the earth under the Hayden Planetarium, at 8lst St. and Central Park West. It asks that, if convenient, such visitors first notify the Optical Division by telephoning as follows: Mon.; Tues., Wed., Thurs., Fri., 10 to 4, Endicott 2-8500, Ext. 478; 6 to 9, Ext. 509. Sat. and Sun., 2 to 9, Ext. 509. Even if you lack opportunity to make such arrangement, the planetarium guard may by luck find someone in the optical workshop. Planetarium performances. which you may wish to combine with this visit, take place weekdays at 2:00, 3:30, 8:30. Saturdays at 11:00, 1:00, 2:00, 3:00, 4:00, 5:00, 8:30. Sundays and holidays at 2:00, 3:00, 4:00, 5:00, 8:30. An exhibition of products of amateur astronomers' activities will be held in an adjacent building from July 30 to Aug 20, inclusive, and a convention of amateur astronomers will be held on August 19 and 20.

In "Amateur Telescope Making-Advanced," the sequel book to "Amateur Telescope Making," H. E. Dall describes the camera obscura, which is essentially a refracting telescope for use from indoors. The first since then to make one is William R. Crosby, 566 Glide St., Rochester, N. Y., a member of the thriving group of amateurs in that city. Figures 2, 3 and 4 show the design and construction. At the top, in Figure 2, is an elevation showing a roof and, projecting above it, a flat both pivoted and rotatable. This picks up rays from any chosen object and directs them downward through the objective lens shown, which projects an image on a horizontal viewing table several feet beneath. Here the image may be viewed with the eye or, if still higher magnification is desired, with a microscope called a viewing telescope. The lower elevation shows the apparatus from another quarter.

Crosby, however, did not build his camera obscura into his house but into his automobile trailer. Figure 2, taken outside, shows the trailer roof and the raised flat, while Figure 4 shows the interior part of the instrument, also the interior of the trailer (this Crosby also built).

Your scribe has sat in this trailer, rotated the flat and examined the landscape roundabout, and can testify that the spectacle came far above best expectations. Because the observer sits in the dark, his eye is not sky-flooded. Therefore the large image on the table appears to be vividly colored-even more vividly than the actual landscape. A long, continuous panorama of greens and sky-blues sweeps across the viewing table which in this case is the dining table of the trailer, and several persons can examine it at one time. For astronomical work the camera obscura equates with a refractor of the enclosed observing room type.

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