First of all, a piece of optical glass is cut down by sawing, chipping, or grinding to an approximate sphere perhaps 7/8 inch in diameter. The source of glass may be an old lens or prism or a chunk of rolled optical glass-borosilicate crown. (War surplus prism blanks sell for a few cents.)

If handy, a wet glazier's abrasive belt sander will make quick work of it. Or the chunk may be rubbed to a rough ball on another piece of glass or metal with coarse abrasive grains and water.

The ball is now ground to a perfect sphere by the method which has been used in China for thousands of years to make quartz gazing crystals. The method is still used by German lapidaries to make agate marbles. The rough sphere is ground in between two brass tubes charged with Carbo. The tubes should have walls from about 1/16-inch to 1/32-inch thick and have a diameter two thirds that of the rough sphere of glass. One tube is fastened to a vertical spindle and rotated at moderate speed by a motor. The ball is placed on top of this tube. The other tube is of the same size and is held on top of the sphere at an angle of about 15 degrees. The sphere is charged by means of a paint brush dipped in coarse Carbo and water. Carbo embeds itself in the soft brass a cuts the glass on a curve. The ball then rotates and is cut on all surfaces by the two tubes. This is the principle of the lens generator, the modern machine which makes spherical surfaces, and of the centerless grinder.

The more the ball rolls the smaller and rounder it becomes. If you did a poor job of shaping the original rough sphere and tried to work a jagged chunk having sharp corners and deep fissures you may take solace in the original Chinese treatise on gazing balls which, translated, states: The going may be a little rough at first but it will soon settle down."

To those who may ask whether the ball gets smaller faster than it gets rounder I can offer the accompanying table of figures on one I made. Grinding was started at 9:30 o'clock and proceeded with interruptions for changing abrasive and making measurements as shown. "Max. diam." was the diameter in inches across the longest axis and "Min. diam." the shortest. "Difference" is the difference between the two measurements and shows how nearly round the sphere was. The table shows that while the sphere loses size it gets

rounder much faster than it gets smaller. By continuing the process a reasonable length of time the shape should become perfect so far as can be measured. The table also shows the approximate length of time needed to grind a sphere 3/4-inch in diameter.

Polishing was done as follows. The lower tube was cleaned and heated with a flame until pitch would melt on it, then a thick rim of hollow pitch was built up around the edge. While the pitch was still soft a washer of felt, cut out of an old hat, was pressed down on the rim and held in shape in shape by ground sphere. The top tube was later treated the same way so that both tools now had a rim of felt attached to working surfaces. To polish, the felt charged with rouge and water, and polishing went as with grinding, but more smoothly. Ten minutes gives a good polish.

Now, we have a perfect glass sphere which could be used "as is," especially if it is a smaller size. But for larger sizes (and I suggest trying these first) it had better be made into a Coddington lens. The right cylinder diameter for a Coddington 3/4-inch long is about 3/8 inch. To make this, a piece of wooden dowel 3/8-inch in diameter was concaved on one end to fit the sphere and the sphere was cemented to it with hard pitch. Now a "cookie cutter" charged with medium Carbo was brought down on it in a drill press and a cylinder was cut out of the sphere.

THE FINAL operation, after removing the waste from the sphere, is cutting the Coddington field stop. Leaving the cylinder of glass still cemented to the dowel, the glass is rotated against a thin edge of metal or other sheeting armed with abrasive and a thin groove is cut around the cylinder to a constant depth, stopping when the groove begins to infringe on the field. This groove, when blackened with India ink or paint, acts as an aperture stop, giving a sharp edge to the field and removing the poorly lighted and poorly defined marginal rays.

It is interesting to see how small an ocular can be made by this method. If anyone completes one less than 4 mm. in diameter I hope he will let me know and I will send him the straitjacket I am now wearing.

I found that long-focus Coddingtons, which are easier to make, were poor. The one made from a 3/4-inch sphere had a field of about 40 degrees of which only about 20 degrees was good. There was much aberration and color at the edges. Eye relief was satisfactory at about 1/2 inch and seeing in the center was fine. The best one I made has an effective focal length of .31 inch, an angular field of 40 degrees, an eye relief of .2 inch and is usable with eyeglasses. The definition is good, there is no color, and it is grooved down to about a .3-inch waist. It is the best short focus ocular I have used. The shortest I have completed is .22-inch focus but it didn't turn out well, though it is usable provided you really want an ocular of that short focal length. If we now go a step further and regard the solid ocular as the "triplet" field lens of an orthoscopic ocular and add a plano-convex lens, flat side toward the eye, we have a fairly good imitation orthoscopic design with a larger usable field and more eye relief. For example, if the solid ocular made from the 3/4-inch sphere mentioned above is used in conjunction with a plano-convex lens of about 1 1/2-inch focal length and with a spacing of about 1/4 inch between the two, a considerable improvement will be noted in the marginal definition. The resulting eyepiece will be no world beater, but usable.

End of Holeman's contribution.

If the Coddington is approached with great expectations, the probable reaction, unless the worker is objective in his estimate, will be that it is no good at all. But if no miracle is expected the result may equal or surpass the expectation. The project was undertaken by Holeman as a kind of sporting adventure to see whether the very old-fashioned ocular might not have more merit than might be thought, and so it proved. Bell, in The Telescope, rates the Coddington as somewhat better than a simple lens.

ANOTHER adventure in the unusual is the modern attempt to duplicate some of the tiny high-power eyepiece lenses made by the great Sir William Herschel (1738-1822). These were described in the Transactions of the Optical Society (London), Volume 26, by W. H. Steavenson, F.R.A.S., a prominent member of the mainly amateur British Astronomical Association. Steavenson has kindly furnished two of the photographs originally published in that periodical. He visited the old Herschel home, examined and carefully tested many of Herschel's mirrors and flats, also the famous "seven-foot" reflector (7 feet, 2 3/4-inch focal length, 6.2 inch aperture) so often mentioned in Herschel's writings. In the Transactions Steavenson writes:

"Herschel's claims to have used powers between 1,000 and 6,000 on his seven-foot scopes have been the subject of some controversy and not a little incredulity in the past. It therefore seemed to be of particular interest to find out whether he really possessed eyepieces which would yield such powers as these. Quite a short search sufficed to lay any doubts at rest by revealing no less than nine of these eyepieces whose focal lengths and powers (on the telescope used by Herschel) found by careful measurement to be as given in the table. "All these are well-formed bi-convex lenses, with the exception of D₂₁ [identifying notation-Ed.], which is a simple sphere. Two or three of them, including the smallest, were found to be somewhat astigmatic, and in some cases there were signs of devitrification of the surfaces, but in general they gave sharp images in the micro-focometer used, and their focal lengths were quite readily measured.

"All were tested on celestial objects with a six inch Wray refractor and were found to form recognizable (though of course dim and diffuse) images of stars and planets. Even D₂₆, despite astigmatism and excessive power (about 10,000 diameters) showed the spurious disk of Vega, with portions of the first diffraction ring, and also exhibited the general outlines of the planet Saturn. The field of view was, however, only about 20 seconds of arc in diameter, and the image could hardly have been examined, or even held in view, without the help of a good clock drive. And yet Herschel, in his experiments on high powers, had nothing better than an altazimuth stand with hand driven slow motions!

"We are told nothing of the methods employed in making these tiny lenses, the smallest of which is only 1/45-

inch in diameter. Compared with this, the front lens of a modern 2 mm. oil-immersion objective is a large and clumsy object. It would be interesting to know exactly how a present-day optician would proceed, if required to make a duplicate of the most powerful of Herschel's eyepieces, fashioned in the 18th Century."

In the illustration of the eyepiece, what may look like the lens is the eye end of the eyepiece shell. On that shell is a raised nipple. On the tip of the nipple is a tiny dot, just visible. This dot is the wee lens shown in the other illustration, its diameter one third that of a pinhead.

Herschel's trick eyepieces may have been made by him partly as a stunt. So thinks his granddaughter, Constance A. Lubbock, editor of The Herschel Chronicle. Herschel was ever a case of "once an amateur always an amateur," which carries with it an incurable interest in stunts done purely for the fun of it. He probably wanted to see just how small a lens he could make, and probably as he made those described he chuckled, "I'll give 'em 160 years to beat me on these."

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