THE FOLLOWING IS A continuation of the article on making objective lenses without recourse to an optical flat begun last month by Patrick A. Driscoll, of Rochester, N. Y.

So far we have tested only for radius of curvature. In testing for figure or irregularities we revert to the small pinhole and we may use the edge of the frosted glass as the knife-edge.

When the test plate has been polished to a curvature of -935.51 and corrected for irregularities (we want the even gray shadow of the sphere) we turn it over and on a new polisher proceed exactly as before to polish the -2884.64 curve. This will not in any affect the figure of the first side.

The-666.46 curve of the flint glass will be our next surface to polish. Since this surface is to be used not only as a temporary test plate, but is also our flint component, it then follows that it must be polished completely, leaving no pits. A scratch on any of the surfaces should be ignored, as the time expended to regrind or polish it out is not worth the infinitesimal amount of light lost in transmission.

TEST PLATE READING-Having all our minus curves polished, we are now ready to polish the plus curves, and now at last comes the application of test plates. We shall start to polish the back side of our flint component (the +2884.64 side). As soon as we have a reasonable polish on it we are ready to test, and we must now enter into a short study of the behavior of interference fringes, or Newton's rings. We place the-2884.64 side of the test plate on the flint lens and, using a sodium light ("A.T.M.," page 244), if we have ground tightly to our gage, and if we have entirely freed both surfaces from dirt or lint which would prevent the ring pattern from forming, we shall immediately see the rings.

We place the thumb and forefinger of each hand on the edge of the test plate, making a roughly four-cornered application of pressure, and press lightly The rings, under pressure, will appear to well up, or roll into the center of the lens. If the rings well up out of the center and roll to the edge, the lens is "high," too convex, or overcorrected. If, under pressure, they roll in toward the center and seem to disappear down into the center, then the lens is "low," too concave, or undercorrected. The simple and easily remembered rule of test plates can be memorized by this mnemonic: "Since high is up and low is down, then it follows: if the rings come up the lens is too high, if the rings go down the lens is too low."

If the lens is high, or too convex, we must polish the center more than the edge. If low or too concave we must polish the edge more than the center.

On this job all surfaces are permitted to be as much as five rings higher or lower than the test plate that fits them. This will give us a ten-ring range for smoothing out irregularities. For every ring (consisting of one bright band plus one dark band) we are 1/100,000" away from our test plate curve.

If the rings are even and concentric, then the lens also will be spherical and have no zones.

If, under Foucault's test, the test plate showed no zones, then any zones that show in the rings will be on the lens surface we are testing.

Now for the contour test, or using the test plate to show what the surface is by an actual picture. It is not necessary to know this method but it will help the worker in understanding how his test actually looks. We place the test plate on the lens and, when the rings have appeared, apply pressure with one finger on one side of the lens at the edge. The rings appear to roll either toward or away from the point of pressure. They roll apparently right off the edge of the lens and then leave curved bands (Figure 3).

If the rings roll away from the point of pressure they will show such a series of bands as in 1; if toward the point of pressure they will show a pattern such as in 2.

If we regard the drawings as round, framed pictures and the point of pressure as its bottom, then 1 is a picture of a lens that is too low and 2 a picture of a lens that is toe high.

Again, 3 is a lens that is high, with a hole or low zone in the center, and 4 a lens that is low, with a still lower hole or zone in the center.

CENTERING-When the lenses or components have all been polished and show approximately five or less concentric rings above or below the test plate (ignore turned edge if not over 1/8" wide, as the cell mounting will hide it), we are ready to center or true them up and bring them to equal diameter. I believe that a centering spindle should be vertical, to minimize chance of the lens falling off while its optical axis is being lined up to make it run true with the axis of the spindle. The flint lens will be centered first, as the possibilities of developing prism or "wedge" in it are much greater than in the crown. Unless our measurements around the edges of the components, to eliminate wedge in grinding, were very lax, the crown will be so close that it will unquestionably center down to the flint, whereas the flint usually loses more in centering, due to the long radius of curvature of its back.

The spindle may be anything that ingenuity may devise. Figure 4 gives an idea of one such setup. The target used in alining the optical axis of the lens may be an ordinary flashlight with a 'T' cut in a black paper mask placed over its end. Spindle speed should be about 250 rpm. The target should be about 2' above the spindle.

The flint lens is mounted on the spindle in the following manner: Take it upstairs to the kitchen and, over the little woman's or girl friend's protests, place it in the cold oven. With the gas turned low bring it slowly to a temperature that could be called "good and warm''-say, 140 degrees, Fahrenheit.

Next, heat the spindle with a bunsen burner, smear ordinary sealing wax around its edge, and place the warm lens on it. Rotate it slowly and note the reflections of the target light. There will be two, one from each surface of the lens, and these reflections will appear to rotate as long as the optical axis of the lens remains eccentric with the axis of the spindle. Revolve the spindle and move the lens this way and that (warming the spindle occasionally with the burner to keep the wax soft) until the reflections are stationary. Now the respective optical axes coincide but, unless the lens edge was ground more uniformly to thickness all around than is likely in spite of care, the lens itself is off center and must be centered. The actual centering is accomplished by applying slow abrasive pressure on the lens edge, by means of a brass band controlled by a turnbuckle, using No. 180 emery or Carborundum. If you prefer a smoother finish, give the edge about two minutes additional of No. 500.

Stop the centering of the flint as soon as the edge shows no flat spots. Stop the crown as soon as the diameter of the flint lens is reached. The remainder of the excess diameter can be taken up by the cell mounting, as this lens is calculated to give the very best results with a finished diameter close to 4-7/8". It is unnecessary to grind off excess diameter. Removing glass that can do no harm to the objective's ultimate performance can be a long and tedious process.

CEMENTING-Procure from your druggist a piece of pure Canada balsam and a half pint of alcohol. Place both lenses in the cold oven and warm them slowly to a point at which the balsam, under trial, will melt instantly upon application. This will be very hot, since we do not want the balsam to be merely glue-like; it must be liquid.

Remove the flint lens from the oven, place it on a clean paper and apply about one half teaspoonful of melted balsam to the center of the concave.

Now, using gloves, remove the crown lens from the oven and place it straight down on the flint. Do not slide it. Press down with considerable pressure. The balsam in the center will spread evenly out to the edge and the excess will run down on the paper. Pressure on one side or the other will persuade bubbles to disperse to the edges and disappear.

The crux of the operation is heat and lots of it but do not hurry it: apply it slowly. A good way to keep the lenses hot while eliminating trapped air bubbles is to rest them on the flat, level face of an inverted and braced electric flatiron.

Clamp the achromat around the edge with a simple metal band to keep the lenses from shifting off-side and let it cool until cold-naturally, not assisted, since to hurry the cooling is to ask for a cracked lens.

When it is cold, remove the band and clean the excess balsam from the edge with alcohol and, behold, you are now the proud possessor of a perfect telescope objective!

ANOTHER DESIGN-I chose the specifications named because I have produced this achromat and found it gave excellent results on all tests for all aberrations, with a minimum of secondary spectrum. As an alternative I submit specifications for a smaller but equally good lens for the amateur who does not wish to tackle a larger achromat as a first attempt.

NO READER need be left wondering whether Driscoll claims to have originated the above-described test plate method (sometimes it is called proofplating). Fearing this might happen, he asks that its true antecedents be made as plain as day. This basic method of reproducing lenses was used long ago by Zeiss and others. It is described in Dévé "Optical Workshop Principles," also in Twyman's "Prism and Lens Making"-not, however, with added instructions for making objective lenses. Driscoll's true contribution to fellow amateurs consists essentially-in addition to the valuable specifications for two achromatic objectives which he donates-in the preparation and offering for publication of a rounded, organized systematic sequence of operations for making objective lenses for refracting telescopes by amateurs, employing the test plate method as its most characteristic part. Nor has anybody actually published such a procedure for that specific method, so far as is known. (But it does the public no practical good to be told by someone in a commercial industry, after a given method has been published, that it has previously been in use in a given shop no one in that industry has ever bothered to make it available to all.

When Driscoll's manuscript was received, it was shown to Dr. D. Everret Taylor, 191 Prospect St., Willimantic, Conn., author of a chapter on the refractor in "A.T.M.A." He took an immediate interest in it and, with a copy of the manuscript constantly at hand made a 4" objective lens strictly according to the steps of the testplate procedure. His comment while making it was: "The Ronchi bands, which permit of being reduced to one in number, thus revealing almost maximum distortion, are straight." At the completion of the job he wrote: "Shop and indoor testing of the unmounted lens for star image and definition give this method a high rating. It offers every opportunity for the most precise craftsmanship, since at all times one can know the exact condition of the surfaces. It eliminates two big items: iron tools (difficult to machine accurately) and an optical flat. In short, after following it through, I would switch to Driscoll's procedure if tomorrow I were to start the most important objective of my experience."

FURTHER notes on the Driscoll procedure will follow next month, including details of a method of reducing the size of the objectives he has specified, if this is desired.

OPTICAL glass specified in these articles cannot yet be had-needed for war. Have to wait a little.

NEWS note: Thus far two Maksutov telescopes have almost been completed, out of the 19 entrants. Most of the 19 are doing war work, which comes first. Have to wait a little here.

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