Professor Thomson's activities have been almost legion but he never has forgotten that he is an amateur telescope maker, one of the kind described by Professor Hale in "A.T.M." (page 180). At present he is "amateuring" a rather larger job than any amateur seems likely to tackle for sometime to come-the fused quartz disk for the 200-inch telescope. However, most of Dr. Thomson's activities have been in the electrical field. He is one of the founders of the General Electric Company, and is the recognized "dean of electrical engineers." He has taken out more than 700 patents the third largest number granted to any individual. His account is as follows:

"AFTER having acquired experience in the making of two or three smaller objectives, including one of eight inches aperture, I determined about 1899 to construct a telescope of ten inches aperture, and mount it in a suitable observatory building.

"To this end, I obtained two guaranteed disks of glass made by Mantois in Paris, from their agents in New York. These disks of flint and crown glass respectively were 10-1/2 inches in diameter. The crown glass disk had a density of 2.543, and the index of refraction for the D line (sodium) of the spectrum was 1.51709; (A line = 1.51139; C = 1.51446; F = 1.52335; G = 1.52848; mean dispersion C to F = .00892). For the flint glass disk, these values were: D line = 1.62842 (A = 1.60621; C = 1.61158; F = 1.62842; G = 1.63866; mean dispersion C to F = .01684).

"It was decided to follow a form given by Steinheil, which gave a focal length of 135 inches, approximately, the flint disk to be the outer one. R₁ is convex, R₂ concave, R₃ convex and R₄ convex.

"Four disks of heavy plate glass (porthole glass) were secured for grinders, and a machine driven by a small electric motor set up with means for reciprocating an iron grinder for roughing, and below this on a vertical shaft, a face plate about 7-1/2 inches in diameter to which the disk in work was cemented by three spots of pitch.

"A CIRCULAR iron catch box open at top contained the wet grinding powder; in this case, Carborundum No. 40 or 60. This catch box, E, was 16 inches in diameter, and to it was attached an adjustable edge grinder of copper, D, for truing the edges of the lenses of glass disks. The small iron grinder disk, I, (about 4-1/2 inches in diameter) could be given a long or short stroke and set to move over either the center or the diameter of the revolving disk cemented to the face plate, or over chords of it. In this way, by changing the stroke, the roughing out could be made to produce either a concave or a convex form at will, and the radius of curvature could likewise be varied as needed. This small iron tool was used only for roughing out, as it could be given a stroke which would work on one part without unduly wearing down the other parts.

"There was also a third beveling grinder, C, set to bear up under the edges of the lens disks so as to produce a level edge. The figure shows the general arrangement. All three grinders are adjustable. The iron disk can rock on the pin, which is carried by the reciprocating wooden beam B; the grinder under the edge at C can be raised as it cuts away the under edge of the disk; while the edger at D is made to swing toward and from the disk edge, gradually reducing it to complete circular form. The progress toward proper curves was watched either by a spherometer or by templates.

"A very convenient way of making templates of desired radii is to affix a glazier's diamond to a radius bar and cut them from ordinary glass plates, such as window glass or photo plates. When long radii are concerned, this can be done on an open flat floor. In any case, the cut curved edges of the glass plates are ground together on a flat board until they match. This is a speedy way of securing such templates as are needed. In using such templates, the curved edges can be blackleaded by a pencil and applied to the lens surface, with a slight motion in the length, which will cause a mark to be made on the ground lens surface where the actual meeting contact is.

"After the disks have been roughed out and brought to approximate radii, the work is transferred to the proverbial barrel, around which one walks. Care must be taken to support the lens disks evenly and arrange stops (generally of wood) to bear on the edges of the blank to keep it in place. Calipering the edge to secure even thickness all around is a necessity, and the grinding must be governed to do more work on the thicker parts until all is uniform.

"In the case of my 10-inch, the surface R₁-the outer surface of the flint glass disk-was ground to an approximate radius of 74.65 inches, but allowed to become considerably shorter; below 73 inches. This was to enable color correction to be better obtained, as estimation showed the likelihood of this provision with the glass used. The rough shaping was by the use of No. 60 Carborundum, changing to No. 120 when near the desired curve.

"One of the plate glass grinders, ground down to present a convex side of about the radius desired for the concave side of the flint lens and the concave side R₂ of the flint lens, was worked on the barrel with this deep convex tool and soon brought to a fit. The curve was adjusted by care, with the aid of the spherometer as a guide, to approach a value of 28.98 inches = R₂. Local grinding at edge or center by small glass grinders enabled such adjustments to be made. Heavy glass tools, except for the rough grinding, were always used. They were generally a little less in diameter than the lens itself. A surface too concave is ground by strokes which avoid the center, while one too convex is worked so as to avoid the edges. In all cases the two fitting, surfaces-those respectively of the lens and of the glass disk about eight inches in diameter which is used to produce the surface-are brought to exact fit before measuring. In this way, I have altered long radii of surfaces by small fractions of an inch at a time.

THE crown glass disk was carefully brought to an even thickness all around, and the surface R₃ was brought to 28.56 inches radius, using on it the deep concave grinder which was used to form it, with No. 120 Carborundum. The last or back convex surface of the crown lens was then shaped to approximately R₄ = 230 inches.

The next step was the 'smoothing' or fine grinding of all four surfaces, without permitting change of radii. Fine washed grades of Carborundum were used for the last 30 minutes during smoothing.

"Next came the polishing with pitch and rouge. Surface was R2 first worked, then R₃. At this stage, R₁ was given a partial polish. I term this 'glossing.' Though the polish is not more than partial, still it is sufficient to enable optical tests to be made and can be produced in a few minutes.

"The same 'glossing' was given to R₄.

"Tests for color correction and spherical aberration were now made, using an artificial star in the usual way. There was found overcorrection for color and considerable negative spherical aberration. The surface of flint lens R₁ was then reground and given 71.8 inches radius, while R₄ was worked locally for spherical correction, as most of the fault seemed to be in that surface. Tested again, the color correction was found to be satisfactory-pale lilac and soft apple green overhanging. Completion of polish was now given to R1 flint outer surface, leaving R₄ (back of crown) still partly polished. After quite a series of tests involving repeated trials and small corrections, surface R4 received its final polish. Result: ring systems inside and outside of focus good, and definition excellent. Tests of focal length gave 139.5 inches; aperture full ten inches, a ratio of aperture to focus 1:14, very nearly.

"The polishers were iron disks seven inches in diameter, faced with the usual pitch squares and worked with levigated rouge and water. The final measurements of radii gave: R₁ = 71.8"; R₂ = 28.93"; R₃ = 28.5"; R₄ = 229.04".

"The lens was placed in its cell and mounted on a Warner and Swasey steel tube. The equatorial mounting was partly made from my drawings, and many of the patterns for the castings, including the heavy, hollow pillar, I made myself. In the pillar is the drive, which consists of a small fan motor (A.C.) controlled by a sensitive centrifugal governor which I constructed for the purpose and which I found very satisfactory. Through a relay it cuts the motor in and out at about one-half second intervals, and can be regulated closely.

"For the purposes of testing, I had made a 12-inch plane and used it with an artificial star in all the later stages of adjusting and figuring the 10-inch objective. To avoid dust and grit while the delicate polishing was carried on, the work proceeded in a closed room (no open windows) and in the hottest weather, a great advantage of which is that the temperature of the skin is very near that of the glass, and danger of distortion by irregular heating is diminished. Further, the extreme humidity, though very uncomfortable, permits the grinding and polishing to proceed without rapid drying of the water out of the powder being used.

"Tests on the stars showed excellent definition, and the characteristic interference rings inside and outside focus were developed symmetrically and were perfectly circular when a star was observed. The performance of the lens was as good as could be expected. It resolves double stars down to 0.5 second separation, and this is about the theoretical limit for 10-inch aperture. When the members of a pair of small stars are 0.3 second apart, it will show an oval, or when the two are of different magnitude, an egg-shaped, image. Small stars at 0.6 second apart are seen very clearly separated. The outstanding secondary spectrum is at a minimum for the glasses used, and the lens shows stars of small magnitude, with a single interference ring around the image. On steady nights during favorable oppositions of the planet Mars and at times of best seeing, the so-called canali of Mars are plainly visible, with the other markings of that interesting object. The divisions in the rings of Saturn are distinctly observed, and at times a graininess of the inner ring, like the streaks from a paint brush, is seen.

"I have not dwelt on the mechanical details of the mounting, which is in essence the usual equatorial, nor have I dwelt on accessories, such as spectroscopes constructed by me in my workshop, helioscopes and the like-deeming the central problem and the one demanding most skill to be the objective lens itself."

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