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Originally Posted by 99Panhard
By all means, use my description. I'm glad it was understandable since its very difficult to describe a process you are familiar with to someone who isn't. You always leave something out.
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You really did an excellent job. Making it very clear for me (and perhaps others not initiated into the elements of the process).
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I suspect that the drawback to using male/female laps against each other is that both pieces will wear. I am guessing that if the lens maker had a set of gages like I used, which are simply used to check the curve, the lap could be spun in any lathe-like machine and the surface selectively filed or ground to return it to true. As I've said, I held the lap and the gage up to a window and looked for a streak of light between them...a very accurate way of measuring once you have some practice and know what to look for.
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Does not such a file leave a grove in the metal? Is any such groove then polished away, so as to not leave a striation on the glass? (Do you have a photo, or a sketch of any such gauge, as I can't quite picture it. It sounds like a simple device.) A diagram would be invaluable.
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One criticism of the reproduced tool illustrated here. I also suspect that the lenses were either convex on both sides or, more likely, flat on one side and convex on the other (like modern rifle scopes. This would make it much easier to find the optical center of the lens and trim the outer edges accordingly.)
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Yes. This is the predominant lens, and most likely so for Spinoza who argues for it.
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One of my jobs was called "layout" where, using a prescription as a guide, I would mark the desired optical center of the lens with a little sharpened stick dipped in india ink. You'd think this was practically impossible but after a week or so of doing it you can literally get it within a fraction of a millimeter. The little metal piece I described was positioned so that the center hole was directly over the optical center. Again, its sounds almost impossible but with practice you come very close with very little effort.
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Fascinating.
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The power of the lens represents the total of the inside and outside curves so I suspect that the "grinding cup" (for lack of a better term) on this reproduction has much too deep a curve. Its the sort of subtle difference that a period artist might easily fail to understand.
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Very good point. I think they took it straight from the drawing by Manzini:
It is possible that the curvature is only for effect, but then early telescopes were of much shorter focal lengths than later ones. These are like very extended spy glasses. The original telescope had a focal length of a convex-convex lens at 18 inches, and a concave-concave lens at 6 inches. By your eye though, it is still too much?
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I also have the 1723 French edition of Isaac Newton's "Traite du Optique" which I suspect contains much pertinent information from only 100 years after Spinoza. the text may well contain information on how things were done and when advances took place. Unfortunately I can't read French but there are numerous copper plate engraved illustrations. Let me know if you'd like me to try to photograph some of them for you.
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Thank you so very much. Unfortunately I don't read French, but if there is any non-savant (that is, non-inventive), elementary diagram of lens grinding techniques, that would be of interest.
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Another memory just came back...I think that the felt was attached to the lap with fish or hide glue. The lens was checked by holding it up to a light bulb with a single filament. You held it in such a way that the light from the filament reflected off the surface. If there were no breaks or nicks in the reflection, the lens was perfectly true. This could also be done by stretching a hair across a window and picking up the shadow. You could never see the imperfections with the naked eye. Now I'm beginning to feel like a "living history" exhibit!
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This is rather remarkable, I might say, because Rolf Willach (a specialist in the field of antique telescopes and lenses claims that the testing of the lens's sphericality was a major problem for early telescopists, something they could only achieve by putting the lens into the telescope itself and looking at stars:
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Originally Posted by Willach
The Venetian spectacle makers polished their lenses on a rotating felt or deer leather. It was at that time the only known polishing method and we shall discuss it in more detail below. With this method, during the polishing process, the originally spherical ground lens becomes more and more aspherical, beginning first at the edge and then gradually towards the central part. Now the great problem for the spectacle makers was that they had no testing method and, therefore, were unable to control the increasing aspherical deformation. In the end, they had many lenses with an enormous variation in different qualities.
Therefore, Galileo had to examine hundreds of lenses in order to find a few suitable for astronomical use. With the best lenses, the useful central part has a diameter of 20± 25 mm and, with the mediocre lenses, only 10± 20 mm. All other lenses
were trash.
Now, how did Galileo examine his lenses ? He had no other option than to put
each lens in a tube of suitable length and then to look at bright fixed stars on clear nights. Before this test, he had to stop down the aperture of the lens to such an amount that the chromatic aberration remained below the resolving power of the human eye...
...Now it is important to realize that Galileo, with this test, could obtain no
information about the quality of the lenses in the part covered by the aperture stop. It was impossible for him to find any quality difference between, on the one had, a lens with a fine image inside a free aperture of 25 mm but with rapidly increasing asphericity towards the rim and, on the other hand, a lens such as the `single lens ’ in Florence, which was perfect from the centre up to the outer edge.
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Perhaps we are talking about different thresholds of sphericality, but it seems that what you by experience understand, Rolf Willach denies. What do you make of this. (It is a signficant point to my subject.)
Also, when you say "reflects off the surface" what surface do you exactly mean?
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(my 3rd edit) Will this ever stop? The lens was finished in what we called an "edger" which was nothing more than a lathe-like spindle that gripped the little metal piece glued to the lens and spun it against a grinding wheel. These were not the modern clay-based wheels but slow turning natural stone wheels that ran in water, the grinding wheel turning one way and the lens in the opposite direction. In this way the outer edge was gradually reduced in a manner perfectly concentric with the optical center. Even if the metal attachment was slightly off center on the original lump of glass, this process insured that it would be perfectly concentric when finished. You could only remove the metal piece after this was done and you could not replace it perfectly so it was a once-chance-only affair.
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This technique (minus the running water) was I believe similar to the one devised by Christiaan Huygens in his drawings, though it is not sure that he ever built such a thing, perhaps having difficulty with the alignment of the axes. At the time Spinoza argued against the use of any mechanized fixing of the lens, finding a hand held lens the most sure and productive. I don't know if this was due to the state of the art of machine lathes (their imperfection) or simply a matter of Spinoza's own very patient and slow polishing process, but just at this time the mechanization (craftmenlessness) of the lathe was coming into view, begun with Descartes' own fantastical machine.
Thanks for all your views, memories and experiences. Most appreciated.