Spinoza's Lathe

[kvond]

17th Century Lens-Lathing

I am hoping to find someone who knows something about the history of 17th century lathes. I am looking into the possible construction of the philosopher Spinoza's lathe. Some may not know that the famous rationalist Dutch philosopher was a grinder of lenses, and a builder of microscopes and telescopes. I am attempting to make much more clear the kind of mechanism he likely used.

If anyone has an expertise in this area help would certainly be appreciated.
The issues of interest are these:

1. There is at the Spinoza museum in Rijnsburg a reproduction of a 17th century lathe. Some pictures of it can be found here and here, on my weblog. If any one can identfy the type of lathe this likely is, this would be wonderful. (The authenticity of this lathe I cannot establish.)

A sample:

2. I have read much on the history of the lens-grindling lathe at this period. The difficulty is that most of the texts are about experimental models being made by savants. I still do not have a sense of exactly what the average lens craftsman would be doing? For instance, would a blacksmith cast the "forms" for lens-grinding? What trade guilds ruled spectacle production in Holland? If anyone has a vivid sense of the state of the art of lathing for optical glass, from metal forms to finished glass, I would value their opinion.

3. I have also come to the realization that as Spinoza did not likely learn his trade from Guilded Spectacle Makers, he may very well have learned it from diamond polishers in his community. If anyone has any knowledge of the kinds of gem polishing lathes, and techniques that were being used in the 1650''s in Amsterdam, or any ideas for sources on these facts, this would be of great help. Spinoza is known to have been quite adept at polishing his lenses, something that was admired by some of the most brilliant craftsmen of his time, and it may very well be that by using gem-polishing techniques he had advantages that other lens-grinders may not have had.


Thanks in advance for any thoughts that might be given. Here are my weblog on the issue of how Spinoza's lathe-work may have influenced his philosophy:Frames /sing: Spinoza's Foci

 

[L Vanice]

I have seen amateurs grinding lenses for telescopes entirely by hand using two glass disks and abrasive powders. No machine was required. Perhaps that technique was used 350 years ago as well?

I have never seen a lens grinding lathe, but I have noted that the word lathe is used for very different machines by people in different branches of mechanical work. I have seen a modern machine for grinding the concave side of eyeglass lenses, but I don't recall what it was called. It probably was a way of mechanizing the motions of the people I saw making lenses by hand. In some types of work, the name lathe is given to what a machinist would call a bench grinder or buffer.

A lathe of early design, but still recognizable to modern machinists as a lathe, would have been used to make the brass fittings for telescopes and microscopes. Such a lathe would have been about the same as those used for clock and watch work during the same period. In other words, you could study early horological tools, which might give you more information than restricting your search to optical tools.

Larry

 

[kvond]

Thanks Larry,

Spinoza most definitely must have used a lens-grinding lathe. This is an example of an early 17th century lathe from an optical text by Manzini:

You are very right that the kinds of lathes used were related to those employed by clock and watchmakers. In the one depicted above, we have the craftsman holding the glass in the left hand, up against a spinning metal form. By Spinoza's time his lathe was likely foot-driven, and the grinding done at eye-level.

It seems that the details generally allow that iron "templates" were made by grinding convex and concave forms against one another. These forms are then used (by one account), to make "tools" which are iron or brass, and more highly polished "dishes" or "plates". Into these plates are ground the glass blank with a bit of abrasive. It is not altogether clear that this was the universal method, but something like this was done in the lathing of glass lenses.

I still am having a hard time getting the details of this process, if indeed it was very common, and cannot get a grasp on the nature of the lathe at the Spinoza museum, or what techniques diamond polishers may have used which are different.

I do appreciate all thoughts.

 

[L Vanice]

The illustration above is an example of what a machinist would call a grinder or lap, but a dental lab or jewelry maker would call a lathe.

It has little in common with the lathe used for turning round shapes in wood or metal.

An optical instrument maker would have used both kinds of machines.

Diamond work uses circular saws with thin copper blades to divide a rough stone in two. The flat facets are created using laps with tin or copper disks rotating in a horizontal plane. The girdle is the only part of a faceted diamond that has a cylindrical shape and the girdle is not highly polished. Diamonds are not normally finished with spherical or cabochon surfaces.

Larry

 

[kvond]

Thank you Larry, perhaps we are getting somewhere:

This is what I have found regarding the diamond cutting and polishing near the period we are talking about.

"The diamond finishing industry comprised various occupations, which each required separate skills. Rough diamonds were cleaved first, then cut into shape, and finally polished. Cleavers and sawyers were at the top of the diamond workers’ hierarchy, followed by the cutters. The cutters consisted of brilliant cutters and rose cutters. Brilliant cutting was slightly more complex than rose cutting. Master polishers came next, followed by apprentice polishers.

Diamond polishing involved positioning the stones horizontally on a rotating disc attached to a polishing mill. They were soldered onto a so-called cap (a kind of copper retainer). The diamond had to be rotated inside the cap. This was done by turners. Disc sanders sanded these discs. Although disc sanders were not actually part of the diamond industry, they were obviously affected by the ups and downs there."

As you can see, a different person likely did the different processes, and probably with different tools. I am in particular interested in the polisher, the least skilled in the bunch. I believe that only the "rose" cut was evolved at this time (1650):

It is quite like a plano-convex lens. While you may be very right that contemporary polishing is not done with a spherical surface, the text seems to indicate that at the time they may have been. I cannot quite see the technique that is being described. It seems to suggest that the diamond was rotated within a cap (I imagine that the diamond was held by hand, but cannot be sure).

If Spinoza learned his polishing from diamond polishers, it is possible that he also used comparable devices or techniques.

 

[HelEx]

I've done diamond polishing (usually called cutting for some reason, but faceting is a better term) and I can assure you it has very little in common with lens shaping and polishing, which I've also done. On the other hand, in the distant past, semi-precious stones and even precious stones were often cut en cabochon, basically buffing up and rounding the existing shape. Diamond faceting and cutting are done in the one action after it is cleaved and bruted to rough shape - the facet is polished and cut in the same operation. Lenses on the other hand have to go through a sequence of coarse to fine grinding until the final polish which for the highest quality lenses is still done using rouge (actually cerium oxide now) with a pitch lap. Probably glass was ground to shape using garnet sand or emery. The lap could have been made from wood, probably the end grain - the sand would penetrate into the wood and protect it from wear to some extent. The Chinese jade "carvers" sometimes used wooden tools on their lathes. There is no real need for blacksmiths to be involved in lens making.

- Mike -

 

[kvond]

Thank you mike.

I believe the blacksmiths Guild was involved in lens making to some degree due to there being no outright guild for lens making right off. But the role they would have would be in casting, or shaping the metal forms that lenses were ground in. At least in 1610 these forms were pounded out and shaped by hammers. It could be that turners did this job as well, I just don't know. But Spinoza does say in a letter that someone else makes his metal forms ("dishes").

As for the relation between possible diamond polishing and lens polishing, Spinoza was known for his remarkable polish of the lens. Part of the process of polishing was the "trade secret" of just what kind of abrasive to use, and when. By Spinoza's time it seems that the best process was the use of tripoli rubbed into paper that had been pressed/glued into a metal mould. It could very well be that Spinoza would have learned other techniques of polishing (for instance a paste of diamond dust? I am guessing.) certain recipes or materials which would have given him the edge in either polishing his metal forms, or the glass itself. Do you find this improbable, by your experience?

This is perhaps the closest grinding lathe I have found:

 

[99Panhard]

When I was in my 20s I worked for a couple of years at the Peerless Optical Co in Providence, Rhode Island, making lenses for glasses. While much of the work was automated to a degree there was still a little corner of the shop where very special lenses were ground. Because I was actually interested in the work, that became my department.
The lenses were ground against iron forms, called "laps" (either convex or concave) using a variety of progressively finer abrasives. The final polish was achieved by gluing a thick disc of felt to the lap and using a much finer polishing media. The lap spun in a bucket-like contraption that worked very much like a potters wheel. The lens was kept in contact with the lap by means of a hinged arm with an adjustable pin. The arm was held in place with the left hand, the pin pushing against the lens, while you added abrasive to the lap with the right hand. To secure the lens without damaging it, a small flat piece of metal with a center hole was "glued" to it using thick green pitch, exactly like the "sealing wax" used before the invention of gummed envelopes. We melted the pitch onto the lens with a bunsen burner. It was removed by chilling the whole piece, at which point the pitch would harden and fall off the glass.
Other than the motor that spun the lap, there isn't a thing about this whole process that any 17th century mechanic would find surprising. Also, with particularly difficult lenses, I would have to forgo the hinged arm and hold the lens against the lap with my hand.

In our case, a special purpose-built machine re-cut the laps when they wore...I had a beautiful engraved set of brass gauges which I used to check them (by holding the gauge and lap up to a window) and which must have been 100 years old or more when I was using them. I can see where a lathe of some sort would be essential for making the laps, a primative lathe would suffice, but I can't see it being used to actually make the lens itself.

The machine illustrated in the post above this one is very much like what I am describing. In fact, other than the hand operation it would be instantly recognizable to anyone who was making lenses in the manner I was. I actually made a couple of lenses for an antique telescope on this equipment...they worked perfectly.

Joe Puleo

 

[rivett608]

You might want to take a look through this....

Abell,Leggat & Ogden "A Bibliography of the Art of Turning & Lathe & Machine Tool History"

This is the most complete source of early books that mention the lathe..... I am away from my office so I can't look through mine... it can be had (I think still) from these folks...

http://www.the-sot.com/

And for some reason I think Plumier mentions a earlier book on lens grinding or cannon drilling in his L'Art du Tourner, 1701.....

 

[stef.]

Lens Polishing Lathe

I found this image of a lens polishing lathe in a book titled "SCIENTIFIC INSTRUMENTS IN ART AND HISTORY" By Henri Michel. I hope this helps your quest. I put the image in photo bucket but I'm having difficulty resizing to enable a successful download. I can email to someone with the knowledge to resize.

George


The image was taken from a scan.

 

[kvond]

The machine illustrated in the post above this one is very much like what I am describing. In fact, other than the hand operation it would be instantly recognizable to anyone who was making lenses in the manner I was. I actually made a couple of lenses for an antique telescope on this equipment...they worked perfectly.

Joe Puleo

Joe, your description is wonderful. In the last day I actually have found a reproduction of the model you speak of, made for the 400th year anniversary of the telescope:

Would you mind if I use your description as a post in my blog. It just perfectly describes the timelessness of the action, the simplicity of the device.

As for the recutting of the laps, it seems that some savants had attempted to make all in one lathes which could cut the laps and then grind the lenses. The primary means of "truing" the laps (at least when they were first made) was grinding them together with its inverse mould, convex to concave, to create the spericality. I am unsure if this method was used to correct the laps after use though, texts do not say.

It is also of note that in telescopic lenses it is theorized that the use of felt for polishing actually wore down the edges of the lense, and it was not until the discovered use of "tripoli" (a kind of organic "chalk"), was the felt problem solved. But you say that the method worked well, so perhaps the theory about this change is wrong about its importance, or perhaps you had better felt.

I must say that I am quite enthused to be in contact with someone who actually engaged in these practices. Your interests and experiences are invaluable in terms of perspective.

Thank you very much.

 

[kvond]

You might want to take a look through this....

Abell,Leggat & Ogden "A Bibliography of the Art of Turning & Lathe & Machine Tool History"

This is the most complete source of early books that mention the lathe..... I am away from my office so I can't look through mine... it can be had (I think still) from these folks...

http://www.the-sot.com/

And for some reason I think Plumier mentions a earlier book on lens grinding or cannon drilling in his L'Art du Tourner, 1701.....

Thanks R608, That source looks quite useful. I will look into it.

 

[kvond]

I found this image of a lens polishing lathe in a book titled "SCIENTIFIC INSTRUMENTS IN ART AND HISTORY" By Henri Michel. I hope this helps your quest. I put the image in photo bucket but I'm having difficulty resizing to enable a successful download. I can email to someone with the knowledge to resize.

George


The image was taken from a scan.

George, sure, email it to me: [email protected] . I'd be glad to see it.

 

[stef.]

Lens Polishing Lathe

KVOND:

Will do. About 10:00 PM tonight when I get to my desk top.

Regards, George

 

[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. I'll add that chalk was well understood as a polishing agent for metal in at least the early 18th century and very likely earlier. For instance, the locks and barrels of military firearms were customarily very highly polished. In the field soldiers would use powdered brick dust on a wet rag but chalk is frequently described as the polishing agent actually used in manufacturing.
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.

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.) 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.

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. It would actually be quite easy to replicate the process I used for experimental purposes and I think you'll find that much less curve is in order. I once made a pair of sunglasses out of the bottoms of Coke bottles...terrible glass, soft and full of bubbles but it could actually be done!

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.

I haven't thought about this stuff in more than 30 years buts its really odd how it comes back.

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 filiment. You held it in such a way that the light from the filiment 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!

(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.

Joe Puleo

 

[L Vanice]

It has not been mentioned, so I will point out that one way to generate a spherical surface is with a round tube. Balls made of crystal, agate or other materials can be made by lapping them while held between the ends of two intersecting rotating tubes of about two-thirds or three-quarters the diameter of the finished ball. The skewed axes of the rotating tubes keep the ball in constant motion while various grits of lapping paste are used. If you consider the geometry, you will understand how the ball can be generated. The early plano-convex lenses could have been made by slicing a bit off the side of a sphere.

That kind of ball-making machine is probably of fairly early origin. Certainly crystal balls and agate marbles have been around for a long time. I could not find an example with Google, but there must be some pictures out there somewhere.

Another way to make a spherical form, from material like wood, ivory or soft metals, is with a hand-held tool of hard steel that has a sharp edged hole somewhat smaller than the finished ball shape. First you turn the shape in the lathe as near spherical as you can, by eye, using a hand chisel or graver. Then the true spherical surface is generated when the tool with the sharp hole is swung around the work rotating in a lathe while pressing against the end of the work. This method could create the convex iron forms used for making lens laps and is pretty low-tech.

By the way, the "thick green pitch" mentioned by Joe above is called dop wax by gem polishers. It is indeed very similar to sealing wax. The handle or fixture rod attached to the gem is called a dop stick. I bought an assortment of semiprecious gem rough, dop wax and dop sticks from Sears back in the 1950's. I did not get so far as getting a lapping machine, though, so I have little personal knowledge of the art. I did take a university class in gems, and learned quite a bit there.

Larry

 

[HelEx]

Spectacle lenses are regarded as having a poor quality finish because of the felt laps. On a fine scale they have an "orange peel" finish. The polish with a felt lap is achieved more quickly than with a pitch lap, but makers of precision lenses always use pitch which is shaped to fit the lens (or a pattern of the same shape). Once the pitch has solidified it has grooves cut in it, often in a grid pattern, but the pattern varies depending in how the lens is to be "figured". The grooves serve to hold the polishing abrasive which is put on as a slurry and must be kept wet or the lens will attach itself to the pitch and it is very difficult to remove. The reason why pitch gives a better finish is that it doesn't comply to the finely ground finish on the glass, but only takes off the high spots until finally all the high spots are removed. For the best finish (not usually available commercially) the optician will then polish for twice as long again to remove sub-surface damage. This has no connection with what Spinoza would have done of course.....

- Mike -

 

[99Panhard]

If I remember correctly, we had to be careful not to get any sort of "ripple" effect in the surface of the lens. This could also happen by polishing too long so the perfect result came from just the right amount of rouge and careful pressure on the lens. The gentleman who founded the company I worked for - my uncle actually - was known up and down the east coast for the quality of his lenses.. In fact, although most optical labs are basically local, we got special lenses to make from as far away as Maine and Florida. This was very unusual at the time. Fast delivery was accomplished by taking them around the corner to the bus station and putting the package on a bus... there was no FedX then. I don't doubt that even our best fall short of what is required in the optical business today but I suspect it was probably as good as someone could do in the early 17th century.
Whether they used felt or not, I've no idea but Amsterdam was the center of the world felt hat making trade at the time so some form of that material may have been available although it would have been very expensive. 17th century felt hats were made from beaver fur (hence the immense profits to be made in the fur trade) but whether beaver felt would work I also don't know. The felting process itself can be applied to almost any fiber so I wonder if there weren't special ones for polishing. Such a degree of specialization is well within the customs of trade at the time.

I'd think that getting the "optical grade" glass would be the biggest problem.

Joe Puleo

 

[rivett608]

Another long shot might be some of the Russian books that were translated into English...... do some searching here on the PM for Nartov , he was Peter the Great's mechanicain and we have disscuess this set of books in some threads on him I think. I am about 5000 miles from my studio so I can't be of much help right now. Another place you might inquire is with Bruce Bradly at the Linda Hall libaray in Kansas City, Mo..... we did an exhibit there years ago on books of lathe work.... of the almost 50 books, only 3 were 19th century.... everything else was earlier... some much earlier. Ask him for the catalog or list.

 

[kvond]

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.

You really did an excellent job. Making it very clear for me (and perhaps others not initiated into the elements of the process).

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.

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.

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.)

Yes. This is the predominant lens, and most likely so for Spinoza who argues for it.

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.

Fascinating.

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.

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?

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.

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.

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!

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:

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.

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?

(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.

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.