[ATM] how many errors can you spot in this passage on Foucault?

[Guy Brandenburg]

Some of the readers on this list found some of the errors, but here are a few others that were not found, or at least not yet reported on so far

(1) Foucault did not claim that he invented the method of silvering glass. He gave credit to Liebig and Steinheil on that.

(2) The only astronomical (sort-of) use of rear-reflecting mirrors anywhere near that time frame (that I know of) was the Mangin mirror, and neither Herschel nor the Earl of Rosse nor anybody else was using those for telescopes. Instead, they were used for searchlights.

(3) Foucault's real innovations were in methods for testing the mirrors. He essentially invented what we call the Ronchi test today, among other things. [Bob May correctly points out that the Couder/Foucault numerical knife-edge test that we use today is a serious modification of Foucault's original knife-edge test.] However, Foucault really was the one, I think, who realized that you needed to produce a parabola, or something very close to it, if you wanted to produce good mirrors, and was the one wo figured out way to guarantee that.

The author of that paragraph with SOO many errors was Amir Aczel, in the book "Pendulum: Leon Foucault and the Triumph of Science" which I got cheap at Daedalus books. I am generally astonished at the large number of errors that Aczel makes in every book that I have read of his. Why does he keep getting a pass on that? If nothing else, a science writer should get the facts right, no?

PS if you look at my webpage, the first link is to a translation of Foucault's article.

The points that several writers got right are:

(1) Speculum metal mirrors were probably only 2 or 3 times as dense as glass.

(2) The really huge disadvantage of speculum metal mirrors was not - despite what either Foucault or Aczel said - that they were too heavy or would 'collapse the telescope' (what a completely nutty idea), but that they didn't reflect very much, tarnished quickly, and once they tarnished, had to be polished and resurfaced in an extremely time-consuming manner that essentially meant that one was re-figuring the original mirror all over again. (Just imagine how many telescope mirrors would be in customary use today if you had to go through all of the laborious steps of figuring your mirror every 6 weeks!). With a glass mirror that had a chemically silvered and mechanically smoothed surface, the tarnished reflecting layer can be removed chemically as needed when it becomes not reflective enough, and a new reflective layer can be put on without changing the figure of the mirror in the slightest. But, as I said, this idea was not original with Foucault.

(3) Azcel doesn't understand reflecting telescopes at all.
Guy



Guy Brandenburg <gfbrandenburg at yahoo.com> wrote: Boys and girls,

How many factual and conceptual errors can you spot in this passage, written about Leon Foucault by a writer who is often considered to be one of the major science/math popularists of the day? (The spelling mistakes, if any, are all mine.)

"Foucault's great discovery in the area of astronomical instrumentation was a method of silvering the mirrors for reflecting telescopes. Reflecting telescopes that had been made up to that time used bulky, heavy mirrors. This limited the potential size of these telescopes because the weight of the mirror could collapse the telescope. Foucault inaugurated a new method of applying a layer of silver directly to the front of the telescope's mirror, rather than a mercury amalgam that was typically applied to the back of the mirror. Foucault's telescopes built this way were lighter and of better light-gathering quality than earlier telescopes, as evidenced by their use today, a century and a half after his time."




Guy Brandenburg, Washington, DC
My home page on astronomy, mathematics, education:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
or else 
http://tinyurl.com/r6fh2

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