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

[Bob May]

The mirrors of that time were, of course, of speculum metal.
Thier problem wasn't so much weight as the speculum metal isn't
that much heavier than glass is (you may argue that point if
desired but speculum isn't tten times heavier than glasss is_ but
rather that the metal tarnished rapidly and thus needed to be
reworked to gain back the reflectivity and that meant working the
metal itself.  The silvering process made it possible to put on
and remove a surface of high reflectivity that didn't affect the
shape of the glass which is where the great improvement was made.
In addition, prior to Foucault, the backs of the mirrors weren't
silvered as glass wasn't being used but rather the speculum
metal.
Next, telescopes weren't made delicate enough to "collapse" if
handled slightly roughly as implied by the author.  That is just
general bad engineering practice.
Finally, the silvering process isn't used anymore today as
aluminum coatings have superceeded that process.
Foucault also devised a test where he could test a surface for a
near paraboloidal shape without having to wait for night and do a
star test on the mirror.  He did this by testing the mirror
surrface with the light source at one focus of an ellipsoid and
looking at the image at the other focus of the illipsoid.  He
never really did the test today that we call the Foucault test!
That was done later by others who fully understood the
implications of what Foucault did.
Bob May
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