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

[David Weinshenker <daze39@earthlink.net>]

Guy Brandenburg 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."

Hmmm... well, Foucault is generally credited with introducing the
use of front-silvered glass mirrors (as well as with the method of
testing an optical system by observing the shadows while partially
obstructing the beam with a knife-edge near the focus), but the
previously dominant method of mirror construction for telescopes
was the use of solid metal mirrors made of "speculum metal", an
alloy of copper and tin.

The biggest limitation of those metal mirrors was not their size and
weight (large telescopes, such as Lord Rosse's 72" Newtonian reflector
and the 48" Cassegrain-type instrument installed at Melbourne in the
mid-1800's, had been built with metal mirrors), but the fact that the
reflective character of the surface was provided by the polished
condition of the metal itself - so that renewing the reflectivity
of the surface once it became tarnished and darkened required that
the work of final testing and figuring be repeated with each
re-polishing, and if this could not be accomplished with the
same precision as the initial figuring, the optical performance
of the instrument would be lost. (This, for example, was the fate
of the 48" Melbourne telescope.)

With a coated glass mirror (using either the chemically deposited
silver coatings, or the vacuum-deposited aluminum coatings that
eventually replaced them), the reflectivity is provided by the
coating rather than the figured surface of the substrate itself,
If the coating eventually deteriorates, it can be etched away and
renewed without disturbing the figured surface of the substrate,

-dave w

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