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2) The cast-versus-ground argument is very weak. For years it has been
possible to cast optically-perfect surfaces. Many eyeglass lenses are
cast, and there are other important technological applications for
precision casting.
Any old piece of lumpy glass with a smooth surface (a cast drinking
glass, for example) ... After all, one can still see an image through a
cast drinking glass, though a distorted one.
An excellent point. But I wonder how much of that has to do with the
adaptive focusing powers of the eye? When I try to use a lumpy drinking
glass to form a recognizable image on a passive image-plane (like an index
card), it doesn't work very well.
[Leigh]For a plastic lens this means it can be formed from sheet stock.
If the plastic is good enough to implement micron-scale
figuring _within_ a
given ring, is it not good enough to implement micron-scale control over
the step height?
Well, that is a question you can think about if you know something
about creep or thermal expansion, or if you have ever seen a
Fresnel lens made of a flexible sheet material that, because it is
nonrigid, can't possibly be held to micron tolerances across its
lateral extent.