Ben Birdsey wrote:

>         Being one of those physicists, I can tell you that an ordinary
> spherical glass lens lens *will not* actually produce a parallel beam.
> Spherical lenses are actually just an approximation.  This might
> surprise you, but it's true.
>
>         If you really want a parallel beam, use a reflective parabola like
> David Fontaine suggested.
>
>         Ben
>         <><

Spherical aberration is one of the reasons people didn't trust Gallileo's
telescopes.  Philosophers liked to work with pure shapes, so it was commonly
known that glass distorts the truth.  I think the answer might be in a book
down the hall on optics, but I don't feel like looking, because my experience
tells me that even when you know you have seen the answer before, you can't
find it again without looking for a least a few hours.

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