DW> Going back to a topic we discussed here a while ago...

 DW> I had a check-up by my eye-doctor today. He decided that I
 DW> need stronger glasses, so we discussed options. I asked him
 DW> what he thought about photochromic lenses.

 DW> He said he didn't like them, and wouldn't use them himself.
 DW> The glass ones, which are still obtainable here, have to be
 DW> thick and heavy. The plastic ones depend on ultraviolet light
 DW> to darken them, so they won't work, for example, in a car,
 DW> since the glass of the windshield and windows filters out the
 DW> UV.

 DW> I hadn't heard of this problem before.

It's an easily-observed phenomenon for those of us with photochomic lenses.
 Inside the car, the UV is filtered by the car's window glass.  Plus, it's
rare that direct sunlight hits the lenses, so they wouldn't fully darken,
anyway.

 DW> So I'm sticking with my magneted-on shades.

Clip-ons are good idea (or, as you have, magneted-ons).  But, they don't
have to be an alternative -- they can be used with the photochromic lenses.

I have photochromic progressive lenses.  I have a cheap pair of polaroid
clip-ons that cost me $8 US at a drug store.  They stay in my wife's car. 
I'll put them on when I'm riding in or driving her car.

I also have a pair of glasses with polaroid lenses used for driving only
(no bifocal).   They stay in my car at all times, and I use them only when
I'm driving or riding in mu car.

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