The message
from Anthony E Anson contains these words:

The message
from Janet Baraclough contains these words:

I wash my African violets under the tap (tepid water not cold) to
rinse dust off their hairy leaves, and they don't seem to mind at all.
You can probably get rid of the greenfly after a few washes. Just let
the plants dry off out of the sun so the water drops don't make scorch
marks.

Scorch marks due to droplets of water is an old - er - partner's tale,
Janet. You cannot concentrate the sun's rays (either direct or diffused)
with a sphere or an hemisphere.

I remember Geoffrey Smith pointing that out on GQT ages ago, and
thinking about it, he was quite correct.

Sometimes real life contradicts physics theory :-).I have a
collection of those clear solid glass spheres with a few air bubbles
inside the glass. When we moved here I set them on a sunny south
windowsill, until I found that when the sun shines through the window
and through the glass balls, there's a focussed hotspot; hot enough that
you wouldn't want to keep your hand there for long.

Janet