I've just got myself a Rocket blower to clear the sensor on my newish 30D of
a couple of dust spots. I've only got one lens so there hasn't been much
opportunity for muck to get inside as the lens has never been off the
camera. However, when I was peering inside I did notice that on the mirror
there is a central square/rectangle that is clear and spotless but all
around the edges for a few mm there is a sort of clouded appearance, like
the gradual film of muck that accumulates inside a car windscreen. I haven't
noticed a problem through the viewfinder but I wondered whether there was a
good reason for this murky appearance or whether I should attempt to clean
it off. Whatever it is does not get shifted by blowing - it looks like some
physical contact is required to shift it. But I have no idea what the mirror
is made of and whether I risk scratching it. I don't have any sensor
cleaning pads but I wondered if I should try an ordinary lens brush. Should
I leave well alone or is it safe to try to clean the edges, if indeed it is
even necessary?

Thanks,
Tim.