Ian, another solution (in electrics as in mechanics, there is always at
least two solutions), pick up a heavy-duty wire-wound potentiometer of about
75 to 100 ohms and rated at 25 to 50 watts. The one you want is going to be
less than a couple of inches in diameter and maybe an inch deep, not
counting the shaft that allows you to adjust the resistance (in a past life,
I used these things a lot and I think the brand was "Ohmite", made of
ceramic, heavy wire and fused glass). The Ohmite (or equivalent) should be
available from any good electronic component supplier of electronic parts.

Just wire it in series (one end to the wiper) with your problem lamp and
adjust the brightness as you please. Mount it on any convenient surface (if
I remember, it'll mount thru about a 3/8 dia. hole and is secured with a
nut. Add a standard knob and that way, you can twist the knob to get any
intermediate resistance value to set the brightness exactly as you want it
without having to compromise - because fixed value resistors only come in -
what else - fixed values, and might force you to compromise on either too
dim or too bright.

The thing will get warm but so will a fixed resistor but by using the
wattage rating above, it'll last forever.

Cheers,

Dan Schaefer