Thats what I suspected, the sets in the house are in sixes and 30 watts must be sufficient. There must be instances where smaller numbers are used, some research on domestic light dimmers is worth doing. The dimmers were originally designed for tht 50 watt halogen lights which were used long before LED’s were invented. I believe it was a Japanese researcher who first managed to get the white LED when others doubted if the mix of colours could ever be manufactured.

Edit: you prompted me to check up on modern dimmer switches, and I found they are 3-250W.

I have no interest in GU4 bulbs. I am interested in using a GU10 bulb with the appropriate fitting together with the correct dimmer switch and the earth connections for the microscope body will be retained. How would you know what fitting the microscope uses for its 12V 100W bulb, I did not mention one.

I wanted to rid myself of the 60’s electrics as the bulb gets very hot and the transformer hums. I would use a domestic wall mount dimmer switch removed from the plate which would fit easily either where the on off switch fits or where the reostat for the voltage adjustment goes. The gu10 wiring couldn’t be simpler, live to C, L1 to the bulb and from there to neutral. I might keep the original on-off switch on the live side to isolate the dimmer completely when not in use, but that might be overkill. The Screwfix LED dimmer switch 94816 lookslike a suitable one for a single GU10.

I have made adaptors to use cheap LED torches in my Olympus BHC which has both epi and normal condenser light paths. Works well enough for me, and stays closer to white rather than a filament bulb. The torches I use have 18650 lithium cells and three light levels along with the usual beam focussing and charge directly from USB C.