Guide below describe how to set up 38mm condenser on Dorsey Metrology 16H optical comparator.

Insert a 2mm hex wrench down into the top of the current optical comparator condenser ring and loosen the setscrew holding the 51mm condenser. You can access the setscrew by removing the optical comparator lamp house cover and inserting the hex wrench through the small notch in the front of the lamp house.

Remove the 51mm condenser and install the 38mm condenser (with adapter ring) in its place. The convex side of all Dorsey condensers face towards the comparator. Once in place you will need to adjust the optical comparator condenser for best focus. To set the condenser focus hold a small mirror about 3 inches in front of the condenser. Turn the lamp on and slide the condenser in or out until you can see a sharp image of the bulb filament focused on a wall or ceiling about 25 feet away (further away is ok). Once the bulb filament is in focus tighten the set screw. The condenser is now ready for use.

You can use this focusing procedure to re adjust the 51mm condenser if it is switched back.