I'm not quite sure, but I think what you are calling the "third port" may be the photo tube. If that is correct, I think I see a knob on the right side of the head (if that is the correct term) of the microscope. I'm going to guess that if you pull that out, you will be able to see.

As far as your adjustment of the interpupillary width is concerned, you said you adjusted it for comfort, then you were getting double images. Have you tried adjusting it so the two images become superimposed into a single image?

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John Karlsson
Hope Valley, RI

Don't count your chickens until they have burned their bridges behind them.