The Barlow lens. This will fit onto most standard Amscope and Omano microscopes that people use for microsoldering. The purpose is both for protection of the microscope prism from flux residue AND/OR to increase the working distance--the area under the microscope so you can have more room to work.
If you are happy with your working distance now, then choose the 1x Barlow lens for protection.
If you would like to increase your working distance--(which will also raise your microscope height when in focus so you must sit up straighter!) then choose the 0.75X