It looks like your microscope may have only 3 objectives. If the lowest power is 10X, which is common for that configuration, you might be well served to get a much lower power (e.g., 2X or 3X) and replace the oil immersion objective. Do save the latter in case you ever want to use the scope as a transmission microscope again for such specimens as blood smears.
How do you plan to get that ring lamp fitted to that scope?