Lower down the sub-stage condenser, then remove the existing condenser and replace with MA914 Darkfield condenser applying immersion oil on top of the condenser's lens.
Adjust the center of condenser in the field of view with two adjusting screws by using regular 10X objective. Try to move Darkfield condenser up or down to find out the position to illuminate field of view most evenly bright.
Next, apply immersion oil to the specimen on the slide and change to 50X and 100X (oil) with iris diaphragm objectives. By using fine focus enables to see the specimen clearly. If it is still dark, make the light source brighter.
If there isn't enough light, adjust brightness and darkness with an iris on the objective (close iris for darkfield, and open it for brightfield).
NOTE: The most important thing is when you observe with regular 10X objective, the center of light in the darkfield should be centered in the specimen by using two centering screws.