• Not adjusting the oculars and
• Having the condenser too far down.

Keep in mind that these scopes are used by lots of students, so yours is constantly being readjusted or otherwise messed with. The previous persons eyes are almost certainly not going to be the same as yours.

Here's what to do:

• Put a slide on the stage.
• Adjust the intensity of the light source to a point that is comfortable for you. Don't fry your retinae.
• Adjust the distance between the oculars so you see the image as a circle.
• Close your left eye and use the coarse and fine focus knobs to bring the image into sharp focus.
• Now close your right eye and open your left and twist the ring on the adjustable, left, ocular to focus the image.
  • Don't touch the coarse or fine focus knobs.

• Now both eyes should be seeing the image in focus, and it will stay that way as you change objectives and slides.

Now and again someone will have messed up the condenser.