Working with high-magnification eye-pieces My telescope (Newtonian reflector) has a focal length of 900 mm. So, using a 25 mm eye-piece, I get 900 mm / 25 mm = 36X magnification. Using a 10 mm eye-piece, the magnification would be 90X.

My problem is that I cannot see a thing with the 10 mm eye-piece. If I would insert the 25 mm eye-piece, find a bright star, and adjust the eye-piece distance from the secondary mirror, I would get the things ready after some trial and error. With the higher magnification eye-piece, it's just all black. What should I do to get around this?

The aperture is 130 mm if it matters.
Try the moon first.

If you see nothing but black, assuming that you don't have a lens cap on or something, then most likely, you are zoomed in on, well, relative blackness. The star you were viewing is probably off to the side now. Or, you may just be looking through the eye-piece at the wrong angle or something. The moon is too big a target to miss, and you have nice rough features to use to tune in your focus. And if you see black when you are looking at the moon, it's much easier to troubleshoot. You should definitely see something!

Now, secondarily the reason you expect to see something, but instead see nothing may be a viewfinder problem. If you have a small, low magnification viewfinder scope attached to your telescope, you usually can't just trust that what you see in the center of that view is exactly what your telescope will be aimed at. You may have to align it. Search "align telescope viewfinder" for help with this. From your description, it sounds like you are not using a viewfinder, but this seems to be a common problem, so I thought I should mention it.