Ange20, not all condensers/ setups can do darkfield with a patchstop with a 40x objective... so dont panic (achieving this is often more of a 'sport' than a routine method)
I think is the alignment of the objective?? How can I Check it??
as above ...i.e. find a bit of something on a slide ..centre it into the middle of the view with the 10x and see if it is in the view of the 40x (?) if it isnt , centre it in the view of the 40x , switch back to the 10x and make an estimate of how far across the view it is (in a perfect world it should be in the middle).
If it is badly out; check that all your objectives are tight in the nosepiece and that the nosepiece doesnt 'wobble' noticably, also check that the revolving nosepiece is changing objectives with some definite 'certainty' ...old grease can get a bit thick and then the position selected by the nose can become more (sometimes too) uncertain.
As a check of the the alignment of your condenser to your lower power objectives, focus on the top of a blank slide with a 4x then close the iris on the condenser and then lower it ...you should at some point see the 'hole' of the closed iris diapragm in focus ... and it should be pretty much in the centre of your view... is it? You might be able to do this with the 10x too.
These suggestions are not specific to your microscope, others will know much more about what is likely to go out of line !
