It will just become blurry. The resolution of a light microscope is around 0.2 μm, or 200 nm. This means that it cannot distinguish two points closer than 200 nm. One nm, or nanometre ...

You should not look through the microscope to do this. Line it up so that the specimen - if you can see it - is in the centre of the stage, where the light passes through. Focus the slide by ...