We have an expensive compound light microscope at work and for some reason only the 100X objective will not focus completely. I've tried switching out the objective with a new one and experience the same problem, which leads me to believe it is the microscope. However, how would only the 100X objective be blurry if all the objectives use the same optical path? One thing I noticed recently is that the small glass lens on the tip of the fiber optic light source has a speckled appearance to it, like tempered glass when it shatters. The set screw on the microscope could very well have been overtightened thereby shattering the lens. But would a refracted light source cause higher magnification images (e.g. 100X) to be out of focus? Or maybe not enough light is present with the shattered lens? The image appears more than bright enough though, it just won't focus completely.

 

this may be a silly question, but you are using oil immersion at 100x? because without oil image will be blurry at this magnification.

 

Not related but last week I was on a cruise ship. Pulled out a pair of binoculars from my backpack to look at the shore in the distance. Couldn't see a thing because the lenses were all fogged up. The backpack just came from the air-conditioned cabin while the outside humidity was high. The temperature of the binoculars was well below the dewpoint of the surrounding air.

Have you checked that all the optics are clean and not damaged?

 

Vibration???????