When I compare the images from my right and left eyes, by covering each in turn, one is higher vertically than the other. It is the same with or without glasses. I normally compensate for this quite adequately, but when I am tired - after a long walk - the images separate. Is this common? Can glasses be made to correct this, such as with a prism-like deflection?
Sounds like you might have hyper- or hypophoria. This means that the eyes don't quite align vertically, but only sometimes, like when one is covered up. (So its different to a permanant deviation where the eyes are always out of line, when someone has a squint) It can lead to occasional double vision especially when you're tired or have been reading for a long time. Often prism in your glasses can help so I would go and see your Optometrist for another eye test and see what they think.
Excellent and accurate reply.
I have nothing to add to that.
Thanks very much for your reply - now I know how to describe it when I have my eyes tested.