I haven't tested what I've been saying about microscopes much with own because I find it stressful for some reason (and I can't light it internally although I may be able to get that fixed). However, I think, looking into it now (lit from the mirror), what one is seeing is the lenses of the microscope as well as the lenses of the eye. The squinting alters the focal length, bringing the lenses of the eye and within the microscope into prominence rather than the translucent material on the glass slide. This seems to be confirmed by the fact that when I remove the slide the image remains the same.