Distance of eye from lenses and their thickness

This might be the point being made by a diagram in a 1960s physics textbook I was looking at in a second bookshop recently (unfortunately, it had gone yesterday when I went back to buy it): 

Even if one could see the  objects on the slide – ie, if one’s view were not obviously focusing on something within the microscope – they would appear smaller than one viewed by the eye alone because the effect of looking at something from a distance through glass is to reduce,  rather than increase, its size, the more so the greater the thickness of the glass, I assume again because of the effect of the glass barrier on focal length.