I use glass to glaze locos and coaches, but not microscope slides, they're far too thick. Microscope slide cover slips are what you need and they come in various thicknesses - mine are all 0.13 - 0.17mm, or so sez the label on the boxes. They cut easily with a diamond pen on a piece of MDF, and with a little practice circular spectacles and radius-cornered windows come easily. I reckon my success rate is now well over 70%.
My two preferences for sticking them are impact adhesive, but you've got to master the art of twiddling the stringy bits with a cocktail stick to keep it away from the centre of the glass, or canopy glue which I've found works just as well on glass as plastic glazing (and sticks well to painted metal). The advantage with the latter is that it's easy to remove any excess that gets on the visible part by leaving it to set for 20 minutes or so, then with a cocktail stick sharpened to a chisel shape and lightly moistened with water, you can scrape it all away. One big advantage of glass is if you get the weathering spray on the glass, a thinners-moistened cotton bud will wipe it away, and if you get the action right, it looks as though it's been done by the little people.
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