Microscope cover glass comes in different thicknesses designated numerically. Larger numbers correspond to thicker glass:

Number	Theoretical thickness	Range*
#0	100 μm	80-130 μm
#1	150 μm	130-170 μm
#1.5	170 μm	160-190 μm
#2.0	220 μm	190-250 μm

Source: https://microscopy.duke.edu/guides/coverslip-thickness

* Some vendors specify tighter tolerances