You can measure the size of your fully dilated pupil by holding a ruler next to your
dark-adapted eye and taking a picture of your reflection in a mirror.
But the light from M27 is mainly from narrow emission lines (nearly pure colors), which affect what the
dark-adapted eye perceives.
Plastic tarps, canvas, denim, duck cloth, and similar materials may look dense by day, but at night with
dark-adapted eyes it's surprising how much light gets through them.
But with
dark-adapted eyes and using averted vision, you should be able to see several stars close together - perhaps even the seven main bright stars which give the cluster its name.
Then, to our
dark-adapted eyes, we would see them glow in a variety of colors.
With nothing more than
dark-adapted eyes, a red flashlight, and a star map from this magazine, you can begin to enjoy astronomy by learning to identify the constellations.