Noticed when examining resists_magm() while working on anti-magic
traps: if you were polymorphed, you would be granted magic resistance by
keeping a cloak of same or gray dragon scales/mail in your quiver slot,
your alternate weapon slot, or main weapon slot (ie, by wielding it). And
you obtained light-based blindness resistance regardless of whether you
were polymorphed if you carried a potion of blindness in any of those slots.