lenticle

[len-ti-kuh l] /ˈlɛn tɪ kəl/
noun
1.
a window in a clock case revealing the motion of the pendulum bob.
Origin
< Latin lenticula lentil, equivalent to lenti- (stem of lēns; see lens) + -cula -cle1
British Dictionary definitions for lenticle

lenticle

/ˈlɛntɪkəl/
noun
1.
(geology) a lens-shaped layer of mineral or rock embedded in a matrix of different constitution