Here's why:
* Metonymy is a figure of speech where a word or phrase is used to refer to something else that is closely associated with it.
* In this case, "eyes" are being used to represent the whole person, specifically their soul.
* The "scintillating" quality of the eyes is meant to convey a deeper emotional state or characteristic of the person's soul.
Essentially, the phrase uses the physical attribute of the eyes (scintillating) to represent a more abstract and internal quality of the person (their soul).