Maṣqūlah is a passive participle from the root ṣ-q-l (ص ق ل), meaning polished, smoothed, or made lustrous. Sajanjal refers to a mirror or shining surface. The two expressions work within one sensory field: smoothness, polish, reflected light.
The particle ka- introduces a simile. The compared subject is the upper-chest surface, the comparator is the polished mirror, and the intended shared quality is brightness and smoothness—not every property of a physical mirror.
Identifying the specific point of likeness is central to sound rhetorical analysis. A simile does not transfer every conceivable trait of its comparator to the subject.