、。
As it grew lighter, things gradually became visible.
Literal
Bright become-begin [and], things [subject-が] gradually become-visible-become.
Chains two ~なる constructions: 明るくなりはじめて ('begin-to-become-bright') and 見えるようになった ('come-to-be-visible'). The pattern [verb]ようになる expresses 'come to do/be' — marking a gradual onset of capability or habit. Japanese loves these 'become' layers for expressing gradual change.