By degrees
Definition 1 of 1
Pro Tip 1/3Subtle Progress
Use 'by degrees' to emphasize gradual, almost unnoticeable change or improvement. 
The days grew longer by degrees as summer approached.
Pro Tip 2/3Non-Instantaneous
Implies that the change is not instant, but takes time and patience. 
Her fear subsided by degrees, allowing her to speak more confidently.
Pro Tip 3/3Emotional Changes
'By degrees' can be used to describe changes in feelings or attitudes over time. 
He fell in love by degrees, his affection growing stronger every day.