Before revision: Color schlieren image of the thermal plume from a burning candle, disturbed by a breeze from the right Schlieren (from German; singular "Schliere" , meaning "streak") are optical inhomogeneities in transparent material not necessarily visible to the human eye.
After revision: Color schlieren image of the thermal plume from a burning candle, disturbed by a breeze from the right Schlieren (from German; singular : schliere , meaning "streak") are optical inhomogeneities in transparent medium that are not necessarily visible to the human eye.