The vital elements are often momentary, change-sent things...

The vital elements are often momentary, change-sent things ... a gleam of light on water, a trail of smoke from a passing train, a cat crossing the threshold. Sometimes they are a matter of luck, sometimes of patience, waiting for an effect to be repeated that you have seen. It is usually some incidental detail that heightens the effect of a picture, stressing a pattern, deepening the sense of atmosphere.