Mirth is short and transient, cheerfulness fixed and permanent† Mirth is like a flash of lightning that breaks through a gloom of clouds, and glitters for a moment: cheerfulness keeps up a kind of day-light in the mind, and fills it with a steady and perpetual serenity.

-Addison,Joseph
  In The Spectator, no.381,17 May.

A little while and I will be gone from among you, whither I cannot tell. From nowhere we came, into nowhere we go.What is Life? It is a flash of a firefly in the night. It is a breath of a buffalo in the winter time. It is as the little shadow that runs across the grass and loses itself in the sunset.

-Haggard, Sir (Henry) Rider
  Dying words of the  African chief Umbopa in King Solomon's Mines.  John Peter Turner in The North-West Mounted Police (1950) credited them to Crowfoot (c.1830^1890), chief of the Blackfoot Indians, who died in his teepee overlooking the Bow River,  Alberta, 25  Apr1890, and this attribution gained popular acceptance.

Roughly speaking it was the sensation of being at the centre of an explosion† I fancy you would feel much the same if you were struck by lightning. I knew immediately that I was hit, but because of that seeming bang and flash, I thought it was a rifle nearby that had gone off accidentallyand shot me.

-Orwell, George pseudonym of  Eric Arthur Blair
  Of his wounding during the Spanish Civil War. Homage to Catalonia, ch.12.

We will watch the sun set againömany times, and perhaps we'll see the Emerald Drop, the green flash that brings good fortune.

-Rhys,Jean pseudonym of  Ellen Gwendolen Rees Williams
  Wide Sargasso Sea, pt.2.

It came by a lightning flash like knowledge from the gods.

-Wilson, Edward O(sborne)
  On the1953 discovery of the DNA molecule. Sociobiology.