Lonesome Dove, Volume 2: Comanche Moon by Larry McMurtry

WITH NO WAY to shade his pupils, Scull began to pray for rain-or, if not rain, at least a cloud, anything that might bring his eyes relief. Even on cool days the white light of the sun at noon brought intense headaches. The light was like a hot needle, stabbing and stabbing into his head. Rolling his eyes downward brought a few moments of relief, but not enough-day after day the white light ate at his optic nerve. Even though he heard the caballero Carlos Diaz tell Ahumado that the Texans had agreed to send the cattle for his ransom, Scull felt little hope. He might be blind or insane before the cattle arrived; besides, there was no certainty that Ahumado would honor the ransom anyway. He might take the cattle and kill the Texans-if he respected the bargain it would be mere whim.

From the noon hour each day until the sun edged behind the western cliffs, Scull felt himself not far from madness, from the pain in his eyes. The only thing that saved him, in his view, was that the season was young and the days still fairly short; also, Ahumado had pitched his camp in a canyon, a deep slot in the earth. In the canyon the sun rose late and set early; it only burned at his eyes for some six hours a day, and often spring thunderheads drifted over the canyon and brought him some minutes of relief.