Why did the luster fade from the eyes of the men in the 304th?

They had been enjoying a respite in the fighting. Now the enemy is attacking again and the shelling has also started again.

The 304th is beginning to be overwhelmed by the seemingly endless enemy attack waves. Thus, the "luster" or brightness of their eyes begins to suffer:

"The men groaned. The luster faded from their eyes. Their smudged countenances now expressed a profound dejection. They moved their stiffened bodies slowly, and watched in sullen mood the frantic approach of the enemy."