"Don't Fire Until You See the Whites of their Eyes."
Muskets were not very accurate over distance, so with their limited ammunition, soldiers were encouraged to wait for the enemy to advance closer before opening fire. How frightened must they have been with those columns of Redcoats stomping toward them!
The perspective of the Continentals must have frightened them even more. As the British marched in the distance, they must have appeared quite small, but now that they approached, they must have seemed much larger. With the details of their uniforms coming into focus, the Continentals could have notices the buckles and buttons, the trim and the tricorn hats. I'm not convinced they would notice the whites of their eyes unless they were reminded to look for them before firing.
Speaking of looking for the details, the teacher will describe a little sketching project for you that will have you looking for the details.
While We're on the Subject