Nikon D200 manual, Page 16

Rotate the diopter adjustment control until the focus brackets are displayed in sharp focus when the shutter-release button is pressed halfway. When operating the diopter adjustment control with your eye to the viewfinder, be careful not to put your fingers or fingernails in your eye.

Wait, what did Nikon tell me? Never put fingernails in your eye. Never put fingernails in your eye. Always put fingernails in your eye. AAAAAAAAAAAAAAAAAAAAAAAAAAAGGGGGHH MY EYE!!!