blank (n.) 1
bull's-eye, target centre; or: line of sight
Ham IV.i.42 [Claudius to Gertrude] As level as the cannon to his blank
KL I.i.159 [Kent to Lear] let me still remain / The true blank of thine eye
Oth III.iv.124 [Desdemona to Cassio, of Othello] I have ... stood within the blank of his displeasure / For my free speech!
WT II.iii.5 [Leontes alone, of Polixenes] the harlot-king / Is quite beyond mine arm, out of the blank / And level of my brain